Priručnik

Transcription

Priručnik

L220: Advanced Linux
System Administration II
course materials
originally released under the GFDL by LinuxIT
modified and released under the GFDL by University of Zagreb University Computing Centre SRCE
(“the publisher”)
University of Zagreb University Computing Centre SRCE
________________________________________________________________________________
Copyright (c) 2005 LinuxIT.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2
or any later version published by the Free Software Foundation;
with the Invariant Sections being History, Acknowledgements, with the
Front-Cover Texts being “released under the GFDL by LinuxIT”.
Copyright (c) 2014 SRCE.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2
or any later version published by the Free Software Foundation;
with the Invariant Sections being History, Acknowledgements, with the
Front-Cover Texts being “modified and released under the GFDL
by University of Zagreb University Computing Centre SRCE”.
see full GFDL license agreement on p. 133.
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________________________________________________________________________________
Acknowledgments
The original material was made available by LinuxIT's technical training centre www.linuxit.com.
The original manual is available online at http://savannah.nongnu.org/projects/lpi-manuals/.
The modified version of this manual is available at http://www.srce.unizg.hr/linux-akademija/.
History
CVS version 0.0 January 2004, Adrian Thomasset <[email protected]>.
Reviewed/Updated April 2004, Andrew Meredith <[email protected]>.
Review/Update May 2005, Adrian Thomasset <[email protected]>.
February 2014. Title: L220: Advanced Linux System Administration II (version 1.0). Revised and modified at
University of Zagreb University Computing Centre SRCE (“the publisher”) by Vladimir Braus.
Notations
Commands and filenames will appear in the text in bold.
The <> symbols are used to indicate a non optional argument.
The [] symbols are used to indicate an optional argument
Commands that can be typed directly in the shell are highlighted as below
command
No Guarantee
The manual comes with no guarantee at all.
3
________________________________________________________________________________
University Computing Centre SRCE
As the major national infrastructural ICT institution in the area of
research and higher education in Croatia, the University
Computing Centre SRCE is providing a modern, sustainable
and reliable e-infrastructure for research and education
community.
This includes computing and cloud services, high performance
computing, advanced networking, communication systems and
services, middleware, data and information systems and
infrastructure. At the same time SRCE acts as the computing
and information centre of the largest Croatian university – the University of Zagreb, and is responsible
for the coordination of the development and usage of e-infrastructure at the University.
Furthermore, by applying cutting edge technologies SRCE continuously enriches academic and
reserach e-infrastructure and its own service portfolio. This enables the active participation of Croatia
and Croatian scientists in European and global research and higher education area and projects.
Since its founding in 1971 as a part of the University of Zagreb, at that time the only Croatian university,
SRCE has provided an extended advisory and educational support to institutions and individuals from
the academic and research community in the use of ICT for education and research purposes.
From its beginnings, and still today, SRCE has been recognized as an important factor of the
development of modern e-infrastructure at the national level, participating in different projects and
providing services like Croatian Intenet eXchange (CIX).
SRCE has a 41 year old tradition of organizing professional courses from the field of ICT.
University Computing Centre SRCE
Josipa Marohnića 5
10000 Zagreb
Croatia
http://www.srce.unizg.hr
e-mail: [email protected]
phone: +385 1 6165 555
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Table of Contents
________________________________________________________________________________
Table of Contents
DNS .............................................................................................................................................. 7
1. Basic Bind Configuration ....................................................................................................... 7
1.1 The Logging Statement ....................................................................................................... 8
1.2 The Options Statement ..................................................................................................... 10
1.3 The Zone Statement ......................................................................................................... 11
1.4 The Access Control Lists (acl) Statement.......................................................................... 12
2. Create and Maintain Zone Files............................................................................................ 13
3. Securing a DNS Server ......................................................................................................... 15
3.1 Server Authentication ........................................................................................................ 15
3.2 DATA Integrity and Authenticity ........................................................................................ 17
MAIL AND LISTS ....................................................................................................................... 19
1. Using Sendmail ..................................................................................................................... 19
1.1 Configuration Settings ....................................................................................................... 19
1.2 Virtual Hosting................................................................................................................... 21
2. Configuring Mailing Lists ..................................................................................................... 22
2.1 Majordomo and Sendmail ................................................................................................. 22
3. Managing Mail Traffic ........................................................................................................... 24
3.1 Mail Filtering with Procmail................................................................................................ 24
WEB SERVICES ........................................................................................................................ 27
1. Implementing a Web Server ................................................................................................. 27
1.1 Installing Apache............................................................................................................... 27
1.2 Monitoring apache load ..................................................................................................... 27
1.3 Using Apachectl ................................................................................................................ 28
1.4 Basic Configuration Options .............................................................................................. 29
1.5 Restricting Client Access .................................................................................................. 31
1.6 Client Basic Authentication ............................................................................................... 31
2. Maintaining a Web Server..................................................................................................... 32
2.1 HTTPS Overview .............................................................................................................. 32
2.2 SSL Virtual Hosts .............................................................................................................. 33
2.3 Managing Certificates ....................................................................................................... 34
2.4 Virtual Hosts ..................................................................................................................... 35
3. Implementing a Proxy Server ............................................................................................... 37
3.1 Getting Started .................................................................................................................. 37
3.2 Access Lists and Access Control ...................................................................................... 37
3.3 Additional Configuration Options ....................................................................................... 39
3.4 Reporting Tools................................................................................................................. 40
3.5 User Authentication (using PAM) ...................................................................................... 42
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Table of Contents
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NETWORK CLIENT MANAGEMENT ......................................................................................... 44
1. DHCP Configuration ............................................................................................................. 44
1.1 Default DHCP Configurations............................................................................................ 44
1.2 Dynamic DNS ................................................................................................................... 46
1.3 DHCP Relay ..................................................................................................................... 48
2. NIS Configuration ................................................................................................................. 49
2.1 Master Server Configuration ............................................................................................. 49
2.2 Slave Server Configuration ............................................................................................... 50
2.3 Client Setup ...................................................................................................................... 50
2.4 Setting up NFS home directories....................................................................................... 51
2.5 Basic NIS Administration ................................................................................................... 51
3. LDAP Configuration .............................................................................................................. 52
3.1 What is ldap ...................................................................................................................... 52
3.2 OpenLDAP server configuration ........................................................................................ 53
3.3 Client configuration files .................................................................................................... 54
3.4 Migrating System Files to LDAP ........................................................................................ 54
3.5 LDAP Authentication Scheme ........................................................................................... 57
4. PAM Authentication .............................................................................................................. 59
4.1 PAM Aware Applications ................................................................................................... 59
4.2 PAM Configuration ............................................................................................................ 60
SYSTEM SECURITY .................................................................................................................. 61
1. Iptables/Ipchains ................................................................................................................... 61
1.1 The Chains ....................................................................................................................... 61
1.2 The Tables ........................................................................................................................ 62
1.3 The Targets ...................................................................................................................... 62
1.4 Example Rules .................................................................................................................. 63
2. Differences with Ipchains ..................................................................................................... 64
3. Security Tools ....................................................................................................................... 65
3.1 SSH .................................................................................................................................. 65
3.2 LSOF ................................................................................................................................ 67
3.3 NETSTAT ......................................................................................................................... 68
3.4 TCPDUMP ........................................................................................................................ 68
3.5 NMAP ............................................................................................................................... 70
Vježbe (Exercises) ................................................................................................................... 73
GNU Free Documentation License ........................................................................................ 133
6
DNS
________________________________________________________________________________
DNS
NOTICE
Computer name resolution can be performed in a number of ways, including /etc/hosts
file and DNS.
/etc/hosts file is a convenient way to manage name resolution for a small number of
computers, such as a small home network with just two or three machines. /etc/hosts
must be updated on every computer on a network whenever any machine’s name or IP
address changes or whenever a computer is added to or removed from the network.
In addition to /etc/hosts and DNS, several other name resolution systems exist, including
Network Information Service (NIS), Windows Internet Name Service (WINS), and more.
1. Basic Bind Configuration
The configuration file for a Bind server is /etc/named.conf. This file has the following main
entries:
Main entries in named.conf
logging
Specify where logs are written too and what needs to be logged
options
Global options are set here (e.g the path to the zone files)
zone
Defines a zone: the name, the zone file, the server type
acl
Access control list
server
Specific options for remote servers
Let's look at a typical configuration file for a caching only server. We will add entries to it as
we go to create new zones, logging facilities, security, etc.
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DNS
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Skeleton named.conf file
options {
directory "/var/named";
datasize 100M;
};
zone "." IN {
type hint;
file "named.ca";
};
zone "localhost" IN {
type master;
file "localhost.zone";
allow-update { none; };
};
zone "0.0.127.in-addr.arpa" IN {
type master;
file "named.local";
allow-update { none; };
};
1.1 The Logging Statement
The syntax for logging is:
logging {
channel "channel_name" {
file "file_name";
versions number_of_files;
size log_size;
syslog < daemon | auth | syslog | authpriv | local0 -to- local7 | null >;
severity <critical | error | warning | notice | info | debug | dynamic >;
print-category yes_or_no;
print-severity yes_or_no;
print-time yes_or_no;
};
category "category_name" {
"channel_name";
};
};
The channel defines where logs are sent to (file, syslog or null). If syslog is selected then
the facility and the log level can be specified too.
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The category clause defines the type of information sent to a given channel (or list of
channels). The type of channel is given then the default logging facility is used
category default { default_syslog; default_debug; };
Example:
We choose not to use the syslog daemon and log everything to a file called “LOG” that will
be created in the same directory as the zone files (default /var/named/). For this we will
create the channel foo_channel. Next we want to log queries using this channel.
The entry in named.conf will look like this:
logging {
channel foo_channel {
file "LOG";
print-time yes;
print-category yes;
print-severity yes;
};
category "queries" {
"foo_channel";
};
};
Categories such as queries are predefined and listed in the named.conf(5) manpages.
However some of the names have changed since BIND 8, so we include as a reference
the list of categories for BIND 9 below:
BIND 9 Logging Categories
default
Category used when no specific channels (log levels, files ...) have
been defined
general
Catch all for messages that haven't been classified below
database
Messages about the internal zone files
security
Approval of requests
config
Processing of the configuration file
resolver
Infornation about operations performed by clients
xfer-in or xfer-out Received or sent zone files
notify
Log NOTIFY messages
client
Client activity
update
Zone updates
queries
Client Queries
dnssec
DNSEC transactions
lame-servers
Transactions sent from servers marked as lame-servers
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DNS
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1.2 The Options Statement
The global options for the server are set at the beginning of named.conf. The syntax is:
options{
option1;
option2;
....
};
We next cover the most common options.
version
Manpage says “The version the server
should report via the ndc command. The
default is the real version number of this
server, but some server operators prefer
the string (surely you must be joking )”
version " (surely you must be
joking) ";
directory
The working directory of the server
directory "/var/named";
fetch-glue (default yes) - obsolete
Prevent the server from resolving NS records (the additional data section). When a record
is not present in the cache BIND can determine which servers are authoritative for the
newly queried domain. This is often used in conjunction with recursion no.
notify (default yes)
Send DNS NOTIFY messages to the slave servers to notify zone changes (helps speed
up convergence)
recursion (default yes)
The server will perform recursive queries when needed
forward (only or first)
The default value is first and causes the sever to query the forwarders before attempting
to answer a query itself. If the option is set to only the server will always ask the
forwarders for an answer. This option has to be used with forwarders.
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DNS
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forwarders (list)
List of servers to be used for
forwarding. The default is an empty
list.
datasize
Limit the size of the cache
forwarders { 10.0.0.1; 10.0.0.10;};
datasize 512M;
allow-query (list)
A lists of hosts or networks that may query the server
allow-recursion (list)
List of hosts that can submit recursive queries
allow-transfer (list)
List of hosts (usually the slaves) who are allowed to do zone transfers
1.3 The Zone Statement
The syntax for a zone entry in named.conf is as follows:
zone domain_name {
type zone_type;
file zone_file;
local_options;
};
We first look at the local_options available. Some of these are the same options with the
same syntax as the global options we have just covered (with some additional ones). The
most common ones are notify, allow-transfer and allow-query. Additional ones are
masters (list of master servers) or dialup.
The domain_name is the name of the domain we want to keep records for. For each
domain name there is usually an additional zone that controls the local in-addr.arpa zone.
The zone_type can either be
master
slave
hint
stub
the server has a master copy of the zone file
the server has a version of the zone file that was downloaded from a
master server
predefined zone containing a list of root servers
similar to a slave server but only keeps the NS records
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DNS
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The zone_file is a path to the file containing the zone records. If the path is not an absolute
path then the path is taken relatively to the directory given earlier by the directory option
(usually /var/named).
Example master zone entries, allowing zone transfers to a slave server at 10.1.2.3:
zone seafront.bar {
type master;
file "seafront.zone";
allow-transfer{10.1.2.3;);
};
zone 2.1.10.in-addr.arpa {
type master;
file "10.1.2.zone"
allow-transfer{10.1.2.3;);
};
The next example is the corresponding named.conf zone section for the slave server,
assuming the master has the IP 10.1.2.1:
zone "seafront.bar" IN {
type slave;
masters {10.1.2.1;};
file "slave/seafront.zone";
};
type slave;
masters {10.1.2.1;};
file "slave/10.1.2.local";
};
1.4 The Access Control Lists (acl) Statement
Rather than use IPs it is possible to group lists of IP addresses or networks and assign a
name to this grouping.
Exmaple acl:
acl internal_net {10.0.0.0/8; };
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DNS
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There are built-in ACLs as follow:
any
all hosts
none
no host
localhost
all IP address for the local interfaces
localnets
network associated to the localhost interfaces
The Server Statement
This statement is used to assign configuration options for a specific server. For example if
a server is giving bad information it can be marked as bogus. One can also set the keys
associated with a server for hosts authentication when using DNSSEC (see section 4.
Securing a DNS Server)
2. Create and Maintain Zone Files
The format of the zone files is defined in RFC 1035 and contains resource records (RR) for
the administered domain or sub-domain.
The types of resource records are:
1 – Start Of Authority (SOA)
root-name TTL IN SOA name-server email-address (
serial number;
refresh;
retry;
expire;
minimum;
)
The SOA record includes the following details:
 the primary name server for the domain
 the responsible party for the domain
 a timestamp (serial number) that changes whenever you update your domain
 the number of seconds before the zone should be refreshed
 the number of seconds before a failed refresh should be retried
 the upper limit in seconds before a zone is considered no longer authoritative
 the negative result TTL (for example, how long a resolver should consider a negative
result for a subdomain to be valid before retrying).
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DNS
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The root-name is often replaced with an “@” symbol which resolves to the name of the
zone specified in named.conf.
Example:
$TTL
@
1D
86400
IN
SOA
ns.seafront.bar. root.seafront.bar. (
46
; serial (d. adams)
1H
; refresh
15M
; retry
1W
; expiry
1D )
; minimum
2 – Records defining the name-servers for this domain, NS records
domain-name IN NS name-server
Example:
IN
NS
ns
NOTICE
1. If the name of the domain is missing then @ is assumed
2. The fully qualified name of the name-server is ns.seafront.bar.. A host name that
doesn't end with a dot will automatically have the domain-name '@' appended to it.
Here for example
ns becomes ns.seafront.bar.
3 – Records defining the mail-servers for this domain, MX records
domain-name IN MX PRI mail-server
The PRI entry is a priority number. If several mail-servers are defined for a domain then
the servers with the lowest priority number are used first.
4 – Authoritative information for hosts on the domain, called A records
host-name IN A IP-address
Authority Delegation
When defining the name-servers responsible for another sub-domain additional NS
records are added as well as some glue records which are simple A records resolving the
DNS servers.
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DNS
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Example:
devel.myco.com.
ns1
IN NS
IN A
ns1.devel.myco.com
192.168.21.254
Reverse zone files
5 – Authoritative PTR records, resolving IP addresses
n
IN PTR
host-name
3. Securing a DNS Server
In 1995, following major security flaws discovered in DNS, a new topic called DNSSEC
was started within the IETF. This DNSSEC protocol is described in a sequence of three
draft documents known as RFC2535bis and proposes to handle server authentication as
well as data authenticity.
3.1 Server Authentication
DNSSEC attempts to handle vulnerabilities that occur during unauthorised dynamic
updates as well as spoofed master impersonations. These involve host-to-host
authentications between either a DHCP or a slave server and the master server.
The dnssec-keygen tool is used to generate a host key on the master server that can
then be transferred on a slave server. This authentication mechanism is call TSIG and
stands for Transaction Signature. Another mechanism is SIG0 and is not covered in these
notes.
Master Configuration
1. First generate the host key on the master server called seafront.bar:
dnssec-keygen –a HMAC-MD5 -b 256 -n host seafront.bar.
This will create the following public and a private key pair:
Kseafront.bar.+157+49196.key
Kseafront.bar.+157+49196.private
Notice:
These keys must NOT be inserted in the zone files (there is an IN KEY
section in the public key that is misleading, looks like a RR).
The public and the private keys are identical: this means that the private key
can be kept in any location. This also means that the public key shouldn't be
published.
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The content of the Kseafront.bar.+157+49196.key is:
seafront.bar. IN KEY 512 3 157 QN3vIApnV76WS+a2Hr3qj+AqZjpuPjQgVWeeMMGSBC4=
2. In the same directory as the server's named.conf configuration file create the file
slave.key with the following content:
key "seafront.bar." {
algorithm hmac-md5;
secret "QN3vIApnV76WS+a2Hr3qj+AqZjpuPjQgVWeeMMGSBC4=";
};
3. Apply the following changes in named.conf:
include "/etc/slave.key";
type master;
allow-transfer { key seafront.bar.; };
};
zone 2.1.10.in-addr.arpa {
type master;
file "10.1.2.zone"
allow-transfer { key seafront.bar.; );
};
Slave Configuration
Copy the slave.key file to the slave server in the directory containing named.conf. Add
the following server and include statements to named.conf:
server 10.1.2.1 {
keys {seafront.bar.;};
};
(this is the IP for the master server)
Troubleshooting
Restart named on both servers and monitor the logs. Notice that DNSSEC is sensitive to
time stamps so you will need to synchronise the servers (using NTP). Then run the
following command on the master server in the same directory where the dnssec keys
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where generated:
dig @10.1.2.1 seafront.bar AXFR -k Kseafront.bar.+157+49196.key
3.2 DATA Integrity and Authenticity
This aspect of DNSSEC is above the level of this manual and is simply a summary of the
concepts involved.
Data authenticity may be compromised at different levels.
The recognised areas are:
 altered slave zone files
 cache impersonation
 cache poisoning.
New RR records
The integrity and authenticity of data is guaranteed by signing the Resource Records using
a private key. These signatures can be verified using a public DNSKEY. Only the validity
of the DNSKEY needs to be established by the parent server or “delegation signer” DS.
So we have the following new RRs in the zone files:
RRSIG
DNSKEY
DS
the signature of the RR set
public key used to verify RRSIGs
the Delegation Signer
Signing Zone Records
These are the basic steps:
1. Create a pair of public/private zone signing keys (ZSK)
dnssec-keygen -a DSA -b 1024 -n zone
You should get two files such as these:
Kseafront.bar.+003+31173.private
17
seafront.bar.
DNS
________________________________________________________________________________
2. Insert the public key into the unsigned zone file:
cat
>> seafront.bar
3. Sign the zone file
dnssec-signzone
-o
seafront.bar
Kseafront.bar.+003+31173
You should see a message such as:
WARNING WARNING WARNING WARNING WARNING WARNING WARNING WARNING WARNING
WARNING
WARNING
WARNING
WARNING
WARNING This version of dnssec-signzone produces zones that are
WARNING
WARNING incompatible with the forth coming DS based DNSSEC
WARNING
WARNING standard.
WARNING
WARNING
WARNING
WARNING WARNING WARNING WARNING WARNING WARNING WARNING WARNING WARNING
seafront.zone.signed
This is due to the fact that the dnssec-signzone tool doesn't support the -k switch which
would allow to make use of a key signing key (KSK) which is then forwarded to a parent
zone to generate a DS record.
If you want to make use of this signed zone, change the filename in named.conf to
“seafront.bar.signed”
18
Mail and Lists
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Mail and Lists
A wide variety of SMTP servers can run on Linux. The most popular servers are:
Sendmail
This server has long dominated Internet mail delivery. sendmail has also earned a
reputation for a difficult-to-master configuration file format. Fortunately, tools to create a
configuration file from a simpler file are common.
Postfix
This server is comparable to sendmail in popularity. Postfix uses a series of small
programs to handle mail delivery tasks, as opposed to the monolithic approach used by
sendmail. Its configuration is much easier to handle than is sendmail’s.
Exim
This mail server is not quite as popular as sendmail or Postfix, but it is still a popular Linux
mail server. Like sendmail, Exim uses a monolithic design, but Exim’s configuration file is
much more intelligible. This server includes extensive pattern - matching tools that are
very useful in fighting spam.
