Reverse DNS lookup

The opposite direction from a normal lookup

An ordinary DNS lookup turns a hostname into an address. Reverse DNS does the opposite: given an address, it finds a hostname, using a PTR record published under a special reversed-address zone (in-addr.arpa for IPv4, ip6.arpa for IPv6).

Why it often returns nothing

Unlike the forward direction, a PTR record has to be configured deliberately by whoever controls that IP address - usually the hosting provider or ISP, not the domain owner. Plenty of legitimately-used addresses have no PTR record at all, which is not an error, just a choice nobody made.

What it is actually used for

• Mail server reputation - many receiving mail servers check that a sending server's IP has a PTR record, and that it roughly matches the sending domain, before accepting mail from it.
• Reading server logs - turning a raw visitor IP into a readable hostname when investigating traffic.
• Sanity-checking hosting - confirming an IP is actually associated with the provider you expect.

Related tools

For the forward direction, see A lookup and AAAA lookup.