Domain Name System

• Human cant rmb ip addresses, we rmb domain name.
• DNS act as Internet phonebook, translate google.com → 142.250.xxx.xxx

DNS Resolution Flow

1. Open browser and search www.xxx.com
2. Check Browser Cache (RAM in browser) whether already knows IP for this domain.
3. If no, check OS DNS Cache
   • Linux: systemd-resolved or nscd
     • Also checks /etc/hosts
   • macOS: mDNSResponder
     • Also checks /etc/hosts
   • Windows: ipconfig /displaydns
     • Also check C:\Windows\System32\ drivers\etc\hosts
4. If no, check Recursive Resolver (a type of DNS Resolver), the “middleman”
   1. DNS Resolver will go through Root, TLD, Authoritative servers until get final answer.

1. Root Name Server
   • Knows where .com lives
   • Responds: provide .com TLD servers
2. TLD Name Server (.com)
   • Knows who manages google.com
   • Responds Authoritative DNS server
3. Authoritative Name Server
   • The final source of truth
   • Returns actual IP
4. Cache the domain and ip.

[Your Computer]
        |
        | asks “IP of www.google.com”
        v
[Recursive Resolver (ISP / Public DNS)]
        |
        | checks cache, if missing:
        v
[Root Name Server] → [.com TLD Server] → [Authoritative Google DNS]
        |
        v
[IP Address returned to Resolver]
        |
        v
[Resolver caches IP, sends it to your Computer]
        |
        v
[Browser connects to the IP]

Common DNS types