Linux Iptables Firewall: Log IP or TCP Packet Header

Posted on in Categories Howto, Iptables, Linux, Networking, Security last updated January 9, 2008

Iptables provides the option to log both IP and TCP headers in a log file. This is useful to:
=> Detect Attacks

=> Analyze IP / TCP Headers

=> Troubleshoot Problems

=> Intrusion Detection

=> Iptables Log Analysis

=> Use 3rd party application such as PSAD (a tool to detect port scans and other suspicious traffic)

=> Use as education tool to understand TCP / IP header formats etc.

How do I turn on Logging IP Packet Header Options?

Add the following command to your iptables script beo:

iptables -A INPUT -j LOG --log-ip-options
iptables -A INPUT -j DROP

How do I turn on Logging TCP Packet Header Options?

Add the following command to your iptables script:

iptables -A INPUT -j LOG --log-tcp-options
iptables -A INPUT -j DROP

You may need to add additional filtering criteria such as source and destination ports/IP-address and other connection tracking features. To see IP / TCP header use tail -f or grep command:
# tail -f /var/log/messages

Recommended readings:

Force iptables to log messages to a different log file

Posted on in Categories Iptables, Linux, Monitoring, Security last updated October 3, 2006

According to man page:
Iptables is used to set up, maintain, and inspect the tables of IP packet filter rules in the Linux kernel. Several different tables may be defined. Each table contains a number of built-in chains and may also contain user defined chains.

By default, Iptables log message to a /var/log/messages file. However you can change this location. I will show you how to create a new logfile called /var/log/iptables.log. Changing or using a new file allows you to create better statistics and/or allows you to analyze the attacks.

Iptables default log file

For example, if you type the following command, it will display current iptables log from /var/log/messages file:
# tail -f /var/log/messages
Output:

Oct  4 00:44:28 debian gconfd (vivek-4435): Resolved address "xml:readonly:/etc/gconf/gconf.xml.defaults" to a read-only configuration source at position 2
Oct  4 01:14:19 debian kernel: IN=ra0 OUT= MAC=00:17:9a:0a:f6:44:00:08:5c:00:00:01:08:00 SRC=200.142.84.36 DST=192.168.1.2 LEN=60 TOS=0x00 PREC=0x00 TTL=51 ID=18374 DF PROTO=TCP SPT=46040 DPT=22 WINDOW=5840 RES=0x00 SYN URGP=0
Oct  4 00:13:55 debian kernel: IN=ra0 OUT= MAC=ff:ff:ff:ff:ff:ff:00:18:de:55:0a:56:08:00 SRC=192.168.1.30 DST=192.168.1.255LEN=78 TOS=0x00 PREC=0x00 TTL=128 ID=13461 PROTO=UDP SPT=137 DPT=137 LEN=58

Procedure to log the iptables messages to a different log file

Open your /etc/syslog.conf file:
# vi /etc/syslog.conf
Append following line
kern.warning /var/log/iptables.log
Save and close the file.

Restart the syslogd (Debian / Ubuntu Linux):# /etc/init.d/sysklogd restartOn the other hand, use following command to restart syslogd under Red Hat/Cent OS/Fedora Core Linux:# /etc/init.d/syslog restart

Now make sure you pass the log-level 4 option with log-prefix to iptables. For example:
# DROP everything and Log it
iptables -A INPUT -j LOG --log-level 4
iptables -A INPUT -j DROP

For example, drop and log all connections from IP address 64.55.11.2 to your /var/log/iptables.log file:
iptables -A INPUT -s 64.55.11.2 -m limit --limit 5/m --limit-burst 7 -j LOG --log-prefix '** HACKERS **'--log-level 4
iptables -A INPUT -s 64.55.11.2 -j DROP

Where,

  • –log-level 4: Level of logging. The level # 4 is for warning.
  • –log-prefix ‘*** TEXT ***’: Prefix log messages with the specified prefix (TEXT); up to 29 letters long, and useful for distinguishing messages in the logs.

You can now see all iptables message logged to /var/log/iptables.log file:
# tail -f /var/log/iptables.log

Updated for accuracy.