Overview
This lecture explains how DNS (Domain Name System) resolves domain names to IP addresses, describes key DNS server types, and details the step-by-step DNS name resolution process.
DNS Basics and Configuration
- DNS converts human-readable domain names into computer-friendly IP addresses.
- Name resolution is the process of mapping a domain name to an IP address using DNS.
- A host on a modern network must have its IP address, subnet mask, gateway, and DNS server configured.
- DNS servers make using domain names easier for humans, though not strictly required for computer operation.
Types of DNS Servers
- Five main types: caching name servers, recursive name servers, root name servers, TLD (Top Level Domain) name servers, and authoritative name servers.
- DNS servers can fulfill multiple roles, especially caching and recursive functions.
- Caching name servers store DNS results temporarily to speed up future lookups.
- Recursive name servers perform full DNS resolution processes, often combined with caching.
DNS Lookup Example and Caching
- When a domain is queried for the first time, the name server performs a full recursive lookup and caches the result.
- Subsequent queries for the same domain within the cache's TTL are responded to from the cache, speeding up access.
- TTL (Time to Live) determines how long a DNS entry is cached; longer in the past, now typically minutes to hours.
- DNS changes propagate only after the TTL expires across caches.
Full DNS Resolution Process
- The local recursive server starts by asking a root name server for guidance.
- There are 13 root name server authorities globally, distributed using Anycast for reliability.
- Root servers direct the query to the appropriate TLD name server (e.g., .com).
- TLD name servers reply with the correct authoritative name server for the specific domain.
- Authoritative name servers provide the actual IP address for the requested domain.
- The hierarchy ensures reliability and security for DNS responses.
DNS Caching Hierarchy
- Both local network name servers and individual computers maintain temporary DNS caches to minimize repeated full lookups.
- Caching improves network efficiency by avoiding unnecessary traffic and delays.
Key Terms & Definitions
- DNS (Domain Name System) — system that translates domain names into IP addresses.
- Name Resolution — process of converting a domain name to an IP address.
- TTL (Time to Live) — duration a DNS entry is cached before requiring a refresh.
- Anycast — networking method that routes traffic to the nearest or best node out of several possible ones.
- TLD (Top Level Domain) — final part of a domain name, representing the domain's category (e.g., .com, .org).
- Authoritative Name Server — DNS server with final authority over a particular domain's records.
Action Items / Next Steps
- Review the hierarchy and role of each DNS server type.
- Practice tracing DNS resolution steps for a sample domain.
- Understand the function and impact of TTL on DNS record updates.