A PTR record (Pointer Record) is a DNS resource record used in reverse DNS lookups to map an IPv4 or IPv6 address back to a hostname or FQDN.
A PTR Record (Pointer Record) is a specialized DNS resource record defined in IETF RFC 1035 that performs Reverse DNS (rDNS) resolution. While standard forward DNS records (A and AAAA) map human-readable domain names to machine-routable IP addresses, PTR records perform the exact opposite function—mapping an IPv4 or IPv6 address back to its canonical Fully Qualified Domain Name (FQDN).
PTR records play a vital role in email security (anti-spam SMTP verification and FCrDNS), network infrastructure diagnostics, server logging, and cybersecurity forensics.
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| Specification | IPv4 Reverse DNS | IPv6 Reverse DNS |
|---|---|---|
| Standard Reference | IETF RFC 1035 / RFC 2317 | IETF RFC 3596 / RFC 1886 |
| DNS Root Zone | .in-addr.arpa |
.ip6.arpa |
| Record Name Format | Reversed 4-octet quad (e.g. 4.3.2.1.in-addr.arpa) |
Reversed 32-nibble hex string (e.g. 1.0...8.b.d.0.1.0.0.2.ip6.arpa) |
| Authority Delegation | Regional Internet Registries (ARIN, RIPE, APNIC) & ISPs | Regional Internet Registries (RIRs) & ISPs |
| Primary Use Cases | Email deliverability (FCrDNS), server identification, traceroute | IPv6 node verification, network troubleshooting, anti-abuse |
Because the global DNS namespace is structured hierarchically from right to left (TLD down to hostname labels), querying an IP address directly would require searching every DNS zone in existence. To solve this, the ARPA domain architecture reorganizes IP addresses into standard hierarchical trees:
(Root .)
│
[.arpa]
┌───┴───┐
[in-addr] [ip6]
│ │
[198] [2.0.0.1]
│ │
[51] [0.d.b.8]
│ │
[100] ...
│ │
[42] (PTR Record) [1] (PTR Record)
│ │
▼ ▼
mail.example.com node.example.org
.in-addr.arpa): The octets of an IPv4 address (e.g., 198.51.100.42) are reversed: 42.100.51.198.in-addr.arpa..ip6.arpa): The 128-bit IPv6 address is expanded into 32 hexadecimal nibbles, reversed, and punctuated with dots: 1.0.0.0...8.b.d.0.1.0.0.2.ip6.arpa.Major mail service providers (Google Gmail, Yahoo Mail, Microsoft 365, Apple iCloud) mandate Forward-Confirmed Reverse DNS (FCrDNS) on all connecting SMTP servers to filter out botnets and spoofed spam sources:
Step 1 (Reverse Lookup):
Connecting SMTP IP: 198.51.100.25 ──(PTR Query)──> Resolves to "mail.example.com"
Step 2 (Forward Verification):
"mail.example.com" ───────────────(A Query)────> Resolves to "198.51.100.25"
Match? YES ➔ Mail Accepted for SPF/DKIM Processing
Match? NO ➔ 550 SMTP Permanent Rejection (Spam Rejected)
Without a valid PTR record matching your mail server's forward A record and HELO/EHLO banner, outbound emails will be rejected or routed straight to spam folders.
dns/promises)import { promises as dns } from 'dns';
async function performReverseDns(ipAddress: string) {
try {
const hostnames = await dns.reverse(ipAddress);
console.log(`Resolved PTR for ${ipAddress}:`, hostnames);
return hostnames;
} catch (err) {
console.error(`Reverse DNS failed: ${(err as Error).message}`);
return [];
}
}
// Example: Google Public DNS
performReverseDns('8.8.8.8'); // Output: ['dns.google']
Yes. DNS allows multiple PTR records to be associated with a single IP address. However, for outbound email servers, it is best practice to configure exactly one PTR record matching the server's primary FQDN to avoid SMTP handshake ambiguity.
PTR records must be published in the authoritative .in-addr.arpa zone owned by the entity that owns the IP block (your ISP, cloud hosting provider, or datacenter). You must configure PTR records via your cloud provider (e.g. AWS Elastic IP reverse DNS, Google Cloud PTR, DigitalOcean droplet settings) rather than your domain registrar.
Enter any IPv4 or IPv6 address into our IP to Hostname Lookup to resolve PTR records, check reverse zone syntax, and verify query response times.
Free, browser-based utilities to test, generate, and inspect PTR Record (Pointer Record & Reverse DNS) payloads directly.
Reverse DNS lookup — resolve PTR records to find hostnames for any IPv4 or IPv6 address.
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