72

I have this rule in my iptables:

iptables -A INPUT -s 192.168.11.0/24 -j LOG

My question is:

Where is the iptables log file, and how can I change that?

4 Answers 4

82

These logs are generated by the kernel, so they go to the file that receives kernel logs: /var/log/kern.log.

If you want to redirect these logs to a different file, that can't be done through iptables. It can be done in the configuration of the program that dispatches logs: rsyslog. In the iptables rule, add a prefix that isn't used by any other kernel log:

iptables -A INPUT -s 192.168.11.0/24 -j LOG --log-prefix='[netfilter] '

Following the example set by 20-ufw.conf, create a file under /etc/rsyslog.d/00-my_iptables.conf containing:

:msg,contains,"[netfilter] " -/var/log/iptables.log
& stop

Putting the rule early (the file names in /etc/rsyslog.d are used in lexicographic order) and adding &stop causes these logs to go only to the specified location and not to the default location as well.

Rsyslog has to be restarted for the config changes to take place.

8
  • i dont have installed ufw, so i cannot find logs in syslog,kern.log or iptables.log
    – pylover
    Sep 21, 2013 at 19:47
  • 3
    @pylover UFW was just an example. I know you don't have iptables.log, the point of my answer is to show you how to create it. You may not have /var/log/kern.log if you're running a different version of Ubuntu (I think recent versions no longer use this file and put kernel logs in /var/log/syslog instead), but it doesn't matter. Oh, but if you're running an older version of Ubuntu, you may need to install the rsyslog package. Sep 21, 2013 at 19:50
  • To make this work on 12.10, I had to adjust the rsyslog.d file to have the following additional char: ":msg,contains,"[netfilter] " -/var/log/iptables.log"
    – Daniel
    Feb 26, 2014 at 15:10
  • 2
    one more thing, i needed to name the file like 00-my_iptables.conf, otherwise iptables still were logging into kern.log Jan 3, 2015 at 12:02
  • 2
    Maybe you could mentioned the & stop command as well. That way you avoid duplicates in the kern.log file, duplicates that could imper your ability to see other important kernel logs. Oct 21, 2016 at 20:08
19

I know that's far too late and the answer is already marked as the accepted one. I just have a piece of new info to give.

The log file of the LOG action is found at either /var/log/syslog (Ubuntu and similar OSs) or /var/log/messages (CentOS and similar OSs).

13

If you are in trouble finding the right file you may try like this:

find /var/log -mmin 1

This will find any file modified in the last 1 min inside the /var/log and below. You may find out that the -j LOG may update more than just a single file.

For instance on Ubuntu 18, both the /var/log/kern.log and /var/log/syslog are impacted with netfilter logging.

1
  • 2
    I was doing ls /var/log/ to find the log file and it didn't show until I did sudo ls /var/log/. Your answer helped me, thank you. Aug 15, 2021 at 10:34
1
# on my computer !
# i wrote at the top of the script

iptables -F

iptables -X

# 
iptables -A INPUT -m state --state NEW -j LOG \ 
--log-prefix='[iptables_input] '



iptables -A OUTPUT -m state --state NEW -j LOG \
 --log-prefix='[iptables_output] '

# and found the results in /var/log/syslog
# the LOG instruction are executed only when other iptables instructions 
# are not registered before
1
  • 6
    Welcome to Ask Ubuntu. A script without any instructions and explanations is not a good answer.
    – user68186
    Sep 27, 2020 at 16:17

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