Kernel ≥ 2.4         iptables
   Kernel 2.2:          ipchains
   Kernel 2.0:          ipfwadm

Chains for network packets: INPUT OUTPUT FORWARD Chains are combined into tables: filter (default) NAT mangle Each of the chains filters data packets based on Source and Destination IP Source and Destination Port number Network interface State of the packet Target for the rule: ACCEPT DROP REJECT QUEUE RETURN LOG

FILTER TABLE Check iptables filter configuration: /sbin/iptables -L -t filter Reference: packet-filtering-HOWTO