There are two secrets to setting up Infinitum DSL from Telmex under Linux. The first is realizing that Telmex is using PPPOE, that is a PPP connection over an ethernet network. If you have just a single linux box connected to your DSL modem, you just need to put your username and password in the right config file. This file will vary from distribution to distribution, so you will need to look through your documentation to find out where it is. In my case (SuSE 8.0) it is at /etc/sysconfig/network/providers/dsl-provider0, and it contains the following:
PROVIDER="DSL provider"
DSLSUPPORTED="yes"
MODEMSUPPORTED="no"
ISDNSUPPORTED="no"
USERNAME="put your username here"
PASSWORD="put your password here"
IDLETIME="300"
DEMAND="yes"
DNS1="200.23.242.201"
DNS2=""
I have mine configured for dial on demand, which make a new connection whenever I want to use the internet for something. Unlike dial-up this is almost instantaneous.

Now, if you are running a network behind your dsl connected box, as I am, there is one more wrinkle that took me several days of research to uncover. The symtom is that some www sites cause our system to hang, for no apparent reason. This was maddening, since it basically worked well, except all of a sudden I couldn't check my credit card balance at www.mbna.com. Like I said, it took a while to get to the bottom of this, and the problem is described in great detail at: http://sdb.suse.de/en/sdb/html/cg_pmtu2.html, but the crux of it is that you need to reset all of the MTUs (Maximum Transfer Units) of all the ethernet cards in your network to (probably 1492) the size of the MTU on your PPP0 connection. HUH? you say. Here is the output /sbin/ifconfig on the linux box that acts as my router:

eth0      Link encap:Ethernet  HWaddr 00:80:C8:8B:C2:6C  
          inet6 addr: fe80::280:c8ff:fe8b:c26c/10 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1492  Metric:1
          RX packets:528744 errors:0 dropped:0 overruns:0 frame:0
          TX packets:523438 errors:0 dropped:0 overruns:0 carrier:0
          collisions:43 txqueuelen:100 
          RX bytes:315335468 (300.7 Mb)  TX bytes:60519670 (57.7
Mb)
          Interrupt:3 Base address:0x300 

eth1      Link encap:Ethernet  HWaddr 00:48:54:C0:FA:0E  
          inet addr:192.168.0.9  Bcast:192.168.0.255 
Mask:255.255.255.0
          inet6 addr: fe80::248:54ff:fec0:fa0e/10 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1492  Metric:1
          RX packets:432677 errors:0 dropped:88 overruns:0 frame:85
          TX packets:442528 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:100 
          RX bytes:81874769 (78.0 Mb)  TX bytes:314233462 (299.6
Mb)
          Interrupt:9 Base address:0x320 

lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:81688 errors:0 dropped:0 overruns:0 frame:0
          TX packets:81688 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:8268570 (7.8 Mb)  TX bytes:8268570 (7.8 Mb)

ppp0      Link encap:Point-to-Point Protocol  
          inet addr:200.66.229.178  P-t-P:148.223.111.237
Mask:255.255.255.255          
          UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:1492 
Metric:1
          RX packets:426557 errors:0 dropped:0 overruns:0 frame:0
          TX packets:421216 errors:0 dropped:234 overruns:0
carrier:0
          collisions:0 txqueuelen:3 
Notice that on the third line of the ppp0 output is MTU:1492. Normally ethernet cards have an MTU of 1500, but PPPOE uses 8 bytes of that space for its own special header. Now eth0 is the card that is connected to the DSL modem, and eth1 is the card that is connected to the rest of my network. If you have a similar setup, chances are the MTU on both eth0 and eth0 are 1500, but you need to change them to whatever is the MTU for ppp0. You will also need to do this on every computer that is connected to eth1, ie, your local network. Again how you do this will depend on your distribution, but somewhere along the line it needs to be set. I got desperate and resorted to brute force:
ifconfig eth1 latitude mtu 1492 up
ifconfig eth0  mtu 1492 up
Where latitude is the name of my router, and these commands run whenever the network is started. So there you have it. It is running like a charm, as evidenced by:
henry@latitude:~> uptime
9:36pm  up 78 days, 12:02,  5 users,  load average: 0.16, 0.05,
0.01
-----------^^^^^^^ - Microsoft users! Eat your hearts out!
As my friends at SuSe would say: Have a lot of fun!

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