DNS

Resource Record

• Name
• Type (name and number)
  • A, MX, CNAME (see Resource Record Types)
• RDATA
  • Type specific data
    • A: IP address
    • MX: priority and hostname
    • CNAME: hostname
    • ...
• RDATALENGTH
  • Length of RDATA
• TTL
• Class
  • IN - Internet (typical)
  • CH - Chaosnet (deprecated)
  • HS - Hesiod

Resource Record Types

• A - IPv4 address (32 bits)
  • 52.32.44.21
• AAAA - IPv6 address (128 bits)
  • 2001:0db8:85a3:0:0:8a2e:0370:7334
• CNAME - domain name
  • Cannot set to zone apex (e.g. "example.org")
  • If set for subdomain ("alfa.example.org") no other record can be set for ("alfa.example.org")
• MX - priority and domain name
  • 10 mail.example.org
• NS - domain of the name server for the hosted zone 
  • ns-1.example.com
• PTR (Pointer)
  • Used for mail server verification
  • mail.example.com
• SOA (Start of Authority)
  
  • Base DNS configuration
    • Host that created record
    • Email address of the administrator
    • Revision number
    • Minimum TTL (useful for NXDOMAIN caching)
• SPF (Sender Policy Framework)
  • deprecated - use TXT for SPF instead
• SRV (Service Locator)
  • Provides information on available services
  • Name follows special convention
    • priority, weight, port, domain name 
      • e.g. "10 5 80  _ldap._tcp.tuesday.mydirectory.com"
  • Used in AD Connector scenario to look-up AD
• TXT
  • Max 255 characters
  • case sensitive
  • Used for misc purposes (SPF, DKIM, domain validation)
• CAA
  • Specify CA that can issue certificates for your domain/subdomain
• NAPTR
  • Name Authority Pointer
  • Typically used in Telephony

NXDOMAIN

• Record returned when domain does not exists
• "DNS Hijacking" service provider return IP address for NXDOMAIN to redirect to their site
• Response is cached (SOA minimum TTL)

EDNS

• Extension to DNS
• Expose /24 (first 3 octets) of the client address ("subnet")
• Improves accuracy of geolocation (especially in case of using global DNS resolvers like 8.8.8.8)