1. Using Sendmail
Sendmail is a general purpose internetwork email routing facility that supports many kinds
of mail-transfer and delivery methods, including the Simple Mail Transfer Protocol
(SMTP) used for email transport over the Internet.
1.1 Configuration Settings
DNS Settings
1. We first want to make sure that mail will be sent to our machine. We assume that we
have properly configured a domain called seafront.bar with BIND 8 or 9. Let's make
sure that the zone file for this domain has an MX record pointing to our system.
For example if our machine is called test1 and has the IP 192.168.246.12 then
we need the following lines:
seafront.bar.
IN
MX 10
test1.seafront.bar.
test1.seafront.bar.
IN
A
192.168.246.12
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Mail and Lists
________________________________________________________________________________
2. Next we need to make sure that this information is read by the resolvers, so we add the
following at the top of the file /etc/resolv.conf:
nameserver 127.0.0.1
domain seafront.bar
Sendmail Settings
We go into sendmail's main configuration directory /etc/mail. Here we need to do the
following:
1. By default sendmail is configured to listen for connections ONLY for the 127.0.0.1
interface. In order to make sendmail listen to all interfaces we need to comment out the
following line in /etc/mail/sendmail.mc using 'dnl' which stands for “do next line”:
dnl
DAEMON_OPTIONS(`Port=smtp,Addr=127.0.0.1, Name=MTA')dnl
2. Once this is done run:
m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cf
Notice: Make sure /etc/sendmail.cf isn't also there, if it is, delete it.
3. Restart sendmail and try the following:
telnet test1.seafront.bar 25
Warning: If you get a connection then sendmail is responding. This doesn't mean that
sendmail will deliver mail (relay) for you!
3. To configure sendmail to relay for you you need to add the IP for your machine to the
/etc/mail/access file:
192.168.246.12
RELAY
4. Finally, we also need to tell sendmail to accept mail for @seafront.bar addresses.
For this, add the domain name to /etc/mail/local-host-names:
seafront.bar
Restart sendmail and send a mail to an existing user. If you have a user tux on the
machine then check the output of the following:
mail -v -s "test seafront domain" [email protected] < /etc/passwd
20
Mail and Lists
________________________________________________________________________________
1.2 Virtual Hosting
We want the server seafront.bar to accept mail for the city.bar domain. For this
we follow the following steps.
The DNS entries
We need to add an MX record for the city.bar domain. Here is the whole block for clarity:
seafront.bar.
IN
MX 10
test1.seafront.bar.
city.bar.
IN
MX 10
test1.seafront.bar.
test1.seafront.bar.
IN
A
192.168.246.12
Reload the zone file:
rndc reload
Sendmail Settings
1. We need to make sendmail accept mail for users at @city.bar. For this we add the next
line to the local-host-names file:
city.bar
If mail is sent to [email protected] and tux is a valid user on test1.seafront.bar
then mail will be delivered to the local user tux.
To avoid this we can use the /etc/mail/virtusertable database.
2. If you want to forward mail onto another account here are example entries for the
virtusertable database:
[email protected]
@city.bar
[email protected]
[email protected]
administrator
local-list
Here mail for user tux is diverted to [email protected], the user administrator is
the catchall account and lists are redirected to local lists (this needs to point to a valid
list defined in the aliases).
21
Mail and Lists
________________________________________________________________________________
2. Configuring Mailing Lists
2.1 Majordomo and Sendmail
Pre-installation Configuration
1. In the Makefile, replace /bin/perl with the path to the perl binary on your system (usually
/usr/bin/perl):
PERL = /usr/bin/perl
To make things easier we will leave the W_HOME as is:
W_HOME = /usr/test/majordomo-$(VERSION)
You need to create the directory /usr/test
mkdir /usr/test
Create a group called majordomo with GID 45, and add a user called majordomo with
UID 123
groupadd -g 45 majordomo
useradd -g 45 –u 123 majordomo
2. In the sample.cf file we need to define our domain (for example seafront.bar). This is
also where the path to the sendmail binary is set:
$whereami = "seafront.bar";
$sendmail_command = "/usr/sbin/sendmail";
Now we can run
make install
make install-wrapper
Finally you can test the configuration as suggested with the following:
cd /usr/test/majordomo-1.94.5; ./wrapper config-test
If all goes well you will be prompted to register to the majordomo mailing list. Since we
do not have a valid email address, answer NO to the question.
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Sendmail Configuration
The sendmail configuration involves adding appropriate entries in /etc/aliases for each
mailing list we create. But before that we need a symbolic link in /etc/smrsh pointing to the
majordomo wrapper binary, and here is why.
In order to limit the number of programs mail can be piped to (using a '| command' instead
of an email address) sendmail defines a set of commands known as “sendmail restricted
shells” or smrsh. The list of restricted shells is contained in /etc/smrsh which are symbolic
links to the actual binaries we allow mail to be piped to.
We will make the wrapper binary available, which is located in /usr/test/majordomo-1.94.5,
with the following:
ln -s
/usr/test/majordomo-1.94.5/wrapper /etc/smrsh
Before adding the entries to /etc/aliases we need to decide on a name for our first list, and
we choose ... test.
Remember that before sending mail to the list [email protected] we first need to subscribe
to this list by sending a mail to [email protected] with the contents subscribe
test. Some work needs to be done for this to work.
Creating the list “test” (as documented in NEWLIST):
1. Create an empty file called test and a file containing information about the list called
test.info in the directory /usr/test/majordomo-1.94.5/lists/
2. Create the following aliases in /etc/aliases:
majordomo:
test:
test-list:
test-request:
owner-test:
test-approval:
"|/usr/test/majordomo-1.94.5/wrapper majordomo"
"|/usr/test/majordomo-1.94.5/wrapper resend -l test test-list"
:include:/usr/test/majordomo-1.94.5/lists/test
"|/usr/test/majordomo-1.94.5/wrapper request-answer test"
tux
tux
3. Run newaliases and restart sendmail.
Majordomo Test
Send an email to [email protected] with the content:
subscribe test
If all goes well you will receive a response with further steps to be taken.
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3. Managing Mail Traffic
3.1 Mail Filtering with Procmail
Procmail is a program for filtering electronic mail. It is very useful for presorting and
preprocessing large amounts of incoming mail. You can use it to sort out mail from mailing
lists, to dispose of junk mail, to send automatic replies, or even to run a mailing list.
The Procmail is generally not started from the command line. It is usually invoked by mail
delivery subsystems (like Sendmail or Postfix) or from a mail retrieval agent (such as
fetchmail). The companion tool formail allows Procmail to be used in batch-processing on
mail that already is in a user's mailbox.
The primary use of Procmail is to filter messages into several mailboxes, based on the
headers. This filtering is done based on the rules set down in your ~/.procmailrc file.
Procmail is normally installed on most distributions by default. Run which procmail to
find out where Procmail is located (usually that is /usr/bin/procmail).
In depth information can be found in the procmail, procmailrc and procmailex
manpages. Here are a few examples taken from procmailex(5).
The Procmail agent uses recipes, to determine where to deliver the various mail
messages.
A promailrc file is a sequence of recipes of the form:
:0 [flags] [ : [locallockfile] ]
<zero or more conditions (one per line)>
<exactly one action line>
Each recipe that Procmail uses consists of flags, conditions and action. The next tables
cover the main flags, conditions and actions available.
Flags Description
H
Egrep the header (default).
B
Egrep the body
E
This recipe only executes if the immediately preceding recipe was not executed.
e
This recipe only executes if the immediately preceding recipe failed
w
Wait for the filter or program to finish and check its exit code
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Mail and Lists
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The conditions are extended regular expressions with the additional conditions below:
Conditions
Description
!
Invert the condition
$
Evaluate the remainder of this condition according to sh(1) substitution rules
inside double quotes, skip leading whitespace, then reparse it
?
Use the exitcode of the specified program
<
Check if the total length of the mail is shorter than the specified (in decimal)
number of bytes
>
Check if the total length of the mail is larger than the specified (in decimal)
number of bytes
The action line can start with one of
Action line
Description
!
Forwards to all the specified mail addresses
|
Starts the specified program
{
Followed by at least one space, tab or newline will mark the start of a
nesting block
Anything
else
interpret as a mailbox (file or directory relative to current directory or
MAILDIR)
Examples:
Sort all mail coming from the lpi-dev mailing list into the mail folder LPI:
:0:
* ^To.*lpi-dev
LPI
Forward mails between two accounts main.address and the-other.address. This rule is for
the procmailrc on the main address account. Notice the X-Loop header used to prevent
loops:
:0 c
* !^X-Loop: [email protected]
| formail -A "X-Loop: [email protected]" | \
$SENDMAIL -oi [email protected]
The c option tells Procmail to keep a local copy.
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Mail and Lists
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Filtering Spam
Spam filtering generally takes place using SpamAssassin. This program analyzes your
mail for its likely level of "spamminess", and gives it a numeric score; anything greater than
5 is generally considered spam, and is marked up as such, by adding an "X-Spam-Status:
yes" header.
The new mail server automatically runs all emails through SpamAssassin. You can filter
messages that SpamAssassin marks as spam by adding the following recipe to your
.procmailrc file:
:0:
* ^X-Spam-Status: yes
spam
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Web Services
1. Implementing a Web Server
1.1 Installing Apache
The apache source code can be downloaded from www.apache.org.
There are two versions of the apache server: 1.3 and 2.x
The configure script allows us to customise the installation. In particular we can choose
which modules we want to compile etc. Modules can either be
- statically compiled with
--enable-MODULE (where MODULE is the Module Indentifier ) or
--enable-modules=”MOD1 MOD2 ...”
- dynamically compiled with
--enable-mods-shared=”MOD1 MOD2 ...”
-disabled with
--disable-MODULE
1.2 Monitoring apache load
SNMP
Create a read-only SNMP community and restart the snmpd daemon:
/etc/snmp/snmp.conf
rocommunity lifesavers
Restart the snmpd service:
/etc/init.d/snmpd restart
Check that you can browse information about your system using the community name
lifesavers:
snmpwalk -v 1 -c lifesavers localhost ip
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MRTG
MRTG stands for “multi-router traffic grapher” and uses SNMP to get information about the
system.
cfgmaker
--output=/etc/mrtg/seafront.cfg \
-ifref=ip --global "workdir: /var/www/mrtg/stats" \
lifesavers@localhost
This will create a file called /etc/mrtg/seafront.cfg. We next update the information in
/var/www/mrtg/stats with the following command:
mkdir /var/www/mrtg/stats
mrtg /etc/mrtg/seafront.cfg
This should be run at regular intervals so it should be run through a cron job.
Task: The graphical output for MRTG will be saved in /var/www/mrtg/stats as an HTML
document. This is not a usual place to keep files for the apache server. After the next
section, we will make the appropriate changes to httpd.conf to make this directory
accessible through the webserver.
Many other tools are available such as Webalizer which analyse the access logs of the
apache server.
1.3 Using Apachectl
The apachectl script is used to control the httpd daemon. It takes the following options:
apachectl option Description – extract from apachectl(8)
start
Start the Apache httpd daemon. Gives an error if it is already
running. This is equivalent to apachectl -k start
stop
Stops the Apache httpd daemon. This is equivalent to apachectl -k
stop
restart
Restarts the Apache httpd daemon. If the daemon is not running, it is
started. This command automatically checks the configuration files as
in configtest before initiating the restart to make sure the daemon
doesn’t die. This is equivalent to apachectl -k restart
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fullstatus
Displays a full status report from mod_status. For this to work, you
need to have mod_status enabled on your server and a text-based
browser such as lynx available on your system. The URL used to
access the status report can be set by editing the STATUSURL
variable in the script.
status
Displays a brief status report. Similar to the fullstatus option,
except that the list of requests currently being served is omitted
graceful
Gracefully restarts the Apache httpd daemon. If the daemon is not
running, it is started. This differs from a normal restart in that
currently open connections are not aborted. This is equivalent to
apachectl -k graceful
configtest
Run a configuration file syntax test. It parses the configuration files
and either reports Syntax Ok or detailed information about the
particular syntax error. This is equivalent to apachectl -t
1.4 Basic Configuration Options
Section 1: General Options
KeepAlive on/off
Allows a client to perform multiple requests through a single
connection
MaxKeepAliveRequests 100
Maximum number of requests during a persistent connection
KeepAliveTimeout 15
Number of seconds to wait for a next request on the same
connection
Single Threaded Server
The httpd daemon is a single threaded process which needs to fork child daemons to deal
with multiple connections – only with apache2 is it possible to build a multi threaded
server.
StartServers 8
Number of httpd servers to start
MinSpareServers 5
Minimum number of spare servers to keep loaded in memory
MaxSpareServers 20
Maximum number of spare servers to keep loaded in memory
MaxClients 150
Maximum number of server processes allowed at any one time
MaxRequestsPerChild 1000
Maximum number of requests before a child is “retired”
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Multi Threaded Server
These options are available only for apache2 and onwards. You need to recompile apache
to enable threads. Most current apache2 binary distributions are still single threaded
because of conflicts with most dynamic modules which don't support multi threading yet.
StartServers 2
Notice that this is much lower than the single threaded server
MinSpareThreads 25
Minimum number of spare threads
MaxSpareThreads 75
Maximum number of spare threads
ThreadsPerChild 25
Number of worker threads per child
MaxClients 150
Maximum number of server processes allowed at any one time
MaxRequestsPerChild 0
Never retires?
Listen 80
Specify which port to listen on.
Can be of the form IP:port
LoadModule MODULE INDENTIFIER /PATH-TO/MODULE
Section where dynamic modules
are loaded
Include FILE
Read extra configuration options
from FILE. Apache2 has a conf.d
directory for this
Section 2 :Server Configuration
ServerName
The name of the server – can be different
User
Name of the user the server runs as
Group
Name of the group the server runs as
DocumentRoot
The directory the where HTML files are kept
<Directory>
Specify options (access control,...) for directories containing HTML files
Alias
URL alias for a given directory
AliasScript
Same as “Alias” option but for directories containing CGI scripts
DirectoryIndex
Set the name of the file which will be used as an index
Section 3: Virtual Hosts
We will cover virtual hosts when configuring SSL servers later in this chapter. For now we
distinguish two concepts:
<VirtualHost IP:PORT>
IP based virtual host
<VirtualHost HOSTNAME:PORT>
Name based virtual
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1.5 Restricting Client Access
Host based control is available using the keywords Order, Deny from and Allow from on
directories
<Directory PATH-TO-DIRECTORY> ... </Directory>
or locations
<Location URL> ... </Location>
The next configuration paragraph will allow anybody to access the directory /var/www/safe
except the host with IP 192.168.3.101:
<Directory /var/www/safe>
Order allow,deny
Deny from 192.168.3.101
Allow from all
</Directory>
Alias /safe /var/www/safe
Notice: The Order keyword is important. If we reverse the above order to Order
deny,allow then the following would happen: host 192.168.3.101 would first be denied
access because of the Deny rule but the Allow rule is read last and will subsequently grant
it access. The default access is given by the last argument in the order directive. I.e.
“Order allow,deny” has a default of “deny”.
1.6 Client Basic Authentication
The htpasswd tool is used to create passwords for users. For example, we create a new
file in the ServerRoot directory called passwords-for-directory1 with a password for
user gnu:
htpasswd -c passwords-for-directory1 gnu
If we choose to implement client authentication for the directory /var/www/html/seafront we
need to add the following paragraph to httpd.conf:
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Web Services
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<Directory /var/www/html/seafront>
AuthType basic
AuthName "protected site"
AuthUserFile conf/seafront.passwd
Require user gnu
</Directory>
Notice: Alternatively, with httpd2 configurations we could create a file called seafront.conf
with the above content and save it in the /etc/httpd/conf.d directory.
Reread the configuration file with:
apachectl graceful
2. Maintaining a Web Server
2.1 HTTPS Overview
The secure socket layer protocol SSL allows any networked applications to use
encryption. This can be thought of as a process which wraps the socket preparing it to use
encryption at the application level.
In the case of HTTPS, the server uses a pair of keys, public and private. The server's
public key is used by the client to encrypt the session key, the private key is then used to
decrypt the session key for use.
The public key is published using certificates. A certificate contains the following
information:
- Name and Address, Hostname, etc.
- Public Key
- TTL
- (optional) ID + Signature from a certificate authority (CA)
The certificate will be used to establish the authenticity of the server. A valid signature
from a known CA is automatically recognised by the client's browser. With Mozilla for
example these trusted CA certificates can be found by following the links: Edit ->
Preferences -> Privacy & Security -> Certificates then clicking on the “Manage
Certificates” button and the Authorities tab.
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Web Services
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1
Start SSL Handshake
2 Send Certificate
client
server
3
Send encrypted session key
4
Encrypt HTTP session with session key
On the other hand communications would be too slow if the session was encrypted using
public key encryption. Instead, once the authenticity of the server is established, the client
generates a unique secret session key which is encrypted using the servers public key
found in the certificate. Once the server receives this session key it can decrypt it using the
private key associated with the certificate. From there on the communication is encrypted
and decrypted using this secrete session key generated by the client.
2.2 SSL Virtual Hosts
A separate apache server can be used to listen on port 443 and implement SSL
connections. However most default configurations involve a single apache server listening
on both ports 80 and 443.
For this an additional Listen directive is set in httpd.conf asking the server to listen on
port 443. Apache will then bind to both ports 443 and 80. Non encrypted connections are
handled on port 80 while an SSL aware virtual host is configured to listen on port 443:
<VirtualHost _default_:443>
SSL CONFIGURATION
</VirtualHost>
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Web Services
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The SSL CONFIGURATION lines are:
SSLEngine on
SSLCipherSuite ALL:!ADH:!EXPORT56:RC4+RSA:+HIGH:+MEDIUM:+LOW:+SSLv2:+EXP
SSLCertificateFile PATH_TO_FILE.crt
SSLCertificateKeyFile PATH_TO_FILE.key
We need to generate the servers private key (FILE.key) and certificate (FILE.crt) to
complete this configuration.
2.3 Managing Certificates
The keys and certificates are usually kept in subdirectories of /etc/httpd/conf called
ssl.crt and ssl.key.
There should also be a Makefile that will generate both a KEY and a CERTIFICATE in
PEM format which is base64 encoded data.
Using the Makefile
For example if we want to generate a self-signed certificate and private key simply type:
make mysite.crt
The Makefile will generate both files mysite.key (the private key) as well as mysite.crt (the
certificate file containing the public key). You can use the following directives in
httpd.conf:
SSLCertificateFile ... mysite.crt
SSLCertificateKeyFile ... mysite.key
Certificate Requests
On a production server you would need to generate a new file called a “certificate request”
with:
openssl
req -new
-key mysite.key -out mysite.csr
This file can be sent to a certificate authority (CA) to be signed. The certificate authority
will send back the signed certificate.
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Pass Phrases
A private key can be generated with or without a passphase, and a private key without a
passphrase can be constructed from an existing private key.
A passphrased file: If a private key has a passphrase set then the file starts with
-----BEGIN RSA PRIVATE KEY----Proc-Type: 4,ENCRYPTED
DEK-Info: DES-EDE3-CBC, ---- snip ---.....
this means that the file is protected by a pass-phrase using 3DES. This was generated by
the line /usr/bin/openssl genrsa -des3 1024 > $@ in the Makefile. If the -des3
flag is omitted NO passphrase is set.
You can generate a new private key (mysite-nophrase.key) without a passphrase from the
old private key (mysite.key) as follows:
openssl rsa -in mysite.key -out mysite-nopass.key
2.4 Virtual Hosts
Name based virtual hosts
We will first discuss the situation where only one IP has been assigned to the server but
there are several A records or CNAME records pointing to the same IP.
Example: Modify the zone files to include a new CNAME record for
test1.seafront.bar to point to the actual name of the web server.
e.g.
test1.seafront.bar. IN CNAME
server1
IN A
server1.seafront.bar.
192.x.x.x
In httpd.conf it will be enough to create the following:
<VirtualHost test1.seafront.bar:80>
ServerAdmin [email protected]
DocumentRoot /var/www/html/test1
ServerName test1. seafront.bar
</VirtualHost>
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Web Services
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Example 2: Create an SSL aware VirtualHost for test1
- Make the certificate and the key: make host1.seafront.bar
- Add these lines to httpd.conf:
<VirtualHost 192.168.3.200:443>
SSLEngine on
SSLCipherSuite ALL:!ADH:!EXPORT56:RC4+RSA:+HIGH:+MEDIUM:+LOW:+SSLv2:+EXP
SSLCertificateFile /etc/httpd/conf/test1.seafront.bar.crt
SSLCertificateKeyFile /etc/httpd/conf/test1.seafront.bar.out
ServerAdmin [email protected]
DocumentRoot /var/www/html/test1
ServerName test1.seafront.bar
</VirtualHost>
Notice that the certificate that is presented once you connect to the https://test1 site is
incorrect. This is because test1.seafront.bar resolves to the servers IP address and the
server will start the SSL handshake before looking at the HTTP request. The next section
will fix that.
IP Based Virtual Hosts
Example: We will directly create a series of virtual SSL aware hosts and verify that they
present the client with the correct certificate.
