What

Convert human-readable names to computer-usable addresses

Why

Because remembering 2001:0db8:85a3::8a2e:0370:7334 is hard
Load balancing by converting the same domain into multiple IPs
Service discovery by adding certain records (e.g. MX) to a domain
Hierarchical nature makes it scalable

Concepts

• Hierarchy:
  • Root (.) → TLD (.com) → Second-level (example.com) → Subdomains www.example.com
• DNS records:
  • A: IPv4
  • AAAA: IPv6
  • MX: Mail
  • CNAME: Alias
  • NS: Name server
  • PTR: Reverse DNS
  • TXT: Text (abused for a ton of stuff)
  • SOA: Authority
• Recursive resolver
  • Client-side program that recursively queries name servers until it finds an answer
  • Does the full lookup work, like following referrals
• Iterative resolver:
  • Returns referrals
  • "I don't know the answer, but you should ask X"
• Authoritative nameserver:
  • Server that has the definitive answers for a certain domain
• Root servers: the servers at the top of the hierarchy
  • a.root-servers.net to m.root-servers.net
• Caching and Time To Live are used to reduce latency and load
• Zone: administrative area of control
• Glue records: A records needed to prevent circular dependencies
  • Query example.com
  • Root: "ask ns1.example.com"
  • But to query ns1.exmaple.com, we need to ask the nameserver for example.com for the IP
  • Instead, add A/AAAA records to the parent zone for the nameservers
  • So .com TLD will have records for ns1.example.com
  • Glue records are only needed when the nameserver is within the domain:
    • NEEDED: example.com. NS ns1.example.com
    • NO GLUE NEEDED example.com. NS ns1.hosting-provider.net

How

1. User types query
2. Check local cache
3. Query root server, will return NS record for the next level down
4. Query TLD server, will return NS record again
5. Query authoritative NS for domain in question
6. Get result