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Quick question about the DNS protocol, which I didn't really understand. Let's take a website, say I type the address in my browser.

The DNS resolver will check the root servers, the GTLD servers. Which will say, if I understand well, "hey, check to get your answer".

But in that case, how does the resolver know the IP for, if the answer is held by the servers themselves ?

disco:~ Disco$ dig +trace

; <<>> DiG 9.6.0-APPLE-P2 <<>> +trace
;; global options: +cmd
.           50181   IN  NS
.           50181   IN  NS
;; Received 449 bytes from in 55 ms

com.            172800  IN  NS
com.            172800  IN  NS
;; Received 495 bytes from in 165 ms     172800  IN  NS     172800  IN  NS     172800  IN  NS     172800  IN  NS
;; Received 164 bytes from in 219 ms     345600  IN  A
;; Received 48 bytes from in 70 ms


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up vote 0 down vote accepted

This apparent cyclic dependency is resolved by using what are known as "glue" records.

See this answer.

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You have authoritative servers (on which the zones are defined) but root servers know which domains are registered on which servers.

In short, you get the "where is" from

$ dig

; <<>> DiG 9.6.1 <<>>
; (2 servers found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 19654
;; flags: qr rd; QUERY: 1, ANSWER: 0, AUTHORITY: 4, ADDITIONAL: 4
;; WARNING: recursion requested but not available

;                        IN      A

;; AUTHORITY SECTION:             172800  IN      NS             172800  IN      NS             172800  IN      NS             172800  IN      NS

;; ADDITIONAL SECTION:         172800  IN      A         172800  IN      A         172800  IN      A         172800  IN      A

;; Query time: 139 msec
;; WHEN: Tue Oct 19 09:39:25 2010
;; MSG SIZE  rcvd: 164
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Thanks a lot for your help :) – Pierre Oct 19 '10 at 7:43

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