Understanding DNS: How Domain Name Resolution Works Step by Step

The Domain Name System (DNS) is one of the most critical components of the internet. Every time you visit a website, send an email, or connect to an API, DNS is working behind the scenes to translate human-readable domain names into IP addresses that computers can understand.

How DNS Resolution Works

When you type example.com into your browser, a series of steps occur:

1. Browser Cache: The browser checks its own DNS cache first.
2. OS Cache: If not found, the operating system checks its resolver cache.
3. Recursive Resolver: The query goes to your ISP's or configured DNS resolver.
4. Root Name Server: The resolver asks a root server which TLD server handles .com.
5. TLD Name Server: The .com TLD server points to the authoritative name server for example.com.
6. Authoritative Name Server: Returns the actual IP address for the domain.
7. Response: The IP address travels back through the chain to your browser.

DNS Record Types

Understanding record types is essential for managing domains:

• A Record: Maps a domain to an IPv4 address (e.g., example.com → 93.184.216.34)
• AAAA Record: Maps a domain to an IPv6 address
• CNAME Record: Creates an alias pointing to another domain name
• MX Record: Specifies mail servers for the domain
• TXT Record: Stores text data (used for SPF, DKIM, domain verification)
• NS Record: Specifies authoritative name servers for the domain
• SOA Record: Contains administrative information about the zone
• SRV Record: Specifies servers for specific services
• PTR Record: Reverse DNS lookup (IP to domain)

DNS Troubleshooting Tools

dig

# Basic query
dig example.com

# Query specific record type
dig example.com MX

# Query specific DNS server
dig @8.8.8.8 example.com

# Trace the full resolution path
dig +trace example.com

# Short answer only
dig +short example.com

# Reverse DNS lookup
dig -x 93.184.216.34

nslookup

# Basic lookup
nslookup example.com

# Specify record type
nslookup -type=MX example.com

# Use specific DNS server
nslookup example.com 8.8.8.8

DNS Caching and TTL

TTL (Time to Live) controls how long DNS records are cached:

• Low TTL (300s): Records update quickly but increase DNS query load
• High TTL (86400s): Reduces query load but changes propagate slowly
• Before migration: Lower TTL to 300s days in advance, then change records, then raise TTL back

# Flush local DNS cache
# Linux (systemd-resolved)
sudo systemd-resolve --flush-caches

# macOS
sudo dscacheutil -flushcache && sudo killall -HUP mDNSResponder

# Windows
ipconfig /flushdns

DNS Security

Protect your DNS infrastructure with these measures:

• DNSSEC: Digitally signs DNS records to prevent tampering
• DNS over HTTPS (DoH): Encrypts DNS queries in HTTPS traffic
• DNS over TLS (DoT): Encrypts DNS queries using TLS
• Response Rate Limiting: Prevents DNS amplification attacks
• Split-horizon DNS: Provides different answers for internal vs external queries

Common DNS Issues and Solutions

• Propagation delays: DNS changes can take up to 48 hours to propagate globally. Lower TTL before making changes.
• Misconfigured records: Use dig +trace to identify where resolution fails.
• Missing reverse DNS: Can cause email delivery issues. Always set PTR records for mail servers.
• DNS hijacking: Enable DNSSEC and use reputable DNS providers.

DNS is the backbone of internet connectivity. Understanding how it works and how to troubleshoot it will help you resolve networking issues faster and configure domains correctly the first time.