Linux Modem sharing mini-HOWTO

Author:  Friedemann Baitinger,  baiti@herrenberg.netsurf.de
last update: V1.00, 06/12/97


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 -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.

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. 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 <URL: http://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 serer 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.