Serial Laplink Mini-Howto

Willem J. Atsma

  v1.0, 22 Aug 2000

  How to create a serial connection between two computers? This document
  details how to setup a "laplink" connection between two computers.
  Having had a bit of trouble getting the information to get this to
  work myself, this document should make it easy for you.
    _________________________________________________________________

  Table of Contents
  1. [1]Introduction

       1.1. [2]Distribution policy

  2. [3]On the server side

       2.1. [4]Configure pppd
       2.2. [5]A getty-like installation of pppd
       2.3. [6]Start the server when needed
       2.4. [7]Serving MS Windows clients

  3. [8]On the client side

       3.1. [9]Start the client
       3.2. [10]Connecting to an MS Windows server
       3.3. [11]Oddly enough

  4. [12]Connecting...
  5. [13]Internet through the serial port
  6. [14]Other useful documents

1. Introduction

  This very brief document describes how to set up a serial "laplink"
  connection between two Linux machines or a Linux and a Windows
  machine. The serial link consists of a "null-modem" cable and a
  communication protocol: PPP. I use such a setup to send files to and
  from my laptop; it can also quite nicely be used to install new
  software from a cd-rom when a laptop doesn't have one. I had more
  trouble than I should have had when setting up my serial laplink.
  Hopefully this document will solve your problems in one go (the ones
  to do with the serial cable anyways).

  Both server and client side of the setup I will describe could be
  either a Linux machine or a different operating system, as long as it
  supports the PPP protocol on a null-modem. The MS Windows
  implementation, as it turns out, is slightly different from standard,
  requiring some special attention. This document has two main sections.
  The first describes how to setup the server side. This is the main
  station with a serial port dedicated to a PPP connection.
  Alternatively the connection can be started when needed from the
  command line. The second part describes the configuration of the
  client that connects to the server to retrieve or upload files.

  This documentation was prepared for pppd version 2.3.11, on a version
  2.2.16 kernel.

  Please let me know if you are having any particular difficulties and
  suggestions for improvements on this document; my email address is
  listed at the bottom.
    _________________________________________________________________

1.1. Distribution policy

  Copyright 2000 Willem J. Atsma

  This HOWTO is free documentation; you can redistribute it and/or
  modify it under the terms of the GNU General Public License as
  published by the Free Software Foundation; either version 2 of the
  License, or (at your option) any later version.

  This document is distributed in the hope that it will be useful, but
  without any warranty; without even the implied warranty of
  merchantability or fitness for a particular purpose. See the GNU
  General Public License for more details.

  You can obtain a copy of the GNU General Public License by writing to
  the Free Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139,
  USA.
    _________________________________________________________________

2. On the server side

2.1. Configure pppd

  The communication will be through a serial port. On the server we will
  connect through the "/dev/ttyS1" (COM2 in DOS/Windows) device. On your
  computer this may be different and you should modify the examples
  below accordingly. I force authentication because I use the
  configuration to provide a way to access files on a file server for a
  group of people. After logon they can use the Samba file shares to
  copy files to and from a laptop. If you are not concerned with
  security you can comment out the relevant options.

  PPP (Point-to-Point Protocol) communication provides TCP/IP across a
  serial link. In other words: when you want to do internet-based
  browsing through a modem, you are likely to be using it. In Linux PPP
  is implemented by the PPP daemon "pppd". Its configuration is done
  through files in the "/etc/ppp/" directory. We will be using the
  following files:

  /etc/ppp/options
         contains all general options for PPP connections

  /etc/ppp/options.ttyS1
         contains PPP options specific to connections through
         "/dev/ttyS1"

  /etc/ppp/pap-secrets
         contains authentication information

  /etc/inittab
         starts initial system services

  The "/etc/ppp/options" file should look more or less like this:
#/etc/ppp/options
lock
#auth forces authorization from peer
#login makes authentication use the system password file
#NOTE: my pap-secrets allows anyone access, so if this is not specified
#      anyone could connect! If this is a machine on which you dial out
#      as well, then comment auth and login out and move them to
#      /etc/options.ttySn
auth
login

  The lines starting with a '#' are comments. The "lock" parameter
  indicates that a lock file will be created to ensure exclusive access
  to the serial device, the "auth" parameter indicates that the client
  will need to authenticate itself, and "login" tells pppd to use the
  system user names and passwords for authentication.