- Assign new IP addresses to the eth0 interface: ifconfig eth0:0 X.X.X.X
- For each IP enter a new A record: www1 IN A X.X.X.X
- For each host create a self signed certificate.
- Enter a <VirtualHost X.X.X.X:443> paragraph in httpd.conf.
Notice: You may have to change the existing SSL virtual host from
<VirtualHost _default_:443>
to
<VirtualHost 127.0.0.1:443>
This prevents the default host certificate from being presented irrespective of the site
hostname.
Test that https://www1 and https://www2 do present the proper certificates.
Notice that if you permanently accept a certificate it will be added to the list of CA
certificates on your browser!
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3. Implementing a Proxy Server
3.1 Getting Started
You can verify that the squid proxy server is installed using:
rpm -q squid
Most versions will install the /etc/init.d/squid rc-script that creates the initial caching
directories. If this is not the case squid can initialise these cache directories with the -z
switch.
squid –z
NOTICE
You may need to add an access rule in the squid configuration file before being able to
rebuild the cache (see the next section “Access Lists and Access Control”)
The configuration file is /etc/squid/squid.conf. The syntax of this file can be checked
using the -k switch:
squid -k check
As with most network services the /etc/init.d/squid rc-script is used to start the service.
3.2 Access Lists and Access Control

Access Lists (acl)
In squid.conf the access lists have the following format:
acl
aclname
acltype
string/file
In the simplest cases an acl defines a list of hosts, networks or domains and is given a
name. This list can then be granted or denied access using the access control command
http_access described in the next paragraph.
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Web Services
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The next line defines an access list name called localnet corresponding to the local LAN:
acl localnet src 192.168.2.0/255.255.255.0
The main ACL types are listed below:
acltype
description
src
IP/netmask or IP1-IP2/netmask (client's IP address)
dst
IP/network (URL requested)
arp
MAC address
srcdomain
.example.com (client addresses)
dstdomain
.example.com (URLs requested)
time
range of times
port
space separated list of ports or range of the form p1-p2

Access control (http_access)
With http_access a particular access list is either allowed or denied access via the proxy.
The format is as follows:
http_access
allow|deny
aclname
The http_access requests are read in sequence and the first rule matched is used. To
allow access to all computers on the network insert the following before the http_access
deny all line:
http_access allow localnet
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3.3 Additional Configuration Options
The following table is a list of additional options available to further control the squid proxy.
Option
Description
http_port
the port squid uses to listen for requests (default 3128)
cache_peer
specify another proxy server to query whenever an object
isn't cached
cache_mem
limit the amount of additional memory used to cache objects
(this parameter doesn't limit the maximum process size)
cache_swap_low
percentage of swap utilisation. Once this limit is passed
objects start to be cached to disk
cache_swap_high
percentage of swap utilisation. Once this limit is approached
objects start getting evicted from the proxy cache
maximum_object_size
objects larger than this will not be cached
maximum_object_size_in_memory objects larger than this will not be kept in the memory cache
Memory Management (from the SQUID FAQ section 8)
“This version of SQUID stores incoming objects only in memory, until the transfer is
complete. At that point it decides whether or not to store the object on disk. This means
that when users download large files, your memory usage will increase significantly. The
squid.conf parameter maximum_object_size determines how much memory an in-transit
object can consume before we mark it as uncachable. When an object is marked
uncachable, there is no need to keep all of the object in memory, so the memory is freed
for the part of the object which has already been written to the client. In other words,
lowering maximum_object_size also lowers Squid-1.1 memory usage.”
“If your cache performance is suffering because of memory limitations, you might consider
buying more memory. But if that is not an option, There are a number of things to try:





Try a different malloc library [compile SQID with a different malloc]
Reduce the cache_mem parameter in the config file. This controls how many ''hot''
objects are kept in memory. Reducing this parameter will not significantly affect
performance, but you may recieve some warnings in cache.log if your cache is busy
Turn the memory_pools off in the config file. This causes Squid to give up unused
memory by calling free() instead of holding on to the chunk for potential, future use.
Reduce the cache_swap parameter in your config file. This will reduce the number
of objects Squid keeps. Your overall hit ratio may go down a little, but your cache
will perform significantly better
Reduce the maximum_object_size parameter (Squid-1.1 only). You won't be able to
cache the larger objects, and your byte volume hit ratio may go down, but Squid will
perform better overall”
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3.4 Reporting Tools
Most log analysis tools available for squid are listed on the following site:
http://www.squid-cache.org/Scripts/
The main logfile for squid is the /var/log/squid/access.log file. Next is a short overview of
calamaris and webalizer. Also notice that webmin produces log reports based on
calamaris.
cachemgr.cgi script
The current squid package installs a CGI script in /usr/lib/squid called cachemgr.cgi.
One can copy this across to the /var/www/cgi-bin directory where all CGI scripts can run
from. It is recommended however to set up a separate directory with htaccess
authentication.
Calamaris
The code is GPL and can be downloaded from http://cord.de/tools/squid/calamaris. You
can generate reports as follow:
cat /var/log/squid/access.log | calamaris
# Summary
lines parsed:
invalid lines:
parse time (sec):
221
0
0
# Incoming requests by method
method
request
%
Byte
%
sec kB/sec
--------------------------------- --------- ------ -------- ------ ---- ------GET
221 100.00 1244262 100.00
3
1.68
--------------------------------- --------- ------ -------- ------ ---- ------Sum
221 100.00 1244262 100.00
3
1.68
# Incoming UDP-requests by status
no matching requests
# Incoming TCP-requests by status
status
request
%
Byte
%
sec kB/sec
--------------------------------- --------- ------ -------- ------ ---- ------HIT
35 15.84
42314
3.40
0
6.11
MISS
182 82.35 1197840 96.27
1
4.97
ERROR
4
1.81
4108
0.33 120
0.01
--------------------------------- --------- ------ -------- ------ ---- ------Sum
221 100.00 1244262 100.00
3
1.68
In order to get information on webpage requests per host one can use the -R switch: There
are many more switches available (check the manpages for calamaris).
40
Web Services
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There are also a number of scripts that can run hourly or monthly reports. These scripts
are included in the EXAMPLES file distributed with calamaris.
calamaris -R 5 /var/log/squid/access.log
# Incoming TCP-requests by host
host / target
request
hit-%
Byte
hit-% sec
kB/sec
--------------------------------- --------- ------ -------- ------ ---- ------192.168.2.103
72
0.00
323336
0.00
0
10.24
*.redhat.com
35
0.00
126726
0.00
0
10.44
*.suse.co.
20
0.00
63503
0.00
0
13.15
*.lemonde.fr
6
0.00
109712
0.00
1
16.39
207.36.15.*
5
0.00
8946
0.00
0
3.94
*.akamai.net
4
0.00
12428
0.00
1
4.43
other: 2 requested urlhosts
2
0.00
2021
0.00
1
0.71
192.168.2.101
63
0.00
295315
0.00
1
4.65
cord.de
17
0.00
115787
0.00
0
20.86
*.doubleclick.net
13
0.00
26163
0.00
1
2.07
*.google.com
10
0.00
30646
0.00
1
3.71
*.squid-cache.org
8
0.00
51758
0.00
1
6.53
<error>
4
0.00
4290
0.00
0
10474
other: 6 requested urlhosts
11
0.00
66671
0.00
5
2.28
--------------------------------- --------- ------ -------- ------ ---- ------Sum
135
0.00
618651
0.00
1
6.51
Webalizer
This tool is often installed by default on some Linux distributions. It is also GPL'ed and can
be downloaded from http://www.mrunix.net/webalizer/.
By editing the /etc/webalizer.conf file one can choose between apache access logs, ftp
transfer logs or squid logs.
Example graphics generated with Webaliser:
41
Web Services
________________________________________________________________________________
3.5 User Authentication (using PAM)
To prevent unauthorised users browsing on the Internet you can setup squid to ask for a
username and password.
IMPORTANT: You cannot have user authentication and transparent proxy at the same
time! The work around is to block all outgoing requests on port 80, except the ones from
the Squid proxy itself. Users are then forced to manually set up their browsers to use the
proxy.
Configuration settings for PAM authentication:
Here are the list of options you need to set in the squid.conf file:
squid.conf
PAM authentication
settings
[Older versions]
authenticate_program /usr/lib/squid/pam_auth
[Squid V2.5]
auth_param basic program /usr/lib/squid/pam_auth
auth_param basic children 5
auth_param basic realm Anvil Internet Proxy
auth_param basic credentialsttl 2 hours
acl password proxy_auth REQUIRED
http_access allow password
The PAM configuration in /etc/pam.d:
Here we register squid to use the Pluggable Authentication Module.
This is done by adding a file in /etc/pam.d/ called squid with the following content
/etc/pam.d/squid
auth required /lib/security/pam_stack.so service=system-auth
auth required /lib/security/pam_nologin.so
account required /lib/security/pam_stack.so service=system-auth
password required /lib/security/pam_stack.so service=system-auth
session required /lib/security/pam_stack.so service=system-auth
session required /lib/security/pam_limits.so
This is a standard policy description on what to do when a person logs on.
The login session is abstracted into 4 part: auth, account, password and session.
PAM then uses a specific library function which handles each stage.
Notice that most lines request the system-auth service which is the
42
Web Services
________________________________________________________________________________
/etc/pam.d/system-auth file.
Also note the following from the pam_auth man page.
When used for authenticating to local UNIX shadow password databases the program
must be running as root or else it won't have sufficient permissions to access the user
password database. Such use of this program is not recommended, but if you absolutely
need to then make the program setuid root
chown root pam_auth
chmod u+s pam_auth
Please note that in such configurations it is also strongly recommended that the program is
moved into a directory where normal users cannot access it, as this mode of
operation will allow any local user to brute-force other users passwords. Also note the
program has not been fully audited and the author cannot be held responsible for any
security issues due to such installations.
43
Network Client Management
________________________________________________________________________________
1. DHCP Configuration
Dynamic Host Configuration Protocol (DHCP) is network protocol for automatically
assigning TCP/IP information to client machines. Each DHCP client connects to the
centrally-located DHCP server that returns the client's network configuration including IP
address, gateway, and DNS servers.
WARNING: You should not attempt to run a DHCP server unless you are certain not to
interfere with the network you are currently using – The safest option for this section is to
be totally isolated from the network and use a hub or a switch to connect the classroom
together.
1.1 Default DHCP Configurations
The basic communication process between a client workstation joining a TCP/IP network
and the DHCP server is depicted below.
The DHCPDISCOVER request is sent using the broadcast 255.255.255.255
The DHCP server can use two methods to allocate IP addresses:
1. A dynamic IP is assigned for a client host chosen from a range of IPs
2. A fixed IP is assigned for a specific host (identified using the MAC address, similar to
bootp)
44
________________________________________________________________________________
Since a single DHCP server can be used to administer IPs over several networks, the
dhcpd.conf configuration file is composed of global options followed by network sections:
Example network block:
subnet 10.0.0.0 netmask 255.0.0.0 {
....
}
In the next example we will assign both dynamic IP addresses and a fixed IP address:
subnet 10.0.0.0 netmask 255.0.0.0 {
range 10.5.5.10 10.5.5.200;
host
proxy {
hardware ethernet 00:80:C6:30:0A:7E;
fixed-address 10.5.5.2;
}
}
For each subnet it is possible to give information on network services, such as
- the default gateway
- the DNS domain name and the NIS domain name
- the DNS servers
In the subnet section above these directives would look like this:
option
option
option
option
routers
nis-domain
domain-name
domain-name-servers
10.254.254.254;
"nisdomain";
"seafront.bar";
10.0.0.2;
The database of dynamically assigned IP addresses is stored in
/var/lib/dhcp/dhcpd.leases.
This file should not be modified by hand. DHCP lease information for each recently
assigned IP address is automatically stored in the lease database. The information
includes the length of the lease, to whom the IP address has been assigned, the start and
end dates for the lease, and the MAC address of the network interface card that was used
to retrieve the lease.
45
________________________________________________________________________________
NOTICE
When the DHCP server is started for the first time, it fails unless the dhcpd.leases file
exists. Use the command touch /var/lib/dhcpd/dhcpd.leases to create the file if it does
not exist.
If the same server is also running BIND as a DNS server, this step is not necessary, as
starting the named service automatically checks for a dhcpd.leases file.
To start the DHCP service, use the command /sbin/service dhcpd start. To stop the
DHCP server, use the command /sbin/service dhcpd stop.
If more than one network interface is attached to the system, but the DHCP server should
only be started on one of the interfaces, configure the DHCP server to start only on that
device. In /etc/sysconfig/dhcpd, add the name of the interface to the list of
DHCPDARGS:
DHCPDARGS=eth0
1.2 Dynamic DNS
We assume that we still have the private/public key used for the seafront TSIG
authentication, we will use this same key to allow the DHCP server to update the zone files
on the DNS server.
Additional Configurations on the DHCP Server
On the DHCP server add the following to the dhcpd.conf file
ddns-update-style interim;
ignore client-updates;
key seafront.bar. {
algorithm hmac-md5;
secret QN3vIApnV76WS+a2Hr3qj+AqZjpuPjQgVWeeMMGSBC4=;
};
zone seafront.bar. {
primary 192.168.3.100;
key seafront.bar.;
}
zone 3.168.192.in-addr.arpa. {
primary 192.168.3.100;
key seafront.bar.;
}
46
________________________________________________________________________________
Optionally, it is possible to set a specific host name and domain name for a given host with
the keywords
ddns-hostname host_name
ddns-domain-name domain_name
If the ddns-hostname option is not present then the DHCP server will try and use the
name provided by the client. The domain on the other hand cannot be set by the client, so
if ddns-domain-name is not present then the DHCP server will use the value given by the
domain-name option.
Additional Configurations on the DNS Server
On the DNS server we need to do the following:
1. If you are using DNSSEC signed zone files then we need to use the unsigned zones.
2. Add the allow-update option to the seafront.bar entry:
type master;
allow-update { key seafront.bar.; };
allow-transfer { key seafront.bar.; };
};
and do the same with the in-addr.arpa zone:
type master;
file "192.168.3.local";
allow-update { key seafront.bar.; };
allow-transfer { key seafront.bar.;};
};
Testing with nsupdate
nsupdate is a utility used to request the name server to update its database. nsupdate
takes commands like nslookup does, if run without arguments.
47
________________________________________________________________________________
The following commands are good to know:
server [server address] - Sets the target server for who to send updates.
key [keyname] [secret]
- Tell nsupdate what your key is.
zone [zonename]
- Explicitly choose a zone to send updates for. If unspecified,
nsupdate will guess.
update [...]
- Request an update to record.
send
- Send updates.
show
- Show updates that haven't been sent.
Client Configuration
On Linux clients it is possible to set the DHCP_HOSTNAME variable in the interface setup
script. In Redhat-like variants this would be in the /etc/sysconfig/network-scripts/ifcfg-ethX
files. Notice that this is simple a hostname, the domain name will be appended to that
name on the DHCP sever.
1.3 DHCP Relay
The DHCPDISCOVER packets from clients reach the server through the broadcast
255.255.255.255, however broadcasts are blocked by routers.
So in a configuration with multiple networks and a single DHCP server each router needs
to be able to relay DHCPDISCOVER broadcasts from a given network to the DHCP
server.
For a Linux router this is done using the dhcp-relay or dhcrelay (more recent) tool. Both
tools take a mandatory single argument which is th IP of the DHCP server.
By default the relay tools will listen on all network interfaces for DHCP requests. One can
specify an interface with the -i option:
dhcrelay -i eth0
IP_FOR_DHCP_server
48
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2. NIS Configuration
2.1 Master Server Configuration
On a Linux system the network information system (NIS) server is called ypserv (package
name: ypserv). The RPM package has the same name and installs the following main files:
Files installed with ypserv
Description
/etc/rc.d/init.d/yppasswdd
script for the daemon allowing users to change passwords
/etc/rc.d/init.d/ypserv
script for ypserv daemon
/etc/rc.d/init.d/ypxfrd
script for daemon used to speed up transfers to slave
servers
/etc/ypserv.conf
main configuration file for ypserv
/var/yp/Makefile
Makefile for database files – should only be used on the
master server
1. Choose a nisdomain name
In /etc/sysconfig/network set the variable NISDOMAIN. For example we can set the
nisdomain to linis as follows\
NISDOMAIN=linis # entry in /etc/sysconfig/network
The file /etc/sysconfig/network will be sourced by the ypserv initscript.
2. Make sure the master server will push map changes to the slave servers. For this you
need to edit the file /var/yp/Makefile and put
NOPUSH=false
3. Start the ypserv daemon
/etc/init.d/ypserv restart
4. Check that the nisdomain has been properly set
nisdomainname
linis
49
________________________________________________________________________________
5. Create the databases, the -m option to ypinit is to indicate the server is a master server
/usr/lib/yp/ypinit –m
Enter the list of slave servers you will run on this domain. This will create a number of
DBM files in /var/yp/linis as well as a file called /var/yp/ypservers.
2.2 Slave Server Configuration
On the slave server, we need to install the ypserv package too. This time we run ypinit
and point it to the the master server:
/etc/rc.d/init.d/ypserv start
/usr/lib/yp/ypinit -s MASTER_IP
Also make sure to leave the line NOPUSH=true in /var/yp/Makefile
2.3 Client Setup
On the client the main service is called ypbind (package name: ypbind). This daemon is
responsible for binding to a NIS server and successfully resolves names and passwords
as needed.
The main configuration file is /etc/yp.conf.
If the NISDOMAIN variable is set in /etc/sysconfig/network which is sourced by the rcscript /etc/init.d/ypbind then the NIS server will be detected using the broadcast. One can
also configure yp.conf and specify. Once this is set one can start ypbind
/etc/init.d/ypbind start
Make sure that the nis keyword is added to /etc/nsswitch.conf.
50
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2.4 Setting up NFS home directories
Once the NIS server and clients are setup as above, anybody with an account on the NIS
server can log onto a client machine with ypbind pointing at the correct server.
All that is needed is for the user is to access a home directory. This can be done in a
number of ways. We will describe one implementation using NFS.
We assume that all the home directories are on a single server with the following IP
address 10.0.0.1
All the clients are on the 10.0.0.0/8 network.
On the NFS server
Edit /etc/exports and add
/home 10.0.0.1/8(rw)
Notice that root_squash will apply automatically.
On the client
Edit /etc/fstab and add
10.0.0.1:/home
/home
defaults
0
0
2.5 Basic NIS Administration
With the latest versions of ypserv a number of default maps are created using source files
in /etc. It is possible to alter the YPPWDDIR and YPSRCDIR variables in the Makefile to
build maps from alternative files from custom locations.
Updates are made with the Makefile in /var/yp. The targets are all, passwd, group ...
Copy the new maps to /var/yp/linis and run yppush to update the slave servers:
yppush MAP_NAME
51
________________________________________________________________________________
Additional Commands
Command
Description
ypcat
get values from a database, for example ypcat passwd
ypwhich
return the name of the NIS server on the network
3. LDAP Configuration
3.1 What is ldap
LDAP stands for Lightweight Directory Access Protocol. The protocol allows access to
data in a tree-like structure using attributes. LDAP can be thought of as a specialised
database which handles trees. Since directories are also trees, navigating LDAP fields is
like navigating a directory. Added to this LDAP has been designed mainly for optimal
access. This clarifies the words Directory and Access.
The Distinguished Name
An item in the database can be referenced using a unique Distinguished Name (dn). This
is similar to a file's full path in a directory. Each intermediate subfolder is called a Relative
Distinguished Name.
Distinguished Name
dc=example, dc=com
ou=People
ou=Aliases
cn=Tux
dn: cn=Tux, ou=People , dc=Example, dc=com
More Terminology
DIT
DN
RDN
LDIF
The Data Information Tree
Distinguished Name
Relative Distinguished Name
LDAP Data Interchange Format
52
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Attributes:
dc
cn
c
l
o
ou
sn
st
uid
Domain Component
Common Name
Country
Location
Organisation
Organisational Unit
Surname
State
User id
3.2 OpenLDAP server configuration
The server is called slapd (Standalone LDAP daemon) and its configuration file is:
/etc/openldap/slapd.conf
We will cover each section of this file in more detail.
Importing schemas
There is an include clause in slapd.conf which tells the LDAP server which schemas
should be loaded.
We need at least the following:
include
include
include
include
include
/etc/openldap/schema/core.schema
/etc/openldap/schema/misc.schema
/etc/openldap/schema/cosine.schema
/etc/openldap/schema/nis.schema
/etc/openldap/schema/inetorgperson.schema
Database Definition
Available DBMs (Database Managers) are ldbm or the more recent bdb.
