Re: comcast problem

Larry Finger wrote: 
hazzmat wrote:

On Wed, 18 Jan 2006 13:05:26 -0800, d wrote:


hazzmat wrote:

This is not a Linux specific problem but I was hoping people with Comcast
experience could suggest some ideas...
My brother has had a working Comcast highspeed cable modem internet
service working with his Mac for months and months. Now it's not working
and I took a Linux system over (also win2k) to try to help diagnose the
problem.
In Linux the symptoms are these (and as far as I can tell, it's the same
with winblows). System set to dhcp (which i know works because I tested it
in my home network before schlepping it over), Linux gets an IP address
(69.something.something.something) it appears to get (according to 'route
-n') a default GW, and it gets 2 ISP provided nameservers, too. But no
pinging is possible. Not Comcast nameservers, not default GW address,
nothing. Switching ethernet cables, resetting modem, powercycling modem
makes no difference.

I know nothing about Comcast service
setup--nothing about what ethernet cables should be used between
"Scientific Atlanta" modem and NIC. I am assured that no special login
software is needed. According to tech support, says my brother, "Anybody
with a laptop should be able to come over, plug in and surf through the
modem." What should I be looking at?

--
Get Big Brother out of my email to reply.


1. Is the machine directly connecting to the cable modem?



I tried it that way(direct), and when direct connection to the cable
modem failed, I then tried plugging a netgear router in, which has been
used successfully at the location with the cable modem previously. (when
using the Netgear, the linux system got a 192.168.0.X address and could
ping the Netgear router, but nothing beyond. Nameserver and default gw
were successfully set by DHCP to the Netgear's internal IP address when
the Netgear was in the chain. Like I said, in a *normal* network context
the DHCP enabled connection of the Linux system works perfectly (windows
does too). In this context however with the Comcast connection all those
things get set up by DHCP in an apparently correct manner, but pinging and
nameresolution just do not work. It would seem that packets go into the
cable modem from our client end, but they aren't getting out. I tried to
ping an external machine at a fixed IP address, which was set to log ICMP.
Looking at the log I see that nothing came in from the Comcast network
block we were getting IPs from.


2. If not, and you have a router, reset the 


router.


3. Does your Linux machine have iptables/ipchains on? 4. If so, turn the 


service off.

There was no firewall script in place--all chains accept, no natting or
anything. This setup worked flawlessly just as it is in my home Linux
based network. When this system was booted into w2k there was a Sygate
personal firewall running, but again, that configuration also worked
flawlessly in my own simple network. There should be no reason that the
distribution's default iptables service would prevent the system from
using DHCP in any context, but if I get another chance to work at the site
tonight I'll trying flushing all iptables rules.


5.
What type of ping response are you receiving?




No response at all: 100% packet loss. (Unless the Netgear router is
interposed between the Linux box and the cable modem. Then as I mentioned
there is no problem when pinging the Netgear's LAN interface--but that's
is as far as it will go.)


Did something happen to that Netgear router. I don't use Comcast, but have a similar setup with Time Warner cable. The cable modem plugs into the WAN port of a Linksys WRT54G wireless AP/router. The Linksys box is a DHCP client to the Time Warner network and a server for the internal network. The internal hosts use 192.168.1.1 (the router's internal address) as both gateway and name server. The Linksys then NATs them out through the WAN port.

One thing to note is that the modem must be on before the router is turned on. As a first step, I would turn both off, modem on, then router on. If that doesn't fix it, keep reading.

Does the Netgear have an HTTP configuration interface? If so, you should be able to connect your browser to http://192.168.0.1 and get a login screen. Once you get logged in, you should be able to check if the router has obtained an IP number from the network. If not, either the WAN port on the router, the modem, or the cable between them is defective.

It is true on my network that if a computer configured for dynamic IP addressing is plugged in, it is immediately ready to go.

Larry

Sounds more like a bad cable modem, I have had two go bad on me already. The symptoms are what he is talking about.

.

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