DNS Records: A, AAAA, CNAME, MX, TXT and NS

A DNS record is one fact about a name, and its type says what kind of fact it is: an address, another name, a mail server or a piece of text. A domain's records together are its zone, written in a zone file one record per line: the name, the TTL in seconds, the class (almost always IN, for internet), the type and the value.

A few rules decide how they are used.

A and AAAA are the answers a browser wants. A name can have several of each, and clients try them in turn or spread their connections across them.

A CNAME replaces everything at its name. It says "this name is really that one", so a resolver asking for any type at www.example.com follows the alias and asks again for example.com. Because of that, a name with a CNAME may hold no other records at all: no MX, no TXT, nothing. This is why CNAMEs suit names like www or a hosted service's subdomain, and cause trouble at the root of a domain.

MX names a host, not an address. Mail servers look up the MX for the part after the @ in an email address, then the A or AAAA record for the host it names. The number is a priority: lower is tried first.

TXT carries text for other systems. Mail providers check TXT records for sending policies, and services ask a domain owner to publish a TXT token to prove they control it.

NS records delegate. The parent zone, .com here, holds NS records pointing to example.com's servers, which is how the referrals in a lookup work.

So when adding a record, decide first what kind of answer the name should give, an address, another name or mail, and the type follows.