We will use bdb:
database
bdb
53
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You need to specify the root or base for the LDAP directory, as well as the directory where
the database file will be kept. This is done below:
suffix
directory
"dc=example,dc=com"
/var/lib/ldap/
The following lines are only needed when modifying the LDAP server online. You can then
specify an administrator username/password. Use the slappasswd to generate an
encrypted hash (see 3.4 Migrating System Files to LDAP):
rootdn
rootpw
"cn=Manager,dc=example,dc=com"
{SSHA}KiXS5htbnVEQp7OrjoteQZHHICs0krBO
3.3 Client configuration files
There are two configuration files called ldap.conf. Here is what they do:


The /etc/ldap.conf file is used by the nss_ldap and pam_ldap modules
The file /etc/openldap/ldap.conf is used by the tools ldapsearch and ldapadd
For example, to save time typing:
ldapsearch -b “dc=example,dc=com”
-x
you can add the next lines to /etc/openldap/ldap.conf
BASE
HOST
dc=example, dc=com
127.0.0.1
So far we have configured slapd and the configuration file for ldapsearch in particular.
Once we have populated an LDAP directory we will be able to test our setup by typing:
ldapsearch –x
3.4 Migrating System Files to LDAP
There are two methods available to populate an LDAP directory.


If the ldap daemon slapd is stopped, we can do an offline update using slapadd
While slapd is running, it is possible to perform an online update using ldapadd or
ldapmodify
54
________________________________________________________________________________
We will also use migration tools which can be downloaded from
http://www.padl.com/OSS/MigrationTools.html.
Creating LDAP directories offline
We are going to work in the directory containing the LDAP migration Perl scripts which we
have downloaded from www.padl.com.
Notice: Some distributions may include the migration tools with the LDAP server package.
You should have the following files:
migrate_automount.pl
CVSVersionInfo.txt
Make.rules
MigrationTools.spec
README
ads
migrate_netgroup_byhost.pl
migrate_netgroup_byuser.pl
migrate_networks.pl
migrate_passwd.pl
migrate_profile.pl
migrate_protocols.pl
migrate_rpc.pl
migrate_services.pl
migrate_slapd_conf.pl
migrate_base.pl
migrate_common.ph
migrate_fstab.pl
migrate_group.pl
migrate_hosts.pl
migrate_netgroup.pl
migrate_aliases.pl
migrate_all_netinfo_offline.sh
migrate_all_netinfo_online.sh
migrate_all_nis_offline.sh
migrate_all_nis_online.sh
migrate_all_nisplus_offline.sh
migrate_all_nisplus_online.sh
migrate_all_offline.sh
migrate_all_online.sh
First edit migrate_common.ph and change the $DEFAULT_BASE variable to:
$DEFAULT_BASE = "dc=example,dc=com";
NOTICE
When migrating the /etc/passwd file one can either use shadow passwords or not. When
using shadow passwords an added objectClass called shadowAccount is used in the
LDAP record and there is no need to migrate the shadow password file.
We create our first LDIF file called base.ldif to serve as our root:
/migrate_base.pl > base.ldif
This flat file will be converted into bdb (or ldbm) files stored in /var/lib/ldap as follows:
slapadd -v < base.ldif
55
________________________________________________________________________________
We next choose to migrate the password without shadow passwords as follows:
./migrate_passwd.pl /etc/passwd passwd.ldif
The entries in passwd.ldif should look like this:
dn: uid=test,ou=People,dc=example,dc=com
uid: test
cn: test
objectClass: account
objectClass: posixAccount
objectClass: top
userPassword: {crypt}$1$FGrRfa0u$lo5XwA9xxssmjboNB2Z361
loginShell: /bin/bash
uidNumber: 505
gidNumber: 506
homeDirectory: /home/test
Now let's add this LDIF file to our LDAP directory:(remember that LDAP is stopped so we
are still offline)
slapadd -v -l passwd.ldif
or
slapadd -v < passwd.ldif
NOTICE:
Make sure all the files in /var/lib/ldap belong to user ldap.
TESTING:
Restart the LDAP server:
/etc/init.d/ldap restart
Search all the entries in the directory:
ldapsearch -x
If the ldap server does not respond, or the result from ldapsearch is empty, it is possible
to show the content of the LDAP databases in /var/lib/ldap with the slapcat command.
56
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Creating LDAP Directories Online
The LDAP server can be updated online, without having to shut the ldap service down. For
this to work we must specify a rootdn and a rootpw in /etc/openldap/slapd.conf.
The password is generated from the command line as follows
sldappasswd
New password:
Re-enter new password:
{SSHA}XyZmHH1RlnSVXTj87UvxOAOCZA8oxNCT
We next choose the rootdn in /etc/openldap/slapd.conf to be
rootdn
rootpw
"cn=Manager,dc=example,dc=com"
{SSHA}XyZmHH1RlnSVXTj87UvxOAOCZA8oxNCT
The next line will update the LDAP entries
ldapmodify -f passwd.ldif -x -D ''dc=example,dc=com'' –W
Enter LDAP Password:
3.5 LDAP Authentication Scheme
Server Configuration
We assume that the LDAP server has been configured as above.
The passwords in the LDAP directory can also be updated online with the ldappasswd
command.
The next line will update the password for user tux on the LDAP server.
ldappasswd -D "cn=Manager,dc=example,dc=com"
"uid=tux,ou=People,dc=example,dc=com"
-S -x -W
\
The -S switch is used to configure a new password.
We assume that the IP address for the server is 10.0.0.1 and that the domain component
is “dc=example,dc=com”
57
________________________________________________________________________________
You may allow users to change their passwords on the LDAP server as follows:
1. Copy the passwd PAM file /etc/share/doc/nss_ldap-version/pam.d/passwd to
/etc/pam.d
2. Add the following access rule in /etc/openldap/slapd.conf
access to attrs=userPassword
by self write
by anonymous auth
by * none
Client Configuration
The clients need to have the nss_ldap package installed (some distributions have a
separate pam_ldap package with the PAM related modules and files). The following files
and libraries are installed:
/etc/ldap.conf
set the hostname and the domain component of the LDAP
server used for authentications
/lib/libnss_ldap-2.3.2.so
an ldap module for the NameService Switch
/lib/security/pam_ldap.so
the PAM ldap module
/usr/lib/libnss_ldap.so
a symbolic link to /lib/libnss_ldap-2.3.2.so
/usr/share/doc/nss_ldap-207/pam.d
sample files for programs using PAM
If we don't use SSL certificates then /etc/ldap.conf is as follows:
The /etc/ldap.conf file
host 10.0.0.1
base dc=example,dc=com
ssl no
pam_password md5
Next in /etc/pam.d replace the file called login with /usr/share/doc/nss_ldap207/pam.d/login. This will tell the authentication binary /bin/login to use the pam_ldap.so
module.
Finally the /etc/nsswitch.conf needs to have the following line:
passwd ldap files
Check the /var/log/ldap/ldap.log file on the server to follow the authentication process.
58
________________________________________________________________________________
4. PAM Authentication
Services or applications which need authentication can use the pluggable authentication
module (PAM) mechanism which offers a modular approach to the authentication process.
For example, if a new hardware authentication scheme is added to a system, using smart
cards or prime number generators, and if corresponding PAM library modules are
available for this new scheme, then it is possible to modify existing services to use this
new authentication scheme.
4.1 PAM Aware Applications
Services which use pluggable authentication modules have been compiled with libpam.
For example sshd is such a service:
ldd `which sshd` | grep pam
libpam.so.0 => /lib/libpam.so.0 (0x00941000)
These applications will scan the PAM configuration files which in turn tell the application
how the authentication will take place.
59
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4.2 PAM Configuration
PAM configuration is controlled with the single file /etc/pam.conf. This file contains a list of
services and a set of instructions, as follows:
service
type
control
module-path
module-arguments
However, if the directory /etc/pam.d exists then pam.conf is ignored and each service is
configured through a separate file in pam.d. These files are similar to pam.conf except
that the service name is dropped:
type
type:
control
module-path
module-arguments
defines the “management group type”. PAM modules are classified into four
management groups which define different aspects of the authentication process:
account:
check the validity of the account (eg. does the users have a UNIX
account, is the user authorised to use the application ...)
auth:
the authentication method. This points to a module(s) responsible for
the challenge-response
password: defines how to change user passwords, if at all
session: modules that are run before and after a service is granted
control: defines what action to take if the module fails. The simple controls are:
requisite: a failure of the module results in the immediate termination of the
authentication process
required: a failure of the module will result in the termination of the
authentication once all the other modules of the same type have been
executed
sufficient: success of the module is sufficient except if a prior required module
has failed
optional: success or failure of this module are not taken into account unless it is
the only requirement of its type
module-path: the path to a PAM module (usually in /lib/security)
module-arguments: list of arguments for a specific module
60
System Security
________________________________________________________________________________
System Security
1. Iptables/Ipchains
What is a Packet Filter?
A packet filter is a piece of software which looks at the header of packets as they pass
through, and decides the fate of the entire packet. It might decide to DROP the packet (i.e.,
discard the packet as if it had never received it), ACCEPT the packet (i.e., let the packet go
through), or something more complicated. - from the “Packet Filtering HOWTO” by Rusty
Russell
For more in depth information see the HOWTOs at www.netfilter.org.
In this section we introduce the iptables concepts of chains, tables and targets. We then
look at some examples to illustrate network address translation (NAT) as well as the
special cases of masquerading and transparent redirections.
1.1 The Chains
A chain is a list of rules which by considering criteria found in the packet's header will
make decisions about the type of action to take (target). There are five chains
corresponding to different stages in the netfilter framework: PREROUTING, INPUT,
FORWARD, POSTROUTING and OUTPUT.
Below is a diagram of the progression of a packet through the kernel netfilter framework:
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System Security
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1.2 The Tables
There are three built-in tables (the IP Tables) which allow to carry out different tasks as
listed below.
filter: this is the default table and the packets are never altered. Packets are available from
the following chains:
INPUT
for packets coming into the box itself
OUTPUT
for locally-generated packets
FORWARD
for packets being routed through the box (check the value of
/proc/sys/net/ipv4/ip_forward)
nat: this table only deals with network address translations (NAT) it is consulted when a
packet creating a new connection is encountered. Packet headers connected with routing
can be altered here. The following chains are considered:
PREROUTING
alters the packets as they come in
POSTROUTING
alters packets as they go out
OUTPUT
alters locally generated packets before routing
mangle: used for specialized packet alterations. Targets in this table allow the TOS or TTL
field to be modified.
Until kernel 2.4.17 it could only interact with two chains:
PREROUTING
for altering incoming packets before routing
OUTPUT
for altering locally-generated packets before routing
Since kernel 2.4.18, the three other chains are also supported:
INPUT
for packets coming into the box itself
FORWARD
for altering packets being routed through the box
POSTROUTING
for altering packets as they are about to go out
1.3 The Targets
The part of the filtering rule which determines what action to take if the rule is matched is
called a target and is preceded by a -j flag (jump). Here is an overview of available targets
for a given table:
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System Security
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all tables: ACCEPT, REJECT, DROP, LOG, ULOG, TCPMSS, MIRROR
filter: (nothing individual to this chain)
nat: DNAT, SNAT, MASQUERADE, REDIRECT
mangle: TOS, MARK, DSCP, ECN
There are more targets, but they come as part of additional extension kernel modules.
1.4 Example Rules
1. Example filter rules:
Drop incoming icmp-request as well as outgoing icmp-reply packets
iptables
-A INPUT -p icmp --icmp-type echo-request
iptables
-A OUTPUT -p icmp --icmp-type echo-reply
-j DROP
-j DROP
Notice: The protocol extension flags allow you to specify more information about a
specific protocol. In the case of TCP packets for example you may have:
-p tcp –tcp-flags ALL SYN,ACK
ALL stands for SYN ACK FIN RST URG and PSH. This rule says that all flags must be
examined and of those, if the SYN and ACK flags are set, the rule is true.
2. Example Destination Network Address Translation (DNAT):
All requests on port 80 for host 192.168.3.100 are redirected to the host 10.1.1.1 on port
80:
iptables -t nat -A PREROUTING -p tcp -i eth0 -d 192.168.3.100 \
--dport 80 -j DNAT --to 10.1.1.1:80
3. Example Source Network Address Translation (SNAT):
The SNAT target is used to change the Source Address. For example, in the case
where a router switches from the address on all outgoing packets leaving through ppp0
to its own (public) IP address. The line would look like this:
iptables -t nat -A POSTROUTING -o ppp0 -s 192.168.3.0/24 -d 0/0 \
-j SNAT –to ROUTER_IP
This rule can also be written using the MASQUERADE target:
iptables -t nat -A POSTROUTING -o ppp0 -s 192.168.3.0/24 -d 0/0 -j MASQUERADE
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System Security
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4. Example Redirection:
A redirection is a special case of DNAT where the –-to host is the same host. For
example if a proxy server is running on a router, all requests through port 80 can be
PRE-routed through port 3128 with:
iptables -A PREROUTING -t nat -i eth0 -p tcp --dport 80 -j REDIRECT --to-port 3128
TASK: At this stage if you want to implement a transparent proxy with the previous
redirection rule you will have to change the configuration file squid.conf and add the
following:
httpd_accel_host virtual
httpd_accel_port 80
httpd_accel_with_proxy on
httpd_accel_uses_host_header on
Remember that if you have implemented an authentication scheme with squid you may
have to disable it for the transparent proxy to work.
2. Differences with Ipchains
Some of the main improvement over ipchains:
With iptables, each filtered packet is only processed using rules from one chain rather than
multiple chains. In other words, a FORWARD packet coming into a system using ipchains
would have to go through the INPUT, FORWARD, and OUTPUT chains in order to move
along to its destination. However, iptables only sends packets to the INPUT chain if they
are destined for the local system and only sends them to the OUTPUT chain if the local
system generated the packets. For this reason, you must be sure to place the rule
designed to catch a particular packet in the correct chain that will actually see the packet.
The advantage is that you now have finer-grained control over the disposition of each
packet. If you are attempting to block access to a particular website, it is now possible to
block access attempts from clients running on hosts which use your host as a gateway. An
OUTPUT rule which denies access will no longer prevent access for hosts which use your
host as a gateway.
Additional Matching Extensions
Matching extensions are implemented in iptables as modules. Modules are invoked with
the -m switch.
For example the state module makes it possible to distinguish new packets and packets
from an established connect. The packet is tested for a matching state. Particular state
64
System Security
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values are NEW, ESTABLISHED, RELATED or INVALID.
iptables -A INPUT -p tcp -m state –-state ESTABLISHED -j ACCEPT
iptables -A OUTPUT -p tcp -m state –-state NEW,ESTABLISHED -j ACCEPT
Matching extension modules are listed below.
Module
Description
Option (example)
connrate
matches the current connection
rate
--connrate [!] [from]:[to]
dstlimit
This module allows you to limit
the packet per second (pps) rate
on a per destination IP or per
destination port base
--dstlimit avg
icmp
this extension is loaded if
‘--protocol icmp’ is specified
--icmptype [!] typename
iprange
specify a range of IPs
--src-range IP-IP
length
matches the length of the packet
--length length
mac
match the MAC source
--mac-source [!] address
state
determine the state of a packet
–state state
(NEW,ESTABLISHED,RELATED,
INVALIDE)
3. Security Tools
3.1 SSH
For a first description of the ssh client and sshd server see the section on “Basic Security”
in the lpi-manuals document for LPI 102. For an in depth presentation see the Internet
draft “The SSH (Secure Shell) Remote Login Protocol” at http://www.free.lp.se/fish/rfc.txt.
This section covers the server configuration file and briefly discusses other mechanisms
that the SSH protocol offers such as X11 forwarding and port forwarding.
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System Security
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sshd_con fig overview
Port 22
Specify which port to listen on. Multiple “Port” options
can be used
Protocol 2,1
Specify version 1 or version 2 SSH protocol. Can be a
comma separated list. If both are supplied, they are
tried in the order presented.
DenyUsers [USER]@HOST
Deny users from a specific host. Wild cards such as *
can be used
IgnoreRhosts yes/no
Default is yes – Ignore the ~/.rhosts and ~/.shosts files
PermitEmptyPasswords yes/no
Default is no – Allow login with an empty passwords
when password authentication is allowed
PermitRootLogin yes/no
Allow or disallow root access
X11Forwarding yes/no
Instructs the remote end to route X11 traffic back
through the ssh tunnel to the user's X session. Unless
disabled, the xauth settings will be transferred in order
to properly authenticate remote X applications
Port Forwarding
It is possible to do port forwarding with the SSH client. This is often used to provide a
simple mechanism to encrypt a connection. For example one can open a local (-L) port
(1234) pointing to the remote host (www.google.com) on another port (80) as follows:
ssh -L 1234:www.google.com:80 127.0.0.1
Quick VPN
This is a user-space VPN as opposed to other types of VPNs which are kernel based.
/usr/sbin/pppd noauth pty \
"ssh SOME_HOST -l root '/usr/sbin/pppd notty noauth
192.168.0.1:192.168.0.2'"
\
192.168.0.2:192.168.0.1
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System Security
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3.2 LSOF
lsof shows open files used by processes. Traditionally it is used to list PIDs of processes
running on a given directory:
lsof +D DIRECTORY
lsof will output the following information:
NAME:
name of the process
PID:
process ID
USER:
name of the user to whom the process belongs
FD:
File desciptor (e.g u = read write, r = read, w = write)
TYPE:
The file type (e.g REG = regular file)
DEVICE: Major/Minor number (e.g 3,16 =/dev/hda16 )
SIZE:
Size or offset of the file
NODE:
Inode of the file
NAME:
The name of the file
lsof can also be used to display network sockets. For example the following line will list all
internet connections:
lsof -i
You can also list connections to a single host:
lsof -i @HOST
For example if a host TOFFY is connected to your localhost on port 1234, the following
would display information about the connection:
lsof -i @TOFFY:1234
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System Security
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3.3 NETSTAT
netstat is used to print network connections, routing tables, etc.
Main options are:
-r
-C
display routing tables
display route cache
-l
only listening services
--inet restrict to network sockets
Protocol types are:
-t
-u
select tcp
select udp
3.4 TCPDUMP
tcpdump dumps traffic on a network.
From the man page:

The TCP Packet
The general format of a tcp protocol line is:
src > dst: flags data-seqno ack window urgent options
Src and dst
- the source and destination IP addresses and ports.
Flags
- some combination of S (SYN), F (FIN), P (PUSH) or R (RST) or a single
‘.’ (no flags).
Data-seqno
- describes the portion of sequence space covered by the data in this
packet (see example below).
Ack
- sequence number of the next data expected in the other direction on
this connection.
Window
- the number of bytes of receive buffer space available in the other
direction on this connection.
Urg
- indicates there is ‘urgent’ data in the packet.
Options
- tcp options enclosed in angle brackets (e.g., <mss 1024>)
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System Security
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
Capturing TCP packets with particular flag combinations (e.g SYN-ACK,
URG-ACK, etc.)
There are 8 bits in the control bits section of the TCP header:
CWR | ECE | URG | ACK | PSH | RST | SYN | FIN
Let’s assume that we want to watch packets used in establishing a TCP connection.
Recall the structure of a TCP header without options:
0
15
31
----------------------------------------------------------------|
source port
|
destination port
|
----------------------------------------------------------------|
sequence number
|
----------------------------------------------------------------|
acknowledgment number
|
----------------------------------------------------------------| HL
| rsvd |C|E|U|A|P|R|S|F|
window size
|
----------------------------------------------------------------|
TCP checksum
|
urgent pointer
|
-----------------------------------------------------------------
A TCP header usually holds 20 octets of data, unless options are present. The first line
of the graph contains octets 0 - 3, the second line shows octets 4 - 7 etc.
Starting to count with 0, the relevant TCP control bit are contained in octet 13:
0
7|
15|
23|
31
----------------|---------------|---------------|---------------| HL
| rsvd |C|E|U|A|P|R|S|F|
window size
|
----------------|---------------|---------------|---------------|
| 13th octet
|
|
|
Let’s have a closer look at octet no. 13:
|
|
|---------------|
|C|E|U|A|P|R|S|F|
|---------------|
|7
5
3
0|
These are the TCP control bits we are interested in. We have numbered the bits in this
octet from 0 to 7, right to left, so the PSH bit is bit number 3, while the URG bit is
number 5.
Recall that we want to capture packets with only SYN set. Let’s see what happens to
octet 13 if a TCP datagram arrives with the SYN bit set in its header:
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System Security
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|C|E|U|A|P|R|S|F|
|---------------|
|0 0 0 0 0 0 1 0|
|---------------|
|7 6 5 4 3 2 1 0|
Looking at the control bits section we see that only bit number 1 (SYN) is set.
Assuming that octet number 13 is an 8-bit unsigned integer in network byte order, the
binary value of this octet is
00000010
and its decimal representation is
7
6
5
4
3
2
1
0
0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2 =
2
We’re almost done, because now we know that if only SYN is set, the value of the 13th
octet in the TCP header, when interpreted as a 8-bit unsigned integer in network byte
order, must be exactly 2.
This relationship can be expressed as
tcp[13] == 2
3.5 NMAP
nmap is the network exploration tool and security scanner.
The scanner makes use of the fact that a closed port should (according to RFC 793) send
back an RST. In the case if a SYN scan, connections that are half opened are immediately
close by nmap by sending an RST itself.
