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Comment by shawnps | Hacker News Reader

Could someone clarify this for me, please? On this page:

http://cr.yp.to/djbdns/intro-dns.html

under the "Multiple Servers" section, he says

"To protect against computer failure, there are actually several root servers, several .to servers, and two yp.to servers."

I don't understand, is he saying there are only two DNS servers in the world that you can contact to resolve yp.to?

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Yes, that's true for most domains. All the other servers have to cache responses from those two servers. (For the record, the two yp.to servers are currently djb's machines dancer and forcewall.)

What would happen if he suddenly decided to unplug dancer and forcewall forever, would all the other DNS servers in the world fail to resolve yp.to after their cached responses expire?

Yes.

Is there a way to find out for any given domain how many actual servers hand out responses to other DNS servers to hold in their caches?

You can use WHOIS or dig, e.g.

$ dig yp.to ns

; <<>> DiG 9.4.3-P3 <<>> yp.to ns ;; global options: printcmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 63068 ;; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION: ;yp.to. IN NS

;; ANSWER SECTION: yp.to. 184700 IN NS b.ns.yp.to. yp.to. 184700 IN NS f.ns.yp.to.

dnstracer can show you that and much more. A great tool, shame nobody's heard of it.

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