Textbook of Linux

dig ("domain information groper") is the de-facto DNS diagnostic tool on Linux. It performs DNS queries and prints the raw response (question, answer, authority, and additional sections), making it invaluable for debugging DNS problems.

dig example.com                  # A record
dig example.com MX               # mail servers
dig example.com AAAA             # IPv6
dig example.com NS               # authoritative nameservers
dig @8.8.8.8 example.com         # query a specific resolver
dig +short example.com           # just the answer, no ceremony
dig +trace example.com           # follow the delegation chain from the root
dig -x 93.184.216.34             # reverse lookup (PTR)

dig comes from the BIND project. Alternatives: host (simpler output), nslookup (older, still widely used), and drill (from ldns, common on Alpine).