The main scan types are:
SYN or Half-open: -sS
nmap will send a synchronisation packet SYN asking for a connection. If the remote host
send a RST/ACK it is assumed that the port is closed. If the remote host sends a
SYN/ACK this indicates that the port is listening.
UDP: -sU
UDP is connectionless. So there is no need for a 3 way handshake as with TCP. If a port
is closed the server will send back a ICMP PORT UNREACHABLE. One then deduces
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System Security
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that all the other ports are open (not reliable in the case were ICMP messages are
blocked).
TCP NULL: -sN
TCP packet with no flags set. Closed port will send a RST when receiving these packets
(except with MS Windows).
TCP Xmas: -sX
TCP packet with the FIN+URG+PUSH flags set. The remote host should send back a RST
for all closed ports when receiving a Xmas packet.
71
Vježbe
Vježbe
__________________________________________________________
Vježba 1: Upoznavanje s radnim okruženjem
1. Prijavite se na sustave kojima ćete se služiti u vježbama. Zabilježite njihovu
trenutačnu konfiguraciju:
Mreža/maska: _________________
IP-adresa: __________________
IP-adresa: _________________
Hostname (FQDN): _________________
Gateway: _________________
DNS:
_________________
server1
Mreža/maska: _________________
IP-adresa: _________________
IP-adresa: _________________
Hostname (FQDN): _________________
Gateway: _________________
DNS:
_________________
server2
Napomena: Nije nužno da su sve postavke na početku tečaja postavljene ili ispravne
(ipak ih zabilježite). Neke postavke će se tijekom tečaja mijenjati.
74
Vježbe
__________________________________________________________
Vježba 2: DNS
Jednostavna konfiguracija (forward only)
1. Na sustavu server1 instalirajte pakete bind i bind-utils
(yum install bind bind-utils).
2. U datoteku /etc/named.conf unutar sekcije options (na primjer iza retka s oznakom
directory) upišite:
forward only;
forwarders {
161.53.2.69;
161.53.2.70;
};
Obrišite ili označite kao komentar redak: listen-on-v6 port 53 { ::1; };
Pokrenite naredbu /usr/sbin/named-checkconf kako biste provjerili postoje li greške
u konfiguracijskoj datoteci /etc/named.conf.
3. Pokrenite named i postavite da se pokreće prilikom svakog pokretanja sustava:
service named start
chkconfig named on
Napomena: Ako ne postoji datoteka rndc.key, skripta /etc/init.d/named će je
stvoriti, što može potrajati i nekoliko minuta.
Provjerite radi li named ispravno:
dig @127.0.0.1 www.srce.hr
ili
nslookup www.srce.hr 127.0.0.1
Definiranje zona
4. Dodajte sljedeći tekst na kraj datoteke /etc/named.conf:
zone "tecaj.hr" IN {
type master;
file "zone/tecaj.hr";
};
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Vježbe
__________________________________________________________
type master;
file "zone/192.168.1";
};
type master;
file "zone/192.168.2";
};
5. Provjerite postoji li definicija za root-zonu:
zone "." IN {
type hint;
file "named.ca";
};
6. Promijenite forward only; u forward first;
Napomena: Ova izmjena ne utječe na izvođenje vježbe, ali dobro ju je napraviti zbog
preglednosti koda.
7. Stvorite direktorij u koji će se upisivati podaci za zone:
mkdir /var/named/zone
chown named:named /var/named/zone
chmod 770 /var/named/zone
8. U direktoriju /var/named/zone stvorite datoteke tecaj.hr, 192.168.1 i 192.168.2
na sljedeći način:
Neka datoteka tecaj.hr ima sljedeći sadržaj (na mjestu opcije serial umjesto
2014030201 možete upisati oznaku koja odgovara aktualnom datumu):
$TTL 1D
@
localhost
server1
server2
IN
SOA
NS
tecaj.hr. root.tecaj.hr. (
2014030201
1D
1H
1W
3H )
server1.tecaj.hr.
A
A
A
A
127.0.0.1
192.168.1.100
192.168.2.201
192.168.2.202
76
;
;
;
;
;
serial
refresh
retry
expire
minimum
Vježbe
__________________________________________________________
Neka datoteka 192.168.1 ima sljedeći sadržaj:
$TTL 1D
@
100
IN
SOA
tecaj.hr.
NS
root.tecaj.hr. (
2014030201
1D
1H
1W
3H )
server1.tecaj.hr.
PTR
server1.tecaj.hr.
;
;
;
;
;
serial
refresh
retry
expire
minimum
;
;
;
;
;
serial
refresh
retry
expire
minimum
Neka datoteka 192.168.2 ima sljedeći sadržaj:
$TTL 1D
@
201
202
IN
SOA
tecaj.hr.
NS
root.tecaj.hr. (
2014030201
1D
1H
1W
3H )
server1.tecaj.hr.
PTR
PTR
server1.tecaj.hr.
server2.tecaj.hr.
Provjerite s /usr/sbin/named-checkconf postoje li greške u datoteci named.conf.
Provjerite postoje li greške u zonama:
/usr/sbin/named-checkzone tecaj.hr /var/named/zone/tecaj.hr
/usr/sbin/named-checkzone 1.168.192.in-addr.arpa /var/named/zone/192.168.1
/usr/sbin/named-checkzone 2.168.192.in-addr.arpa /var/named/zone/192.168.2
9. Pokrenite service named reload.
Provjerite je li sve u redu:
nslookup server1.tecaj.hr 127.0.0.1
(Upišite naredbu nslookup server1.tecaj.hr 192.168.2.201. Koji ste rezultat
dobili i zašto?)
U datoteci /etc/init.d/named proučite što radi naredba service named reload.
Na koje načine ona pokušava ostvariti da named ponovno učita konfiguracijsku datoteku?
Koji je prvi način? Kada će se pokrenuti drugi način?
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Vježbe
__________________________________________________________
10. U datoteci /etc/named.conf (u sekciji options) izmijenite redak u kojem piše
listen-on port 53 { 127.0.0.1; };
tako da on glasi:
listen-on port 53 { any; };
U sekciji options izmijenite postojeći ili dodajte novi (ako ne postoji) redak s
direktivom allow-query tako da piše:
allow-query
{ 192.168.1.0/24; 192.168.2.0/24; localhost; };
Pokrenite service named reload (ili service named restart). Provjerite je li sve
u redu:
nslookup server1.tecaj.hr 192.168.2.201
Postavke na klijentima
11. Na sustavu server1 izmijenite datoteku /etc/resolv.conf tako da njezin sadržaj
bude:
search tecaj.hr
Sadržaj datoteke /etc/sysconfig/network neka bude:
NETWORKING=yes
HOSTNAME=server1.tecaj.hr
GATEWAY=192.168.1.1
Provjerite sadržaj datoteka /etc/sysconfig/network-scripts/ifcfg-eth0 i
/etc/sysconfig/network-scripts/ifcfg-eth1 i napravite izmjene ako je
potrebno (posebno provjerite vrijednosti varijabli IPADDR, PREFIX i GATEWAY).
Ponovno pokrenite sustav. (Aktivacija načinjenih izmjena moguća je i bez ponovnog
pokretanja sustava - dovoljno je ponovno pokrenuti odgovarajuće servise. Ponovnim
pokretanjem sustava provjerit ćemo prije nastavka rada jesu li načinjene izmjene trajne.)
Napomena: Ako želite, možete isključiti Network Manager
(yum -y remove NetworkManager; service network restart).
12. Na sustavu server2 izmijenite datoteku /etc/resolv.conf tako da njezin sadržaj
bude:
search tecaj.hr
78
Vježbe
__________________________________________________________
Provjerite radi li naredba nslookup server1.
Sadržaj datoteke /etc/sysconfig/network na sustavu server2 neka bude:
NETWORKING=yes
HOSTNAME=server2.tecaj.hr
GATEWAY=192.168.2.201
Provjerite sadržaj datoteke /etc/sysconfig/network-scripts/ifcfg-eth0 i
napravite izmjene ako je potrebno.
Ponovno pokrenite sustav.
79
Vježbe
__________________________________________________________
Vježba 3: Sekundarni DNS
1. Na sustavu server1 izmijenite konfiguracijsku datoteku za DNS (/etc/named.conf)
tako da u sekciju options (na primjer nakon retka s allow-query) dodate:
allow-transfer { 192.168.2.202; };
U definicije svih zona (datoteke tecaj.hr, 192.168.1 i 192.168.2 u direktoriju
/var/named/zone) dodajte server2.tecaj.hr kao dodatni name server, a serial
number povećajte za jedan
Na primjer, zaglavlje zone tecaj.hr izgledat će ovako:
@
IN
SOA
NS
NS
tecaj.hr.
root.tecaj.hr. (
2014030202
1D
1H
1W
3H )
server1.tecaj.hr.
server2.tecaj.hr.
;
;
;
;
;
serial
refresh
retry
expire
minimum
Ponovno pokrenite named. (Da smo izmjene radili samo na zonama, tada bi bilo dovoljno
da pokrenemo naredbu rndc reload.)
2. Na sustavu server2 instalirajte paket bind (umetnite instalacijski medij u optički uređaj,
potražite odgovarajući paket u direktoriju Packages te instalirajte ga naredbom rpm, na
primjer: rpm -i bind-9.8.2-0.17.rc1.el6_4.6.i686.rpm).
3. Na sustavu server2 izmijenite datoteku /etc/named.conf tako da u sekciji options
bude:
listen-on port 53 { any; };
forward first;
forwarders {
192.168.2.201;
};
allow-query {
192.168.1.0/24;
192.168.2.0/24;
localhost;
};
80
Vježbe
__________________________________________________________
Na kraj datoteke /etc/named.conf dodajte podatke o zonama:
zone "tecaj.hr" IN {
type slave;
masters { 192.168.2.201; };
file "slaves/tecaj.hr";
};
type slave;
masters { 192.168.2.201; };
file "slaves/192.168.1";
};
type slave;
masters { 192.168.2.201; };
file "slaves/192.168.2";
};
4. Provjerite postoji li direktorij /var/named/slaves. Ako ne postoji napravite ga. Ako
postoji i nije prazan, obrišite njegov sadržaj (rm /var/named/slaves/*).
Provjerite s /usr/sbin/named-checkconf postoje li greške u datoteci named.conf i
nakon toga pokrenite named:
service named start
chkconfig named on
Provjerite s nslookup server1.tecaj.hr localhost je li sve u redu.
Proučite sadržaj direktorija /var/named/slaves. Što se dogodilo?
5. Na sustavu server2 izmijenite sadržaj datoteke /etc/resolv.conf tako da bude:
search tecaj.hr
Na sustavu server1 zaustavite named: service named stop.
Na sustavu server2 pokrenite nslookup.
81
Vježbe
__________________________________________________________
Provjerite rezultate sljedećih upita:
> server
Odgovor bi trebao biti:
Default server: 192.168.2.201
Address: 192.168.2.201#53
Default server: 192.168.2.202
Address: 192.168.2.202#53
> server1
Nakon kratkog čekanja, odgovor bi trebao biti:
Server:
Address:
192.168.2.202
192.168.2.202#53
Name:
server1.tecaj.hr
Address: 192.168.1.100
Name:
server1.tecaj.hr
Address: 192.168.2.201
Što možete zaključiti iz rezultata upita?
Ponovno pokrenite named na sustavu server1 (service named start).
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Vježbe
__________________________________________________________
Vježba 4: DNSSEC
1. Na sustavu server1 (u nekom direktoriju koji nije dostupan ostalim korisnicima, na
primjer u home-direktoriju ili direktoriju /var/named/zone) pokrenite naredbu:
dnssec-keygen -a HMAC-MD5 -b 256 -n host tecajhr
Napomena: Na sporijim sustavima kreiranje ključa može trajati i desetak minuta.
U radnom direktoriju stvorit će se dvije nove datoteke, na primjer:
Ktecajhr.+157+45497.key
Ktecajhr.+157+45497.private
Primjer sadržaja datoteke key-datoteke (Ktecajhr.+157+45497.key):
tecaj.hr. IN KEY 512 3 157 WmcHtBlHmTyw7HNM9+h6Brlp+EjGmqLmHW6Y6tMvXJg=
Primjer sadržaja private-datoteke (Ktecajhr.+157+45497.private):
Private-key-format: v1.3
Algorithm: 157 (HMAC_MD5)
Key: WmcHtBlHmTyw7HNM9+h6Brlp+EjGmqLmHW6Y6tMvXJg=
Bits: AAA=
Created: 20140302191527
Publish: 20140302191527
Activate: 20140302191527
U ovom primjeru konkretna vrijednost ključa je:
WmcHtBlHmTyw7HNM9+h6Brlp+EjGmqLmHW6Y6tMvXJg=
Napomena: U obje je datoteke vrijednost ključa ista jer je riječ o simetričnom algoritmu za
kriptiranje. Vrijednost ključa je tajni podatak koji ne bi trebao biti dostupan
ostalim korisnicima na sustavu.
2. Stvorite datoteku /etc/slave.key sljedećeg sadržaja:
key "tecajhr" {
algorithm hmac-md5;
secret "WmcHtBlHmTyw7HNM9+h6Brlp+EjGmqLmHW6Y6tMvXJg=";
};
(Umjesto "WmcHtBlHmTyw7HNM9+h6Brlp+EjGmqLmHW6Y6tMvXJg=" upišite vrijednost
Vašeg ključa.)
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Postavite odgovarajuće postavke na datoteku /etc/slave.key:
chown root:named /etc/slave.key
chmod 640 /etc/slave.key
3. U datoteku /etc/named.conf (na primjer iza include "/etc/named.root.key";)
dodajte redak:
U sekciju koja opisuje zonu tecaj.hr dodajte:
allow-transfer { key tecajhr.; };
Provjerite da u sekciji options piše:
dnssec-enable yes;
dnssec-validation no;
Provjerite s /usr/sbin/named-checkconf postoje li greške u datoteci named.conf.
Ponovno pokrenite named: service named restart
Napomena: Zone 1.168.192.in-addr.arpa i 2.168.192.in-addr.arpa su radi
potreba ove vježbe namjerno ostale su nepromijenjene. U stvarnom slučaju
primijenili bismo isti postupak i na njih (tj. dodali bismo redak
allow-transfer { key tecajhr; }; u sve sekcije koje opisuju zone).
4. Na sustavu server2 zaustavite named (service named stop) i obrišite podatke u
direktoriju /var/named/slaves (na primjer: rm /var/named/slaves/*).
Ponovno pokrenite named: service named start
Provjerite sadržaj direktorija /var/named/slaves. Koje su se zone kopirale? Nedostaje
li koja?
Provjerite što se dogodilo:
nslookup server1.tecaj.hr localhost
nslookup 192.168.2.201 localhost
4. Kopirajte datoteku /etc/slave.key sa sustava server1 na sustav server2 (na
primjer, naredbom scp ili neposredno kopirajući tekst (copy&paste) iz jednog u drugi
terminalski prozor).
Postavite datoteci /etc/slave.key (na sustavu server2) odgovarajuća prava
pristupa:
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chown root:named /etc/slave.key
chmod 640 /etc/slave.key
U datoteku /etc/named.conf na sustavu server2 dodajte sljedeće retke:
server 192.168.2.201 {
keys tecajhr;
};
Ponovno pokrenite named.
Pogledajte sadržaj direktorija /var/named/slaves. Što se promijenilo?
Potražite redak s ključnom riječju TSIG u datoteci /var/log/messages.
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Vježba 5: Sendmail i Postfix
U ovoj vježbi postavit ćemo mail-poslužitelje (MTA) na sustavima server1 i server2. Na
sustavu server1 postavit ćemo Sendmail, a na sustavu server2 Postfix.
Obzirom da je na sustavu server1 već instaliran Postfix, prvo ćemo Postfix zamijeniti sa
Sendmailom ...
Instaliranje Sendmaila
1. Aplikacija alternatives služi za stvaranje i upravljanje alternativnim softverskim
paketima na sustavu. Na sustavu server1 provjerite trenutačnu situaciju s MTA:
alternatives --display mta
Napomena: Uvid u trenutačnu situaciju možete dobiti i sa
ls -l /etc/alternatives/mta* | cut -f10- -d" "
odnosno
( cd /etc/alternatives ; ls -l mta* | cut -f10- -d" " )
Ukoliko sendmail nije instaliran, instalirajte ga:
yum install sendmail sendmail-cf m4
Ako je potrebno, zaustavite postfix i onemogućite njegovo pokretanje prilikom
pokretanja sustava:
service postfix stop
chkconfig postfix off
Zamijenite Postfix sa Sendmailom:
alternatives --set mta /usr/sbin/sendmail.sendmail
Kako biste dobili detaljniji uvid u načinjene promjene, ponovno pokrenite
( cd /etc/alternatives ; ls -l mta* | cut -f10- -d" " )
U datoteci /etc/mail/sendmail.mc dodajte riječ dnl na početak retka koji glasi:
DAEMON_OPTIONS(`Port=smtp,Addr=127.0.0.1, Name=MTA')dnl
Nakon izmjene, taj redak izgleda ovako:
dnl DAEMON_OPTIONS(`Port=smtp,Addr=127.0.0.1, Name=MTA')dnl
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Napomena: Riječ je o retku kojim se određuje da sendmail prihvaća konekcije samo na
adresi 127.0.0.1. Ovime smo osigurali da se taj redak kod sljedećeg
stvaranja nove konfiguracijske datoteke za sendmail ignorira (sendmail
će prihvaćati konekcije na svim mrežnim sučeljima).
Stvorite novu konfiguracijsku datoteku:
mv /etc/mail/sendmail.cf /etc/mail/sendmail.cf.orig
Provjerite u čemu se razlikuju nova i stara konfiguracijska datoteka:
diff /etc/mail/sendmail.cf /etc/mail/sendmail.cf.orig
Napomena: Izmjene smo mogli napraviti i neposredno na datoteci sendmail.cf, na
primjer:
sed -i.orig 's/Addr=127.0.0.1,//' /etc/mail/sendmail.cf
Pokrenite sendmail:
chkconfig sendmail on
service sendmail start
2. Pošaljite poruku korisniku root (mail [email protected]). Provjerite je li
isporučena.
Proučite sadržaj direktorija /var/spool/mail.
3. Ako je potrebno, instalirajte telnet (yum install telnet).
Pokrenite telnet:
telnet localhost 25
i upišite sljedeće:
HELP
EHLO pogodi-tko-sam.hr
MAIL From: [email protected]
RCPT To: [email protected]
DATA
Subject: Opasna prijetnja
Pozdrav od hakera.
.
QUIT
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Vježbe
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Je li isporučena nova poruka za korisnika root?
Proučite zapise u log-datoteci /var/log/maillog.
MX-zapisi (DNS), local-host-names i aliasi
4. U zonu tecaj.hr (datoteka /var/named/zone/tecaj.hr) dodajte sljedeći MX-zapis:
tecaj.hr.
IN
MX
10
server1.srce.hr.
Pokrenite rndc reload.
Provjerite ispravnost nove konfiguracije: nslookup -query=mx tecaj.hr
5. U datoteku /etc/mail/local-host-names upišite sljedeći redak:
tecaj.hr
Ponovno pokrenite sendmail: service sendmail restart.
Pošaljite poruku na adresu [email protected]. Provjerite je li isporučena.
6. U datoteku /etc/aliases dodajte redak:
ime.prezime:
root
Pokrenite newaliases.
Pošaljite poruke na [email protected] i [email protected]:
mail -s "Ovo je /etc/passwd" [email protected] < /etc/passwd
mail -s "Ovo je /etc/passwd" [email protected] < /etc/passwd
Provjerite jesu li poruke isporučene.
Mail queue
7. Popis poruka koje čekaju u redu za isporuku (mail queue) može se ispisati naredbama
sendmail -bp i mailq. Provjerite ima li na sustavu server1 neisporučenih poruka koje
čekaju u redu.
Pokušajte poslati sljedeću poruku:
mail -s "Oprostite na smetnji" [email protected] < /dev/null
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Vježbe
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Provjerite je li poruka ostala čekati u redu za isporuku:
mailq
Neisporučene poruke nalaze se u direktoriju /var/spool/mqueue. Pogledajte sadržaj
tog direktorija i usporedite ga s prethodnim rezultatom naredbe mailq:
ls -l /var/spool/mqueue
Proučite sadržaj kontrolne datoteke (s prefiksom qf):
more /var/spool/mqueue/qf*
Pronađite u log-datoteci /var/log/maillog zapise vezane uz isporuku poruke koje ste
poslali.
Pokušajte odgovoriti na sljedeća pitanja:
 Kojem je poslužitelju u domeni srce.hr naš poslužitelj pokušao proslijediti poruku?

Koji je SMTP reply-kod naš poslužitelj dobio kad je pokušao isporučiti poruku?

Zašto je poruka ostala u redu (nije isporučena primatelju niti se vratila pošiljatelju)?
