[SOLVED] cable modem requires reboot because one site falls off DNS?

[David Miller]

Very interesting.  Thanks for sharing this with the list.  I'm going to make
that change here and disable the policy routes that I was using to force
traffic to certain websites out our t1.  To see if the problem continues.
--
David

On Mon, Sep 20, 2010 at 11:15 AM, Greg Rundlett (freephile) <
greg at freephile.com> wrote:

> This problem happened again today.  We were unable to access a website that
> we access repeatedly for business.  This time we couldn't access our
> intranet.  I called Comcast and they said right away that they would disable
> the "Smart Packet Detection" setting in the modem.  The support person
> couldn't tell me what "Smart Packet Detection" was, but did say that nearly
> 100% of the time it resolved problems like this.  Now that I know what the
> problem was, I can find similar stories all over the Internet going back
> several years.
>
> Quote from one story:
>
> "Our Comcast business class cable modem (SMC make, model escapes me at the
> moment) has a packet dropping feature called "Gateway Smart Packet
> Detection".
>
> I've yet to see any documentation on this "feature" but when left
> unchecked, as is by default, you may black hole certain IP addresses simply
> by visiting them too much. Or uploading to them too often. Our web team was
> editing a site and this feature would constantly black hole the IP they were
> sending to when this feature was left enabled."
>
>
> http://nwlinux.com/blog/comcast-business-class-router-and-smart-packet-detection/
> http://lmgtfy.com/?q=smart+packet+detection
>
> Greg Rundlett
>
>
>
>
>
> On Fri, Sep 3, 2010 at 4:32 PM, Dan Jenkins <dan at rastech.com> wrote:
>
>>   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.
>>
>>
>>
>>
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