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6.1. PPP on Linux
On Linux, PPP functionality is split into
two parts: a kernel component that handles the low-level protocols
(HDLC, IPCP, IPXCP, etc.) and the user space pppd daemon that handles
the various higher-level protocols, such as PAP and CHAP. The current
release of the PPP software for Linux contains the PPP daemon pppd
and a program named chat that automates the
dialing of the remote system.
The PPP kernel driver was written by
Michael Callahan and reworked by Paul Mackerras. pppd was derived
from a free PPP implementation for Sun and 386BSD machines that was
written by Drew Perkins and others, and is maintained by Paul
Mackerras. It was ported to Linux by Al Longyear.
chat was written by Karl Fox.PPP is implemented by a special line
discipline. To use a serial line as a PPP link, you first establish
the connection over your modem as usual and subsequently convert the
line to PPP mode. In this mode, all incoming data is passed to the
PPP driver, which checks the incoming HDLC frames for validity (each
HDLC frame carries a 16-bit checksum), and unwraps and dispatches
them. Currently, PPP is able to transport both the IP protocol,
optionally using Van Jacobson header compression, and the IPX
protocol.pppd aids the kernel driver, performing the initialization and
authentication phase that is necessary before actual network traffic
can be sent across the link. pppd's behavior may be
fine-tuned using a number of options. As PPP is rather complex, it is
impossible to explain all of them in a single chapter. This book
therefore cannot cover all aspects of pppd, but only gives you an
introduction. For more information, consult
Using &
Managing PPP or the pppd
manpages, or READMEs in the pppd source
distribution, which should help you sort out most questions this
chapter fails to discuss. The PPP HOWTO might
also be of use.Probably the
greatest help you will find in configuring PPP will come from other
users of the same Linux distribution. PPP configuration questions are
very common, so try your local usergroup mailing list or the IRC
Linux channel. If your problems persist even after reading the
documentation, you could try the
