[lug] tcpdump, routing, ppp, and cable modem questions(s)

D. Stimits stimits at comcast.net
Mon Mar 15 14:46:07 MST 2004

Chip Atkinson wrote:
> Greetings all,
> I recently signed up for internet access through Comcast, the cable TV
> provider and am trying to get my machine connected to it.  During the
> process I came across/caused a couple problems and also can't connect out
> to anything past the cable modem.
> Here are the problems:
> 1) the ppp0 interface has disappeared.  It was working that morning but
> now is gone.
> Using kppp I get an error saying
> ppp: no such interface (don't remember the exact wording)
> and
> ifconfig ppp0 up
> returns ppp0: unknown interface: No such device.
> 2) I was using tcpdump to watch network traffic on eth0.  When there was
> no default route set, I saw tons of ARP requests from other machines, but
> when I set the default route: route add default gw (the
> cable modem's IP), the ARP request traffic wasn't visible suddenly.  When
> I deleted the default route, the traffic was visible again.
> I thought tcpdump grabbed all of the traffic, regardless of routing
> tables.
> 3) I'm unable to get to a web server on the Comcast network to register
> and get to the internet and I fear that it's due to something weird that
> also caused ppp and the weird tcpdump behavior.  I can ping the WAN end of
> the cable modem, but nothing else.
> Does anyone have any similar experiences or ideas?  Right now I'm kind of
> stumped.

I can't answer much of what you ask, but I can give you some info. I use 
Comcast, and the Motrola Surfboard 4200 cable modem. Registration was 
via a windows based computer runing IE.

Setup on the linux side was purely DHCP on ethernet. In other words, the 
interface set up by DHCP was completely standard. I happen to use 
multiple network cards so I can use a safer/more secure communications 
to my local machines without going past the cable modem, but essentially 
it is as plain vanilla as you can get. A redhat 
/etc/sysconfig/network-scripts/ifcfg-eth0 that works is:

Then make sure your firewall doesn't block the wrong thing. In 
particular, some DHCP replies come through non-routable IP's. If you 
didn't register the modem first though, you'll never get a reply.

As for no ppp0 interface, it sounds like the kernel changed or the 
module driver for the modem is gone. You won't use ppp0 though unless 
you are still using the phone line/56k, it should have no effect on 
cable. I am guessing you used some sort of gui tool which unconfigured 
ppp0 at the same time you did something to default route or eth0.

D. Stimits, stimits AT comcast DOT net

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