EN_CS_DNS - somaz94/DevOps-Engineer GitHub Wiki

Q3: How DNS Works

Question: Explain how DNS resolves a domain name to an IP address using Recursive and Iterative Query methods. Also cover key DNS record types and their practical use cases.


Key Terms

Term Description
DNS Distributed hierarchical system that resolves domain names to IP addresses
Root DNS Top-level DNS servers (13 clusters worldwide)
TLD DNS Manages top-level domains like .com, .net, .kr
Authoritative DNS Holds the actual IP address for a domain
Recursive DNS Performs the full lookup on behalf of the client; holds cache
TTL Time To Live — duration (in seconds) a DNS record is cached

DNS Hierarchy

Root DNS (.)
├── .com (TLD)
│   ├── google.com (Authoritative)
│   └── example.com (Authoritative)
├── .net (TLD)
└── .kr (TLD)

DNS Resolution Process

Recursive Query — Standard in Practice

The client asks the Recursive DNS once; Recursive DNS performs the full lookup.

Client ──① www.example.com?──→ Recursive DNS
                                    ├──② .com NS?──→ Root DNS
                                    ├──③ example.com NS?──→ TLD DNS
                                    └──④ www.example.com A?──→ Authoritative DNS
Client ←──⑤ 93.184.216.34──────── Recursive DNS

Iterative Query

Client queries each DNS server directly in sequence. Rarely used in practice.


DNS Caching

Layer Location TTL
Browser cache Client ~60s (Chrome default)
OS cache Client Follows TTL
Recursive DNS ISP / Corporate Minutes to hours

Cache hit = IP returned in 1 RTT.


DNS Record Types

Record Description Example
A Domain → IPv4 address www.example.com → 93.184.216.34
AAAA Domain → IPv6 address www.example.com → 2606:2800::1
CNAME Domain alias (points to another domain) blog.example.com → example.com
MX Mail server with priority example.com MX 10 mail.example.com
TXT Arbitrary text (SPF, DKIM, domain verification) v=spf1 include:_spf.google.com ~all
NS Authoritative nameserver for domain example.com NS ns1.example.com
SOA Zone metadata (Serial, Refresh, Retry, Expire) —

CNAME Restriction

  • Cannot use CNAME on root domain (example.com) — conflicts with other records
  • Only valid on subdomains (www.example.com)

Advanced DNS Features

Feature Description
GeoDNS Return different IPs based on user location (AWS Route 53 Geolocation)
Weighted Routing Traffic split by weight (Canary deployments)
Failover Routing Auto-switch to secondary on health check failure
DNSSEC Digital signatures on DNS responses to prevent spoofing

Troubleshooting Commands

dig www.example.com              # Basic lookup
dig @8.8.8.8 www.example.com     # Query specific DNS server
dig +trace www.example.com       # Trace full resolution path
dig -x 93.184.216.34             # Reverse lookup (IP → domain)

# Flush DNS cache
sudo dscacheutil -flushcache && sudo killall -HUP mDNSResponder  # Mac
sudo systemd-resolve --flush-caches                               # Linux

Reference

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