There’s a lot to think about when setting up an Internet gateway for a small LAN. One thing that you definitely don’t want it to do is hold the line open indefinitely if you forget that you’re connected – and maybe run up a big bill with your ISP (Internet Service Provider) in the process!

Fig.14: the "modemtool" utility is used to create the link between your modem and the file /dev/modem by selecting the relevant serial port.

Similarly, you don’t want it repeatedly dialling out, connecting and disconnecting, if there is something wrong at your ISP’s end. This could be a real problem if you run a program that automatically triggers dial-outs according to a preset schedule – eg, an email server that regularly connects to send and retrieve email.

A 20-cent local call mightn’t sound like much but several thousand calls made at 1-minute intervals over a long-weekend will give you a real pain in the wallet. Do the sums – ouch!

This Linux gateway overcomes the above problems by automatically disconnecting after a preset "idle" time (ie, when there has been no Internet traffic) and by limiting the number of unsuccessful connection attempts. You can adjust both the idle time and the number of connection attempts to your liking.

Another important thing to be considered is security. There are plenty of bad guys out there on the Internet, just waiting to hack their way in and wreak all kinds of havoc. We’ll tackle that problem as well.

Actually, we’re going to cover a fair bit of ground this month to get the gateway working. First, we’re going to set up the modem, then we’re going to get pppd (the point-to-point protocol dialler) working. After that, we’re going to cover demand dialling, IP masquerading, IP forwarding and firewalling.