Name-address resolution in DNS

A client (the resolver) must convert a domain name such as www.example.com into an IP address. DNS is a hierarchical, distributed database of servers: root servers → TLD servers (.com) → authoritative servers (example.com), plus a local (recursive) DNS server of the ISP or organisation. The lookup can be done in two ways.

1. Recursive resolution

The client asks a DNS server to do the entire job. If that server does not know the answer, it contacts other servers on the client's behalf and returns only the final answer (or an error).

Client -> Local DNS: "IP of www.example.com?"
           Local DNS -> Root -> TLD -> Authoritative (chain)
           <- final IP address
Client <- Local DNS: 93.184.216.34

Characteristics:

• Burden is on the queried server; the client sends one query and gets one answer.
• The server usually caches the result, speeding up later queries.
• Client-to-local-server queries are normally recursive.

2. Iterative resolution

The client (or the local server) asks a server, and if the server cannot answer it returns a referral: the address of another server that is closer to the answer. The asker then queries that next server itself, repeating until the answer is found.

Local DNS -> Root server:  "www.example.com?"
          <- referral: ask the .com TLD server
Local DNS -> TLD server:   "www.example.com?"
          <- referral: ask ns1.example.com
Local DNS -> Authoritative: "www.example.com?"
          <- 93.184.216.34

Characteristics:

• Burden is on the requester; each server does minimal work.
• Servers reply immediately with the best information they have.
• Root and TLD servers normally answer only iteratively (to protect them from overload).

Comparison

Basis	Recursive	Iterative
Who does the work	The queried server resolves fully	The requester follows referrals itself
Reply to the requester	Final answer (or error)	Answer or a referral to another server
Load on the server	High	Low
Number of queries by the client	One	Several
Typical use	Client → local DNS server	Local DNS server → root / TLD / authoritative servers
Caching benefit	Server caches the final answers	Requester caches each step

In practice the two are combined: the host sends a recursive query to its local DNS server, which then performs iterative queries against the root, TLD, and authoritative servers and returns the final IP address to the host.