8.10. More Advanced PPP Configurations

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

8.10.1. 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:[1]

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:

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:

# 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 pppd drive the modem control lines of the serial port. You should always turn this option on when using pppd with a modem. The crtscts option turns on hardware handshake.

Besides these options, you might want to force some sort of authentication, for 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 Section 4.6.1” 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 like 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.

8.10.2. 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.

Traditional Linux solutions have 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. You must use a modern kernel for this to work, too. Any of the later 2.0 kernels will work just fine.

To configure pppd for demand dialing, all you need to do is add options to your options file or the pppd command line. The following table summarizes the options related to demand dialing:

Option Description

This option specifies that the PPP link should be placed in demand dial mode. The PPP network device will be created, but the connect command will not be used until a datagram is transmitted by the local host. This option is mandatory for demand dialing to work.

active-filter expression

This option allows you to specify which data packets are to be considered active traffic. Any traffic matching the specified rule will restart the demand dial idle timer, ensuring that pppd waits again before closing the link. The filter syntax has been borrowed from the tcpdump command. The default filter matches all datagrams.

holdoff n

This option allows you to specify the minimum amount of time, in seconds, to wait before reconnecting this link if it terminates. If the connection fails while pppd believes it is in active use, it will be re-established after this timer has expired. This timer does not apply to reconnections after an idle timeout.

idle n

If this option is configured, pppd will disconnect the link whenever this timer expires. Idle times are specified in seconds. Each new active data packet will reset the timer.

A simple demand dialing configuration would therefore look something like this:
holdoff 60
idle 180
This configuration would enable demand dialing, wait 60 seconds before re-establishing a failed connection, and drop the link if 180 seconds pass without any active data on the link.

8.10.3. Persistent Dialing

Persistent dialing is what people who have permanent dialup 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 twenty-four 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:

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 manual page, if you'd like to pursue it.



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