Question

Under what circumstances is DNS iterative lookup used?

Answer 1

Answer:

DNS queries can be classified according the manner in which a complete request is processed. Generally queries can be classified as follows.

recursive query
iterative query OR Nonrecursive query
Inverse queries
In an iterative query, the name server, will not go and fetch the complete answer for your query, but will give back a referral to other DNS server's, which might have the answer. In our previous example our DNS server 172.16.200.30, went to fetch the answer on behalf of our resolver, and provided us with the final answer.

But if our DNS server 172.16.200.30 is not a recursive name server(which means its iterative), it will give us the answer if it has in its records. Otherwise will give us the referral to the root servers(it will not query the root server's and other servers by itself.).

Now its the job of our resolver to query the root server, .COM TLD servers, and authoritative name server's, for the answer.

Lets go through the steps involved.

STEP 1: You enter www.example.com in the browser. So the operating system's resolver will send a DNS query for the A record to the DNS server 172.16.200.30 .

STEP 2: The DNS server 172.16.200.30 on receiving the query, will look through its tables(cache) to find the IP address(A record) for the domain www.example.com. But it does not have the entry.

STEP 3: Now instead of querying the root server's, our DNS server will reply us back with a referral to root servers. Now our operating system resolver, will query the root servers for the answer.

Now the rest of the steps are all the same. The only difference in iterative query is that

if the DNS server does not have the answer, it will not query any other server for the answer, but rather it will reply with the referral to DNS root server's
But if the DNS server has the answer, it will give back the answer(which is same in both iterative and recursive queries)
in an iterative query, the job of finding the answer(from the given referral), lies to the local operating system resolver.

It can be clearly noted from the above figure, that in an iterative query, a DNS server queried will never go and fetch the answer for you(but will give you the answer if it already has the answer). But will give your resolver a referral to other DNS server's(root server in our case).