42 comment

1. 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”.

   1. Try google searching for iptables logging. The main reason *nix people do not explain every detail is that the assumption is that a good sysadmin will man/google search. Though in this case the writer offered the answer to your question with the code block above the –log-level text As to why. Iptables does not log by default you therefore must tell it to log. You want to log if you care to see what was blocked. Very useful when users complain that they can’t get to some of your servers resources.

      1. this is really frustrating after manually enter every iptables rules, what should i do know? re enter all the rules just for enable logging? and os official manual are as less help as it can be, all are non official what so ever people talking about unreliable stuff

2. 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.

   1. Yes, it will log all “warning” level events from kern in the ‘/var/log/iptables.log’ file. So potentially (==certainly) events other than iptables.

3. 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 🙂

4. 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.

5. 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.

6. 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

7. 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.

8. 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 !!!!!!!!

9. # 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

10. 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.

11. sombatsombat,

    The post has been updated to fix formatting. Thanks for the heads up.

12. 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.

13. 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

14. 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.

    1. I know this is an old post, but thanks this worked perfectly.

15. 10x Vivek Gite

16. in general, this worked fine for me, even though i used the log-levels names instead of numbers.
    after i had everything set-up, the settings didnt seem to have any effect until i noticed i had
    to explicitly restart the kernel log daemon (/etc/init.d/klogd restart). i dont know if this is true for any other distributions than debian, which i’m running.
    a nice documentation of the predefined iptables targets, such as LOG, can be found here:
    http://www.faqs.org/docs/iptables/targets.html
    under 6.5.4. LOG target, the topic discussed here is also explained another time for the –log-level option

17. Mario:

    don’t need to mock Paul like that… I feel for him and his frustration in getting “there” as well as I do for myself many times.
    Routines are there for something, we all hope that it is to make your life easier… but if in order to put the routines at your service you have to put yourself in the shoes of the programmer all the time (for the same ammount of time) then there is no point… and it really gets to you… Its indeed a proper critic in the linux scene.
    Now, we don’t need to immediately go to the other side of fence, even the best intended, most researchful guy out there gets some scratches jumping the fence.
    Neither autoshit from MS nor ultimate control from some pseudo-eleet.
    Linux diversity lacks in sinthesys!

    Respect kid.

18. Don’t know if you guys had it, but i added that line is syslog.conf . then restarted it.
    added those lines in iptables then restarted it.
    the iptables log starts dumping on the active console.
    I had to log in rescue mode and undo the stuffs.
    may be cause i didn’t have the file /var/log/iptables ?
    later today, i will try touch the file and redo the whole thing.

    any alternate suggestions?

19. sorry, it itself created the file (iptables.log)
    but it also kept dumping on the active konsole.
    any idea?

20. Tawfiq: You can disable the console logging by commenting out this line in your syslog.conf

    #kern.* /dev/console

21. Or you can use ulog.
    Install ulogd (in ubuntu: sudo apt-get install ulogd)

    If you use fwbuilder to configure your firewall then it’s simple.
    In fwbuilder, double click on your firewall –> Firewall Settings –> Logging –> use ULOG

    then compile and reinstall the policy and that’s it.
    The logs from iptables will be in /var/log/ulog/syslogemu.log

22. This is just absurd. Loglevel is not meant to differe services, you should use filters or facility instead, syslog-ng makes things a bit easier.

23. Thanks for this clear guide

24. HI,
    its cool, but for addition how do i get hostnames/FQDN of local nodes instead of IP address. i have dns on firewall system for all dynamic dhcp nodes. since logs are useful to get history of nodes, and all nodes are getting ip from dhcp. and this registers ip address to dns server.

    thanks.

25. Hi
    i have configured 2 log files
    1.) iptable.log 2.)iptable1.log

    i want to write monitor or write log for an ip say 64.55.11.2 in iptable.log and another ip say 202.20.10.80 in iptable1.log is that possible and how to do this?. Thanks in advance…

26. can i have a iptables logs on web page. i have installed “Iptables log analyzer 0.4 beta” but i think its for suse only and it doesn’t put logs to mysql db as it should do according to documentation.

27. Hi ,
    I want to know is it possible to log 5 packets for every 2 minutes if it is possible may i know the command.
    Thanks in advance…

28. for ubuntu 9.10 users edit /etc/rsyslog.d/50-default.conf and dont forget to remove *.=warn part otherwise it will still write to var/log/messages. If anyone has a better method please suggest..

29. In order to filter firewall logs from /var/log/messages use the below (in previous examples, “=” (equal sign) was used, but it is not working on RHEL for example.

    kern.warning /var/log/iptables.log
    *.info;kern.!warning;mail.none;authpriv.none;cron.none /var/log/messages

30. Sound interesting.
    But I have a question? And if I would like to filtering mac address limiting the upload and download for any particular mac address?
    How I can do that?

    Thanks

31. looks like in ubuntu 10.04, the file is now at /etc/rsyslog.d/50-default.conf

    at least i think it is.

32. I would be careful with sentences like “iptables to log messages …”. First of all iptables can’t an doesn’t log anything, it’s just a command line tool to manage rules inside the kernel called “netfilter”. Netfilter and kernel itself does not log anything either, since log daemon log things. And the most important thing: as far as I see, it’s always a bit unfortunate to use the LOG target. Yeah, you can separate messages with log level, prefix etc, to have “different” one than other messages from the kernel which are logged by some log daemon then, but personally I think ULOG target is much more useful, and you don’t need to “hack” your log daemon either. You only need to run ulogd daemon which will log messages then into file or even MySQL table, and so on.

33. It help me to clear.. LOG basics concept
    Thanks

34. Everything is working great, thank you.

    How do I set up the new /var/log/iptables.log file to archive/create new files when it grows to a certain size?…

    1. @awoms:
       http://linuxcommand.org/man_pages/logrotate8.html

35. on ubuntu/xubuntu add this to your rsyslogd.d/50-default.conf

    :msg,contains,”NETFILTER” /var/log/iptables.log

    when you want to log just add –log-prefix NETFILTER (personaly i split NETFILTER-IN NETFILTER-OUT and NETFILTER-FWD

36. Hi!
    Thanks spongebob. I add some info. In debian you can add the following 2 lines in file /etc/rsyslog.conf (beginning of section RULES) or you can add a conf file in directory /etc/rsyslog.d/

    I use the string Shorewall but you can use any string (–log-prefix STRING as spongebob said).

    :msg, contains, "Shorewall"       /var/log/iptables.log
    :msg, contains, "Shorewall"     ~
    

    The first line sends messages that contain “Shorewall” to the file iptables.log. The second line discards messages that contain “Shorewall” so that they are not logged in the /var/log/messages.

    More info:
    http://www.rsyslog.com/doc/rsyslog_conf_filter.html

37. For Centos 6 onwards please note that the syslog.conf file is now the rsyslog.conf file found in the same direcotry

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