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DNS Resolver

lastmod 2025-12-11created 2025-12-112 edits

A DNS resolver is a client-side component that finds the IP address for a domain name when a user enters it. The DNS resolver is an important part of the Domain Name System (DNS), handling the mapping between domain names and IP addresses so that users can access websites. The DNS resolver acts as an intermediary between the user and DNS servers, using cached information or sending queries to multiple DNS servers to find the domain's IP address.

How a DNS resolver works

  1. Receiving the user's request: When a user enters a domain name (e.g., www.example.com) in the browser, the DNS resolver starts working to find the IP address for that domain.
  2. Checking the cache: The DNS resolver first checks whether its own cache has information about the domain. If the IP address is in the cache, it returns the IP address to the user right away. This can greatly reduce query time.
  3. Recursive Query:
    • If the information is not in the cache, the DNS resolver performs a recursive query. In this process, the DNS resolver sends queries to multiple DNS servers, obtaining information about the domain step by step to find the final IP address.
    • This process goes through the root name servers, top-level domain (TLD) servers, and authoritative name servers in sequence.
  4. Returning the IP address: Once the IP address is finally found, the DNS resolver stores the information in its cache and returns it to the user. The user can then connect to the website through this IP address.

Types of DNS resolvers

  1. Stub Resolver:
    • A simple resolver built into the user's device (e.g., PC, smartphone).
    • Sends queries to a recursive resolver run on the local network or by an internet service provider (ISP).
    • Does not handle recursive queries itself; it mainly waits for the recursive resolver's response.
  2. Recursive Resolver:
    • Runs on the DNS servers of an ISP or network, receiving user requests and handling the entire query process.
    • Sends queries to multiple DNS servers to find the IP address.

Key features

  • Caching: To reduce repeated queries, the DNS resolver stores the IP address of a domain it has looked up in its cache for a certain period. This lets it reduce network load and respond quickly when requests for the same domain come in.
  • Recursive queries: When needed to obtain the final IP address, the DNS resolver sends recursive queries to multiple DNS servers, automatically handling the complex DNS lookup process on the user's behalf.
  • Use of UDP and TCP: DNS resolvers generally send queries over UDP, but use TCP when the data is large. They switch to TCP when receiving large responses or using security extensions such as DNSSEC.

See also

  • DNS
  • Forward DNS

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