Reverse DNS Lookup
Look up the PTR record for any IP address. Verify reverse DNS configuration and check FCrDNS for email delivery.
PTR Records and Why Reverse DNS Still Matters
Reverse DNS answers the opposite question to a normal lookup: given an IP address, what hostname does its owner want it known by? The mechanism is a PTR record stored in a special part of the DNS tree — for IPv4 the octets are reversed and appended to in-addr.arpa, so 8.8.8.8 becomes 8.8.8.8.in-addr.arpa, and for IPv6 each hex nibble is reversed under ip6.arpa. Querying that name returns the PTR answer, which for 8.8.8.8 is dns.google.
The reason most people meet reverse DNS is email. Receiving mail servers apply Forward-Confirmed reverse DNS (FCrDNS): the sending IP must have a PTR record, and the hostname in that PTR must resolve back to the same IP. Gmail's sender guidelines require valid PTR records for sending IPs, and Outlook.com and countless corporate gateways reject or spam-folder mail that fails the check, because the ability to pass FCrDNS proves someone administratively controls both the address and the name — something botnets on residential connections cannot fake.
A frequent point of confusion is who can change a PTR. It is not your domain registrar: the in-addr.arpa and ip6.arpa zones are delegated along the IP allocation chain, so only the holder of the address block — your ISP, hosting provider, or cloud platform — can publish PTR records for it. Cloud providers expose this as self-service (a public PTR setting in GCP, an rDNS request in AWS, naming your droplet after its FQDN in DigitalOcean), while traditional ISPs often handle it through support tickets.
Key Reverse DNS Concepts
PTR recordFCrDNS matchGeneric vs custom rDNSSetting PTR in the clouddig -xMismatch problemsFrequently Asked Questions
What is reverse DNS?
Reverse DNS resolves an IP address back to a domain name using PTR records. While forward DNS maps domain → IP, reverse DNS maps IP → domain. Used for verification, logging, and email delivery.
What is a PTR record?
A PTR (Pointer) record maps an IP address to a hostname. For example, 8.8.8.8 → dns.google. PTR records are stored in in-addr.arpa (IPv4) or ip6.arpa (IPv6) zones, managed by the IP address owner.
Why does reverse DNS matter for email?
Many mail servers require a valid PTR record that matches forward DNS (FCrDNS — Forward-Confirmed rDNS). Missing or mismatched PTR records cause emails to be rejected or marked as spam.
How do I set a PTR record?
PTR records are set by the IP address owner — your ISP, hosting provider, or cloud platform. For AWS: VPC settings in Route 53. For GCP: VM Instance network interface settings. For dedicated servers: contact your host.
Why can't I set a PTR record at my domain registrar?
Because PTR records live in the in-addr.arpa and ip6.arpa zones, which are delegated along the IP allocation chain — from IANA to the Regional Internet Registries to whoever holds the address block — not along the domain name hierarchy. Your registrar only controls your domain's forward zone. The PTR for an IP can be set solely by the party the address space is delegated to: your ISP, hosting company, or cloud provider, which is why each provider exposes its own reverse DNS form or API instead of a normal DNS editor.
What is FCrDNS and which mail providers require it?
Forward-Confirmed reverse DNS means the PTR record of your sending IP resolves to a hostname, and that hostname's A or AAAA record points back to the same IP — proving the IP holder and the hostname owner agree. Gmail's sender guidelines require sending IPs to have valid PTR records, and Outlook.com along with many corporate gateways reject or heavily penalise mail from IPs that fail the check. If you operate your own mail server, configure FCrDNS before sending a single message.
Can an IP address have multiple PTR records?
Technically yes — DNS permits several PTR records on one in-addr.arpa name — but it is discouraged in practice. Resolvers return the set in no guaranteed order, so FCrDNS verification becomes unpredictable, and some mail servers treat multiple PTRs as a misconfiguration. The convention is one PTR per IP pointing at a canonical hostname that resolves back to that address; if you need many names on one IP, put them in forward DNS instead.
How long do PTR record changes take to propagate?
Two delays stack up. First, your provider must publish the change — effectively instant with self-service consoles like AWS, GCP, or DigitalOcean, but potentially a business day or more when an ISP handles it as a support ticket. Second, resolvers cache the old answer until its TTL expires, commonly anywhere from one to twenty-four hours. When testing, query the authoritative nameserver directly with dig -x so you can confirm the new value without waiting for caches to clear.
Monitor DNS Configuration Continuously
CertNotify monitors your domains for DNS, SSL, and delivery issues around the clock — so misconfigurations get caught before they cost you email or uptime.