IP filtering firewalls (iptables)

Packet filtering is implemented through Linux Kernel.

Linux network packet filtering tools:

   Kernel 2.4 and 2.6:  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