Obrišite poruke koje čekaju u redu:
rm /var/spool/mqueue/*
8. Pokušajte poslati sljedeću poruku:
mail [email protected] < /dev/null
Što se dogodilo? Nemojte je obrisati iz reda za isporuku. Iskoristit ćemo je u nastavku ove
vježbe.
Postfix
9. Provjerite je li na sustavu server2 instaliran i pokrenut Postfix:
service postfix status
Upoznajte se s aktualnom konfiguracijom:
postconf mail_version
postconf inet_interfaces
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Proučite koje su standardne postavke programa Postfix izmijenjene zapisima u main.cf:
postconf -n
U datoteci /etc/postfix/main.cf postavite vrijednost varijable inet_interfaces
na all:
inet_interfaces = all
Obrišite redak u kojem piše inet_interfaces = localhost ili ga pretvorite u
komentar: # inet_interfaces = localhost
Ponovno pokrenite Postfix:
service postfix restart
Napomena: Obzirom da je u datoteci main.cf promijenjena vrijednost varijabla
inet_interfaces (promijenjene su postavke koje se odnose na mrežno
sučelje), nije bilo dovoljno pokrenuti postfix reload.
10. Na sustavu server1 provjerite je li poruka koju smo pokušali poslati korisniku
[email protected] još uvijek u redu za isporuku (mailq).
Ručno pokrenite obradu poruka koje čekaju u redu:
sendmail -q
Provjerite je li poruka još uvijek u redu. Provjerite je li ju primatelj na sustavu server2
primio.
Mail relaying
Obzirom da server2 za sada još uvijek ne može pristupati sadržajima na Internetu (za to je
potrebno implementirati NAT na sustavu server1, što ćemo napraviti kasnije), postavit
ćemo da sva pošta sa sustava server2 ide preko sustava server1. Isto tako, postavit
ćemo da sustav server1 prihvaća i prosljeđuje svu poštu za sustav server2.
Također, postavit ćemo (putem datoteke .forward) da se pošta za korisnika
[email protected] prosljeđuje i korisniku root na sustavu server1.
11. Na sustavu server2 promijenite sadržaj datoteke /etc/postfix/main.cf tako da
piše:
relayhost = [server1.tecaj.hr]
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Pokrenite postfix reload.
Provjerite jesu li izmjene prihvaćene: postconf relayhost.
12. Na sustavu server1 u datoteku /etc/mail/access dodajte redak:
Connect:192.168.2
RELAY
Pokrenite naredbu
makemap hash /etc/mail/access.db < /etc/mail/access
(ili ponovno pokrenite sendmail).
13. Na sustavu server2 pokušajte poslati poruku korisniku [email protected]:
mail [email protected] < /dev/null
Na oba sustava (server1 i server2) pokrenite naredbu mailq i provjerite na kojem je
sustavu ova poruka završila u redu za isporuku (trebala bi biti na sustavu server1).
Što znači dobiveni rezultat?
14. Na sustavu server2 u home-direktoriju korisnika root stvorite datoteku .forward
sljedećeg sadržaja:
\root
[email protected]
(\root obavezno mora biti u prvom retku)
Postavite odgovarajuća prava pristupa:
chmod 644 .forward
Pošaljite poruku korisniku [email protected] i provjerite je li ona isporučena na
obje adrese navedene u datoteci .forward.
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Vježba 6: Majordomo
Instaliranje Majordoma
1. Ako na sustavu server1 nije instaliran kompajler za C, instalirajte ga
(yum install gcc).
Preuzmite s Interneta Majordomovu distribucijsku arhivu i raspakirajte je u direktoriju
/tmp:
cd /tmp
wget http://www.greatcircle.com/majordomo/1.94.5/majordomo-1.94.5.tar.gz
tar xvf majordomo-1.94.5.tar.gz
cd majordomo-1.94.5
2. U datoteci Makefile načinite sljedeće izmjene:
PERL = /usr/bin/perl
W_HOME = /usr/local/majordomo
TMPDIR = /var/tmp
(Vrijednost varijable TMPDIR mogla je i ostati nepromijenjena. Naime originalna vrijednost
varijable TMPDIR [direktorij /usr/tmp] je simbolički link na /var/tmp.)
U datoteci sample.cf načinite sljedeće izmjene:
$whereami = "tecaj.hr";
$sendmail_command = "/usr/sbin/sendmail";
(Druge postavke za varijablu sendmail_command treba obrisati ili pretvoriti u komentar.)
Stvorite direktorij /usr/local/majordomo te korisnika i skupinu majordomo:
mkdir /usr/local/majordomo
groupadd -g 45 majordomo
useradd -g 45 -u 123 majordomo
Pokrenite prevođenje i instalaciju izvršnog koda:
make install
make install-wrapper
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Napravite provjeru (na pitanje: „Shall I send email to [email protected] to register this version?“ odgovorite s no):
cd /usr/local/majordomo
./wrapper config-test
Omogućite da sendmail pokreće Majordomov wrapper:
ln -s /usr/local/majordomo/wrapper /etc/smrsh
U datoteku /etc/mail/sendmail.mc dodajte sljedeći redak (uočite da su prvi
navodnici backquote):
define(`confDONT_BLAME_SENDMAIL',`GroupWritableDirPathSafe,GroupWritableIncludeFile')
i pokrenite
Ponovno pokrenite sendmail (service sendmail restart).
Napomena: Oni koji se odluče za neposredno editiranje datoteke
/etc/mail/sendmail.cf trebaju dodati sljedeći redak u datoteku
sendmail.cf (potražite gdje se u datoteci sendmail.cf nalazi ključna
riječ DontBlameSendmail i na tom mjestu napravite izmjenu):
O DontBlameSendmail=GroupWritableDirPathSafe,GroupWritableIncludeFile
Napomena: Ovime smo isključili dio sigurnosnih ograničenja implementiranih u program
sendmail koja ne dozvoljavaju da rad s datotekama i direktorijima koji su
group-writable.
U datoteku /etc/aliases dodajte nove aliase:
majordomo: "|/usr/local/majordomo/wrapper majordomo"
majordomo-owner: root
i pokrenite naredbu newaliases.
Provjerite radi li ispravno ono što ste do sada napravili:
echo help | mail -s "help" majordomo
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Vježbe
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Definiranje lista
3. Ako ne postoji korisnik tux stvorite ga (tux će biti vlasnik liste koju ćemo stvoriti u
nastavku vježbe): useradd -c "Tux the Penguin" tux
Podesite sve što je potrebno za novu listu koja će se zvati moja-lista.
Prvo je potrebno stvoriti praznu datoteku /usr/local/majordomo/lists/moja-lista:
touch /usr/local/majordomo/lists/moja-lista
chgrp majordomo /usr/local/majordomo/lists/moja-lista
chmod 664 /usr/local/majordomo/lists/moja-lista
Sljedeći je korak stvaranje datoteke moja-lista.info. U njoj će se nalaziti uvodne
informacije o listi:
cat > /usr/local/majordomo/lists/moja-lista.info
Ovo je lista za sve moje obožavatelje.
^D
(control-D)
Stvorite nove aliase. U datoteku /etc/aliases dodajte:
moja-lista: "|/usr/local/majordomo/wrapper resend -l moja-lista moja-lista-outgoing"
moja-lista-outgoing: :include:/usr/local/majordomo/lists/moja-lista
moja-lista-request: "|/usr/local/majordomo/wrapper request-answer moja-lista"
owner-moja-lista: tux
moja-lista-approval: tux
i pokrenite naredbu newaliases.
Pošaljite poruku Majordomu i proučite odgovor:
echo lists | mail -s "" majordomo
Napomena: Obzirom da nismo stvorili datoteku
/usr/local/majordomo/lists/moja-lista.config, Majordomo će
je po primitku ove poruke sam stvoriti. Možete proučiti njezin sadržaj.
Predbilježbe i slanje poruka
4. Pokušajte se predbilježiti na listu:
echo subscribe moja-lista | mail -s "" majordomo
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Vježbe
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Dobit ćete odgovor s uputama što dalje trebate učiniti (a to je da na adresu
[email protected] pošaljete poruku poput:
auth 35219738 subscribe moja-lista [email protected]).
Pošaljite poruku kojom potvrđujete zahtjev za predbilježbom:
mail -s "" majordomo
auth 35219738 subscribe moja-lista [email protected]
.
Pročitajte odgovor koji ste dobili.
Pogledajte sadržaj datoteke /usr/local/majordomo/lists/moja-lista.
Pogledajte koju je poruku dobio tux (more /var/spool/mail/tux).
5. Konačno, pošaljite poruku na listu:
echo "Pozdrav svima." | mail -s "Juhuhu ..." moja-lista
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Vježba 7: Procmail
1. Provjerite je li na sustavu server1 Procmail postavljen kao lokalni mail delivery agent
(MDA):
grep "^Mlocal" /etc/mail/sendmail.cf
Kada Procmail ne bi bio postavljen kao lokalni mail delivery agent, tada bismo mu poštu
upućivali na filtriranje putem datoteke .forward.
2. Neka se sva pošta koja dođe od korisnika tux automatski arhivira u datoteku tux (u
home-direktoriju).
U tu svrhu kao korisnik root na sustavu server1 stvorite (u home-direktoriju) datoteku
.procmailrc sljedećeg sadržaja:
:0:
* ^From.*tux@server1\.tecaj\.hr
$HOME/tux
U gornjem je primjeru :0 oznaka za početak recipea, a prva zvjezdica (*) u drugom retku
je oznaka za početak uvjeta. Ostatak uvjeta je regularni izraz (regular expression) kojim
se opisuje traženi uzorak. U našem slučaju to je uzorak na početku retka (^) koji
započinje s From nakon čega slijedi proizvoljan niz znakova koji završava s
[email protected].
Navedeni izraz odgovara različitim varijantama From-zaglavlja poput:
From: [email protected]
ili
From: Tux the Penguin <[email protected]>
Neka Vam tux pošalje neku poruku. Što se s njom dogodilo?
3. Neka se sve poruke koja dođu s liste moja-lista automatski arhiviraju u komprimiranu
datoteku moja-lista.gz. Poruke se trebaju pojaviti i u standardnom mailboxu.
U tu svrhu na početak datoteke .procmailrc dodajte:
:0c:
* ^(From|Cc|To).*moja-lista
| gzip >> $HOME/moja-lista.gz
Pošaljite poruku na adresu moja-lista te provjerite je li se poruka pojavila u mailboxu i
je li arhivirana u datoteci moja-lista.gz.
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4. Korisnik root na sustavu server2 ([email protected]) često šalje svojim
poslovnim kolegama poruke sa šalama. Takve poruke naslovljava s „joke“ ili „funny“.
Obzirom da ne želimo da se takve poruke pojavljuju u našem mailboxu, automatski ćemo
ih pohraniti u arhivu jokes. Te poruke također želimo automatski proslijediti drugom
poznatom šaljivđiji i ljubitelju viceva - korisniku tux na sustavu server1.
U tu svrhu u datoteku .procmailrc dodajte:
:0:
* ^From.*root@server2\.tecaj\.hr
* ^Subject.*(joke|funny)
{
:0c
! [email protected]
:0
jokes
}
Pošaljite poruku kao [email protected]:
[root@server2 ~]# mail -s "joke #1453" [email protected] < /etc/motd
i provjerite radi li sve kako treba.
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Vježba 8: Apache
Jednostavna konfiguracija web-poslužitelja Apache
1. Na sustavu server1 instalirajte web-poslužitelj Apache:
yum install httpd
U datoteci /etc/httpd/conf/httpd.conf (otvorite je u editoru) pronađite vrijednosti
sljedećih varijabli, zabilježite njihovu vrijednost te pročitajte komentare koji ih opisuju:
ServerRoot
__________________
DocumentRoot
__________________
Include
__________________
User
__________________
Group
__________________
PidFile
__________________
Listen
__________________
KeepAlive
_______
MaxKeepAliveRequests
_______
KeepAliveTimeout
_______
StartServers
_______
MinSpareServers
_______
MaxSpareServers
_______
ServerLimit
_______
MaxClients
_______
MaxRequestsPerChild
_______
Promijenite vrijednost varijable ServerName tako da bude www.tecaj.hr (pohranite
izmjene i izađite iz editora).
2. U zonu tecaj.hr (datoteka /var/named/zone/tecaj.hr) dodajete sljedeći zapis:
www
CNAME
server1.tecaj.hr.
i pokrenite naredbu rndc reload (provjerite s nslookup www je li izmjena registrirana).
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Vježbe
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3. Pokrenite web-poslužitelj (service httpd start). Provjerite odaziva li se webposlužitelj na adresi http://www.tecaj.hr.
Ako je sve u redu pokrenite naredbu chkconfig httpd on.
(Što se događa ako pokušate pristupiti adresi http://www.tecaj.hr/index.html?)
4. U direktoriju na koji u datoteci /etc/httpd/conf/httpd.conf upućuje varijabla
DocumentRoot (a to je /var/www/html) stvorite datoteku index.html sljedećeg
sadržaja:
<HTML>
<HEAD>
<TITLE>Pozdrav svijetu</TITLE>
</HEAD>
<BODY>
Pozdrav svima!
</BODY>
</HTML>
Ponovno provjerite što se nalazi na adresama http://www.tecaj.hr i
http://www.tecaj.hr/index.html.
5. Pokrenite naredbu telnet localhost 80 (umjesto localhost također možete
upisati server1 ili www) i upišite:
GET / HTTP/1.1
Host: www.tecaj.hr
[Enter]
(Prazan redak)
Proučite rezultat.
Ponovno pokrenite naredbu telnet localhost 80. Ovaj put upišite samo:
GET /
Proučite rezultat.
Još jednom pokrenite naredbu telnet localhost 80. Ovaj put upišite:
HEAD / HTTP/1.1
Host: www.tecaj.hr
[Enter]
Napomena: Komunikaciju između web-poslužitelja i klijenta moguće je pratiti pomoću
naredbe curl -v. Na primjer:
curl -v http://www.tecaj.hr
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Vježbe
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6. Stvorite direktorij /var/www/html/prijatelji:
mkdir /var/www/html/prijatelji
cp /var/www/html/index.html /var/www/html/prijatelji/index.html
Pokrenite naredbu telnet localhost 80 i upišite:
GET /prijatelji
Rezultat je poruka o greški 301 Moved Permanently i preusmjeravanje na novi URL.
Ponovite naredbu telnet localhost 80, ali ovaj put upišite
GET /prijatelji/
Basic Authentication
7. Stvorite datoteku www.passwd u direktoriju /etc/httpd/conf na sljedeći način:
htpasswd -c -b /etc/httpd/conf/www.passwd root glavni
htpasswd -b /etc/httpd/conf/www.passwd tux sporedni
Napomena: U prvoj smo naredbi rabili prekidač -c koji stvara datoteku, obzirom da prije
pokretanja naredbe ciljna datoteka nije postojala. Da smo u drugoj naredbi
ponovno rabili taj prekidač, izgubili bismo sav prethodni sadržaj.)
Pogledajte što se nalazi u datoteci /etc/httpd/conf/www.passwd.
U datoteku httpd.conf (direktorij /etc/httpd/conf) dodajte (može i na kraj
datoteke):
<Directory /var/www/html/prijatelji>
AuthType basic
AuthName "Restricted Stuff"
AuthUserFile conf/www.passwd
Require user root tux
</Directory>
Pokrenite service httpd reload.
Pokušajte pristupiti URL-u http://www.tecaj.hr/prijatelji.
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Vježbe
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Napomena: U ovom smo konkretnom slučaju umjesto Require user root tux
mogli koristiti i direktivu Require valid-user.
Korisnici se također mogu grupirati u skupine. U tom bismo slučaju, na
primjer, u datoteku www.group upisali:
prijatelji: root tux
a u httpd.conf bismo napisali:
<Directory /var/www/html/prijatelji>
AuthType basic
AuthName "Restricted Stuff"
AuthUserFile conf/www.passwd
AuthGroupFile conf/www.group
Require group prijatelji
</Directory>
8. Pokrenite naredbu telnet localhost 80 i upišite:
GET /prijatelji/
Proučite rezultat.
Kriptirajte (Base64 encoding) string „tux:sporedni“:
echo -n "tux:sporedni" | openssl base64 -base64
Rezultat je kod koji ćemo u nastavku koristi za autentikaciju (na primjer:
dHV4OnNwb3JlZG5p).
GET /prijatelji/ HTTP/1.1
Authorization: Basic dHV4OnNwb3JlZG5p
Host: localhost
[Enter]
Pogledajte kako to isto izgleda s naredbom curl:
curl -v -u tux:sporedni http://www.tecaj.hr/prijatelji/
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Vježbe
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Vježba 9: Virtualni web-poslužitelji
1. Stvorite direktorij /var/www/portal i u njemu datoteku index.html sljedećeg
sadržaja:
<HTML>
<HEAD>
<TITLE>Portal</TITLE>
</HEAD>
<BODY>
Pozdrav posjetiteljima portala!
</BODY>
</HTML>
portal
CNAME
server1.tecaj.hr.
i pokrenite naredbu rndc reload.
3. U datoteku httpd.conf (u direktoriju /etc/httpd/conf) dopišite (na kraj datoteke):
NameVirtualHost *:80
<VirtualHost *:80>
DocumentRoot /var/www/html
ServerName www.tecaj.hr
ServerAlias www
ErrorLog /var/log/httpd/error.log
CustomLog /var/log/httpd/requests.log combined
</VirtualHost>
<VirtualHost *:80>
DocumentRoot /var/www/portal
ServerName portal.tecaj.hr
ServerAlias portal
ErrorLog /var/log/httpd/portal-error.log
CustomLog /var/log/httpd/portal-requests.log common
</VirtualHost>
Pokrenite naredbu service httpd reload.
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Vježbe
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Napomena: S obzirom na to da je virtualni poslužitelj s direktivom ServerName
www.tecaj.hr naveden prvi u konfiguracijskoj datoteci, on ima najviši
prioritet i može se smatrati primarnim (default) poslužiteljem. To znači da ako
web-poslužitelj primi upit koji ne odgovara niti jednoj od direktiva
ServerName (na primjer za poslužitelj server1.tecaj.hr), bit će
poslužen od strane prvog virtualnog poslužitelja navedenog u konfiguracijskoj
datoteci (u našem slučaju www.tecaj.hr).
4. Pistupite (curl, wget ili Firefox) sljedećim URL-ovima:
http://www.tecaj.hr
http://portal.tecaj.hr
http://server1.tecaj.hr
i provjerite radi li sve kako treba.
Sljedeća dva primjera pokazuju kako u praksi funkcionira mehnizam virtualnih poslužitelja.
GET / HTTP/1.1
Host: www.tecaj.hr
[Enter]
Zapamtite rezultat i ponovno pokrenite telnet localhost 80. Ovaj put upišite:
GET / HTTP/1.1
Host: portal.tecaj.hr
[Enter]
Uočite razliku u odnosu na prethodni rezultat. Što je uzrok različitim rezultatima? Koji bi
rezultat bio kad bi se ponovila naredba telnet localhost 80 i upisalo: Host:
server1.tecaj.hr?
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Vježbe
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Vježba 10: HTTPS
U ovoj vježbi stvorit ćemo na sustavu server1 novi sigurni virtualni web-poslužitelj
(https://trgovina.tecaj.hr). U tu svrhu dodijelit ćemo sustavu dodatnu IP-adresu
(192.168.1.110) koju će koristiti samo taj poslužitelj. Certifikat za sigurni web-poslužitelj
sami ćemo kreirati. One koji poslužitelju pristupe putem URL-a
http://trgovina.tecaj.hr preusmjerit ćemo na sigurni poslužitelj.
1. Pogledajte direktorij /etc/httpd/conf.d. U njemu se nalaze dodatne konfiguracijske
datoteke koje se standardno učitavaju zajedno s datotekom httpd.conf prilikom
pokretanja web-poslužitelja (u datoteci httpd.conf postoji odgovarajuća direktiva
Include conf.d/*.conf).
2. Instalirajte paket mod_ssl (yum install mod_ssl).
Nakon instalacije paketa mod_ssl pojavit će se u direktoriju /etc/httpd/conf.d nova
datoteka ssl.conf.
U toj se datoteci nalaze direktive nužne za uporabu SSL-a. U stvarnom slučaju svakako
bismo pregledali i modificirali tu datoteku, međutim za potrebe ove vježbe nećemo je
mijenjati (no, u svakom slučaju, možete je ipak pregledati i upoznati se s direktivama
specifičnim za SSL).
3. Certifikate za novi poslužitelj pohranit ćemo u direktorij /etc/httpd/ssl:
mkdir /etc/httpd/ssl
openssl req -x509 -nodes -days 365 -newkey rsa:2048 \
-keyout /etc/httpd/ssl/trgovina.key \
-out /etc/httpd/ssl/trgovina.crt
Nakon pokretanja gore navedene naredbe, trebat će interaktivno upisati neke podatke
potrebne za stvaranje certifikata. Upišite sljedeće:
Country Name (2 letter code) [XX]:HR
State or Province Name (full name) [Some-State]:.
Locality Name (eg, city) []:Zagreb
Organization Name (eg, company) [Internet Widgits Pty Ltd]:Trgovina d.o.o.
