Connections in to and out of your network are working sporadically. Your router's dmesg is flooded with "ip_conntrack: table full, dropping packet." What do you do?
This condition occurs when the connection tracking table has reached its limit. Connection tracking is a function of Netfilter that stores information like the source and destination IP addresses, port numbers, protocol type, state and timeout of a two-way connection. This facility lets us create sophisticated and informed Netfilter rules in a way that is not possible to accurately derive on a packet header-by-header basis.
The conntrack table takes the form of a memory structure; if there were no constraints on the size of the table it could conceivably start knocking off userspace processes if it became too large (i.e. under DoS conditions). Entries in the conntrack table expire either when their timeout has been reached or the connection has been properly closed. In cases where connections are not being closed according to protocol (poor network connectivity, DoS, spoof attack, etc.) the table can fill rapidly causing an intermittent denial of service condition on your network.
The most prominent symptom of a full connection tracking table is that your old, running connections (secure shell sessions) will continue to function while it becomes impossible to establish new ones. Worse, as the entries continue to time out and the table keeps filling up you may "get lucky" and establish a new connection here and there, making the situation much more confusing.
Depending on the situation you may have one or two options. If you have gobs and gobs of RAM available or the (for example) attack is low-volume you can adjust the entry limit of the table. First, check what the current limit is:
# cat /proc/sys/net/ipv4/ip_conntrack_max 65536
You can see how full the table currently is by running:
# cat /proc/sys/net/ipv4/netfilter/ip_conntrack_count 62168
ip_conntrack_max is determined as a multiple of how much RAM the system boots up with but generally stops at 65536 regardless. You may find that this isn't even enough for a high volume network under normal conditions. We can adjust the limit temporarily thus:
# echo 131072 > /proc/sys/net/ipv4/ip_conntrack_max
If this turns out to be your magic bullet and you're sure no other actions need to be taken to mitigate your particular situation add the following line to /etc/sysctl.conf:
net.ipv4.netfilter.ip_conntrack_max = 131072
To load the value from sysctl.conf run:
# sysctl -p
If you don't have the option of throwing more RAM at the problem you may be forced to make an executive decision in the interest of preserving network services for legitimate clients. You can decrease the load on the conntrack table by removing rules that use stateful logic (i.e. containing "-t nat" or "-m state"). The brute force option is to rmmod the ip_conntrack module:
# rmmod ip_conntrack
However this may not be possible in all environments. The other option is to flush your rules and set the default policy to allow:
# iptables -P # iptables -F
This is also typically the effect of
# /etc/init.d/iptables stop or # /etc/init.d/firewall stop