Linux Modem sharing mini-HOWTO

Linux Modem sharing mini-HOWTO

Author: Friedemann Baitinger, [1][email protected]
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:[email protected]
2. ftp://ftp.innet.be/pub/staff/carl/
3. http://www.tactical-sw.com/