Chapter 7. Serial Line IP

Table of Contents
General Requirements
SLIP Operation
Dealing with Private IP Networks
Using dip
Running in Server Mode

Packet protocols like IP or IPX rely upon the receiver host knowing where the start and end of each packet are in the data stream. The mechanism used to mark and detect the start and end of packets is called delimitation. The Ethernet protocol manages this mechanism in a LAN environment, and the SLIP and PPP protocols manage it for serial communications lines.

The comparatively low cost of low-speed dialup modems and telephone circuits has made the serial line IP protocols immensely popular, especially for providing connectivity to end users of the Internet. The hardware required to run SLIP or PPP is simple and readily available. All that is required is a modem and a serial port equipped with a FIFO buffer.

The SLIP protocol is very simple to implement and at one time was the more common of the two. Today almost everyone uses the PPP protocol instead. The PPP protocol adds a host of sophisticated features that contribute to its popularity today, and we'll look at the most important of these later.

Linux supports kernel-based drivers for both SLIP and PPP. The drivers have both been around for some time and are stable and reliable. In this chapter and the next, we'll discuss both protocols and how to configure them.

General Requirements

To use SLIP or PPP, you have to configure some basic networking features as described in the previous chapters. You must set up the loopback interface and configure the name resolver. When connecting to the Internet, you will want to use DNS. Your options here are the same as for PPP: you can perform your DNS queries across your serial link by configuring your Internet Service Provider's IP address into your /etc/resolv.conf file, or configure a caching-only name server as described under the section called Caching-only named Configuration in Chapter 6,” in Chapter 6."