Loading ...Force iptables to log messages to a different log file
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.
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I have absolutely no idea what you are talking about when you get to this part:
“Now make sure you pass the log-level 4 option with log-prefix to iptables.”
What is a “log-level 4 option”? What is a “log-prefix”? Why do I want to do this?
This is a huge problem in the Linux community, and the main reason most people steer clear of Linux: the people with all the know-how talk over the heads of everyone else, and won’t explain what the reason for doing something is, it’s “just do it, because I said to”.
Question: If I was to add
kern.warning /var/log/iptables.log
to ‘/etc/syslog.conf’ as stated above, does it then append anything at the “warning” level in the file ‘/var/log/iptables.log’, regardless of whether or not it has to do with IPTables?
Thanks.
Paul,
I don’t think this article was written as a ’step-by-step-iptables-how-to’… So the author might assume some familiarity with iptables.
I invite you to read
http://www.linuxguruz.com/iptables/howto/iptables-HOWTO-6.html#ss6.3
Specially the section
“Extensions to iptables: New Targets”
There is the answer to your questions
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
No, it is just that some of us can actually read. The rest should steer clear of Linux.
Just to let you all know. Even though you are setting this to log to /var/log/iptables.log, it still logs into /var/log/messages.
I am working on a fix for this.
Shaun –
There is usually a syslog.conf rule that sends *.info to /var/log/messages
On my system (Fedora), it reads
*.info;mail.none;authpriv.none;cron.none /var/log/messages
I changed it to
*.info;kern.!=warning;mail.none;authpriv.none;cron.none /var/log/messages
and it stopped logging iptables stuff to the messages log.
Note, however, that it will now put ANY “warning” level kernel messages into the iptables.log
Thanks for the info. I used it and it works just as I’d hoped. No more excess info in /var/log/messages!! Thank you.
Excellent article!!!
And a few contrib:
iptables -t nat -I PREROUTING -s Paul -j DNAT http://www.microsoft.com
May be in this website the people is more clear !!!!!!!!
# DROP everything and Log it
iptables -A INPUT -j LOG –log-level 4
iptables -A INPUT -j DROP
the ‘-’ sign infront of log-level 4 should be ‘–’ instead
Sorry, I think there is problem with the web page.
The above post was to tell about double ‘-’ sign.
But the web page changed it to one ‘-’ sign.
sombatsombat,
The post has been updated to fix formatting. Thanks for the heads up.
This technique was working well for me under RedHat 9.0, but under CENTOS 5 the kernel spits out a bunch of warning messages for different situations, including a BIOS map on bootup, that result in a not-so-clean iptables log, and missing entries in /var/log/messages. It looks to me that the real solution is a change to iptables, however that might get accomplished.
I tried to use this method but it didnt work for me.
Here is what I did:
Edit the /etc/syslog.conf file by adding the following:
kern.warn /var/log/fwlog
In the iptables configuration file or using the shell add this command to log all the packets that are dropped (implicitely the log level is 4 by default):
iptables -A INPUT -j LOG
Then run this command in the shell to read the modified file again!
killall -HUP syslogd
Here’s how I did it:
In your firewall script, add “–log-level debug” to any logging commands.
In /etc/syslog.conf :
kern.=debug /var/log/iptables
This seams to work and doesn’t add anything to /var/log/iptables other than the iptables logs, I think mostly because very few other applications use the “debug” log level.