12. cfg_lan: ISDN4Linux in a LAN

12.1 cfg_lan: How can I set up Linux so that other computers in my LAN can access the internet via my Linux computer?

There are several possibilities:

1. Your LAN is an official Class C net with IP addresses valid on the Internet. This case is the easiest of configure. You give each network card on your network one of these addresses and set a default route on the ISDN card that goes to your provider.
2. You'd only like to do http in Internet from your LAN. In this case you can make up IP addresses for your LAN; the only official IP address is that for your ISDN card. Then install a proxy server on your Linux router, and enter it in all of your browsers. In this case you do not need a default route.
3. From your LAN you only want to log in to your Linux ISDN router and FROM THERE do your work on the Internet. This is even simpler, then you don't even need a proxy server.
4. Use ip masquerading. This is the most comfortable one to use, but more difficult to set up. The Linux computer acts as a gateway. The trick is that it hides the ip addresses of the LAN, by giving its own internet address as response address. When receiving the response, it will forward it to the correct computer on the LAN. You can also use masquerading with dynamic ip addresses. If you donīt want to start the ISDN connection from the Linux computer to your internet provider manually, then you can set up dial on demand functionality (see section dod ).

12.2 cfg_lan: How can I allow the users in my LAN to dial out via the ISDN card(s) in my Linux PC (like a modem server)?

On the Linux side use modemd, which is a very short perl script:

#!/usr/bin/perl
select((select(STDOUT), $| = 1)($());
select((select(STDIN), $| = 1)($());
exec "cu","-E","''", "-l", "$ARGV(0)";
die "$0: Cannot exec cu: $!\n";

modem           20006/tcp       modemd  # Modem service via TCP
isdn            20007/tcp       modemd  # ISDN  service via TCP

COMT may be found on Simtel or one of its mirror: ftp://ftp.funet.fi/pub/simtelnet/win3/inet/comt200.zip