cable modem requires reboot because one site falls off DNS?

[Dan Jenkins]

  On 9/3/2010 11:19 AM, David Miller wrote:
>
> On Fri, Sep 3, 2010 at 10:37 AM, Greg Rundlett (freephile) 
> <greg at freephile.com <mailto:greg at freephile.com>> wrote:
>
>     I have a strange problem where one (and only one as far as we
>     know) particular website becomes inaccessible to our office.
>
>     The "fix" for this problem is to reboot the Comcast cable modem,
>     however I don't understand how the modem could be the culprit.
>
>     The website in question is nnerenmls.com <http://nnerenmls.com>
>      and the modem is configured to use Comcast's DNS servers....
>     68.87.71.226
>     68.87.73.242
>
>     One red herring:  It would seem that Comcast changed their DNS
>     servers, because the ones currenty in the modem configuration do
>     not appear in the list
>     http://dns.comcast.net/dns-ip-addresses.php  I thought to myself,
>     "I just switch to Google's Public DNS servers" (8.8.8.8 and
>     8.8.4.4)  I'm pretty sure they are not going to change.  However,
>     it doesn't make sense to me that one website would fail, while
>     general DNS would still be working.  And, at the time of the
>     failures, other people using Comcast can resolve that domain
>     meaning it doesn't even appear to be an issue where the target
>     domain is occasionally falling off the web.
>
>     Any ideas on what could cause this and how to troubleshoot?
>
>
> We have that problem here time to time.  It doesn't appear to be a DNS 
> issue in our case it's always been a very strange routing problem that 
> happens after a bunch of correct hops.
>
> We are lucky enough to have a 2nd internet connection and when we have 
> this problem here I can traceroute from each connection and the 
> comcast one normally will get to the correct datacenter and then take 
> a different hop from our T1.  I've never been able to make any sense 
> out of it.  But for this reason I have a few sites setup to route out 
> our T1 so that it doesn't cause any interruptions in our business.
>
> Rebooting the comcast router in our case has always resolved this 
> routing problem.  I'd be interested in any theories as to what causes 
> the routing to go awry after many hops and outside of comcast's network.
>

We've had this strange routing problem several times over the last year. 
It makes no sense to me either, but power cycling the Comcast-provided 
SMSC cable modem/router has solved the problem in all four cases at 
three different clients. In one instance, the traceroute made it to the 
very last hop before their web server, and then died. I wondered if it 
could be some odd TTL issue. Comcast's tech support has been pleasant, 
but not very informative about the cause. Since power cycling works, and 
is quick, I haven't tried to diagnose it further though I'd like to know 
why it happens. In one of the other cases the route to a specific IP 
number immediately went along a completely different path than another 
IP that was in the same destination network. The other case appeared to 
start playing leap frog half way to the destination (hop E to hop F to 
hop E, etc.). The fourth occurrence I just had them power cycle. There 
may have been more cases, but I documented how to power cycle the cable 
modem for them (they are all small enough that it hasn't been a major 
issue) and haven't heard about recurrences.