Installing and Administering PPP

Chapter 4 81

Common pppd Options

IP Routing Tips

The UNIX host’s IP implementation sees PPP as a point-to-point

network connection between two known addresses. If neither end-point

resides on an IP-based local area network (LAN), packets simply ﬂow in

both directions as soon as the PPP connection is established. When the

PPP link connects a remote host to a LAN, provides a connection

between two LANs, or connects a host or a LAN to the worldwide

Internet, the systems involved must be concerned with routing.

The examples below describe how to establish static routes when the

machines boot. This is cumbersome, because any topology change

requires that all the machines even remotely involved be reconﬁgured by

editing their boot-time shell scripts. An alternative to static routes would

be to use some automated system for updating all the hosts’ routing

tables. This is usually implemented as a daemon running on all the hosts

involved. Many networks use the RIP protocol and run routed on their

UNIX systems. If you use routed to propagate routing information, you

will need to specify the PPP link as leading to a passive gateway.

Some sites prefer other routing protocols, and they use gated instead. If

you use gated, you should invoke pppd with the netmask argument set

to the same value as used on the LAN, if that subnet mask is different

from the default for the class of your network.

Connecting a Host to a LAN

There are two conventional approaches for connecting a set of standalone

hosts to a LAN via PPP.

Separate Network

The method that causes the least confusion is to assign a network or

subnet for use by all remote machines. For example, suppose the

organization’s class B network number is 134.19, and they are

subnetting with a class C-sized network mask of 0xffffff00. The

departmental LAN is subnet 134.19.5.0, populated by hosts alpha

(134.19.5.22) and bravo (134.19.5.41). A modem is attached to one of

alpha’s serial ports, and alpha’s Login shell script is a generic one as

shown below. A laptop named nomad wishes to connect to the network.