DNS - Recursive Resolver

A DNS recursive resolver plays a role in the domain name system (DNS), which is integral to the functioning of the internet. It acts as an intermediary between a user's device and the complex hierarchy of DNS servers. Here's a detailed breakdown of what a DNS recursive resolver does:

1. User Query Initiation: When you type a web address into your browser, like "www.example.com", your computer doesn't inherently know the IPv4 or ipv6 address where this website is located. So, it sends a query to a DNS resolver to find out.

2. Role of the Recursive Resolver:

   • Resolver's Function: The recursive resolver receives the query from the user's computer. Its job is to do the legwork of tracking down the IP address associated with the requested domain name.
   • Process of Resolution: This involves querying multiple DNS servers in a process that may require several steps. The recursive resolver starts by asking the root DNS servers, then moves on to the top-level domain (TLD) servers (like those for .com, .org, etc.), and finally queries the authoritative DNS servers for the specific domain or zone.

3. Caching for Efficiency: Recursive resolvers often cache responses. If a recursive resolver has recently resolved a similar request, it can reply to the user's query using this cached data, significantly speeding up the response time. The cache is maintained according to the TTL (Time-to-Live) values set in the DNS records.

4. Iterative Queries: If the recursive resolver doesn't have the requested information cached, it makes a series of queries to various DNS servers. Each of these servers provides a piece of the puzzle, guiding the resolver to the next server in the chain until it reaches the authoritative server for the requested domain.

5. Final Response to User: Once the recursive resolver has the complete information (i.e., the IP address corresponding to the requested domain), it sends this information back to the user's computer. The browser can then use this IP address to initiate a connection to the host server of the website.

6. Benefits of Using a Recursive Resolver:

   • Simplifies the User Experience: Users don't need to worry about the complexities of DNS; they just enter a domain name, and the resolver handles everything.
   • Improves Performance: By caching frequent queries, recursive resolvers reduce latency and improve the speed of web browsing.

7. Security Considerations: Recursive resolvers can be targets for DNS attacks, such as DNS spoofing or cache poisoning. To counter these risks, measures like DNSSEC (DNS Security Extensions) are used to ensure the authenticity and integrity of DNS responses.

A response to a DNS request that comes from a recursive resolver is considered non-authoritative. Even if the recursive resolver made the request to an authoritative server, and the authoritative server responds with the flag set, when the recursive resolver relays that information, it will set the flag to 0. Thus, such a response will always have its authoritative flag set to 0.

A DNS recursive resolver is a critical network service that takes a human-friendly domain name and resolves it to the machine-friendly IP address needed to connect to the desired web resource. This process, while complex in its full execution, is made seamless and efficient through the use of recursive resolvers.

Links

https://networklessons.com/cisco/ccna-200-301/introduction-to-dns