What is the difference between DNS recursion and DNS iteration?

• A. Recursion uses UDP; iteration uses TCP
• B. Recursion is client-side; iteration is server-side
• C. Recursion is for IPv6; iteration is for IPv4
• D. Recursion asks a DNS server to fully resolve the name; iteration returns referrals to other servers for the client to query.

Answer: (D)

The key idea is how a DNS server handles the query. In recursion, the server takes on the full task of resolving the name for the client. The client asks a DNS server to resolve it, and that server goes out to the root, then the top-level domain, then the authoritative servers, gathers the final answer, and returns it to the client. The client sees one completed answer.

In iteration, the server does not finish the job. It responds with referrals to other servers that might know the answer, and the client must query those servers one by one, following the chain of referrals until an authoritative answer is obtained. The server effectively points the client toward the next step rather than doing all the digging itself.

This distinction matters because recursive resolution centralizes the work and caching on the resolver, while iterative resolution distributes the steps and relies on the client to continue querying along referrals. Transport protocol (UDP vs TCP) or IP version (IPv4 vs IPv6) isn’t what defines the difference.