DNS is the Domain Name System, a hierarchical, distributed database where the keys are domain names.
The primary references are:
- RFC 1034 - Domain Names - Concepts and Facilities
- RFC 1035 - Domain Names - Implementation and Specification
The most common record types found in the DNS are:
- "A" records - the mapping from a domain name to an IPv4 address
- "AAAA" records - the mapping from a domain name to an IPv6 address
- "MX" records - the mapping from a domain name to the host name of an SMTP server
- "NS" records - used to delegate a portion of the hierarchy to specific DNS servers
- "PTR" records - typically used (via
in-addr.arpa.) to map an IPv4 address back to a domain name
- "CNAME" records - used to alias a domain name to its canonical version
DNS packets are conventionally transported over UDP and TCP port 53. UDP is more commonly used, but larger DNS responses and zone transfers require TCP. See RFC 5966.
This Wikipedia article provides an approachable introduction to DNS.