Networking

DNS

Domain Name System resolves human-readable domain names into IP addresses through distributed hierarchical database

DNS (Domain Name System)

  • Resolves human readable domain names into machine-usable IP addresses through a distributed, hierarchical database system
  • Eliminates need for maintaining local host.txt files and memorizing IP addresses
  • Provides structured delegation of domain management across multiple authoritative servers
  • The DNS namespace is a directory tree structure, read from right to left

DNS Hierarchy Structure

  • Root Level (.) - Top of DNS tree, managed by root nameservers (13 worldwide clusters)
  • Top-Level Domains (TLD) - Generic (.com, .org, .net) or Country Code (.us, .uk, .ca)
  • Second-Level Domains - Organization-specific (cisco.com, google.com)
  • Subdomains - Department or service-specific (www.cisco.com, mail.cisco.com)

DNS Record Types

Record Purpose Example
A Maps domain to IPv4 address cisco.com → 198.133.219.25
AAAA Maps domain to IPv6 address cisco.com → 2001:420:1101:1::a
CNAME Creates alias pointing to another domain www.cisco.com → cisco.com
MX Directs email traffic to mail server cisco.com → mail.cisco.com
NS Specifies authoritative DNS servers cisco.com → ns1.cisco.com
PTR Provides reverse DNS lookup (IP to domain) 25.219.133.198.in-addr.arpa
SOA Contains zone metadata Refresh: 3600s, Retry: 900s
TXT Stores arbitrary text data “v=spf1 include:_spf.cisco.com”

DNS Server Types

  • Authoritative Servers - Hold actual DNS records for domains they manage
  • Recursive Resolvers - Query other servers on behalf of clients, cache responses
  • Forwarders - Forward queries to specific DNS servers instead of performing recursion
  • Root Servers - Respond with TLD server information (13 logical servers)

Vocabulary

  • TTL (Time To Live) - Specifies how long records can be cached
  • Negative Caching - Caches NXDOMAIN responses to prevent repeated queries
  • Cache Poisoning - Security concern where malicious data is inserted into DNS cache

Notes

  • DNS uses UDP port 53 for queries, TCP port 53 for zone transfers
  • Configure multiple DNS servers for redundancy
  • Use nslookup or dig commands for DNS troubleshooting
  • Always implement redundant DNS servers in production networks
  • Consider DNS security extensions (DNSSEC) for cryptographic authentication