Moxa RM-1050-T [28/52] Iptables

ioPAC RTU Software RTU Communications

4-3

iptables

The iptables command 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 ioPAC RTU controllers support 3 types of iptables tables: Filter tables, NAT tables, and Mangle tables:

A. Filter tables—includes three chains:

INPUT chain

OUTPUT chain

FORWARD chain

B. NAT tables—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

sub-tables

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 the 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 tables—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.