Putting It All Together

A complete DNS server / resolver:

incoming UDP/TCP packet on :53
  -> parse DNS header
  -> parse question
  -> if RD=1 and we're a recursive resolver:
       check cache
       if miss: do iterative resolution
       cache answer
       return to client
     else (we're authoritative):
       look up zone
       handle CNAME chasing
       return answer or NXDOMAIN
  -> serialize response
  -> send back

Components you've built:

What we glossed over:

• DNSSEC validation — RRSIG verification, NSEC walk for non-existence proof
• AXFR / IXFR — zone transfers between authoritative servers
• Notify (RFC 1996) — primary tells secondaries to refresh
• Dynamic updates (RFC 2136) — programmatic zone modification
• Anycast — same IP advertised from many locations; routing picks nearest
• Geo-IP routing — different answers based on client subnet (EDNS CS)
• Health-checked DNS — dynamic answers based on backend health (Route 53)

You now understand how every DNS thing works: BIND, Unbound, dnsmasq, CoreDNS, systemd-resolved, AWS Route 53, Cloudflare DNS — they all do versions of what you've built.