Organizational Unit Name (eg, section) []:.
Common Name (e.g. server FQDN or YOUR name) []:trgovina.tecaj.hr
Email Address []:[email protected]
Nakon toga pokrenite naredbu:
chmod 600 /etc/httpd/ssl/trgovina.*
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Vježbe
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Novostvoreni certifikat možete pogledati sljedećom naredbom:
openssl x509 -in /etc/httpd/ssl/trgovina.crt -text -noout | more
trgovina
A
192.168.1.110
U zonu 1.168.192.in-addr.arpa (datoteka /var/named/zone/192.168.1)
dodajete sljedeći zapis:
110
PTR
trgovina.tecaj.hr.
U obje zone (u zapisu SOA) povećajte serial number.
Pokrenite rndc reload.
5. Stvorite novu datoteku /etc/sysconfig/network-scripts/ifcfg-eth0:0 sa
sljedećim sadržajem:
TYPE=Ethernet
ONBOOT=yes
BOOTPROTO=none
IPADDR=192.168.1.110
PREFIX=24
GATEWAY=192.168.1.1
IPV6INIT=no
NAME=eth0:0
Ako je potrebno, pokrenite service network restart.
6. Stvorite direktorij /var/www/trgovina i u njemu datoteku index.html sljedećeg
sadržaja:
<HTML>
<HEAD>
<TITLE>Trgovina d.o.o.</TITLE>
</HEAD>
<BODY>
Niske cijene svaki dan!
</BODY>
</HTML>
7. Stvorite datoteku /etc/httpd/conf.d/trgovina.conf u kojoj će se nalaziti direktive
za konfiguraciju novog poslužitelja. Neka ta datoteka ima sljedeći sadržaj:
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Vježbe
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<VirtualHost 192.168.1.110:443>
SSLEngine on
SSLCertificateFile
SSLCertificateKeyFile
ServerName
DocumentRoot
ServerAlias
CustomLog
ErrorLog
</VirtualHost>
/etc/httpd/ssl/trgovina.crt
/etc/httpd/ssl/trgovina.key
trgovina.tecaj.hr
/var/www/trgovina
trgovina
/var/log/httpd/trgovina-access.log common
/var/log/httpd/trgovina-error.log
<VirtualHost 192.168.1.110:80>
ServerName
trgovina.tecaj.hr
ServerAlias
trgovina
Redirect 301 / https://trgovina.tecaj.hr/
</VirtualHost>
8. Pokrenite naredbu service httpd reload. Nakon toga pristupite sljedećim
URL-ovima:
https://trgovina.tecaj.hr
http://trgovina.tecaj.hr
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Vježbe
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Vježba 11: snmpd i MRTG
snmpd
1. Sačuvajte originalnu konfiguracijsku datoteku snmpd.conf:
mv /etc/snmp/snmpd.conf
/etc/snmp/snmpd.conf-orig
2. Stvorite novu konfiguracijsku datoteku /etc/snmp/snmpd.conf sljedećeg sadržaja:
rocommunity
syslocation
syscontact
public
"Srce, Ucionica B"
[email protected]
3. Pokrenite snmpd:
service snmpd start
chkconfig snmpd on
4. Instalirajte paket net-snmp-utils:
yum install net-snmp-utils
5. Pokrenite sljedeće naredbe i proučite rezultate:
snmpwalk -v1 -cpublic 127.0.0.1 syslocation
snmpwalk -v1 -cpublic 127.0.0.1 sysname
snmpwalk -v1 -cpublic 127.0.0.1 system
MRTG
6. Instalirajte paket mrtg:
yum install mrtg
Sačuvajte originalnu konfiguracijsku datoteku mrtg.cfg:
mv /etc/mrtg/mrtg.cfg /etc/mrtg/mrtg.cfg-bak
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Vježbe
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7. Stvorite novu konfiguracijsku datoteku /etc/mrtg/mrtg.cfg sljedećom naredbom:
cfgmaker --output=/etc/mrtg/mrtg.cfg \
-ifref=name --global "workdir: /var/www/mrtg/stats" \
public@localhost
8. Stvorite direktorij /var/www/mrtg/stats:
mkdir /var/www/mrtg/stats
9. Paket mrtg je prilikom instalacije stvorio cron-datoteku /etc/cron.d/mrtg. Pogledajte
njezin sadržaj.
10. Sačuvajte originalnu datoteku /etc/httpd/conf.d/mrtg.conf:
mv /etc/httpd/conf.d/mrtg.conf /etc/httpd/conf.d/mrtg.conf-orig
Stvorite novu datoteku /etc/httpd/conf.d/mrtg.conf sljedećeg sadržaja:
Alias /mrtg /var/www/mrtg
<Location /mrtg>
Options Indexes
</Location>
Pokrenite service httpd reload.
11. Pristupite URL-u http://server1.tecaj.hr/mrtg i proučite što se tamo nalazi.
Proučite i sadržaj poddirektorija stats.
Stvorite datoteku /var/www/mrtg/stats/index.html sljedećom naredbom:
indexmaker --output /var/www/mrtg/stats/index.html \
--columns=1 /etc/mrtg/mrtg.cfg
i posjetite URL http://server1.tecaj.hr/mrtg/stats.
108
Vježbe
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Vježba 12: Squid
Obzirom da ekipa iz mreže 192.168.2.0/24 još uvijek ne može pristupati Internetu (čekaju
da u jednoj od posljednjih vježbi postavimo NAT na sustav server1), omogućit ćemo im da
u međuvremenu pristupaju web-sadržajima putem web-posrednika (web proxy). Pristup
posredniku dozvolit ćemo samo iz naše interne mreže i to samo do web-sadržaja na
standardnim portovima 80 i 443. Načinit ćemo najjednostavniju konfiguraciju koja to
omogućava.
Firefox na sustavu server2 podesit ćemo ručno tako da sadržajima na webu pristupa
putem posrednika.
Na kraju ćemo s alternativnog repozitorija EPEL Repo instalirati paket Clamaris za analizu
sistemskih zapisa.
1. Na sustavu server1 instalirajte Sqiud (yum install squid).
Sačuvajte originalnu konfiguracijsku datoteku:
mv /etc/squid/squid.conf /etc/squid/squid.conf-bak
Stvorite novu konfiguracijsku datoteku /etc/squid/squid.conf sa sljedećim sadržajem:
acl Safe_ports port 80
acl Safe_ports port 443
http_access deny !Safe_ports
acl interna_mreza src 192.168.2.0/24
http_access allow interna_mreza
http_access deny all
http_port 3128
Pokrenite Squid (service squid start).
2. Na sustavu server2 podesite Firefox korisniku root tako da pristupa web-sadržajima
putem posrednika.
Pokrenite Firefox i odaberite Edit / Preferences /Advanced / Network.
Kliknite na dugme Settings... i u prozoru Connection Settings upišite postavke
za posrednika (kao na slici).
109
Vježbe
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3. Pokušajte pristupiti sljedećim URL-ovima (koji se nalaze izvan lokalne mreže
192.168.2.0/24):
http://www.srce.hr
https://abc.srce.hr
4. Na sustavu server1 dodajte EPEL Repo na popis paketnih repozitorija:
rpm -Uvh \
http://download.fedoraproject.org/pub/epel/6/i386/epel-release-6-8.noarch.rpm
Provjerite nalazi li se EPEL Repo na popis paketnih repozitorija:
yum repolist
5. Instalirajte paket Calamaris:
yum install calamaris
110
Vježbe
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6. Pokrenite sljedeće naredbe i proučite rezultat:
cat /var/log/squid/access.log | calamaris
calamaris -R 5 /var/log/squid/access.log
7. Stvorite direktorij /var/www/html/calamaris:
mkdir /var/www/html/calamaris
Pokrenite naredbu:
calamaris -a -F html \
/var/log/squid/access.log > /var/www/html/calamaris/index.html
Posjetite URL http://server1.tecaj.hr/calamaris.
8. Na kraju, možemo isključiti EPEL Repo s popisa paketnih repozitorija.
U datoteci /etc/yum.repos.d/epel.repo sve retke u kojima piše enabled=1
zamijenite s enabled=0.
Napomena: EPEL Repo smo s popisa paketnih repozitorija mogli isključiti i na druge
načine. Na primjer: yum-config-manager --disable epel.
Popis paketnih repozitorija može se ispisati naredbom yum repolist all.
111
Vježbe
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Vježba 13: DHCP
Osnovne postavke DHCP-poslužitelja
1. Na sustavu server1 instalirajte paket dhcp (yum install dhcp).
Provjerite u datoteci /etc/init.d/dhcpd u kojem se direktoriju na Vašem sustavu
treba nalaziti konfiguracijska datoteka za DHCP-poslužitelj (obično je to
/etc/dhcp/dhcpd.conf). Zapamtite taj podatak.
Napravite konfiguracijsku datoteku za DHCP-poslužitelj (/etc/dhcp/dhcpd.conf) tako
da ima sljedeći sadržaj:
subnet 192.168.2.0 netmask 255.255.255.0 {
option routers
192.168.2.201;
option subnet-mask
255.255.255.0;
option domain-name
option domain-name-servers
"tecaj.hr";
192.168.2.201;
range 192.168.2.220 192.168.2.240;
}
Napomena: Ako bismo htjeli da se IP-adresa sustavu server2 dodjeljuje putem
DHCP-a, ali da mu uvijek bude dodijeljena ista IP-adresa, tada bismo unutar
sekcije za mrežu 192.168.2.0 dopisali:
host server2 {
hardware ethernet 08:00:27:A8:09:46;
fixed-address 192.168.2.202;
}
(U gornjem primjeru umjesto 08:00:27:A8:09:46 treba napisati konkretnu
ethernet-adresu navedenog sučelja.)
U primjeru koji slijedi to nećemo napraviti, tj. kod dodjeljivanja IP-adrese
sustavu server2 nećemo utjecati na vrijednost dodijeljene adrese. To znači
da se sustavu server2 može dodijeliti bilo koja adresa iz zadanog raspona
(od 192.168.2.220 do 192.168.2.240).
S obzirom na to da konfiguracijska datoteka upućuje da će DHCP-poslužitelj posluživati
samo mrežu 192.168.2.0/24 na koju je spojeno mrežno sučelje eth1, u datoteci
/etc/sysconfig/dhcpd možemo zadati da se DHCP-poslužitelj veže samo uz to
sučelje.
112
Vježbe
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Stoga neka datoteka /etc/sysconfig/dhcpd sadrži sljedeći redak:
DHCPDARGS=eth1
Provjerite postoje li greške u konfiguracijskoj datoteci DHCP-poslužitelja:
service dhcpd configtest
Pokrenite dhcpd i postavite da se pokreće prilikom svakog pokretanja sustava:
service dhcpd start
chkconfig dhcpd on
Proučite sadržaj datoteke /var/lib/dhcpd/dhcpd.leases.
Osnovne postavke DHCP-klijenta
2. Na sustavu server2 podesite mrežno sučelje eth0 tako da adresu preuzima od
DHCP-poslužitelja.
Prije izmjena sačuvajte trenutačne postavke sučelja eth0:
cp /etc/sysconfig/network-scripts/ifcfg-eth0 /tmp/ifcfg-eth0-nodhcp
Promijenite sadržaj datoteke /etc/sysconfig/network-scripts/ifcfg-eth0 tako
da u njoj piše:
DEVICE=eth0
TYPE=Ethernet
ONBOOT=yes
BOOTPROTO=dhcp
U slučaju potrebe ponovno pokrenite mrežu (service network restart).
Provjerite koju adresu ima sučelje eth0.
3. Ponovno pogledajte sadržaj datoteke /var/lib/dhcpd/dhcpd.leases na sustavu
server1.
4. Pokrenite ping server2 (na bilo kojem od sustava). Što se dogodilo i zašto?
5. Vratite prethodne postavke sučelja sučelja eth0:
cp /tmp/ifcfg-eth0-nodhcp /etc/sysconfig/network-scripts/ifcfg-eth0
(i ako je potrebno ponovno pokrenite naredbu service network restart).
113
Vježbe
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Vježba 14: Dynamic DNS (DDNS)
1. Na sustavu server1 stvorite HMAC-MD5-keyfile s imenom dhcpupdate:
dnssec-keygen -a HMAC-MD5 -b 256 -n USER dhcpupdate
U radnom direktoriju pojavit će se dvije nove datoteke, na primjer:
Kdhcpupdate.+157+26467.key i Kdhcpupdate.+157+26467.private.
Na primjer, neka je sadržaj datoteke Kdhcpupdate.+157+26467.key:
dhcpupdate. IN KEY 0 3 157 S9fpDs3dmJQF7k5kGE6U80skG+DuOlAKZnWo5OQXLh4=
U gornjem je primjeru „S9fpDs3dmJQF7k5kGE6U80skG+DuOlAKZnWo5OQXLh4=“ vrijednost
novog ključa.
Stvorite datoteku /etc/dhcpupdate.key sa sljedećim sadržajem:
key "dhcpupdate" {
algorithm hmac-md5;
secret "S9fpDs3dmJQF7k5kGE6U80skG+DuOlAKZnWo5OQXLh4=";
};
(Umjesto „S9fpDs3dmJQF7k5kGE6U80skG+DuOlAKZnWo5OQXLh4=“ upišite vrijednost
Vašeg ključa.)
Postavite odgovarajuće postavke datoteci /etc/dhcpupdate.key te je kopirajte kao
/etc/dhcp/dhcpupdate.key (potrebno je napraviti dvije datoteke s istim sadržajem
zbog različitih prava pristupa):
chown root:named /etc/dhcpupdate.key
chmod 640 /etc/dhcpupdate.key
cp /etc/dhcpupdate.key /etc/dhcp/dhcpupdate.key
chown root:root /etc/dhcp/dhcpupdate.key
chmod 640 /etc/dhcp/dhcpupdate.key
Napomena: Zadnje tri naredbe moguće je zamijeniti jednom:
install -o root -g root -m 0640 \
/etc/dhcpupdate.key /etc/dhcp/dhcpupdate.key.
2. Na sustavu server1 u datoteku /etc/named.conf dodajte redak:
include "/etc/dhcpupdate.key";
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Vježbe
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U zone za koje želite da se dinamički mijenjaju dodajte:
allow-update { key dhcpupdate; };
U našem slučaju to su zone tecaj.hr i 2.168.192.in-addr.arpa.
Iz zona tecaj.hr i 2.168.192.in-addr.arpa (datoteke tecaj.hr i
2.168.192.in-addr.arpa u direktoriju /var/named/zone) obrišite sve podatke za
server2 (uključivši i one koje ga definiraju kao sekundarnog name-servera).
Ponovno pokrenite named.
Napomena: Obzirom da sustav server2 neće imati stalnu IP-adresu, nije prikladno da
bude sekundarni name-server. Također, njegove smo podatke u potpunosti
izbrisali iz DNS-tablica jer će ubuduće te podatke upisivati DHCP-poslužitelj.
3. Pomoću naredbe nsupdate provjerite je li sve u redu:
nsupdate
> server 192.168.1.100
> key dhcpupdate S9fpDs3dmJQF7k5kGE6U80skG+DuOlAKZnWo5OQXLh4=
(Na ovom mjestu upišite vrijednost Vašeg ključa.)
>
>
>
>
>
>
>
zone tecaj.hr.
update add server5.tecaj.hr. 600 IN A 192.168.2.205
send
zone 2.168.192.in-addr.arpa.
update add 205.2.168.192.in-addr.arpa. 600 IN PTR server5.tecaj.hr.
send
quit
Ukoliko je sve u redu, tada se nakon pokretanja komandi send neće javiti poruka o greški.
Služeći se naredbom nslookup dodatno provjerite radi li sve kako treba:
nslookup server5
nslookup 192.168.2.205
4. Na sustavu server1 na početak datoteke /etc/dhcp/dhcpd.conf dodajte:
ddns-updates on;
ddns-update-style interim;
update-static-leases on;
ignore client-updates;
include "/etc/dhcp/dhcpupdate.key";
zone tecaj.hr. {
primary 192.168.1.100;
key dhcpupdate;
}
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Vježbe
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zone 2.168.192.in-addr.arpa. {
primary 192.168.1.100;
key dhcpupdate;
}
Ponovno pokrenite dhcpd: service dhcpd restart.
5. Na sustavu server2 izmijenite datoteku /etc/resolv.conf tako da je njezin sadržaj
bude:
search tecaj.hr
Na sustavu server2 zaustavite named i onemogućite njegovo pokretanje prilikom
pokretanja sustava:
service named stop
chkconfig named off
Podesite mrežno sučelje eth0 na sustavu server2 tako da adresu preuzima od DHCPposlužitelja. Neka je sadržaj /etc/sysconfig/network-scripts/ifcfg-eth0:
DEVICE=eth0
TYPE=Ethernet
ONBOOT=yes
BOOTPROTO=dhcp
DHCP_HOSTNAME=server2
Napomena: Uočite novu varijablu DHCP_HOSTNAME.
Ako je potrebno, pokrenite naredbu service network restart.
S ifconfig provjerite je li se promijenila IP-adresa na sučelju eth0. Koja je nova
adresa?
Na stroju server1 pogledajte ponovno sadržaj datoteke
/var/lib/dhcpd/dhcpd.leases.
Provjerite s nslookup je li zabilježena promjena IP-adrese stroja server2.
Napomena: Ako ručno radite izmjene u sadržaju zone koja koristi Dynamic DNS (DDNS),
tada prvo zamrznite zonu: rndc freeze tecaj.hr
Nakon što ste završili s izmjenama odmrznite zonu (učitajte izmjene i
ponovno dozvolite DDNS): rndc thaw tecaj.hr
(Sustav će odgovoriti porukom: The zone reload and thaw was
successful.)
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Vježba 15: LDAP
U ovoj vježbi stvorit ćemo LDAP-poslužitelj na sustavu server1 (bez replikacije), prenijet
ćemo podatke o korisnicima u LDAP-ovu bazu i napravit ćemo da se autentikacija korisnika
prilikom prijavljivanja na sustav server2 obavlja putem LDAP-poslužitelja na sustavu
server1.
Napomena: Premda će rezultat ove vježbe biti potpuno funkcionalne postavke na sustavima,
te postavke ne bi bile primjerene u stvarnom okruženju (LDAP-ovu bazu bi u
stvarnom okruženju trerbalo dopuniti, omogućiti replikaciju podataka, uključiti i
sam poslužitelj u sustav autentikacije putem LDAP-a, uspostaviti kriptiranu
komunikaciju između poslužitelja i klijenata, postaviti dodatna sigurnosna
ograničenja i slično). Stoga ovu vježbu ni u kojem slučaju ne treba shvatiti
kao potpune upute kako podestiti LDAP za potrebe autentikacije na
Linuxu. Ova vježba prikazuje samo minimalno programsko okruženje u kojem
se to može uspostaviti.
Osnovne postavke poslužitelja OpenLDAP
1. Na sustavu server1 instalirajte pakete openldap, openldap-servers i
openldap-clients:
yum install openldap openldap-servers openldap-clients
Napomena: Postoje dva načina konfiguriranja poslužitelja OpenLDAP. Stari je način
putem datoteke slapd.conf, a novi je način putem LDIF-datoteka u
direktoriju slapd.d. Mi ćemo se služiti novim načinom.
Napravite kopiju direktorija /etc/openldap/slapd.d:
cp -R /etc/openldap/slapd.d /etc/openldap/slapd.d-orig
2. Editirajte datoteke olcDatabase={2}bdb.ldif i olcDatabase={1}monitor.ldif
koje se nalaze u direktoriju /etc/openldap/slapd.d/cn=config:
cd /etc/openldap/slapd.d/cn\=config
Izmijenite sadržaj datoteke olcDatabase={2}bdb.ldif
(vi olcDatabase\=\{2\}bdb.ldif) tako da piše:
olcSuffix: dc=tecaj,dc=hr
olcRootDN: cn=Manager,dc=tecaj,dc=hr
olcRootPW: 123
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Vježbe
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Ako u datoteci olcDatabase={2}bdb.ldif postoji redak koji počinje s
olcConfigFile (na primjer: olcConfigFile: /etc/openldap/slapd.conf.bak),
obrišite ga.
Izmijenite sadržaj datoteke olcDatabase={1}monitor.ldif
(vi olcDatabase\=\{1\}monitor.ldif) tako da piše:
olcAccess: {0}to * by
dn.base="gidNumber=0+uidNumber=0,cn=peercred,cn=external,cn=auth"
read by dn.base="cn=Manager,dc=tecaj,dc=hr" read by * none
3. Stvorite datoteku /var/lib/ldap/DB_CONFIG na sljedeći način:
cp /usr/share/openldap-servers/DB_CONFIG.example /var/lib/ldap/DB_CONFIG
Provjerite postavke datoteka u direktoriju /var/lib/ldap i ako je potrebno pokrenite
naredbu chown -R ldap:ldap /var/lib/ldap.
