Linux can route network traffic from one network segment to another. Routing is normally done on a PC with two or more network adapters. This article presents a configuration using a pair of Ethernet adapters. More specifically, the article examines how a Linux computer can link a local network to the Internet through an Ethernet-attached device like a cable modem or a DSL modem. The article examines the basic concepts pertaining to routing, network address translation (NAT), firewalls, and a program called ipchains.

Individual sections address each concept. The last section combines the basics into a sample configuration for linking a local network to the Internet.

Routing 101
Routing can be done using a number of network protocols but the IP protocol is the one that is addressed in this article. IP is also the protocol used on the Internet. The previous articles address configuration of the IP protocol for network adapters on a Linux computer.

Linux can implement very complex routing using multiple network adapters, but most configurations utilize a pair of network adapters. These can be any combination from a pair of Ethernet adapters to an Ethernet adapter and a modem. This article will concentrate on the former, while the next article will address the latter.

The routing software, or simply router, listens at a network adapter for messages, also called packets, addressed to it in the same way as the Apache web server, httpd, listens for Web-page requests. Many services can use a single network adapter without a conflict. Other computers on the network direct messages to the router. The other computers are setup to do this by setting the IP address of the router in the computer's default router or gateway setting.

The router takes incoming messages and checks the destination IP address to determine where to forward the message. It uses routing tables to make this determination. The message may wind up being sent to another network segment or it may be forwarded yet again to another computer acting as a router. This router-to-router handoff is essentially how the Internet works. A router-to-router handoff is called a hop. Messages going in the reverse direction are handled in the same fashion.

The router assumes the network adapter handles the low level transmission of messages. For modem links, like those covered in the next article, often utilize the Point-to-Point Protocol (PPP). Some DSL and cable modems utilize PPP over Ethernet (PPPoE). For this support, check in the next article. This article assumes that DSL and cable modems are connected to an Ethernet adapter.

The Linux route program is used to configure and display the routing tables. The routing support is built into Linux. The basic syntax and semantics for the route program are covered later.

Next: Network Address Translation 101 »