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DNS Overview and Records

Jun 5, 2025

Overview

This lecture introduces the Domain Name System (DNS), explains its hierarchical structure, and details various DNS resource records, including their purposes and configurations.

Introduction to DNS

  • DNS stands for Domain Name System, translating domain names into IP addresses to facilitate network communication.
  • DNS is a distributed, hierarchical database with parts stored on multiple servers worldwide.
  • The database structure starts with 13 root server clusters, extending to top-level domains (TLDs) like .com or country codes (.us, .ca).
  • Domains can be further subdivided (e.g., www.professormesser.com, mail.professormesser.com).

DNS Query Tools

  • The dig command (Linux/Mac) and nslookup (Windows) retrieve DNS record information for specific domains.
  • These tools display resource records like IP addresses, mail servers, and text records.

DNS Resource Records

  • Resource Records (RR) are entries in the DNS database storing key details such as IP addresses, certificates, or aliases.

Address Records

  • A records map hostnames to IPv4 addresses; AAAA (quad A) records map to IPv6 addresses.
  • Records specify a time to live (TTL), controlling how long clients cache results.

Canonical Name (CNAME) Records

  • CNAME records allow multiple names to point to the same server, simplifying administration.

Mail Exchanger (MX) Records

  • MX records define the mail servers responsible for receiving email for a domain.
  • Email delivery checks the MX record to determine the proper mail server, then looks up that server's address.

Text (TXT) Records

  • TXT records store arbitrary, human-readable text, often for verification and authentication.
  • Examples include verification for services and email anti-spam controls.

DNS and Email Security

  • DKIM (DomainKeys Identified Mail): Uses DNS TXT records to publish public keys, enabling digital signatures for email authenticity.
  • SPF (Sender Policy Framework): Lists authorized mail servers in DNS TXT records, helping prevent spoofed emails.
  • DMARC (Domain-based Message Authentication Reporting and Conformance): TXT records define how to handle emails failing SPF/DKIM and provide reporting options.

Key Terms & Definitions

  • DNS — System translating domain names to IP addresses.
  • A Record — Maps hostname to IPv4 address.
  • AAAA Record — Maps hostname to IPv6 address.
  • CNAME Record — Alias pointing one name to another.
  • MX Record — Identifies mail servers for a domain.
  • TXT Record — Stores text information for various uses.
  • DKIM — Method using DNS to verify email authenticity via digital signatures.
  • SPF — Lists allowed email senders for a domain.
  • DMARC — Policy for handling email authentication failures.

Action Items / Next Steps

  • Practice using dig and nslookup to query DNS records.
  • Review configuration options for A, MX, CNAME, and TXT records.
  • Read about SPF, DKIM, and DMARC setup for securing email domains.

Full transcript