Leave a comment

April 23, 2014 by freakychakra

As has been stated here, the DNS resolution works exactly like we human beings work. The search for an address on web is exactly like the search for address in real life.

Let’s assume that http://www.bigrock.com. resolves to the IP address 12.12.12.12. The job of the DNS is to return 12.12.12.12 to the resolver so that your computer can directly communicate 12.12.12.12. How does this miracle happen ?

1) When you type in http://www.bigrock.com, the resolver will first try to find cached information for it on the ISP’s cached data. If its cached, you are displayed the page immediately. However if it is not cached, then the following happens:

2) The resolver then contacts the root nameservers i.e. a.root-servers.net — m.root-servers.net. and asks them the query “Excuse me ! Could you tell me the IP Address of http://www.bigrock.com ?”

Depending on round robin fashion, one of these 13 servers will check the query and will reply : “No son, we don’t have the IP address of http://www.bigrock.com. But we certainly are familiar with .com. I guess you should ask a.gtld-servers.net– m.gtld-servers.net who happen to the authoritative(responsible) nameservers for .com”

3) Armed with this new knowledge, the resolver then contacts the gtld nameservers i.e. a.gtld-servers.net– m.gtld-servers.net and asks the same query “Excuse me ! Could you tell me the IP Address of http://www.bigrock.com ?”

Again depending on round robin fashion, one of these servers will check the query and will reply : “No man, we don’t have the IP address of http://www.bigrock.com. But we certainly are familiar with bigrock.com. I guess you should ask sia1.akam.net, eur2.akam.net, eur4.akam.net, eur6.akam.net who happen to the authoritative nameservers for bigrock.com”

4) Again with this knowledge, the poor resolver contacts these nameservers and asks them the same query “Excuse me ! Could you tell me the IP Address of http://www.bigrock.com ?”

Again depending on round robin fashion, one of these servers will check the query and will reply “Whoa Brother !!! You hit the bulls eye. We certainly are the authoritative servers for bigrock.com and have the zone file of this domain name. We will give you the zone file. Please look if you can find the IP address of http://www.bigrock.com”

5) On being handed the zone file the resolver, checks for an entry against http://www.bigrock.com and it finds the entry to be marked as 12.12.12.12.

Thus the IP address of http://www.bigrock.com is obtained and it is passed on back to the computer. Your computer is now able to make the connection, and the rest is history !!!

If you see, the process is exactly like the example of Postal mail as shown here. The query is delegated from each higher zone to its successive lower zone and narrowed down until it reaches the intended recipient perfectly.

DNS Resolution of a domain name to get the IP address is all about such zone delegations to reach the final zone file of the domain name. The presentation below should help you understand this better.