Art of Codeartofcode
Lesson 14 of 15

4. DNS

1
The picture to keep in your head
2
4.1 Why do we need names at all?
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4.2 What are the parts of a domain name?
4
4.3 What happens, step by step, when a name is looked up?
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4.4 What kinds of DNS records are there?
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4.5 What is a TTL, and why does DNS cache so much?
7
4.6 How can DNS spread users across servers or send them to the nearest one?
8
4.7 Why do DNS changes "take time to propagate"?
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4.8 What happens when DNS fails?
10
Memory card and where the analogy breaks
11
Practice: five levels
12
Interview questions
13
Solutions
14
Sources
15
Code lab
4. DNS

Sources

3 min read+10 XP

Source notes

  • DNS concepts, record format, label and name length limits, UDP and TCP use: RFC 1034 and RFC 1035. DNS over TCP requirements: RFC 7766. Clarifications on CNAME (no other data at a CNAME name): RFC 2181 and RFC 1034.
  • Negative caching (NXDOMAIN): RFC 2308. EDNS Client Subnet: RFC 7871. DNS over TLS: RFC 7858. DNS over HTTPS: RFC 8484.
  • IPv6 addresses are 128 bits: RFC 8200. Documentation address ranges (203.0.113.0/24, 198.51.100.0/24, 2001:db8::/32): RFC 5737 and RFC 3849.
  • Dyn, 21 October 2016: Dyn's analysis (Scott Hilton, Dyn blog, 26 October 2016) reported a DDoS from the Mirai botnet against its managed DNS; widely reported effects on many major sites. See also the summary at https://en.wikipedia.org/wiki/DDoS_attacks_on_Dyn
  • Facebook, 4 October 2021: "More details about the October 4 outage", Engineering at Meta, 5 October 2021, https://engineering.fb.com/2021/10/05/networking-traffic/outage-details/
  • ALIAS / ANAME / "CNAME flattening" are provider features, not part of the DNS standard.
  • Level 2 numbers (10,000 resolvers, 10 ms and 30 ms hops) are assumptions; arithmetic computed 2026-10-05.
  • Level 3 and Level 4 code: tested 2026-10-05 (code/ch04_dns.py, all checks pass).