  Note that pppd will still check the "/etc/ppp/pap-secrets" file for
  user name and password information. A special, single line entry makes
  that pppd will only use the system's user information. The
  "/etc/ppp/pap-secrets" file looks as follows:
#/etc/ppp/pap-secrets
# Secrets for authentication using PAP
# client             server            secret             IP addresses
*                        *                      ""                     *


  This is the most unsafe setting you can have in this file: it allows
  any client (user) connect to the server, without using a password and
  using any IP address. The "login" parameter in the "/etc/ppp/options"
  file, however, makes that the user name and password supplied by the
  client for authorization have to match the "/etc/ppp/pap-secrets" file
  as well as the system user name and password, so the connection will
  only succeed after typing in a valid user name and password.

  Options specific to the serial line you are connecting with are placed
  in "/etc/ppp/options.ttySn", where n is the number of the serial
  device. My server uses "/dev/ttyS1", so the options go into...
  "/etc/ppp/options.ttyS1".
#/etc/ppp/options.ttyS1
asyncmap 0
crtscts
#local indicates that modem lines are not used
local
#silent causes pppd to wait until a connection is made from the other side
silent
#auth forces authorization from peer
#login makes authentication use the system password file
#NOTE: my pap-secrets allows anyone access, so if this is not specified
#      anyone could connect! If this is a server that will never use ppp
#      for dialing out, you should move auth and login to /etc/ppp/options
#auth
#login
#use PAP, not CHAP for authentication
+pap
-chap
115200
#these are entries that exist in the /etc/hosts file
ppp_laplink_host:serial_laplink_client

  All these options are well described in the pppd man page; a few of
  the key ones are explained with comments in the file. The "crtscts"
  parameter tells pppd to use hardware flow control. This is recommended
  because it is the fastest. Alternatively you could specify "xonxoff"
  to use software-based flow control - you would specify this if your
  null modem cable doesn't connect the RTS/CTS lines (unlikely if you
  bought your cable in the store). "115200" specifies the data
  transmission rate - if you have trouble connecting you might want to
  try with a lower speed. Note that "auth" and "login" options are
  commented out here, because they were specified in the general options
  file. If you also use your computer to dial into an ISP, you will want
  to specify them here rather than in "/etc/ppp/options", or you will be
  asking your ISP to authorize itself when you dial in and that probably
  won't succeed. The reason why they are not specified in this file by
  default is because if you have other incoming PPP connections now or
  in the future, you want to make sure they are always authenticated.
  Remember that the "pap-secrets" as presented here gives zero
  protection.

  Finally, the "ppp_laplink_host:serial_laplink_client" entry specifies
  the local and remote IP address after the link is up. You can use
  actual IP numbers here (e.g. 192.168.0.1:192.168.1.1), or entries from
  the "/etc/hosts" file, like I have done. The nice thing of doing the
  latter is that you can use the names to refer to these links later. I
  also recommend you use IP numbers like the ones I used
  (192.168.0.1:192.168.1.1). These addresses are set aside for local
  networks and don't exist on the internet, so you are avoiding possible
  conflicts. After the link is up, the client can refer to the server
  with the IP address of ppp_laplink_host (192.168.0.1) and the server
  refers to the client with serial_laplink_client (192.168.1.1).