Provjerite jesu li sve konfiguracijske postavke ispravno zadane:
/usr/sbin/slaptest
Ako jesu, pokrenite LDAP-poslužitelj:
service slapd start
Napomena: LDAP-poslužitelj u modusu debug možete pokrenuti, na primjer,
na sljedeći način: slapd -u ldap -g ldap -d 255
(popis i objašnjenje svih debugging-razina pogledajte na
http://www.openldap.org/doc/admin24/runningslapd.html).
Provjerite radi li LDAP-poslužitelj. Zadajte naredbu:
ldapsearch -x -H ldap://localhost -b '' \
-s base '(objectclass=*)' namingContexts
Uočite da je u gornjoj naredbi '' prazan string (a ne dvostruki navodnici).
Ako dobijete odgovor koji sadrži:
dn:
namingContexts: dc=tecaj,dc=hr
to je znak da je najvjerojatnije sve u redu.
Ako je sve u redu, nagradite se naredbom chkconfig slapd on.
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4. Zadajte naredbu:
ldapsearch -h localhost -D "cn=Manager,dc=tecaj,dc=hr" \
-w 123 -b "dc=tecaj,dc=hr" -s sub "objectclass=*"
Stvorite novu datoteku /etc/openldap/ldap.conf sljedećeg sadržaja:
BASE
HOST
dc=tecaj,dc=hr
127.0.0.1
Sada možete pokrenuti prethodnu naredbu u njezinom kraćem obliku (bez navođenja
poslužitelja i baze):
ldapsearch -D "cn=Manager,dc=tecaj,dc=hr" -w 123 -s sub "objectclass=*"
5. Trenutačna je administracijska lozinka vrlo jednostavna i upisana je nekriptirana u
datoteku olcDatabase={2}bdb.ldif. Promijenit ćemo lozinku i upisati je u kriptiranom
obliku.
Neka je nova lozinka p@ssword. Kriptirajte je:
slappasswd -s p@ssword
{SSHA}MzZOUVoiW2o2DoSVbE3uGrpwPtoX7ssa
Rezultat naredbe slappasswd -s p@ssword je kriptirana inačica nove lozinke. Nju
ćemo upisati u datoteku olcDatabase={2}bdb.ldif (u direktoriju
/etc/openldap/slapd.d/cn=config), na mjesto stare lozinke 123:
olcRootPW: {SSHA}MzZOUVoiW2o2DoSVbE3uGrpwPtoX7ssa
Pokrenite service slapd restart i pokušajte izvršiti prethodnu naredbu
ldapsearch s novom lozinkom:
ldapsearch -D "cn=Manager,dc=tecaj,dc=hr" -w p@ssword \
-s sub "objectclass=*"
6. Preostalo je još za urediti ispis logova. Na kraj datoteke /etc/rsyslog.conf dodaje
redak:
local4.* /var/log/ldap.log
Pokrenite naredbu: service rsyslog restart i provjerite je li se stvorila datoteka
/var/log/ldap.log.
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Vježbe
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Vježba 16: Autentikacija putem LDAP-a
Kopiranje sistemskih podataka u LDAP (Migration Tools)
1. Instalirajte paket migrationtools (yum install migrationtools).
Izmijenite postavke u datoteci /usr/share/migrationtools/migrate_common.ph
tako da piše:
$DEFAULT_MAIL_DOMAIN = "tecaj.hr";
$DEFAULT_BASE = "dc=tecaj,dc=hr";
2. Za potrebe ove vježbe neka na sustavu server1 postoji korisnik tux čija je korisnička
lozinka 1, a na sustavu server2 korisnici tux s lozinkom 2 i tom s proizvoljnom
lozinkom.
Na sustavu server1 pokrenite sljedeće naredbe:
useradd -c "Tux the Penguin" tux
(echo 1 ; echo 1) | passwd tux
(ako korisnik tux ne postoji već od prije)
Na sustavu server2 pokrenite sljedeće naredbe:
useradd -c "Tux the Penguin" tux
(echo 2 ; echo 2) | passwd tux
useradd -c "Thomas Cat" tom
(echo jerry ; echo jerry) | passwd tom
3. Zaustavite slapd (service slapd stop).
Napomena: Migraciju podataka moguće je napraviti i s pokrenutim poslužiteljem
slapd (on-line). U tom bismo slučaju umjesto naredbe slapadd u nastavku
koristili naredbu ldapmodify. Mi ćemo migraciju napraviti s isključenim
poslužiteljem.
Prije početka kopiranja podataka proučite sadržaj direktorija /var/lib/ldap.
Pokrenite sljedeće naredbe:
/usr/share/migrationtools/migrate_base.pl > /tmp/base.ldif
slapadd -v -b "dc=tecaj,dc=hr" -l /tmp/base.ldif
/usr/share/migrationtools/migrate_passwd.pl /etc/passwd > /tmp/passwd.ldif
slapadd -v -b "dc=tecaj,dc=hr" -l /tmp/passwd.ldif
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Vježbe
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Napomena: U ovom ćemo dijelu vježbe prenijeti samo dio sistemskih podataka u LDAP
(prenijet ćemo podatke o korisnicima). U stvarnoj bismo situaciji vjerojatno
prenijeli sve podatke (skripta migrate_all_offline.sh).
Ponovno proučite sadržaj direktorija /var/lib/ldap. Što se promijenilo?
Promijenite atribute datotekama u direktoriju /var/lib/ldap:
chown ldap:ldap /var/lib/ldap/*
Ponovno pokrenite slapd (service slapd start).
ldapsearch -x -W -D 'cn=Manager,dc=tecaj,dc=hr' \
-b 'ou=People,dc=tecaj,dc=hr' dn cn
ldapsearch -x -w p@ssword -D 'cn=Manager,dc=tecaj,dc=hr' \
-b 'uid=tux,ou=People,dc=tecaj,dc=hr' dn cn uid userPassword
Postavke na klijentima
4. Na sustavu server2 instalirajte paket openldap-clients (umetnite instalacijski medij
u optički uređaj, potražite odgovarajući paket u direktoriju Packages te instalirajte ga
naredbom rpm, na primjer: rpm -i openldap-clients-2.4.23-31.el6.i686.rpm).
Provjerite radi li prethodna naredba i na sustavu server2:
ldapsearch -x -w p@ssword -h server1 -D 'cn=Manager,dc=tecaj,dc=hr' \
-b 'uid=tux,ou=People,dc=tecaj,dc=hr' dn cn uid userPassword
5. Na sustavu server2 instalirajte pakete nscd, pam_ldap i nss-pam-ladpd.
Na primjer:
rpm -i nscd-2.12-1.107.el6.i686.rpm
rpm -i pam_ldap-185-11.el6.i686.rpm
rpm -i nss-pam-ldapd-0.7.5-18.el6.i686.rpm
6. Preimenujte originalnu datoteku /etc/ldap.conf (ako postoji):
mv /etc/ldap.conf /etc/ldap.conf-orig
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Vježbe
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Stvorite novu datoteku /etc/ldap.conf sa sljedećim sadržajem:
base
dc=tecaj,dc=hr
suffix "dc=tecaj,dc=hr"
uri ldap://192.168.2.201
pam_password md5
Izmijenite datoteku /etc/sysconfig/authconfig tako da piše:
USELDAPAUTH=yes
USELDAP=yes
Na kraj datoteke /etc/nslcd.conf dodajte sljedeće retke (ili izmjenite postojeće retke
ako navedene direktive već postoje):
uri ldap://192.168.2.201
base dc=tecaj,dc=hr
Pokrenite naredbe:
chkconfig nslcd on
shutdown -r now
7. Na sustavu server2 izmijenite sadržaj datoteke /etc/nsswitch.conf tako da piše:
passwd:
shadow:
ldap files
ldap files
Pokušajte se prijaviti na sustav server2 kao korisnik tux (ssh tux@server2).
Pokušajte se prvo prijaviti s lozinkom 2. Je li Vam uspjelo? A s lozinkom 1?
Odjavite se sa sustava (kao korisnik tux).
8. Na sustavu server1 zaustavite LDAP-server (service slapd stop). Ponovno se
pokušajte prijaviti na sustav server2 kao korisnik tux. Koja je lozinka ovaj put bila
uspješna. Zašto?
Odjavite se sa sustava (kao korisnik tux).
Ponovno pokrenite slapd na sustavu server1.
9. Provjerite možete li se prijaviti na sustav server2 kao korisnik tom (ssh
tom@server2). Odjavite se sa sustava.
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Vježbe
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Na sustavu server2 izmijenite sadržaj datoteke /etc/nsswitch.conf tako da sada
piše:
passwd:
ldap [NOTFOUND=return] files
Pokušajte se ponovno prijaviti kao korisnik tom. Je li Vam uspjelo?
10. Na sustavu server2 izmijenite datoteku /etc/nsswitch.conf tako da (ponovno)
piše:
passwd:
shadow:
files
files
Napomena: U ovoj smo vježbi sve izmjene na poslužitelju server2 radili ručno. Izmjene
je moguće napraviti i pomoću grafičkog alata
system-config-authentication i(li) pomoću tekstne aplikacije
authconfig-tui.
123
Vježbe
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Vježba 17: iptables
Osnovne postavke vatrozida
1. Na sustavu server1 proučite sadržaj datoteke /etc/sysconfig/iptables-config.
Pohranjuju li se postavke (rules) vatrozida prilikom njegovog zaustavljanja i(li) restartanja?
U slučaju potvrdnog odgovora promijenite te postavke (tako da piše
IPTABLES_SAVE_ON_STOP="no" i IPTABLES_SAVE_ON_RESTART="no").
Proučite konfiguraciju zapisanu u datoteci /etc/sysconfig/iptables.
Pokrenite vatrozid:
service iptables start
Ako nisu pokrenuti, pokrenite servise named, httpd, squid i slapd.
Provjerite možete li sa sustava server2 pristupiti tim servisima na sustavu server1.
(Za provjeru prisutupa LDAP-poslužitelju možete pokrenuti naredbu:
ldapsearch -h 192.168.2.201 -D "cn=Manager,dc=tecaj,dc=hr" \
-w p@ssword -s sub "objectclass=*".
Pristup DNS-u provjerite naredbom nslookup. Za ostalo možete koristiti wget, na
primjer: wget -e http_proxy=192.168.2.201:3128 http://192.168.2.201/)
2. Na sustavu server1 napravite kopiju datoteke iptables:
cp
/etc/sysconfig/iptables /etc/sysconfig/iptables-orig
Na odgovarajuće mjesto u datoteku /etc/sysconfig/iptables (na primjer, odmah
nakon retka za ssh) dodajte sljedeće retke:
-A
-A
-A
-A
-A
INPUT
INPUT
INPUT
INPUT
INPUT
-m
-m
-m
-m
-m
state
state
state
state
state
--state
--state
--state
--state
--state
NEW
NEW
NEW
NEW
NEW
-p
-p
-p
-p
-p
udp
tcp
tcp
tcp
tcp
--dport
--dport
--dport
--dport
--dport
53 -j ACCEPT
389 -j ACCEPT
3128 -j ACCEPT
80 -j ACCEPT
443 -j ACCEPT
Pokrenite naredbu:
service iptables restart
Na sustavu server2 ponovno provjerite možete li pristupiti ranije navedenim servisima.
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Vježbe
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Transparentni web-posrednik
3. Sustav server1 podesit ćemo tako da se ponaša kao transparentan web-posrednik
(samo za port 80). To ćemo napraviti tako da sve web-konekcije (na port 80) koje prolaze
kroz sustav server1 a dolaze iz interne mreže, preusmjerimo na web-posrednika (Squid)
na portu 3128. Squid ćemo podesiti tako da radi kao transparentan posrednik.
Na sustavu server1 na kraj datoteke /etc/sysconfig/iptables dodajte sljedeće
retke:
*nat
-A PREROUTING -i eth1 -p tcp -m tcp --dport 80 -j REDIRECT --to-port 3128
COMMIT
Izmijenite sadržaj datoteke /etc/squid/squid.conf tako da piše:
http_port 3128 intercept
Ako nije uključen, uključite ip-forwarding:
echo 1 > /proc/sys/net/ipv4/ip_forward
Ponovno pokrenite vatrozid i Squid:
service squid restart
4. Na sustavu server2 pokušajte pristupiti web-sadržajima izvan lokalne mreže bez
„vidljive“ uporabe posrednika.
Na primjer:
wget http://www.srce.hr
ili podesite Firefox tako da ponovno ne koristi web-posrednika (pogledajte vježbu 12) i
pokušajte pristupiti nekom sadržaju izvan lokalne mreže (na primjer
http://www.srce.hr).
NAT
5. Na sustavu server1 u datoteku /etc/sysconfig/iptables na odgovarajuće mjesto
(prije direktiva s REJECT, na primjer iza direktiva koje su upisane u koraku 2 ove vježbe)
dodajte sljedeće retke:
-A FORWARD -i eth0 -o eth1 -m state --state RELATED,ESTABLISHED -j ACCEPT
-A FORWARD -i eth1 -o eth0 -j ACCEPT
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Vježbe
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Izmijenite direktive u sekciji nat (one se nalaze na kraju datoteke, pogledajte korak 3 ove
vježbe). Neka ta tablica izgleda ovako:
*nat
-A POSTROUTING -o eth0 -j MASQUERADE
COMMIT
Obrišite ranije unesene direktive za transparentnog web-posrednika:
-A PREROUTING -i eth1 -p tcp -m tcp --dport 80 -j REDIRECT --to-port 3128
service squid stop
6. Na sustavu server2 ponovno pokušajte pristupiti web-sadržajima izvan lokalne mreže
(uočite da je Squid na sustavu server1 isključen).
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Vježbe
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Vježba 18: sshd
Osnovne postavke
1. Na sustavu server2 onemogućite prijavljivanje na korisnički račun root putem
protokola SSH.
U datoteci /etc/ssh/sshd_config promijenite vrijednost varijable
PermitRootLogin tako da ona glasi:
PermitRootLogin no
Pokrenite naredbu: service sshd restart
Pokušajte se putem protokola SSH prijaviti na sustav server2 kao korisnik root.
Sigurno prijavljivanje bez lozinki
2. Omogućit ćemo sigurno prijavljivanje korisnika root sa sustava server1 na korisnički
račun tux na sustavu server2 bez uporabe lozinke.
Na sustavu server1 (kao korisnik root) pokrenite sljedeću naredbu:
ssh-keygen -q -t rsa -f ~/.ssh/id_rsa -C '' -N ''
Pogledajte koje su dvije datoteke nastale u direktoriju ~/.ssh.
Sadržaj datoteke id_rsa.pub kopirajte na kraj datoteke ~tux/.ssh/authorized_keys
na sustavu server2.
Napomena: Pripazite da vlasnik direktorija .ssh i datoteke authorized_keys bude
korisnik tux (i da pripadaju odgovarajućoj skupini) te da prava pristupa na
direktorij .ssh budu 700, a na datoteke authorized_keys budu 640:
chown -R tux:tux ~tux/.ssh
chmod 700 ~tux/.ssh
chmod 640 ~tux/.ssh/authorized_keys
Pokušajte se sa sustava server1 prijaviti na sustav server2 kao korisnik tux:
ssh tux@server2
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Vježbe
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Dodatne informacije
DNS
 BIND9 Master and Slave DNS Server setup on CentOS 6.5 (Dorián Ocsovszki)
http://ocsovszki-dorian.blogspot.com/2013/12/bind9-master-and-slave-dns-serversetup.html
 Using the rndc Utility
http://access.redhat.com/site/documentation/enUS/Red_Hat_Enterprise_Linux/6/html/Deployment_Guide/s2-bind-rndc.html
 DNSSEC verification with dig
http://backreference.org/2010/11/17/dnssec-verification-with-dig/
 Securing Zone Transfers With Bind 9 (John Cartwright)
http://www.grok.org.uk/docs/tsig.html
 HOWTO turn BIND into a Validating Resolver (Rick van Rein)
http://dnssec.surfnet.nl/?p=402
 Validating and Exploring DNSSEC with dig (Mark Bryars)
http://bryars.eu/2010/08/validating-and-exploring-dnssec-with-dig/
Sendmail
 Understanding SMTP Error Messages (Heinz Tschabitscher)
http://email.about.com/cs/standards/a/smtp_error_code.htm
 SMTP reply codes
http://www.greenend.org.uk/rjk/tech/smtpreplies.html
 SMTP Commands (Andy Welter)
http://the-welters.com/professional/smtp.html
 Overriding File Security Checks
http://www.sendmail.com/sm/open_source/tips/DontBlameSendmail/
Postfix
 Postfix Standard Configuration Examples
http://www.postfix.org/STANDARD_CONFIGURATION_README.html
Majordomo
 Majordomo
http://www.greatcircle.com/majordomo/
 Creating a Majordomo E-Mail List
http://www.gsp.com/support/virtual/email/majordomo/list/
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Vježbe
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Procmail
 Procmail (Red Hat Linux 7.2: The Official Red Hat Linux Reference Guide)
http://www.centos.org/docs/2/rhl-rg-en-7.2/s1-email-procmail.html
 Mail Filtering with Procmail (Ian Soboroff)
http://userpages.umbc.edu/~ian/procmail.html
 Introduction to Procmail
http://partmaps.org/era/mail/procmail-presentation.html
 Apache SpamAssassin Used via Procmail
http://wiki.apache.org/spamassassin/UsedViaProcmail
Apache
 How To Create a SSL Certificate on Apache for CentOS 6 (Etel Sverdlov)
http://www.digitalocean.com/community/articles/how-to-create-a-ssl-certificate-onapache-for-centos-6
 How To Set Up Apache Virtual Hosts on CentOS 6 (Etel Sverdlov)
http://www.digitalocean.com/community/articles/how-to-set-up-apache-virtual-hosts-oncentos-6
 VirtualHost Examples
http://httpd.apache.org/docs/current/vhosts/examples.html
 Using User Authentication
http://www.apacheweek.com/features/userauth
 Common Apache Misconfigurations (Paul Waring)
http://wiki.apache.org/httpd/CommonMisconfigurations
HTTP
 Testing Basic Authentication with telnet and openssl
http://community.mcafee.com/thread/41920
SNMP
 Install and configure SNMP on RHEL or CentOS
http://www.it-slav.net/blogs/2008/11/11/install-and-configure-snmp-on-rhel-or-centos/
MRTG
 MRTG 2.17.4 configuration reference (Tobias Oetiker)
http://oss.oetiker.ch/mrtg/doc/mrtg-reference.en.html
 MRTG Implementation Manual (Florin Prunoiu)
http://www.enterastream.com/whitepapers/mrtg/mrtg-manual.html
129
Vježbe
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SQUID
 cachemgr (Cache Manager) configuration for Squid (Nikesh Jauhari)
http://linuxpoison.blogspot.com/2008/06/cachemgr-cache-manager-configuration.html
DDNS
 Dynamic DNS and DHCP (Jordan Sissel)
http://www.semicomplete.com/articles/dynamic-dns-with-dhcp/
 DDNS
http://wiki.debian.org/DDNS
LDAP
 Setting up OpenLDAP on CentOS 6
http://docs.adaptivecomputing.com/viewpoint/hpc/Content/topics/1setup/installSetup/settingUpOpenLDAPOnCentos6.htm
 Minimal LDAP configuration on RHEL6 in stages and details
http://spectlog.com/content/Minimal_LDAP_configuration_on_RHEL6_in_stages_and_details
 Introduction to LDAP (Brad Marshall)
http://quark.humbug.org.au/publications/ldap/ldap_tut.html
 OpenLDAP Software 2.4 Administrator's Guide
http://www.openldap.org/doc/admin24/
 Installing OpenLDAP on Redhat / CentOS 6.3 (Suneet Shah)
http://wiki.openiam.com/pages/viewpage.action?pageId=7635198
 LDAP authentication using pam_ldap and nss_ldap
http://www.tldp.org/HOWTO/archived/LDAP-Implementation-HOWTO/pamnss.html
 CentOS : secure OpenLDAP traffic with SSL
http://blog.wains.be/2007/07/13/centos-secure-openldap-traffic-with-ssl/
LDAP Migration Tools
 MigrationTools
http://www.padl.com/OSS/MigrationTools.html
 LDAP Authentication In Linux
http://www.howtoforge.com/linux_ldap_authentication
PAM
 Pluggable Authentication Modules (PAM)
http://www.centos.org/docs/5/html/Deployment_Guide-en-US/ch-pam.html
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Vježbe
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 How PAM works
http://www.tuxradar.com/content/how-pam-works
 Understanding and configuring PAM (Vishal Srivistava)
http://www.ibm.com/developerworks/linux/library/l-pam/index.html
NIS
 Configuring NIS under Red Hat Linux
http://bradthemad.org/tech/notes/redhat_nis_setup.php
iptables
 NAT with Linux and iptables
http://www.karlrupp.net/en/computer/nat_tutorial
 Fundamentals of iptables (Rajnesh Kumar Siwal)
http://www.youtube.com/watch?v=fQF2vEvqHgU
131
Vježbe
__________________________________________________________
Dodatak: Mrežne postavke na Virtualboxu
132
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