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Module 5: Application Layer & Security

20257m

What is DNS? Explain with a suitable example how the query resolving process is done.

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What is DNS?

The Domain Name System is a distributed, hierarchical database that translates human-readable domain names (e.g. www.example.com) into IP addresses. It uses UDP/TCP port 53 and a hierarchy of root, TLD and authoritative name servers, plus a local (recursive) resolver.

Query resolution example: www.example.com

SVGVector Architecture Diagram (SVG)
Client Local DNS Root TLD (.com) Authoritative 1 query 2 query 3 refer: TLD 4 query 5 refer: auth. NS 6 query 7 IP address 8 IP address Client to Local DNS: recursive. Local DNS to servers: iterative.
  1. The browser asks the local DNS resolver for the IP of www.example.com (recursive query).
  2. If not cached, the resolver asks a root server, which refers it to the .com TLD server.
  3. The TLD server refers it to the authoritative server for example.com.
  4. The authoritative server returns the IP address (an A record).
  5. The resolver caches the answer (for its TTL) and returns it to the client, which connects to that IP.

The resolver-to-server steps are iterative (each server returns an answer or a referral). Common record types: A, AAAA, CNAME, MX, NS.

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