
                        Linux Modem sharing mini-HOWTO
                                       
   Author:  Friedemann Baitinger,  [1]baiti@toplink.net
   last update:
   
   v1.01, 11 February 1997 source now in HTML format, added a reference
   on where
                   to get 'cu', also added a reference on where to get
                   mserver. '-s' parameter added to 'cu' example.
   
   V1.00, 06/12/97 Initial release
   ______________________________________________________________________
   
1. Introduction

   This mini-HOWTO describes how to setup a Linux system in order to
   share a modem attached to this system  with other systems over a
   TCP/IP network.
   
2. The Server Side

   It is assumed that the server is a Linux system with either:
   
     a)  a modem attached to a /dev/ttySx device
   
     b)  an 'isdn4linux'-emulated modem mapped to a /dev/ttyIx device
   
   The easiest setup I can think of uses a five lines perl script to
   implement a 'modem demon':
   
       $ cat /usr/sbin/modemd
   
       #!/usr/bin/perl
       select((select(STDOUT), $| = 1)[$[]);
       select((select(STDIN), $| = 1)[$[]);
       exec 'cu -s 115200 -l /dev/ttyS1';
       die '$0: Cant exec cu: $!\n';
   
   The modem demon is started by the INETD process if a client connects
   to the appropriate port as described below. The 'modemd simply
   connects the socket handle with STDIN and STDOUT of the 'cu' command
   and lets 'cu' handle the actual modem device. In case you don't have
   'cu' on your system, please install the 'UUCP' package, 'cu' is
   usually part of 'UUCP'.
   
   The existence of the modem demon must be made known to the INETD
   process by updating its configuration file, usually /etc/inetd.conf
   like:
   
       #
       # modem daemon
       #
       modem stream tcp nowait root /usr/sbin/tcpd /usr/sbin/modemd
   /dev/ttyS1
   
   In order to make this work, an entry to '/etc/services' needs to be
   added like:
   
       modem           2006/tcp        modemd
   
   This associates a symbolic name with an explicit port, 2006 in the
   example. The portnumber could be any number not already assigned to an
   existing service. After these changes have been made, a signal must be
   sent to the inetd process in order to let inetd re-read and process
   its configuration file:
   
       $ ps |grep inetd
       194  ?  S     0:00 /usr/sbin/inetd
   
       kill -HUP 194
   
   Now the server side is ready to accept requests from clients. The
   correct function can be verified by:
   
       $ telnet localhost modem
   
       Trying 127.0.0.1...
       Connected to localhost.
       Escape character is '^]'.
   
   You are now connected to the modem. You can now issue 'AT' command in
   order to verify the setup:
   
       atz
       atz
       OK
   
       ati1
       ati1
       Linux ISDN
       OK
   
       ^]
       telnet>quit
       $
   
   Instead of using the Perl script as a modem server, there is also a
   program named 'mserver' available on
   [2]ftp://ftp.innet.be/pub/staff/carl/ . In case it is not there
   anymore, you may want to use 'archie' or any other search engine in
   order to locate the mserver archive. I haven't had the opportunity yet
   to install and use mserver.
   
3. The Client Side

   At this time, only Windows client setups are described here.  On the
   client PC, a COM-port redirector for TCP/IP is required. The best
   program for this purpose I have found is 'DialOut/IP' from 'Tactical
   Software' for Windows 3.1 and Windows 95.  (The Windows 3.1 version
   can be used under Windows NT for 16-bit applications only.  A 32-bit
   version for Windows NT is due late summer 1997.)
   
   DialOut/IP presents the shared modem on a new virtual COM port that it
   adds to Windows.  This virtual COM port can be used by Windows
   programs as if the shared modem is directly connected.  Most client
   applications (including Windows 95 dial-up networking) accept this and
   work as if there were a real COM port and modem, with the general
   exception being fax applications or any others that need access to
   UART control lines. DialOut/IP can be configured to provide Telnet
   protocol processing, but that feature applies to certain modem pool
   products and not to the Linux setup described in this file.  Note
   that, despite its name, DialOut/IP can be used also by applications
   that wait for incoming calls.
   
   On [3]www.tactical-sw.com there is a page for downloading a fully
   functional evaluation version that times out in 1-2 weeks.
   Installation and configuration is handled by a setup program, with
   installation details in the README.TXT file.  When you run DialOut/IP,
   you enter the IP address and port number of the shared modem.
   
   DialOut/IP is a commercial product that is licensed on a per-modem
   basis, that is, the price depends on the number of modems that you are
   sharing. The license states that you can install the software on any
   number of PC's that access the shared modems.
   
4. Security Considerations

   If you have only one modem for all your hosts in your local area
   network, there is probably no reason to worry about security here.
   However, if any one or more of the hosts in your LAN are connected to
   the internet by other means than using the modem we have just setup as
   a modem server, then security considerations are required, otherwise
   anybody can do a 'telnet your_host modem' and dial out long distance
   or even international calls at his will.
   
   I suggest to install and configure tcp-wrappers in order to protect
   the modem sevrer against unauthorized access.
   
5. Examples

   I am using the setup as described in (2) and (3) to run Quicken on my
   Windows 95 ThinkPad and do homebanking with the modem attached to my
   Linux machine. The 'modem' in my case is not even a real modem, it is
   an emulated modem on an ISDN-So card. Quicken just sees a COM port, it
   doesn't know that the device attached to the COM port is actually at
   the other end of my Ethernet LAN, nor does it know that it is not a
   standard analog modem but an ISDN device which happens to understand
   'AT' commands.

References

   1. mailto:baiti@toplink.net
   2. ftp://ftp.innet.be/pub/staff/carl/
   3. http://www.tactical-sw.com/
