The mysteries of Comcast DNS

[Winterpool]

For some time now (since at least summer of 2009), we've experienced incredibly dodgy DNS service at my parents' house (southern NJ) where Comcast is their ISP. It seems clearly to be a DNS issue, since I can visit sites via IP addresses directly, but neither web browsers nor ping commands are able to resolve domain names into IP addresses during the outages. Outages last anywhere from a few minutes to hours...

Originally, I blamed Comcast's DNS servers, but the very odd thing is, when I replaced their DNS servers with OpenDNS servers in the router, the problem persisted (though it felt less severe).

During one particularly egregious outage, I tried connecting a computer directly to the cable modem (configuring the computer to use DHCP), and--voilà!-- DNS worked. So I blamed the router, switched it, and... got the same issues. I've tried two Linksys and one D-Link router, and am currently on a new Linksys WRT160N v2 (bought a completely new router out of desperation).

As we were using a frightfully old cable modem (DOCSIS 1.1), I swapped it out for a Motorola SB6120 (DOCSIS 3.0 capable). The combination of the new cable modem and new Linksys router seemed to do the trick, as DNS problems became far more infrequent (perhaps the norm for Comcast crappiness, who knows). But a few weeks ago, the problem returned! D: Aargh!

The local network uses both physical Ethernet and 802.11g/n wireless. There are about half a dozen clients, ranging from iPod touch to iMac and several Windows Vista / 7 machines. As far as I'm aware, there is no malware resident on any systems, and I've completely re-installed Windows on every system during the period of this DNS affliction.

I've Googled but found only the usual complaints about Comcast unreliability, DNS-hijacking, etc. Does the AnandTech community have any ideas?

 

[melchoir]

4.2.2.2
8.8.8.8

 

[Red Squirrel]

My ISP used to have serious DNS issues too. Everytime I'd call them about it they'd go through the same spiel of trying a different phone line and stuff. I was trying to explain to them that this problem was not a layer 1 issue and they just kept going. So I'd just unplug the modem, wait like 5 minutes while the phone is down and say I changed it. 😛

Ended up just setting up my own local DNS cache server that uses the root DNS. No issues since then. The nice thing now is I can make my own local domains too.

 

[Winterpool]

I've already been swapping OpenDNS for Comcast periodically, but jah I should probably give Google DNS a try.

 

[spidey07]

Many times a fubarred ip stack will show these symptoms.

 

[Winterpool]

So I should perhaps try the 'netsh int ip reset' command?

Would there be an equivalent command on Mac OS? There has got to be something better than this. It's UNIX after all...

 

[dmcowen674]

I'm in Chicago and having this issue with Comcast since April 11th.

All they do is generate an E-mail but not doing anything to fix the problem.

 

[Winterpool]

Over on Ars Technica, a couple of punters insist it's to do with crappy Comcast signal. Their argument is that as DNS uses UDP, it's more sensitive to intermittent signal than, say, the TCP packets of HTTP. I'm not a networking expert (I only got as far as CCNA, heh), and I can't recall how robust DNS queries are. An IT pro friend of mine supplised anecdotal support: he experienced similar issues with Comcast until he got them to redo the wiring to his house.

Since switching to Google's DNS servers, the DNS outages at my parents' house are briefer, but still occur. Again, during these outages, I can reach websites via IP addresses, but cannot resolve their domain names, either in browsers or using the 'dig' command (see the Ars Technica forum for more details).

When I feel up to it (and perhaps bolstered by some qualite alcohol), I'll try to wrestle with Comcast support and try to convince them of this problem.

 

[dawks]

Make sure you're changing the DNS servers on the client and not at the router. This will ensure you're avoiding the lazy/sloppy DNS forwarders in the routers.

Try using GRC's DNS benchmark to find a nice, fast server for you. I've found the 4.2.x.x series work well for me, since i have better routing to them than the OpenDNS servers.