When enabled through the use of NAT or other stateful inspection rules, netfilter (iptables) under Linux maintains a list of connections passing through the router. Each connection tracking entry contains defined characteristics of the packet, including the source and destination IP address and port number.
The connection tracking entries are ultimately stored in a hash table with a fixed size. By default on an Imagestream router, the hash table can store 8064 entries. For routers with stateful inspection enabled, the number of connections to track may exceed the total number of connections available in the table. If the router reaches the maximum number of connection tracking entries, it will log an error:
“ip_conntrack: table full, dropping packet”
each time that it is unable to store an entry in the connection tracking table. Each instance of this message represents a connection that the router has discarded, typically meaning that the user whose connection was dropped must re-establish their connection.
The maximum size of the connection tracking table can be increased. The maximum size value is stored in the router’s proc filesystem in the file /proc/sys/net/ipv4/ip_conntrack_max. Increasing the maximum size of the connection tracking table to a value larger than the total number fo connections will eliminate the error message and prevent the router from dropping connections due to a lack of space in the connection tracking table.