Chapter 4: Discovery

Introduction

• Focus on creating reliable, secure channels between processes on different machines
• DNS (Domain Name System) acts as the Internet's phone book
• DNS is a distributed, hierarchical, and eventually consistent key-value store

DNS Resolution Process

1. Browser first checks local cache for hostname resolution
2. If not found, request goes to DNS resolver (typically from ISP)
3. Root name server maps top-level domain (TLD like .com) to appropriate server
4. Resolver queries TLD name server for domain info
5. TLD server maps domain to authoritative name server
6. Authoritative server returns final IP for hostname

Key Technical Details

• Handles subdomains through delegation
• Uses UDP for queries due to low overhead
• Currently transitioning to DNS over TLS for security
• Resolution process requires multiple round trips
• Caching used at multiple levels to improve performance

Caching & TTL (Time To Live)

• Browser, OS, and DNS resolver maintain internal caches
• Each DNS record has a TTL indicating validity period
• TTL Tradeoffs:
  • Long TTL: Slower propagation of changes
  • Short TTL: Higher load on name servers
  • Client might not respect TTL settings

Critical Considerations

• DNS can become single point of failure
• Server unavailability + short TTL = More impacted clients
• Service becomes unreachable if DNS fails to resolve IP
• Industry moving towards DNS over TLS for security