call-back mini HOWTO

by Pawel Skonecki, stona@fizyka.umcs.lublin.pl

v1.0, March 2000


This document describes how to set up call-back by using the Linux system and modem. I would like to thank Anna for her patience.

1. Introduction

2. Procedure


1. Introduction

1.1 OPINION

I'll will be waiting for all opinions about this document. I have tried to gather information as complete as possible. Tell me when your find any mistakes. I'll be grateful to people who will send me any suggestions or corrections. Their contributions will make this document better. I don't mind answering your questions but I'd rather you read the whole article first.

1.2 PUBLISHING

This document can by published under the conditions of Linux Documentation Project. Get in touch with the author if you can't get this license. This document is free.


2. Procedure

2.1 PART I: Net at home ?

Most of us use the Internet in a place of work. However we offen need the net at home or outside the place of work. It may be possible that the work from home is cheaper then from a company building. I think that the best solution is to install call-back software on the Linux server. Call-back makes it possible to re-call index number at the cost of the company. I'll try to present how it works. An entitled person calls modem is varied for the first time in Linux server. Then on the user's side the "hang up modem" is switched on. At the same time Linux calls the user. User is verified again. We have connection and the server is charged. The user pays only for the initiation of connection. The double verification and extra options in the call-back program unable the unsuitable persons to charge our bill. We can restrict the access to the connection only to corporation network or Internet. Call-back is very flexible. Below, I'll try to present the configuration of a call-back server on Linux system and I'll show you how to set up your computer for re-calling the connection.

2.2 PART II: The first steps with modem.

The administrators prefer different modem but while buying a modem we should remember about certain rules:

When we have the suitable modem we have to set it up in our system. We have to check on which com our modem is. Then we have to make a symbolic link to this hardware and /dev/modem. For example, if we have the modem for the 2nd com we write:

ln -s /dev/cua1 /dev/modem 

We check it

lrwxrwxrwx 1 root uucp 9 Sep 19 19:10 /dev/modem -> /dev/cua1

If we have the modem on different com we have to remember that

/dev/cua0 is com1 

/dev/cua1 is com2 

/dev/cua2 is com3 

/dev/cua3 is com4 

For new kernels:

/dev/ttyS0 is com1 

/dev/ttyS1 is com2 

/dev/ttyS2 is com3 

/dev/ttyS3 is com4

Now, we check our configuration using the program minicom.

2.3 PART III Call Linux

The first step to make the call-back on Linux accessible is to set up a suitable parameter in kernel. Then we check whether our kernel serves the protocol ppp. If you don't have ppp in your kernel or in module you will have to compile your kernel and add ppp. You will find more information in Kernel-HOWTO. OK. We have a good kernel. Now, we have to set up software to our system. The call-back program is a part of mgetty-sendfax and ppp. You will find it all in your distribution. Because call-back system we have double verification and we create a user who will be running ppp on the side of server.

pppuser:klkIOM89mn65H:230:PPP Dialin:/home/pppuser:/etc/ppp/ppplogin

Then change the password. This user doesn't have a usual shell but a file /etc/ppp/ppplogin. We have to make it ourselves. For example vi /etc/ppp/ppplogin and we type:

#!/bin/sh 

exec /usr/sbin/pppd -detach 192.168.1.1:192.168.1.2 

where the address 192.168.1.1 is the address of server with modem and the address 192.168.1.2 is the address which we assigned to our modem. We set up executable options for this file. Because we will use the ppp demon we have to set up the options for this demon. We edit file /etc/ppp/options:

netmask 255.255.255.0 

proxyarp 

lock 

crtscts 

modem 

Proxyarp is the most important from the above options, because you can go to Internet by the modem in the server. The remaining options are used control your modem. Your user can work only in local network if you remove proxyarp option. You have to see PPP-HOWTO and man pppd for more information. We will set up our modem now. Our server must be ready to receive a connection after start. We edit file /etc/inittab and we add it's to modem on the 2en com.

s1:2345:respawn:/sbin/mgetty ttyS1 -D /dev/ttyS1 vt100 

For the 1st com line looks as follows:

s0:2345:respawn:/sbin/mgetty ttyS1 -D /dev/ttyS1 vt100

We make init q. If we don't have information about any mistakes in logs we go to the next step. We come back to directory /etc/ppp and create options.ttyS1 (for modem com1 options.ttyS0)

IP_local: IP_remote

for our net it will be

192.168.1.1:192.168.1.2

We have done a lot work so far. Now, we check the file /etc/mgetty+sendfax/login.config. The most important line is:

/AutoPPP/ - a_ppp /usr/sbin/pppd auth -chap +pap login detach kdebug 7 debug

The remaining lines can be marked #.

We have to set up suid for ppp demon, because pppuser has to run pppd and make interface work.

chmod u+s /usr/sbin/pppd 

and its effect is:

-rwsr-xr-x 1 root root 106892 Jan 11 1999 /usr/sbin/pppd

I think that it is a good idea is to add it to cron becouse I had a problem after restart of my server pppd changed preference. For verification we call to Linux. We use scripts for it. If we do this in MS Windows we mark options "call out a terminal after connection". We login as pppuser with its password. I hope that all is OK.

2.4 PART IV Linux calls us

We can already call our Linux. Now it's time Linux called us. It's not very diffucalt. We have to edit only two files. We crate o file /etc/mgetty+sendfax/callback.conf and we leave it empty.

Then we have to ask our users for their phone number. It's time to write the numbers we have connected earlier. In order to do it we edit /etc/mgetty+sendfax/login.conf and add line:

call - - /usr/sbin/callback - S 123456 

where call is a pseudo-user needed to initiate the connection. The line in the /etc/mgetty+sendfax/login.conf puts in motion the program calling the given number (in this case it's 123456). The same procedures can be applied to other users. I'll try to explain how it works. When we call a server. It asks us to give verification. We login as pseudo-user, in this case it calls. The script in our computer hangs up the modem. We wait and the connection is cut off. The program call-back starts working and recalls us. We verify ourselves again as pppuser with password. We combine the connection and interface ppp. It's all. The configuration of work-stations is very simple. When you have the MS Windows, you have to install dal-up for your number. In the modem propriety we find " propriety--->extended---> extra options" where we write.

&c0 s0=1 

We close the window and call. We log in according to the description given above. If we want to use Linux, we must refer to the script. It's difficult to give only one good script for our Linux. A good configuration of ppp in the system is of primary importance. (You can call it as pppuser through the scripts first).The scripts below were whiten by A. Gozdz. I suggest putting everything to catalogue. It is only my suggestion & you don't have to start the scripts here. Detailed information cocernig writing scripts on Linux can found in PPP-HOWTO.

THIS SCRIPTS WORK WELL WITH SLACKWARE

The configuration file of demon ppp (an example for modem on com2) /etc/options


lock

defaultroute 

noipdefault 

modem 

/dev/cua1 

33600 

crtscts 

debug 

passive 

asyncmap 0 

and the specific scripts

THE SCRIPTS WHICH WORK GOOD WITH LINUX RED HAT 6.x

2.5 PART V Summary

The Configuration of call-back is not complicated. The most important thing is a proper arrangement of ppp server on Linux. I don't know a better way of setting up an access - server. The configuration presented above is a result of numerous attempts & it can be done in a different way. That's way I suggest reading all documents concerning this issue man pppd, HOWTO-NET3.