This is my first How-to so bear with me :-)
This guide is written for people who want to use an old Alcatel Speedtouch ADSL modem under linux. The first part of the guide will tell you how to set up the uplink using the modem. After that, I'll explain how to turn the gentoo machine which controls the uplink into a internet router.
Note: this is a copy from the original article I posted on Gentoo Wiki. By now there might be differences in the content but this is the original.
Updated: Reformatted by hand to clean up the HTML to fix the layout.
Right now I have an old linux server (200Mhz Pentium) downstairs running SuSE 6.x which is hooked up to an Alcatel Speedtouch modem working as an internet router. Although an old modem, there seem to be numerous people who are still using this piece of history to connect to the internet.
There used to be a (dutch) site which had a prefab package to setup the modem, the server and set up a firewall. Unfortunately the site is gone now leaving people like me without a fresh installer to configure the Alcatel Speedtouch.
Back then my knowledge of linux was poor at best - I knew a few commands and I thought I was a big shot for getting SuSE to run and even route my ADSL traffic... Looking back I regret that I didn't know what I do now as I would probably be able to monitor the install scripts to be able to copy the behaviour and redo it on my new and shiny gentoo server.
While writing this I'm attempting to get the Speedtouch to work with gentoo so hopefully this how-to is easy to follow as you will be doing the same things as I am.
How this is supposed to work
If you're no tech-wiz, skip the techno talk and simply follow the steps. For everybody else, this is how it should work when we're done.
The modem and the server have their own ethernet connection in the 10.0.0.x range. Over this network they will create a tunnel using ppp (to be exact synchronous pptp). This tunnel transports the actual internet traffic. Therefor, the endpoint of this tunnel is at the server and will have the public ip address. I will assume that the server has 2 network interfaces, one will be used for the network connection with the modem and the other will go towards your internal network.
IMPORTANT: I assume eth0 is the interface with is connected to the modem while eth1 is the interface connected to your lan. Also, the default configuration of the modem is to use address 10.0.0.138, which I will use in this howto.
To provide a bit of a complete manual I'll incorporate the set up of the network forwarding and NAT so you can use the internet from every system in your lan. To make the set up of all your computers easier I will also add a DHCP server.
Please note however that these are basic setups and not advanced guides for firewalling or DHCP servers. Please look to other guides for that kind of information.
WARNING: This implies that your server will be connected to the outside world without intervention of the modem: there is no firewall or anything between your server and the world wide web... Ponder about that for a while and I hope you understand the importance of a firewall or at least a simple iptable script that closes all the ports on the internet uplink.
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