The difference between DNS iterative queries and recursive queries

iterative queries and recursive queries are two types of requests that can be sent to a domain name server. Recursive queries are the most common requests sent to a local domain name server. When the local domain name server accepts the client's query request, the local domain name server will try to find the answer on behalf of the client, while the client is just waiting when the domain name server performs all the work. If the local domain name server cannot answer directly, it searches the branches of the domain tree recursively for answers.

For a recursive query, the DNS server will continue to search until an answer is received. This answer can be the IP address of the host, or it can answer "host does not exist". Regardless of the result, the recursive domain name server returns the result to the client.

If a domain name server is set up to use a forward server, it will send a recursive query to the specified forward host. The host that receives the forward query will also work in recursive mode to answer the query. If the local server is not a subordinate server, it may be able to start the query process on its own, although it still wishes to be answered from its predecessor to the host. If the local domain name server is set up as a subordinate server to a forward server, the working mode will be different. The client can send a recursive query to a subordinate server, but the subordinate server cannot perform a recursive query, essentially it will be the client of its predecessor to the server waiting to be answered. The 3rd chapter introduces the former to the server and the subordinate server.

iterative queries are different. The best example of an iterative query is a local domain name server that sends requests to the root server. When an enterprise's local domain name server queries the root server, the root server does not necessarily represent the local domain name server to assume responsibility for answering inquiries. Another argument is that the root server does not receive recursive queries. In fact, the root server only does one thing for parsing queries: directs the local domain name server to another host to query for answers. This practice is often referred to as the guideline and the result of the desired iterative query. For example, when the root server is asked to query the www.45it.com address, the root server does not go to the ISI domain name server to query the WWW host's address, it simply returns a hint to the local domain name server, telling the local domain name server to the ISI domain name server to continue querying and obtaining results.