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Linux Networking: Using Ipchains
Multiple Machines, A Single Connection
July 24, 2000
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
Individual sections address each concept. The last section combines the basics into a sample configuration for linking a local network to the Internet.
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.