How To Set Up Linux As A Dial-In Server 

Author: Sohail Riaz <sohail AT fastadmins DOT com>
                              <sohaileo AT gmail DOT com> 

1 Overview

This document describes how to attach modems to a Linux box and allow it to receive calls to connect users to the network. Its like being your own ISP (Internet Service Provider). If your Linux box is connected to the Internet, then the users will also be connected to the Internet. Your Linux box becomes a router. This is also known as RAS (Remote Access Services) in the Microsoft world. In the Linux world its called PPP (Point to Point Protocol).

2 Attach A Modem

Attach a modem to COM 1 or COM 2. If you have an internal modem, you need to know what COM port it is. You could just activate all the COM ports and see later which one works by viewing the /var/log/messages file. Then turn off the other 3.

3 Run kudzu

Kudzu is the Linux hardware auto detection that usually runs at boot time. You may reboot the computer or else just type:

kudzu

Hopefully, kudzu will detect your modem and configure it automatically.

4 Configure Modem For mgetty

Mgetty is the program that allows you to login after the modems connect. It is activated in the /etc/inittab file. Use vi to add a line after the mingetty lines (halfway into the file). 

Physical Port            Linux Port                              /etc/inittab line for turning on mgetty
COM 1                       ttyS0
COM 2                       ttyS1
COM 3                       ttyS2
COM 4                       ttyS3

vi /etc/inittab

S0:2345:respawn:/sbin/mgetty -D ttyS0
S1:2345:respawn:/sbin/mgetty -D ttyS1
S2:2345:respawn:/sbin/mgetty -D ttyS2
S3:2345:respawn:/sbin/mgetty -D ttyS3

The -D means data only, no fax machines are allowed to connect. The baud rate is automatic. I hope your modem can handle that. 

5 Turn On PPP Dial In Service

Mgetty by default will not invoke PPP, it is commented out in the /etc/mgetty+sendfax/login.config file. We need this service so IP packets can flow across the dial-in connection. 

vi /etc/mgetty+sendfax/login.config

Look for a line:

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

Change to

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

and remove the first character, the # and save the file. Notice the "debug" option on that line. This logs useful information in /var/log/messages that we will look at later. Also, the "login" option means to authenticate with the /etc/passwd file after "pap" authentication (described below). 

6 Setup PPP Options

When PPP starts up, it reads options from the command line from /etc/mgetty+sendfax/login.config. Then it read more options from the /etc/ppp directory. We will create a new file called options.server where we will put generic options for all modems that dial in. Then we will have an options file for each modem where we can put the IP address we will assign anyone on that modem. That file will be named options.ttyS0 or options.ttyS1. 

vi /etc/ppp/options.server

-detach
asyncmap 0
modem
crtscts
lock
proxyarp                                                     
ms-dns 1.2.3.4           #replace 1.2.3.4 with DNS address Primary                    
ms-dns 3.4.5.6           #replace 3.4.5.6 with DNS address Slave 

vi /etc/ppp/options.ttyS0 

192.168.0.12:192.168.0.100            #serverAddress:clientAdress
netmask 255.255.255.0                    #The netmask of the LAN the server is connected to

7 Add Users To pap-secrets

In order to allow dial in, you will have to define users and passwords to authenticate them with. PPP will authenticate them. First, we must add users and passwords to the /etc/ppp/pap-secrets file.

vi /etc/ppp/pap-secrets

Client (User)      Server       Secret (password)         IP
sohail               *               boby                  *
zain                 *               zain123               * 

8 Create Linux Users

Now, create some regular linux users that correspond to the /etc/ppp/pap-secrets file. Use the same password that has been entered in that file. If you do not want to do this step then you must remove the "login" option from the command line of ppp kept in /etc/mgetty+sendfax/login.config. 

9 Turn On Routing

We now want Linux to be a router and allow packets to flow through it. This is called packet forwarding.

vi /etc/sysctl.conf

net.ipv4.ip_forward  = 1 

sysctl -e -p /etc/sysctl.conf

10 Start Mgetty

Tell the init to re-read its config file (/etc/inittab) and start up mgetty on the modems. 

/sbin/telinit q

11 Test Dial In and View Logs

Have someone try dialing in on Phone number attached to modem and you can watch the logs live by typing this: 

tail -f /var/log/messages

You will see the connection attempts and some useful debugging info.