2. The Server Side

It is assumed that the server is a Linux system with either:

• a modem attached to a /dev/ttySx device

• 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 daemon in /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 daemon 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 daemon 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:

   
bash# ps | grep inetd
194  ?  S     0:00 /usr/sbin/inetd

bash# kill -HUP 194

Now the server side is ready to accept requests from clients. The correct function can be verified by:

bash$ 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 commands in order to verify the setup:

   
atz
atz
OK

ati1
ati1
Linux ISDN
OK

^]
telnet>quit
bash$

Instead of using the Perl script as a modem server, there is also a program named Masqdialer available at http://w3.cpwright.com/mserver/.

With Masqdialer you can export any number of modems connected to your server to any host that can connect the server via TCP/IP on a given port with a binary data stream.

5800 /dev/ttyS1      115200,8,N,1    *.foo.org,192.168.2.1