Best of IRC

Back in full force, it's another edition of Best of IRC: Chronicles of an online phenomenon. The dust has finally settled on the new look and feel of linux.com, leaving a trail of caffeinated penguin mints and countless sleepless nights in its path. Meanwhile, the popularity of #linuxhelp and #linux.com-live! events has been on rapid increase.

This weeks Best of IRC column is about an event that took place in #linux.com-live! not too long ago about TCP/IP Configuration for Highspeed Networks. In short, how to setup your cable or DSL modem. The guest speakers are Brian Richardson from linux.com/hardware/, Chris Campbell from linux.com/sysadmin/ and Jeff McClure. What follows is a transcript of that event:

Chris Campbell:

Well, if you have bell atlantic, you are probably familiar with the fact that they have their own take on PPP called PPPOE. PPPOE is a brand new protocol, and isn't widely supported yet. When the new version of the kernel comes out (version 2.4), then there should be support for it. Basically, PPPOE takes standard PPP packets and wraps them up into ethernet packets. Point to Point Protocol Over Ethernet.

There are different ways to handle PPPOE in linux, but I have found the most success with Roaring penguin (RP). If you wish to have PPPOE in the kernel, that would be located at to http://www.davin.ottawa.on.ca/pppoe/. Now, down to the setup.

I had to do the intial setup to sign on to the account using an NT box. - This is to setup your user profile with the ISP. Now, PPPOE hands out addressing dymaically, you should not use DHCP or anything like that. DO NOT USE DHCP, that messes everyone up.

Brian Richardson:

There's basically three types of DSL modems/adapters the phone company will give a user:

1. Internal PCI card
2. USB External
3. Generic external connecting to RJ45

Each device has pros and cons associated with it. 1 & 2 are cheaper. 2 won't work with linux (yet). 1 may not work with linux. 3 works with anything but is more expensive. 3 is able to connect to a hub (via crossover cable) to get packets to other PCs. All three devices connect to RJ11 at some point to get DSL from the phone line.

The DSL adapter gets the IP to the Internet from the provider (i.e. Bell South) using DHCP, so my adapter has 207.69.x.x. that gets seen by the net. But, only one machine on my network gets to send/recieve stuff from the net using the PPPoE client. So nless you are running some sort of router/ipmasq configuration you only get one PC on the net. That's where the wonders of Linux come in (that's why we're here)

It's pretty much the same as any ipmasq/router/firewall configuration, except for the PPPoE client (Roaring Penguin for Linux). PPPoE looks like a ppp adapter in the networking config.

Let me give you a practcal example ... my network:

I have 4 machines at home, three are clients (2 are Win98, one is Mandrake 7.2). The fourth is a dinky Cyrix with a floppy & two NICs. I run the Cyrix as a router between the DSL adapter and my local net.

• Adapter 1 is a Tulip card, connected to the external DSL adapter via crossover cable
• Adapter 2 is a 3com 3c905 hooked up to my hub

The cyrix system, acting as router/ipmasq/forewall routes from the net to the Adapter on 192.168.0.1. The Cyrix machine is the ONLY machine on this side of the net.

Jeff McClure:

Okay. Actually, most of what Brian described applies to cable modems. However, what I understand as the most common configuration requires you to use DHCP. The cable modem connects to your CATV coax on one side, and the other side is a standard Ethernet connection. The Ethernet connects to a NIC in the Linux box. You set that NIC up to use DHCP to get its IP address. As with the DSL discussion, that IP address is visible to the world.

If the Linux box is the only machine on the network, then that's it. However, to connect other machines, you need a second NIC with an "internal" IP address, usually from that 192.168.x.x range that Brian mentioned. The Linux box then acts as a router/firewall that straddles the two networks, same setup that Brian mentioned. The real difference is the need to use a DHCP client to get the external IP address. I think Roadrunner may require a special client to connect. Some providers need to see your NetBIOS hostname set to a certain value. Most DHCP clients have an option that allows sending that.

Another gotcha: many times, cable modems "learn" the MAC (hardware) address of the NIC to which they are attached. If you connect the cable modem to a different NIC, it may not work until you reset the cable modem. Even better, My cable modem (3Com) won't relearn the MAC just by reset. If I need to use a different network card, I have to call my provider and tell them the MAC so they can force the modem to relearn.

Questions and Answers

Can my neighbour packet sniff my cable modem?
My understanding is that DOCSIS-compliant modems (the standard) filter all packets not destined for you.

So, you have your shiny Ethernet DSL modem. Do you have to do something to that to bring it onto your network?
The external adapters all assume you run a 192.168.0.x network unless otherwise configured. To reconfigure them you'll probably need the windows utility.

Does the hostname of your gateway computer have to be the same as the one your Cable has assigned?
No, the hostname of your computer is irrelevent. You may however have to configure your dhcpcd to provide an identifier which is usually refered to as the NetBIOS name (Although it has little to do with netbios).

Is there anyway to tweak a router with a built-in DSL modem? How do you tweak your ethernet card to get full throughput?
You're talking about something refered to as MTU. Maximum Transmit Unit. This setting controls how large packets can be, you need to find the 'optimal' packet size for your speed. Too large and your packets will suffer fragmentation, too small and you'll limit your speed.

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This column is written for and about you, the linux user. If you've had an experience with #linuxhelp or #linux.com-live! that you want to share with the world then write to us at live@linux.com and we'll make your message heard.