Moxa W325A-LX [41/74] Iptables

W315A/325A Linux User's Manual Managing Communications

4-7

IPTABLES

IPTABLES is an administrative tool for setting up, maintaining, and inspecting the Linux kernel’s IP packet filter

rule tables. Several different tables are defined, with each table containing built-in chains and user-defined

chains.

Each chain is a list of rules that apply to a certain type of packet. Each rule specifies what to do with a matching

packet. A rule (such as a jump to a user-defined chain in the same table) is called a “target.”

The W315A/325A supports 3 types of IPTABLES table: Filter tables, NAT tables, and Mangle tables:

A. Filter Table—includes three chains:

INPUT chain

OUTPUT chain

FORWARD chain

B. NAT Table—includes three chains:

PREROUTING chain—transfers the destination IP address (DNAT)

POSTROUTING chain—works after the routing process and before the Ethernet device process to transfer

the source IP address (SNAT)

OUTPUT chain—produces local packets

Use the following optional parameters to configure the NAT table.

Source NAT (SNAT)—changes the first source packet IP address

Destination NAT (DNAT)—changes the first destination packet IP address

MASQUERADE—a special form for SNAT. If one host can connect to Internet, then other computers that

connect to this host can connect to the Internet when the computer does not have an actual IP address.

REDIRECT—a special form of DNAT that re-sends packets to a local host independent of the destination IP

address.

C. Mangle Table—includes two chains

PREROUTING chain—pre-processes packets before the routing process.

OUTPUT chain—processes packets after the routing process.

It has three extensions—TTL, MARK, TOS.

The following figure shows the IPTABLES hierarchy.