Day 4: DNS: How Names Become IPs | Networking for Developers

DNS Record Types

Record types

A      → name → IPv4 address
       example.com.  A  93.184.216.34

AAAA   → name → IPv6 address

CNAME  → alias → canonical name
       www  CNAME  example.com.

MX     → mail exchange server
       @  MX  10  mail.example.com.

TXT    → arbitrary text
       @ TXT  'v=spf1 include:_spf.google.com ~all'
       @ TXT  'google-site-verification=abc123'

NS     → nameserver for this domain

TTL    → time-to-live: how long to cache the record (seconds)

Debugging DNS

# Basic lookup
dig example.com
nslookup example.com

# Specific record type
dig example.com MX
dig example.com TXT
dig www.example.com CNAME

# Authoritative nameserver
dig example.com NS

# Trace the full resolution
dig +trace example.com

# Check propagation (different DNS servers)
dig @8.8.8.8 example.com    # Google DNS
dig @1.1.1.1 example.com    # Cloudflare DNS
dig @9.9.9.9 example.com    # Quad9 DNS

💡

DNS changes take time to propagate because of TTL. If you change a record with TTL=3600 (1 hour), some users might still get the old IP for up to an hour. Set TTL to 300 (5 min) before making changes. Restore to 3600 after.

📝 Day 4 Exercise

Configure DNS for a Domain

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Day 4 Summary

A record → IPv4. AAAA → IPv6. CNAME → alias. MX → email routing. TXT → verification and SPF.
TTL controls caching. Low TTL = faster propagation but more DNS queries.
dig +trace domain shows every DNS hop from root to authoritative nameserver.
Always lower TTL before DNS changes. Restore it after.

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Day 5: Load Balancers and CDNs

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