Using the Raspberry Pi as a Router?

I have used the Pi for routing, it worked well. As you say, you need at least one more network interface, as the Pi has only one ethernet port. You can add another interface connected to the USB port. I have used a GSM modem and a Wifi stick.

Things to note:

• Use a powered hub. Wifi and GSM devices demand more power than what the Pi:s USB ports can supply.
• If you shop for Wifi devices, get one with good Linux support. You want one that works in access point mode, not all sticks do. Check at linuxwireless.org.

Here is my blog post about routing, and another about setting up a WiFi access point. This guide helped me with the routing setup. The routing stuff is not very specific to the Pi, anything that works on Linux in general should be possible. The details depend a bit on which distribution you use.

Having a little network background, the first thing that comes to mind is use it as a Snort box, and then you don't have to worry about only having one interface. You would setup your switch/router to forward all traffic to the Pi, but also pass it onto the end device. This is known as a switchport monitor, but your device may not support it. You may need a managed switch to do this. Snort would then analyze the traffic and report on it. More information on Snort http://www.snort.org/

The nice thing about this kind of setup is your Pi is sitting off to the side watching everything that is being run through the switch, rather than sitting inline with the data. If you have a software glitch this way, traffic can still flow, just not be monitored. With what you are describing with two interfaces, your program crashes, your network crashes.

This does not use the pi as a router, but does allow you to monitor and report on traffic that is moving across your network.