Baffled by a Cable Modem

[Rick Stevens]

Patrick wrote:
> Did you try another browser to see if it works with the other sites?

Here's one way to troubleshoot it:

Using FireFox, see if you can get to the cable modem's GUI interface.
Most of them will have one of the following four IPs as their default:

	192.168.0.1
	192.168.0.254
	192.168.1.1
	192.168.1.254

The first thing you should do is make sure your NIC can access all of
those addresses without using a gateway.  As root in a console:

	ifconfig eth0 192.168.0.128 netmask 255.255.0.0

This puts your computer on the class B (CIDR /16) network of 192.168.0.0
and you should be able to hit any of the above four addresses directly
(in fact, any IP from 192.168.0.1 through 192.168.255.254).

Next, try "http://192.168.0.1" and see if you get the modem's GUI.  If
you don't, try the other three IPs above.  If it's none of those, check
the manual for the modem and see what the default LAN IP is.  Make sure
you get the LAN IP, _NOT_ the WAN IP (anything on the WAN side is the
cable company's side of the connection).

If you can browse the IP, then you know that at least your machine and
your modem can communicate.  Wander through the modem's user interface
looking for its DHCP server settings.  Make sure the DHCP server is 
enabled and take note of which addresses it will offer (usually called
the "pool").

At this point, you should be able to have the modem give you an IP.
Make sure your system is set up to fetch an IP address via DHCP and
either reboot your system, restart NetworkManager (if you use it):

	service NetworkManager restart

or restart the non-NM service:

	service network restart

Once that's all done, again as root in a console, do:

	ifconfig eth0

and verify it has an IP address that's in the range described by the
pool data you got before.  If that's all good, do:

	netstat -rn

and verify that the line that contains the flags "UG" is the same
IP as the cable modem.  Example:

	0.0.0.0   192.168.1.254   0.0.0.0    UG  0 0    0 eth0
	          ^^^^^^^^^^^^^
	          IP of cable modem

If all that checks out, then you SHOULD have a live connection to the
Internet.  The last thing to check is your /etc/resolv.conf file and
ensure that the "nameserver" lines have legitimate IP addresses for
live DNS servers.  This should be set up when the DHCP server gives you
the IP address and default route, but not always.  If it doesn't set
this up, you could manually add entries.  Here's a couple of entries you
can use:

	nameserver 208.67.222.222
	nameserver 208.67.220.220

Hope that helps a bit.
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- Rick Stevens, Systems Engineer                      ricks at nerd.com -
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