Specifically, users comfortable with ipchains should be aware of the following significant differences between ipchains and iptables before attempting to use iptables:
- Under iptables, each filtered packet is only processed using rules from one chain rather than multiple chains. In other words, a FORWARD packet coming into a system using ipchains would have to go through the INPUT, FORWARD, and OUTPUT chains in order to move along 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. For this reason, you must be sure to place the rule designed to catch a particular packet in the correct chain that will actually see the packet.
- The DENY target has been changed to DROP. In ipchains, packets that matched a rule in a chain could be directed to the DENY target, which silently dropped the packet. This target must be changed to DROP in iptables to have the same effect.
- Order matters when placing options in a chain rule. Previously, with ipchains, it did not matter very much how you ordered the rule options when typing the rule.
- When specifying network interfaces to be used with a rule, you must only use incoming interfaces (-i option) with INPUT or FORWARD chains and outgoing interfaces (-o option) with FORWARD or OUTPUT chains. This is necessary due to the fact that OUTPUT chains are no longer used by incoming interfaces, and INPUT chains are not seen by packets moving through outgoing interfaces.
The advantage is that you now have finer-grained control
over the disposition of each packet. If you are attempting to block access
to a particular website, it is now possible to block access attempts from
clients running on hosts which use your host as a gateway. An OUTPUT rule
which denies access will no longer prevent access for hosts which use your
host as a gateway.
The iptables command is a bit pickier about where some
options may go. For example, you must now specify the source or destination
port after the protocol (ICMP, TCP, or UDP) to be used in a chain's rule.
