At first glance, ipchains and
iptables appear to be quite similar. Both methods of
packet filtering use chains of rules operating within the Linux kernel
to decide what to do with packets that match the specified rule or set of
rules. However, iptables offers a more extensible way
of filtering packets, giving the administrator a greater amount of
control without building a great deal of complexity into the system.
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 processed using rules from only one chain rather than
multiple chains. For instance, a FORWARD packet coming
into a system using ipchains would have to go
through the INPUT, FORWARD, and OUTPUT chains 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, it is important to place the rule
designed to catch a particular packet within the rule that
actually handles 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. This target must be
changed to DROP under iptables.
Order matters when placing options in a
rule. With ipchains, the order of the
rule options does not matter. The iptables
command uses stricter syntax. In iptables
commands, the protocol (ICMP, TCP, or UDP) must be specified before
the source or destination ports.
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 because OUTPUT chains are no
longer used by incoming interfaces, and INPUT chains are not seen
by packets moving through outgoing interfaces.
This is not a comprehensive list of the changes, given that
iptables is a fundamentally rewritten network
filter. For more specific information, refer to the Linux
Packet Filtering HOWTO referenced in Section 18.7 Additional Resources.
