iptables use chains of rules that operate within the Linux kernel to filter packets based on matches with specified rules or rule sets. However,
iptables offers a more extensible way of filtering packets, giving the administrator greater control without building undue complexity into the system.
You should be aware of the following significant differences between
iptables, each filtered packet is processed using rules from only one chain rather than multiple chains.
For example, a FORWARD packet coming into a system using
ipchains would have to go through the INPUT, FORWARD, and OUTPUT chains to continue to its destination. However,
iptables only sends packets to the INPUT chain if they are destined for the local system, and only sends them to the OUTPUT chain if the local system generated the packets. It is therefore important to place the rule designed to catch a particular packet within the chain that actually handles the packet.
- The DENY target has been changed to DROP.
ipchains, packets that matched a rule in a chain could be directed to the DENY target. This target must be changed to DROP in
- Order matters when placing options in a rule.
ipchains, the order of the rule options does not matter.
iptables command has a stricter syntax. The
iptables command requires that the protocol (ICMP, TCP, or UDP) be specified before the source or destination ports.
- Network interfaces must be associated with the correct chains in firewall rules.
For example, incoming interfaces (
-i option) can only be used in INPUT or FORWARD chains. Similarly, outgoing interfaces (
-o option) can only be used in FORWARD or OUTPUT chains.
In other words, INPUT chains and incoming interfaces work together; OUTPUT chains and outgoing interfaces work together. FORWARD chains work with both incoming and outgoing interfaces.
OUTPUT chains are no longer used by incoming interfaces, and INPUT chains are not seen by packets moving through outgoing interfaces.