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
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: SXI ADMIN

The author is the creator of SXI LLC 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.