  You could use different entries in "/etc/ppp/pap-secrets" to only
  allow select users access. I am using the PAP protocol for
  authentication; you could use CHAP if you'd like - the setup is much
  the same, using the "chap-secrets" file. For these and other options
  you can consult the man pages and the documentation mentioned at the
  bottom.
    _________________________________________________________________

2.2. A getty-like installation of pppd

  You can have the PPP daemon (pppd) start when you boot the system and
  have it monitor the serial line of your choice. An elegant way of
  achieving this is to edit the "/etc/inittab" file. This file contains
  information for initializing the system. Add the following to this
  file:
# Start pppd for the serial laplink.
pd:345:respawn:/usr/sbin/pppd /dev/ttyS1 -detach

  This reads as follows: for run-levels 3, 4 and 5 start "/usr/sbin/pppd
  /dev/ttyS1 -detach" and if it dies (at the end of a connection)
  respawn (start a new one). The -detach option makes that pppd stays
  connected to the terminal that started it, rather than forking and
  exiting. This option is necessary because the "init" process would
  respawn a new one immediately otherwise. Other entries in the inittab
  file specify getty processes to run on serial terminals (tty's); their
  initialization looks a lot like this one.

  To activate this new configuration type:
[root@griis /root]# /sbin/init q
    _________________________________________________________________

2.3. Start the server when needed

  If it is only occasionally that you want to connect to your server,
  you might prefer to start the connection manually. All the settings
  remain the same; you can start the server by simply typing:
/usr/sbin/pppd /dev/ttyS1 -detach

  at the command line. The "-detach" option is not really necessary, but
  it makes it easy to kill the connection by pressing "ctrl-c".
    _________________________________________________________________

2.4. Serving MS Windows clients

  Unfortunately the MS Windows implementation is not quite standard.
  Before initiating the PPP connection it requires the exchange of the
  text strings "CLIENT" (from the client) and "CLIENTSERVER" (from the
  server). To accomodate a Windows client the following line has to be
  added to the "/etc/ppp/options.ttyS1" file:
connect 'chat -v -f /etc/ppp/scripts/winclient.chat'

  Then create the scripts directory and the chat file
  "/etc/ppp/scripts/winclient.chat":
TIMEOUT 3600
CLIENT CLIENTSERVER\c

  The connect option allows you to specify a program to deal with the
  string exchange before the connection. Usually the "chat" program is
  used for this; check the manual for more details. The given script
  deals with the Windows connection issue. You don't need it when
  connecting a Linux box.
    _________________________________________________________________

3. On the client side

  After having configured the server, the client is easy. The
  "/etc/ppp/options" file has a single entry:
#/etc/ppp/options
lock

  The serial port on my client is "/dev/ttyS0" (COM1), so I also have to
  create a "/etc/ppp/options.ttyS0" file:
#/etc/ppp/options.ttyS0
115200
crtscts
local
name zaphod
noauth

  Only a few new options in this file. Note that the speed on server and
  client has to be the same (here it is 115200). With "name zaphod" the
  client name is specified. This name has to correspond to an entry in
  the "/etc/ppp/pap-secrets" file of the client and has to be a valid
  user name on the server. The "noauth" option specifies that the peer
  (server) does not have to authenticate itself to the client.

  The last bit: setting the client name (user name) and its secret
  (password) in the "/etc/ppp/pap-secrets" file:
#/etc/ppp/pap-secrets
# Secrets for authentication using PAP
# client             server            secret             IP addresses
zaphod             *                     gargleBlaster

  So user "zaphod" uses a password "gargleBlaster", which should get him
  into the system. Note that these files contain sensitive information,
  make sure you set the permissions on them correctly.
    _________________________________________________________________

3.1. Start the client

  On the command line, type:
/usr/sbin/pppd /dev/ttyS0 -detach
    _________________________________________________________________

3.2. Connecting to an MS Windows server

  What if you want to make a connection to a Windows server? Again we'll
  need the "connect" option and a chat script. Add the following line to
  your "/etc/ppp/options.ttyS0" file:
connect chat -v -f /etc/ppp/scripts/winserver.chat

