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  call-back mini HOWTO
  by Pawel Skonecki,
  v2.1a, 2001-10-06

  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.

  Table of Contents

  1. Introduction

     1.1 OPINION

  2. Procedure

     2.1 PART I: Net at home ?
     2.2 PART II: The first steps with modem.
     2.3 PART III Call Linux
     2.4 PART IV Linux calls us
     2.5 PART V Summary


  1.  Introduction

  1.1.  OPINION

  I 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.


  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 who calls the 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 the 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 the 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. I don't describe configuration of ISDN call-
  back because  I don't use ISDN in my connection to the Internet. If
  you set call-back on ISDN send me your configuration.  I had some
  problem when I changed my kernel from 2.2.x to 2.4.x. I will describe
  new option for a new kernel. Remember that if you change your kernel
  for high version you will have to change pppd, too.  I don't create a
  new section for description of new possibility in kernels 2.4.x but I
  write a new configurations in old section.  I would like to apologise
  to people who asked me about options for new kernels.  I didn't have
  time to write a new version of HOW-TO. I have changed my job and place
  of liveing. Sorry.

  2.2.  PART II: The first steps with modem.

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

  �  If you have USR WinModems see Linmodem-HOWTO.

  �  The externale modem is moust flexible you have more place inste
     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 modem see Plag-and-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.

  2.3.  PART III Call Linux

  The first step to make the call-back on Linux accessible is to set up
  a suitable parameters 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.In the kernels 2.4.x series you have
  to mark follow options:


  After compilation you have to add some lines for  /etc/modules.conf

  alias /dev/ppp          ppp_generic alias char-major-108
  ppp_generic alias tty-ldisc-3       ppp_async alias tty-ldisc-14
  ppp_synctty alias ppp-compress-21   bsd_comp alias ppp-compress-24
  ppp_deflate alias ppp-compress-26   ppp_deflate

  You can't forget that you need new pppd demon for kernels 2.4.x (for
  me it was ppp-2.4.0 this ppp has some mistake you have to get latest
  from in /pub/ppp ).

  OK. We have a good kernel.  Now, we have to set up the 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 the call-back
  system has double verification we create a user who will be running
  ppp on the side of server.  In /etc/passwd you have new user and you
  have to change their shall.

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

  I changed the above line for kernel 2.4.x this line in /etc/passwd (I
  use shadow and you don't see password)

  pppuser:x:6778:44:PPP Dialin:/etc/ppp/:/usr/sbin/pppd

  I don't use a special script for the running of pppd but I run it
  directly while login pppuser.

  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 *

  In the 2.4.x kernels you have to write in /etc/ppp/pap-secrets

  *               *       ""                      *

  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


  exec /usr/sbin/pppd -detach

  where the address is the address of server with modem and
  the address 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





  If it's 2.4.x kernel you write in /etc/ppp/options

    asyncmap 0

  The last 3rd  option is very important. You use only PAP
  authentication ,require-pap. Don't use chap authentication, refuse-
  chap.  You can use ms-dns, If you have M$ Windows system clients you
  can send them information about DNS server. If you wont to send IP of
  DNS server for Linux/UNIX machine you use usepeerdns option, more in
  man pppd.

  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 to control your modem. You can only work on the server 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


  s1:2345:respawn:/sbin/mgetty ttyS1 -s 115200 -D /dev/ttyS1

  For the 1st com line looks as follows:

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


  s0:2345:respawn:/sbin/mgetty ttyS0 -s 115200 -D /dev/ttyS0

  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

  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 7 debug

  If you have 2.4.x kernel you have write in this file:

   /AutoPPP/ -     a_ppp   /usr/sbin/pppd file /etc/ppp/options

  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.  I insert have to insert opinion of Bill
  Staehle "NOTE: Some distributions think they know better than you, and
  delete this permission when their 'admin' tool (yast in SuSE,
  linuxconf in many others) is used. Read the documentation of the
  "tool" to see how to stop this crap."

  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. It's
  normal situation in.  I add opinion of Bill Staehle "COMMENT: This is
  what I was referring to. In Red Hat, look at
  /usr/lib/linuxconf/redhat/perm/ppp (delete or rename the file)."

  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.

  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 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.


  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

  The configuration file of daemon ppp (an example for modem on com2)

  �  /etc/ppp/options









     asyncmap 0

  �  /etc/ppp/pppcallback

     TIMEOUT 5





     ABORT '\nVOICE\r'

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

     TIMEOUT 40

     OK ATDT5376443 CONNECT ''

     ogin:-ogin: ppp-pseudo-user

     TIMEOUT 180

     CONNECT ''

     TIMEOUT 20

     ogin:-ogin: pppuser

     sword:-sword password_for_ppuser

  �  /usr/bin/ppp-call


     teksta="Connection failed"

     tekstb="Probably, You will be connect"

     # /sbin/setserial /dev/ttyS1 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
  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"


  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 and it can be done in
  a different way. That's way I suggest reading all the documents
  concerning the issue man pppd, NET-HOWTO, PPP-HOWTO,ISP-Setup-RedHat-
  HOWTO,Modem-HOWTO .  A special thanks for Bill Staehle.

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