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

George Sexton gsexton at mhsoftware.com
Mon Mar 15 13:25:59 MST 2004

Here is what I understand about Comcast Digital Cable Modems

1)	Your Digital Cable box and Modem all receive Ips.
2)	The IP Addresses assigned to the modem, and to your cable box
are not relevant to your PC. They are used for management only.
3)	The PC should use DHCP via the cable modem to receive an IP

In essence, the cable modem acts like a router with Two NICS, one for
management, and one for your PC to connect to in BridgeMode.

I have a box running RH 9 that has worked flawlessly for about 6 months.
I did do the initial setup with a machine running WinXP, but once that
was done I plugged in the Linux machine.

Personally, you can by broadband routers/modems from CompUSA for about
$10.00 after rebates. Unless you have a pressing need, get one of these
and use it.

-----Original Message-----
From: lug-bounces at lug.boulder.co.us
[mailto:lug-bounces at lug.boulder.co.us] On Behalf Of Ferdinand Schmid
Sent: Monday, March 15, 2004 9:07 AM
To: Boulder \(Colorado\) Linux Users Group -- General Mailing List
Subject: Re: [lug] tcpdump, routing, ppp, and cable modem questions(s)


You will need to use Internet Explorer to get to the registration site
Comcast.  I have gone through this exercise and found that they really
IE.  Also - dhcp in their cable modems doesn't necessarily work with
computer.  I had a terrible time with it on my PC - couldn't get it to
work on
a Dell laptop under Linux or Windows.  However, a D-Link router was able
talk to it.  Strange...  So my approach to get it working was to use a
cable combined with a Windows PC to do the modem registration.  Until
this is
complete you will only be able to get to your cable modem and to
Comcast's DNS

So - find a Windows PC to register the modem and then connect your Linux
system.  If you can't get dhcp working with your particular modem then
pick your IP address manually.  The default route (your cable modem's
IP) never changes anyway.


--On Monday, March 15, 2004 08:25:44 AM -0700 Chip Atkinson
<chip at rmpg.org>

> 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
> to anything past the cable modem.
> Here are the problems:
> 1) the ppp0 interface has disappeared.  It was working that morning
> 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
> no default route set, I saw tons of ARP requests from other machines,
> when I set the default route: route add default gw (the
> cable modem's IP), the ARP request traffic wasn't visible suddenly.
> 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
> and get to the internet and I fear that it's due to something weird
> 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
> stumped.
> Thanks in advance.
> Chip
> _______________________________________________
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Ferdinand Schmid
Architectural Energy Corporation
Celebrating over 20 Years of Improving Building Energy Performance

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