Call-back mini howto

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<A name=s2>2. Procedure</A>

<A name=ss2.1>2.1 PART I: Net at home ?</A>

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.

<A name=ss2.2>2.2 PART II: The first steps with modem.</A>

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

Don't buy the Win-Modem because it doesn't work with Linux
The external modem works faster than the modem which is inside your computer
The internal modem with ISA slot is better then the are with PCI slot (you can use your PCI slot for something different)
Don't use Plug&Play modem. If you have it set off, Plug&Play options and set up modem on free com (read Plug&Play-HOWTO).

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.

<A name=ss2.3>2.3 PART III Call Linux</A>

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. We have to add information abut password in the file /etc/ppp/pap-secrets (more in man pppd)

pppuser * password_for_pppuser *

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. Our server will work as router. We have to enable IP forwarding and we add this line for the file /etc/rc.d/rc.local :

echo "1" > /proc/sys/net/ipv4/ip_forward

If you are RedHat user you can change in /etc/sysconfig/network from FORWARD_IPV4=false to FORWARD_IPV4=true.

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.

<A name=ss2.4>2.4 PART IV Linux calls us</A>

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 create a 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. That's all. The configuration of work-stations is very simple. When you have MS Windows, you have to install dial-up for your number. In the modem propriety we find " propriety--->extended---> extra options" where we write.

&c0s0=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.

THESE SCRIPTS WORK WELL WITH SLACKWARE

The configuration file of daemon 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 first named /etc/ppp/ppp-call

#!/bin/bash 

teksta="Connection failed"

tekstb="Probably, You will be connect" 

# /sbin/setserial /dev/cua1 spd_vhi 

killall -INT pppd 2>/dev/null 

rm -f /var/lock/LCK* /var/run/ppp*.pid 

(/usr/sbin/pppd -detach /dev/ttyS1 115200 \ 

connect "/usr/sbin/chat -v -f /etc/ppp/pppcallback" &) || \ 

(echo $teksta; ls marsss >/dev/null; exit 1) 

echo $tekstb 

exit 0

the second file called /etc/ppp/pppcallback

TIMEOUT 60 

ABORT 'ERROR' 

ABORT 'BUSY' 

ABORT 'NO ANSWER' 

ABORT 'NO DIALTONE' 

ABORT '\nVOICE\r' 

ABORT '\nRINGING\r\n\r\nRINGING\r' 

'' AT&FH0 <p>'OK-+++\c-OK' 'AT&C0S0=1' 

TIMEOUT 75 

OK ATDT123456 

CONNECT '' 

ogin:-ogin: ppp_pseudouser 

'\nNO CARRIER\r' '' 

TIMEOUT 180 

'\nRING\r' AT&C1A 

CONNECT '' 

TIMEOUT 20 

ogin:-ogin: pppuser 

sword:-sword password_for_pppuser

You can ppp-call, now. :)

THESE SCRIPTS WORK GOOD WITH LINUX RED HAT 6.x

/etc/ppp/options

lock 

defaultroute 

noipdefault 

modem 

33600 

crtscts 

debug 

passive 

asyncmap 0

/etc/ppp/pppcallback

TIMEOUT 5 

ABORT 'ERROR' 

ABORT 'BUSY' 

ABORT 'NO ANSWER' 

ABORT 'NO DIALTONE' 

ABORT '\nVOICE\r' 

ABORT '\nRINGING\r\n\r\nRINGING\r' 

'' AT&FH0 'OK-+++\c-OK' 'AT&C0S0=1' 

TIMEOUT 40 

OK ATDT5376443 CONNECT '' 

ogin:-ogin: ppp-pseudo-user 

'\nNO CARRIER\r' '' 

TIMEOUT 180 

'\nRING\r' AT&C1A 

CONNECT '' 

TIMEOUT 20 

ogin:-ogin: pppuser 

sword:-sword password_for_ppuser

/usr/bin/ppp-call

#!/bin/bash 

teksta="Connection failed" 

tekstb="Probably, You will be connect" 

# /sbin/setserial /dev/cua1 spd_vhi

killall -INT pppd 2>/dev/null 

rm -f /var/lock/LCK* /var/run/ppp*.pid 

(/usr/sbin/pppd -detach call ppp_call &) || \ 

(echo $teksta; ls marsss >/dev/null; exit 1) 

echo $tekstb 

exit 0

You can run ppp-call, now. :)

If you have M$ Windows you can use this script for conection. I don't test it (I use terminal) you can ask some more Adrian Debkowski ( <A href="mailto:adrian@cr-media.pl">adrian@cr-media.pl</A>).

proc main

delay 1

waitfor "ogin:"

transmit "call^M"

waitfor "RING"

transmit "ATA^M"

waitfor "CONNECT"

waitfor "ogin:"

transmit "pppuser^M"

waitfor "word:"

transmit "ppp^M"

endproc

<A name=ss2.5>2.5 PART V Summary</A>

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 and it can be done in a different way. That's way I suggest reading all documents concerning this issue man pppd, NET4-HOWTO, PPP-HOWTO.

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