Reverse DNS

A machine has two identities: its address and the name that belongs to it. This tool walks the circle between them in one request: our server asks the name for its addresses (A and AAAA, with a CNAME recorded separately as alias), reverses every public address among them (PTR), and asks every PTR name that came back forward again. In your browser the outcome is read per address — closed, open, no PTR, or not measurable — with every name and address measured along the way named alongside.

Why this counts for mail: RFC 1912 §2.1 asks a zone that PTR and A match — a PTR should point back at a valid A record, not at an alias defined by a CNAME — and RFC 5321 §2.3.5 calls the primary hostname of a host the name that host gives itself, the name it would state in an EHLO. RFC 5321 §4.1.1.1 states the border with it: a receiving server may check name against address, but may not refuse mail on that check alone.

What the tool does not do stands on the card with it: it asks no mail server what it finds, it trusts one resolver of this server (four resolvers side by side is what dns-compare does), it opens no port and sends no message, and it stops at three addresses and two names per address — what fell outside that is named not-inspected and not absent.