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

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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
Output:

8192

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:
net.ipv4.netfilter.ip_conntrack_max=12000
The following will tell you how many sessions are open right now:
# wc -l /proc/net/ip_conntrack
Output:

5000 /proc/net/ip_conntrack

Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin, DevOps engineer, and a trainer for the Linux operating system/Unix shell scripting. Get the latest tutorials on SysAdmin, Linux/Unix and open source topics via RSS/XML feed or weekly email newsletter.

12 comment

  6. 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?

    thanks!

  7. [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.

  8. 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.

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

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

    To print current limit type:
    # sysctl net.nf_conntrack_max
    Output:65536

    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

  12. 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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