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DNS the domain naming service and BIND.

To organise all of these names and numbers, a distributed database is used. What this means is that many computers on the Internet have information as to the IP address of a particular name within a domain. These computers are called DNS servers, and most run a program called BIND, or djbDNS. Of note is D J Bernstein and the incredible tools that he has co-created and promoted for the Internet to increase the safety and security of the Internet.

Several tools are out there to determine the IP address of a name from this DNS system, and the reverse as well. nslookup is an old familiar one, and now a tool called dig serves a similar purpose.

If I want to find the IP address of the webserver for the University of Alberta, I will type in (on Linux): dig www.ualberta.ca and this will be returned

; <<>> DiG 9.2.1 <<>> www.ualberta.ca
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 15401
;; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 3, ADDITIONAL: 1

;; QUESTION SECTION:
;www.ualberta.ca. IN A

;; ANSWER SECTION:
www.ualberta.ca. 1961 IN CNAME duff-5.ucs.ualberta.ca.
duff-5.ucs.ualberta.ca. 1961 IN A 129.128.5.42

;; AUTHORITY SECTION:
ucs.ualberta.ca. 39808 IN NS NAME.ualberta.ca.
ucs.ualberta.ca. 39808 IN NS MENAIK.CS.ualberta.ca.
ucs.ualberta.ca. 39808 IN NS NOM.ualberta.ca.

;; ADDITIONAL SECTION:
MENAIK.CS.ualberta.ca. 3377 IN A 129.128.4.241

;; Query time: 27 msec
;; SERVER: 199.185.220.36#53(199.185.220.36)
;; WHEN: Sun Mar 7 10:43:42 2004
;; MSG SIZE rcvd: 151

By filtering out all the junk, I can see in the “ANSWER” section that this IP address is 129.128.5.42.

We can also do the reverse by typing dig -x 129.128.5.421
; <<>> DiG 9.2.1 <<>> -x 129.128.5.42
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 10832
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 3, ADDITIONAL: 1

;; QUESTION SECTION:
;42.5.128.129.in-addr.arpa. IN PTR

;; ANSWER SECTION:
42.5.128.129.in-addr.arpa. 86400 IN PTR duff-5.ucs.ualberta.ca.

;; AUTHORITY SECTION:
5.128.129.in-addr.arpa. 86400 IN NS NOM.ualberta.ca.
5.128.129.in-addr.arpa. 86400 IN NS NAME.ualberta.ca.
5.128.129.in-addr.arpa. 86400 IN NS MENAIK.CS.ualberta.ca.

;; ADDITIONAL SECTION:
MENAIK.CS.ualberta.ca. 1222 IN A 129.128.4.241

;; Query time: 48 msec
;; SERVER: 199.185.220.36#53(199.185.220.36)
;; WHEN: Sun Mar 7 10:47:46 2004
;; MSG SIZE rcvd: 156

Note that we do not quite get www.ualberta.ca, but we do get duff-5.ucs.ualberta.ca and we could type in http://duff-5.ucs.ualberta.ca and get their webserver. A good thing for the University to do would be to make their webserver switch duff-5.ucs.ualberta.ca back to www.ualberta.ca but perhaps they have a good reason for not doing so.

All Internet based programs such as email, and the world wide web use and query the DNS database for information, in fact, all computers on the Internet have at least two nameservers that tell the computer two or more places to look things up.

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