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Linux Iptables ip_conntrack: table full, dropping packet error and solution

My Red hat Enterprise Linux 5 server reporting the following message in /var/log/messages (syslog):

ip_conntrack: table full, dropping packet.

How do I fix this error?

A. If you notice the above message in syslog, it looks like the conntrack database doesn’t have enough entries for your environment. Connection tracking by default handles up to a certain number of simultaneous connections. This number is dependent on you system’s maximum memory size.

You can easily increase the number of maximal tracked connections, but be aware that each tracked connection eats about 350 bytes of non-swappable kernel memory!

To print current limit type:
# sysctl net.ipv4.netfilter.ip_conntrack_max


To increase this limit to e.g. 12000, type:
# sysctl -w net.ipv4.netfilter.ip_conntrack_max=12000
Alternatively, add the following line to /etc/sysctl.conf file:
The following will tell you how many sessions are open right now:
# wc -l /proc/net/ip_conntrack

5000 /proc/net/ip_conntrack
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{ 12 comments… add one }
  • blink4blogJanuary 12, 2008, 3:30 pm

    How about SuSEFirewall2? Are there any similar way to do the same? Thanks

  • nixCraftJanuary 12, 2008, 3:42 pm


    Above instructions must work on Suse Linux.

  • we3caresSeptember 23, 2008, 4:16 am

    Very Nice Article……..

    Like it….. :)

  • OQiis co.November 12, 2008, 2:44 am

    So nice solution.

    Eng. Mahmoud Al Sayed.

  • Georgi GeorgievMarch 7, 2009, 1:45 am

    It works for me too – I love you :)

  • d0rApril 3, 2010, 9:49 pm

    my server has 16 gb ram – i am hosting some VPS.
    one of the VPS is under syn ddos, the limit of conntrack is already at 300000 but the table is still full.
    i can set the limit to 3000000 and the table is always full.

    actually i use:
    net.ipv4.netfilter.ip_conntrack_max = 9527600
    net.ipv4.ip_conntrack_max = 9527600

    OS: centos 5

    is there a limit of max. conntrack value?


  • d0rApril 3, 2010, 9:52 pm

    [root@host4 ~]# wc -l /proc/net/ip_conntrack
    65143 /proc/net/ip_conntrack

    mhh… seems there is a limit of the max. conntrack value.
    of course i have done sysctl -p and done restarts etc., but it dont help.

  • AmosJune 7, 2011, 7:29 am

    You can also get current count of entries in the connection table by reading /proc/sys/net/ipv4/netfilter/ip_conntrack_count.

    It’s much faster than a “wc -l” and useful for graphing/monitoring with collectd/zabbix/nagios etc.

  • lam seo websiteJuly 10, 2012, 7:48 am

    i try with this introduce but i don’t understand about conntrack, who can explain for me

  • BalleshApril 12, 2013, 3:46 pm

    Good Article, very useful……………………………………..

  • farzinAugust 30, 2014, 3:20 pm

    for centos 6.x it is changed to the following codes :

    To print current limit type:
    # sysctl net.nf_conntrack_max

    To increase this limit to e.g. 100000, type:
    # sysctl -w net.nf_conntrack_max=100000

    To make this settings permanent add the following line to /etc/sysctl.conf file:
    net.nf_conntrack_max = 100000

    The following will tell you how many sessions are open right now:
    # wc -l /proc/net/nf_conntrack

  • DeepakAugust 6, 2015, 12:55 pm

    i am using centos 2.1 and ite not working actually i am not found this: net.ipv4.netfilter.ip_conntrack_max

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