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:

Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin and a trainer for the Linux operating system/Unix shell scripting. He has worked with global clients and in various industries, including IT, education, defense and space research, and the nonprofit sector. Follow him on Twitter, Facebook, Google+.

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