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Memorable DNS IPs

Comcast has ocassionally messed up its DNS servers - last year they had a series of systemwide DNS outages that lasted several days. (My Comcast DNS servers are 68.80.0.12 and 68.80.0.5.)

Today Comcast DNS is being slow - but not out. That is good, because it means I can still get to Google and I can still use it to find other DNS servers.

For future reference: if my DNS servers ever go out, I will use the following DNS servers with easy-to-remember IP addresses - thanks to AT+T and Level 3:

4.2.2.1, 4.2.2.2, 4.2.2.3, 4.2.2.4, 4.2.2.5, 4.2.2.6

Here in Pennsylvania, these servers are all just as close as Comcast's DNS servers to me, < 20ms ping, and they are serving DNS faster today.

Update Apparently Reza also uses the same DNS IP addresses in Seattle with good results, which seems remarkable. Curious, I collected some roundtrip numbers to 4.2.2.1 from various computers I use:

From Philadelphia, PA	18 ms
From Washington, DC	13 ms
From Mountain View, CA	3 ms

The evidence seems to contradict the laws of physics. It takes 32ms for a photon traveling at the speed of light to bounce from the U.S. Northeast to the West Coast and back again; in practice you're doing extremely well if you can get through all the routers in twice that (for example my DC and CA computers have a 94ms ping time between them). But 3+13=16ms is much less than 32ms and a physical impossibility. Even with perfect zero latency routers and perfectly straight-line optical paths, a 16ms round-trip is twice as fast as possible.

Perhaps there is a very clever predictive algorithm in San Jose that knows how to send a reply 40ms before receiving the request? Or perhaps Level 3 has installed top-secret quantum time-warp hardware at 4.2.2.1? Or maybe 4.2.2.1 is using Robert Boyd's magical erbium glass fiber?

It turns out that 4.2.2.1 routed using "anycast," which is a kind of routing that lets servers distributed around the world to be assigned to the same IP address - something that is usually a no-no. Most big web servers use round-robin DNS for load-balancing, which is where there is more than one IP address that can answer to the same DNS address. But since you can't rely on DNS lookup to distribute load for DNS servers themselves, anycast is the alternative. Interesting.

Update 9/5/2006: 4.2.2.2 was having trouble yesterday.

For future reference, the OpenDNS servers are:

208.67.222.222, 208.67.220.220

And they are quite fast! (They also come with some weird features like anti-phishing redirect, etc, which is both a plus and a minus depending on what you want to do.)

Posted by David at May 6, 2006 07:01 AM