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What is DNS Recursion?

Published in DNS Resolution 3 mins read

DNS recursion is a process where a DNS server takes on the responsibility of fully resolving a DNS query on behalf of a client. Instead of the client making multiple requests to different servers, the recursive server does all the work and delivers the final IP address back to the client.

How DNS Recursion Works

In essence, a recursive DNS server acts as an intermediary. When a client sends a DNS request (e.g., asking for the IP address of www.example.com), the recursive server doesn't just respond with what it already knows. Instead:

  • It checks its cache to see if it has already looked up this domain.
  • If not, it begins querying other DNS servers, starting from the root servers, and continues down the hierarchy until it finds the authoritative server that has the answer.
  • Once it finds the answer (the IP address), it stores this in its cache for future queries and sends the IP address to the client.

This contrasts with an iterative DNS query, where the client itself would have to go through this whole process, asking different DNS servers step-by-step.

Key Differences Between Recursive and Iterative DNS Queries

Feature Recursive DNS Query Iterative DNS Query
Responsibility The DNS server resolves the full query The client resolves the full query
Communication Server communicates with multiple servers Client communicates directly with servers
Complexity Server handles the complexity Client handles the complexity
Result Server returns the final IP address Client gets referred to other servers
Efficiency Generally more efficient for clients More efficient for heavily loaded servers

Why is DNS Recursion Important?

  • User Experience: It greatly simplifies the DNS resolution process for end-users, making web browsing faster and more user-friendly. Users don’t need to perform the complicated process of contacting multiple DNS servers.
  • Performance: Caching helps speed up subsequent requests for the same domain.
  • Security: When properly configured, recursive servers provide security features such as DNSSEC validation to protect against DNS spoofing and manipulation.

Practical Insights:

  • Most home and office networks use a DNS resolver from an internet service provider (ISP) or use a public DNS resolver like Google (8.8.8.8) or Cloudflare (1.1.1.1). These servers use recursive lookups.
  • A DNS server that only performs iterative lookups may be considered authoritative for one or more domain names. It gives information about its domain and refers the client to another server if it doesn’t know the answer to the query.
  • It is important to secure recursive resolvers so they cannot be abused to participate in DDoS attacks.

Conclusion

Recursive DNS lookup is a core function of the internet that simplifies and speeds up domain name resolution by having a DNS server handle the entire process on behalf of a client. Understanding how this process works is essential for appreciating the mechanics of online navigation.