Re: Cable Modem Locking

[Jason Healy <jhealy@logn.net>]

At 982940890s since epoch (02/23/01 10:08:10 -0500 UTC), Anonymous wrote:
> I have my cable modem hooked up to my OpenBSD box, it is configured using
> DHCP on ep0 After a random amount of time my connection just like locks.

Does it ever come back up of its own accord, or does it just stay down
forever?

Can you ping the cable modem directly? (some cable modems have an "internal"
address that your machine can get to).

I had a similar problem, but my ISP replaced the modem and the problem went
away, so I wrote it off as a defective modem.  What would happen is that the
modem would start sending packets to my machine with the wrong MAC address.
(That is, the cable modem would send packets to my IP, but use the wrong MAC
address for my machine, so my box would ignore the packets.) A little while
later (after an ARP on the network by either my machine or the modem), the
connection would be fine again.

I blamed my ISP, and they blamed my OpenBSD box.  But like I said, they
replaced the modem, and the problem hasn't happened since.

If you want to check for this problem, wait until the modem dies and then
try something like:

tcpdump -i xl0 -e icmp or arp
 
Once that gets going, start pinging something using another term.  You
should see packets going out:

(timestamp) (your MAC) (CM MAC) ip 98: (your IP) > (dest IP) icmp: echo request

Now, if you see packets coming back (similar to line above, only dest and
source addresses are switched), then that means that you're getting
something.  But if ping doesn't report the packets as coming back, then
you might have the problem I had.

Check those MAC addresses and make sure they agree.  If they don't, I'd love
to hear about it, because it either means that there are some defective
Cable Modems out there, or we have a bug on our hands.

If tcpdump shows no packets as returning, then you really aren't getting
anything and you have a more serious networking problem.

Best of luck, these can be really frustrating.

Jason
--
Jason Healy    |     jhealy@logn.net
LogN Systems   |   http://www.logn.net/