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I haven't read the bind source in a long time, but my best guess is that it's in memory.

http://docstore.mik.ua/orelly/networking_2ndEd/dns/ch02_06.h...

Has some details, and I believe Bind and PowerDNS call it "Smoothed RTT" or SRTT.

interesting, not a ton of details of how. off to the source I go, thanks. sounds like it assigns a 'score' of some sort to each authoritative ns.

BIND assigns a score based on every IP it talks to and a hit rate. (so if ns1.foo.com resolves to 3 IPs, each is tracked individually, same if ns1.bar.com and ns1.baz.com are the same IP) It is a pretty small cache, maybe like 1024?

This is all from fuzzy 5 year old memory.

This might help - section 4.4 has formal details on the Bind server selection algorithms: http://ocw.mit.edu/courses/mathematics/18-996-topics-in-theo...

yep definitely, an interesting implementation, and certainly almost impossible to guess from passive observation. thanks.

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