[SOLVED] cable modem requires reboot because one site falls off DNS?
Bruce Dawson
jbd at codemeta.com
Mon Sep 20 11:28:43 EDT 2010
This is just a guess, but it could some sort of spoofing/caching
mechanism - that's clearly broken in some way they can't determine.
--Bruce
On 09/20/2010 11:15 AM, Greg Rundlett (freephile) 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
> <mailto: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>
> <mailto: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> <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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