cable modem requires reboot because one site falls off DNS?

[David Miller]

On Fri, Sep 3, 2010 at 11:19 AM, David Miller <david3d at gmail.com> wrote:

>
> On Fri, Sep 3, 2010 at 10:37 AM, Greg Rundlett (freephile) <
> 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  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.
>

I guess I should have prefixed that with make sure it's a DNS issue and not
a routing issue.  Because my prior experience with Comcast Business with the
symptom being one site being inaccessible makes me believe that it's a weird
routing issue.

This routing issue is apparently a pretty common issue with Comcast
Business.  The person who made the suggestion to reboot the router was tech
support at SaaS provider that we were having a problem accessing.
 Apparently they have had a few customers with strange routing issues with
Comcast Business where rebooting the Comcast router resolved it.

You can test for this issue by doing a traceroute before and after rebooting
the router and comparing the output.  I think you'll find out that the DNS
resolution is the same before and after you reboot.  At least if it's the
same issue that I've run across with Comcast Business.
--
David
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