kernel error : nf_conntrack: table full, dropping packet.

# tail -1 /etc/lsb-release DISTRIB_DESCRIPTION="Ubuntu 10.10" # uname -a 2.6.35-25-server #44-Ubuntu SMP Fri Jan 21 19:09:14 UTC 2011 x86_64 GNU/Linux

I saw the following errors on my DNS Server runnning unbound.

/var/log/message

kernel: [96324.941657] nf_conntrack: table full, dropping packet.

or you can also see same messages via dmesg

# dmesg | tail -1 [96324.941657] nf_conntrack: table full, dropping packet.

This seems to be caused by a full “iptables connection_table” due to DDoS , a huge amount of traffic.
You may solve this by increasing “ip_conntrack_max” if you have enough Memory available.

the default value is:

# cat /proc/sys/net/ipv4/netfilter/ip_conntrack_max 65536

This value is 32bit integer.

increase this value w/ sysctl command

# sysctl -w net.netfilter.nf_conntrack_max=131072 net.netfilter.nf_conntrack_max = 131072

or edit sysctl.conf to reflect permanently

# egrep conntrack /etc/sysctl.conf net.netfilter.nf_conntrack_max=131072

In my case , after increasing that value from 65536 to 131072 , I haven’t seen above errors :)