Eventually Bitwise

DNS

Overview

DNS (Domain Name System) is the “phonebook of the internet.” It translates human-readable domain names (e.g., google.com) into machine-readable IP addresses (e.g., 142.250.190.46).

Core Concepts

  • Hierarchical Namespace:
    • Root Zone: The top of the hierarchy (.).
    • TLD (Top-Level Domain): .com, .org, .net, .io.
    • Authoritative Name Server: The final server in the chain that holds the actual IP record for the domain.
  • DNS Record Types:
    • A Record: Maps a hostname to an IPv4 address.
    • AAAA Record: Maps a hostname to an IPv6 address.
    • CNAME: An alias for another domain name.
    • MX Record: Specifies the mail server for the domain.
    • TXT Record: Used for verification (e.g., SPF, DKIM).
  • Resolution Process:
    • Recursive Resolver: The server that does the work of searching for the IP (usually provided by the ISP).
    • Iterative Query: The resolver asks the Root, then TLD, then Authoritative server until the IP is found.
  • Caching: To speed up resolution, DNS records are cached at various levels (browser, OS, resolver).

Code Examples

# Digging a domain to see its records
dig google.com A
dig google.com MX
# Checking the local hosts file (overrides DNS)
cat /etc/hosts

Use Cases

  • User Experience: Allowing users to remember names instead of strings of numbers.
  • Load Balancing: Using a single domain name to point to multiple different IP addresses (Round Robin DNS).
  • Service Discovery: Using SRV records to find services in a network.

Gotchas

  • DNS Propagation: When you change a DNS record, it can take hours (or days) to reach all resolvers worldwide due to caching (TTL – Time To Live).
  • DNS Spoofing/Poisoning: An attacker inserting fake records into a resolver’s cache to redirect users to a malicious site.

Related Notes

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