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Linux Modem sharing mini-HOWTO

Author:  Friedemann Baitinger,  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

    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

    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:

    Linux ISDN


Instead of using the Perl script as a modem server, there is also a program named 'mserver' available on 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 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.