  Also create the chat script "/etc/ppp/script/winserver.chat":
TIMEOUT 10
'' CLIENT\c

  This makes the client send the "CLIENT" string before trying to start
  the PPP connection.
    _________________________________________________________________

3.3. Oddly enough

  Oddly enough I created a Linux server that has connect to both Linux
  and Windows computers, so I found myself with the odd situation of
  having to setup both Linux systems such that they would connect to
  Windows servers and clients!
    _________________________________________________________________

4. Connecting...

  The moment of truth: connect the computers with the null modem cable,
  first start the server side (if you are using the "/etc/inittab"
  approach it should be up already), then start the client side. You
  should see something like this on the server:
[root@griis /root]# pppd /dev/ttyS1 -detach
Using interface ppp0
Connect: ppp0 <--> /dev/ttyS1
user zaphod logged in
Deflate (15) compression enabled
local  IP address 192.168.0.1
remote IP address 192.168.1.1

  Note that if you started pppd from the "/etc/inittab" file these
  messages will appear in "/var/log/messages". On the client side you
  should see something like this:
[root@wylg /root]# pppd /dev/ttyS0 -detach
Using interface ppp0
Connect: ppp0 <--> /dev/ttyS0
Remote message: Success
Deflate (15) compression enabled
local  IP address 192.168.1.1
remote IP address 192.168.0.1

  On either side you can ping the other computer to see if the
  connection is active:
[zaphod@wylg zaphod]$ ping 192.168.0.1
PING 192.168.0.1 (192.168.0.1) from 192.168.1.1 : 56(84) bytes of data.
64 bytes from 192.168.0.1: icmp_seq=0 ttl=255 time=31.7 ms
64 bytes from 192.168.0.1: icmp_seq=1 ttl=255 time=20.3 ms
64 bytes from 192.168.0.1: icmp_seq=2 ttl=255 time=19.2 ms
64 bytes from 192.168.0.1: icmp_seq=3 ttl=255 time=20.3 ms

--- 192.168.0.1 ping statistics ---
4 packets transmitted, 4 packets received, 0% packet loss
round-trip min/avg/max = 19.2/22.8/31.7 ms

  You can now access the server through any TCP/IP services it might
  have available, such as FTP, HTTP, TELNET and SAMBA.
    _________________________________________________________________

5. Internet through the serial port

  Suppose you have a laptop and you want to access the internet
  occasionally. Also suppose you have a machine that has an ethernet
  connection and a serial port. You can set the laptop up as a client,
  the other machine as a server, and use IP masquerading to connect to
  the internet through the null modem cable. Check the
  IP-Masquerading-Howto for help on how to do this.
    _________________________________________________________________

6. Other useful documents

  The pppd and chat manual pages

  PPP-HOWTO

  Modem-HOWTO

  Serial-HOWTO

  IP-Masquerade-HOWTO

  author: W.J. Atsma

  email: [email protected]

  revision: 22aug2000s

References

  1. Serial-Laplink-HOWTO.html#AEN11
  2. Serial-Laplink-HOWTO.html#AEN17
  3. Serial-Laplink-HOWTO.html#AEN23
  4. Serial-Laplink-HOWTO.html#AEN25
  5. Serial-Laplink-HOWTO.html#AEN58
  6. Serial-Laplink-HOWTO.html#AEN65
  7. Serial-Laplink-HOWTO.html#AEN70
  8. Serial-Laplink-HOWTO.html#AEN77
  9. Serial-Laplink-HOWTO.html#AEN87
 10. Serial-Laplink-HOWTO.html#AEN91
 11. Serial-Laplink-HOWTO.html#AEN98
 12. Serial-Laplink-HOWTO.html#AEN101
 13. Serial-Laplink-HOWTO.html#AEN110
 14. Serial-Laplink-HOWTO.html#AEN114

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