PTR Record
Pointer Record — Reverse DNS
The mirror image of an A record: instead of turning a name into an IP, a PTR record turns an IP back into a name. This reverse lookup is what mail servers rely on to judge whether a sender is legitimate.
A PTR record (pointer record) provides reverse DNS: given an IP address, it returns the hostname associated with it. Forward DNS answers "what IP is mail.example.com?" with an A record; reverse DNS answers the opposite — "what name belongs to 203.0.113.10?" — with a PTR record. The two directions are independent: having one does not automatically create the other.
How reverse DNS is structured
Reverse lookups happen inside a special zone called in-addr.arpa for IPv4 (and ip6.arpa for IPv6). The IP's octets are reversed and the suffix appended:
; the IP 203.0.113.10 becomes this name:
10.113.0.203.in-addr.arpa. 3600 IN PTR mail.example.com.The octets are reversed because DNS reads names from most-specific to least-specific (right to left), and IP addresses are written most-significant first — so flipping them lets the hierarchy line up with how IP blocks are delegated.
Who controls it — and why that matters
This is the detail that trips people up: you do not set your own PTR record the way you set an A record. Reverse DNS is delegated to whoever owns the IP block — your ISP or hosting provider. To add or change a PTR record you usually request it through their control panel or support, because the authority follows IP allocation, not domain ownership.
The email reputation connection
The single most important use of PTR records is email deliverability. When your server connects to a receiving mail server, the receiver looks up the PTR record of your sending IP. A missing PTR record, a generic ISP-assigned name (like host-203-0-113-10.example-isp.net), or a PTR that does not match your sending hostname are all treated as spam signals. Legitimate mail infrastructure almost always has clean reverse DNS; much spam does not — so receivers lean on it heavily.
Forward-confirmed reverse DNS (FCrDNS)
The gold standard mail servers check for is forward-confirmed reverse DNS: the PTR record gives a hostname, and that hostname's forward A record points back to the same IP. The loop must close in both directions.
| Direction | Lookup | Expected result |
|---|---|---|
| Reverse (PTR) | 203.0.113.10 → | mail.example.com |
| Forward (A) | mail.example.com → | 203.0.113.10 |
If both match, the IP passes FCrDNS and is trusted far more readily.
Looking up a PTR record
# macOS / Linux — reverse lookup an IP
dig -x 203.0.113.10 +short
# Windows
nslookup 203.0.113.10If the reply is a meaningful hostname that also resolves forward to the same IP, reverse DNS is configured correctly. If it returns nothing, the IP has no PTR record — fine for a home connection, but a problem for any server that sends email.