Names and addresses

What a DNS answer tells you

DNS is a distributed naming system. A recursive resolver finds answers on a client's behalf. An authoritative server publishes records for a zone. Cached answers are reused for a limited time rather than fetched anew for every request.

Common record types

• A maps a name to an IPv4 address.
• AAAA maps a name to an IPv6 address.
• CNAME points one name to another name.
• MX identifies mail servers for a domain.
• TXT stores text used by applications, including ownership verification.

Time to live

A record's TTL is the time a resolver may cache that answer. Lowering the TTL does not instantly erase answers already cached under an older, longer TTL. Changes therefore become visible at different times to different clients.

Names are not locations

A name can resolve to multiple addresses. A content delivery network may return different addresses depending on the resolver and network conditions. A DNS response identifies a destination for a connection; it does not reveal the application's full deployment topology.

A lookup is only the first check

When a site fails, separate the questions: did name resolution succeed, could the transport connect, was the TLS certificate valid, and did the application return an expected response? Treating these as independent checks makes failures easier to interpret.