Iterative DNS query definition
An iterative DNS query is a step-by-step process where if the DNS server has no relevant DNS records in its cache, it refers the DNS client to another DNS server. The process repeats (iterates) until one of the queried DNS servers returns the correct IP address for the domain.
Unlike a recursive query, where the DNS server does all the work, in an iterative query the resolver performs each step itself, contacting root servers, TLD servers, and authoritative servers in succession.
See also: DNS query, DNS resolution, DNS client, DNS proxy, authoritative DNS server, recursive DNS server, DNS redirection, DNS failover
How iterative DNS queries work
Step 1: Client sends initial query. The DNS client sends a query to its configured DNS resolver requesting the IP address for a domain name.
Step 2: Resolver queries root server. The resolver contacts a root DNS server, which responds with a referral to the appropriate top-level domain (TLD) server (like .com or .org).
Step 3: Resolver queries TLD server. The resolver then contacts the TLD server, which responds with a referral to the authoritative name server for the specific domain.
Step 4: Resolver queries authoritative server. The resolver contacts the authoritative name server, which holds the actual IP address for the requested domain and returns it to the resolver.
Step 5: Resolver returns result to client. The resolver sends the IP address back to the DNS client, which can now connect to the destination and cache the result.
Real iterative DNS query examples
- Query to root server (first step): One of 13 root servers in the world extracts the top-level domain from the query and provides details for the next server.
- Query to authoritative name server (last step): The ANS returns the correct IP address (or NXDOMAIN if it cannot find the domain) to the DNS resolver.
