Building a Linux Dial-up Server, Part 2

Any Linux distribution comes with all the tools and protocols you need for building your own dial-up and dial-in servers. In this second article of a two-part series, learn how to create your own dial-in server for allowing users to directly dial in to your network.

By Carla Schroder | Posted Jul 22, 2003
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In part 1 we looked at a simple setup for creating and sharing a dial-up Internet connection. Today we'll learn how to build a dial-in server. A dial-in server is useful for remote system administration, remote user access, or building a low-cost WAN. A Linux dial-in server can serve as a gateway for both Linux and Windows boxes.

Getting Gettys

There are three primary elements to a Linux dial-in server:

  • inittab
  • mgetty
  • pppd

A getty – 'get tty' – is a daemon that monitors serial lines. Modems are represented by ttySN — /dev/ttyS0, /dev/ttyS1, dev/ttyS2, and /dev/ttyS3. There are all kinds of different Linux and Unix gettys. mgetty is especially good — it supports data, fax, and voice, and integrates nicely with pppd. If your system does not have mgetty, I recommend getting it.

Configuring mgetty

At root, open /etc/mgetty/login.config for editing. (Note: check your documentation for file locations, as they may vary.) We want to add this line:

/AutoPPP/ - - /usr/sbin/pppd file /etc/ppp/options.server

Note that a similar line may already be present:

/AutoPPP/ - a_ppp /usr/sbin/pppd auth -chap +pap login debug

These represent two different ways of doing the same thing. The first line, which I prefer, puts all the pppd options into a file named /etc/ppp/options.server. You can name this file anything you like. (The docs I learned from use /etc/ppp/options.server, and I'm too lazy to think of something else.) PPP is a peer-to-peer protocol, so our dial-in server options could also go into /etc/ppp/options. Since it's being used as a server, I like this method as it eases the strain on my aging brain.

Configure inittab

mgetty is not run from the command line; it's a daemon. Start it at boot with an entry in inittab:

S0:2345:respawn:/sbin/mgetty ttyS0 /dev/ttyS0

Note the tricky bits — on my system the modem is an external serial modem at /dev/ttyS0. I've selected runlevels 2,3,4, and 5. Use appropriate values for your system. Then run init -q to start it up.

Configure etc/ppp/options.server

One way to configure etc/ppp/options.server is to copy the contents of /etc/ppp/options, comments and all, and then edit it for dial-in server duties. This is the good and educational method. The fast way is to start fresh and copy the following (be sure to have only one command per line):

asyncmap 0
modem
crtscts
lock
require-pap
refuse-chap
proxyarp
192.168.1.10:192.168.1.11

'asyncmap' could be a chapter by itself — set the value to zero to turn off escaping control characters, unless you have a need to manage escaping control characters (now doesn't that inspire some interesting mental images...) Do not leave this out, because then by default, all control characters will be escaped, and nothing will work right.

The 'modem' and 'crtcts' lines enable hardware flow control. I can't imagine using software (xonxoff) flow control, unless you have an unimaginably ancient or bizarre modem. Fortunately, both ancient and bizarre modems are well-supported in Linux, if this is indeed your situation.

'lock' is for locking the serial device so that no other system functions can take it over.

The 'require-pap' and 'refuse-chap' lines are examples of selecting the type of authentication.

'proxyarp' is very important. All machines dialing in to your server must have an IP address. If they don't, proxyarp assigns one to the serial port. The first IP address belongs to the server, and the second one, delimited by a colon, is assigned to the user dialing in. Obviously, you don't want to have any duplicate IPs on the subnet.

Page 2: PAP/CHAP

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