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6.10. More Advanced PPP Configurations

While configuring PPP to dial
in to a network like the Internet is the most common application,
some users have more advanced requirements. In this section
we'll talk about a few of the more advanced
configurations possible with PPP under Linux.

6.10.1. PPP Server

Running pppd as a
server is just a matter of configuring a serial tty device to invoke
pppd with appropriate options when an incoming
data call has been received. One way to do this is to create a
special account, say ppp, and give
it a script or program as a login shell that invokes
pppd with these options. Alternatively, if you
intend to support PAP or CHAP authentication, you can use the
mgetty program to support your modem and exploit
its "/AutoPPP/" feature.

To build a
server using the login method, you add a line similar to the
following to your /etc/passwd file:[10]

[10] The useradd or
adduser utility, if you have it, will simplify
this task.

ppp:x:500:200:Public PPP Account:/tmp:/etc/ppp/ppplogin

If your system
supports shadow passwords, you also need to add an entry to the
/etc/shadow file:

ppp:!:10913:0:99999:7:::

Of course,
the UID and GID you use depends on which user you wish to own the
connection, and how you've created it. You also have
to set the password for the mentioned account using the
passwd command.

The ppplogin script might look like this:

#!/bin/sh
# ppplogin - script to fire up pppd on login
mesg n
stty -echo
exec pppd -detach silent modem crtscts

The mesg command
disables other users from writing to the tty by using, for instance,
the write command. The stty
command turns off character echoing. This command is necessary;
otherwise, everything the peer sends would be echoed back to it. The
most important pppd option given is -detach
because it prevents pppd from detaching from the
controlling tty. If we didn't specify this option,
it would go to the background, making the shell script exit. This in
turn would cause the serial line to hang up and the connection to be
dropped. The silent option causes
pppd to wait until it receives a packet from the
calling system before it starts sending. This option prevents
transmit timeouts from occurring when the calling system is slow in
firing up its PPP client. The modem option makes
Besides these options, you might want to
force some sort of authenticationfor example, by specifying
auth on
pppd's command line or in the
global options file. The manual page also discusses more specific
options for turning individual authentication protocols on and off.

If you wish to use
mgetty, all you need to do is configure
mgetty to support the serial device your modem
is connected to (see Chapter 3
for details), configure pppd for
either PAP or CHAP authentication with appropriate options in its
options file, and finally, add a section similar
to the following to your
/etc/mgetty/login.config file:

# Configure mgetty to automatically detect incoming PPP calls and invoke
# the pppd daemon to handle the connection.
#
/AutoPPP/ -     ppp   /usr/sbin/pppd auth -chap +pap login

The first field is a special piece of magic
used to detect that an incoming call is a PPP one. You must not
change the case of this string; it is case sensitive. The third
column is the username that appears in who
listings when someone has logged in. The rest of the line is the
command to invoke. In our example, we've ensured
that PAP authentication is required, disabled CHAP, and specified
that the system passwd file should be used for
authenticating users. This is probably similar to what
you'll want. Remember, you can specify the options
in the options file or on the command line if
you prefer.

Here is a small checklist of tasks to perform and the sequence you
should perform them to get PPP dial in working on your machine. Make
sure each step works before moving on to the next:

1. Configure the modem for auto-answer mode.
   On Hayes-compatible modems, this is performed using a command such as
   ATS0=3. If you're going to be
   using the mgetty daemon, this
   isn't necessary.

2. Configure the serial device with a
   getty-type of command to answer incoming calls.
   A commonly used getty variant is
   mgetty.

3. Consider authentication. Will your callers authenticate using PAP,
   CHAP, or system login?

4. Configure pppd as
   server as described in this section.

5. Consider routing. Will you need to
   provide a network route to callers? Routing can be performed using
   the ip-up script.

6.10.2. Demand Dialing

When there is IP traffic to be carried
across the link, demand
dialing causes your telephone modem to dial and
to establish a connection to a remote host. Demand dialing is most
useful when you can't leave your telephone line
permanently switched to your Internet provider. For example, you
might have to pay timed local calls, so it might be cheaper to have
the telephone line switched on only when you need it and disconnected
when you aren't using the Internet.

In the past,
Linux solutions used the diald command, which
worked well but was fairly tricky to configure. Versions 2.3.0 and
later of the PPP daemon have built-in support for demand dialing and
make it very simple to configure.

To
configure pppd for demand dialing, all you need
to do is add options to your options file or the
pppd command line. Table 6-2
summarizes the options related to demand dialing.

A simple demand dialing configuration would therefore look something
like this:

demand
holdoff 60
idle 180

This configuration would enable demand dialing, wait 60 seconds
before reestablishing a failed connection, and drop the link if 180
seconds pass without any active data on the
link.

6.10.3. Persistent Dialing

Persistent dialing is what people who have permanent
dial-up connections to a network will want to use. There is a subtle
difference between demand dialing and persistent dialing. With
persistent dialing, the connection is automatically established as
soon as the PPP daemon is started, and the persistent aspect comes
into play whenever the telephone call supporting the link fails.
Persistent dialing ensures that the link is always available by
automatically rebuilding the connection if it fails.

You might be fortunate to not have to pay for your telephone calls;
perhaps they are local and free, or perhaps they're
paid by your company. The persistent dialing option is extremely
useful in this situation. If you do have to pay for your telephone
calls, then you have to be a little careful. If you pay for your
telephone calls on a time-charged basis, persistent dialing is almost
certainly not what you want; unless you're very sure
you'll be using the connection fairly steadily 24
hours a day. If you do pay for calls, but they are not time charged,
you need to be careful to protect yourself against situations that
might cause the modem to endlessly redial. The
pppd daemon provides an option that can help
reduce the effects of this problem.

To enable persistent dialing, you must
include the persist option in one of your
pppd options files. Including this option alone
is all you need to have pppd automatically
invoke the command specified by the connect option
to rebuild the connection when the link fails. If you are concerned
about the modem redialing too rapidly (in the case of modem or server
fault at the other end of the connection), you can use the
holdoff option to set the minimum amount of time
that pppd will wait before attempting to
reconnect. This option won't solve the problem of a
fault costing you money in wasted phone calls, but it will at least
serve to reduce the impact of one.

A typical configuration might have persistent dialing options that
look like this:

persist
holdoff 600

The holdoff time is specified in seconds. In our
example, pppd waits a full five minutes before
redialing after the call drops out.

It is possible to combine persistent dialing with demand dialing,
using idle to drop the link if it has been idle
for a specified period of time. We doubt many users would want to do
so, but this scenario is described briefly in the
pppd manpage, if you'd like to
pursue it.