Reverse DNS Lookup with FCrDNS Check
Find the hostname (PTR record) for an IP and forward-confirm it
How reverse DNS works
A normal DNS lookup turns a name into an address. Reverse DNS goes the other way: IP to hostname. It relies on PTR records published in two special zones — in-addr.arpa for IPv4 and ip6.arpa for IPv6. To look up 8.8.4.4 a resolver actually asks for the PTR record of 4.4.8.8.in-addr.arpa (the octets reversed); IPv6 addresses are expanded to 32 hex digits and reversed nibble by nibble. This tool shows that query name so you can see exactly what was asked.
The important difference from forward DNS is who controls it. You control the A records for your domain, but PTR records belong to whoever owns the IP block — your hosting provider, cloud platform or ISP. To set or change reverse DNS for a server you usually do it in the provider's control panel (many VPS and cloud dashboards have an "rDNS" or "reverse DNS" field) or by opening a ticket. Residential connections normally get a generic PTR generated by the ISP.
Enter an IPv4 or IPv6 address, choose a resolver (Cloudflare 1.1.1.1, Google 8.8.8.8 or Quad9 9.9.9.9) and the tool runs the equivalent of a reverse nslookup from our server. The address you enter goes to that resolver and isn't stored. There's also a "Use my IP address" shortcut to check your own connection's PTR record. Private addresses such as 192.168.1.10 generally have no public PTR — public resolvers answer NXDOMAIN — because their reverse names only exist on your local DNS server.
Forward-confirmed reverse DNS (FCrDNS) and why mail servers care
A PTR record on its own proves little: whoever controls an IP block can make its PTR say anything, including a hostname under someone else's domain. Forward-confirmed reverse DNS closes that gap. After finding the PTR hostname, the tool resolves that hostname back to A (or AAAA for IPv6) records and checks whether the original IP is among them. If it is, the name and address agree in both directions — an "FCrDNS pass" — and since only the domain's owner can publish its A records, the pairing is trustworthy.
This rDNS check matters most for email. Gmail, Outlook and most spam filters reject or heavily penalise mail from IPs with no PTR record or a PTR that doesn't forward-confirm, and many mail servers expect the PTR to match the name they announce in their SMTP HELO/EHLO greeting. If you run your own mail server or a transactional sending IP, a failing FCrDNS check is one of the first things to fix — set the PTR at your provider to a hostname like mail.example.com and make sure that name has an A record pointing back at the same IP.
Reverse DNS is also handy outside email: identifying which network an address in your server logs belongs to, verifying that a crawler claiming to be Googlebot really resolves to googlebot.com (Google's documented verification method is exactly this two-step lookup), or checking that a new server's hostname is set correctly before going live. Because PTR answers are cached like any other DNS record, a change made at your provider can take a while to show up — compare resolvers if a fresh update hasn't appeared yet.
How to use the Reverse DNS Lookup with FCrDNS Check tool
- 1
Enter an IP address
Type an IPv4 address such as 8.8.8.8 or an IPv6 address, or click Use my IP address.
- 2
Pick a resolver
Choose Cloudflare, Google or Quad9 — reverse zones are public, so they should agree.
- 3
Read the PTR result
The hostname (or NXDOMAIN if there's none) is shown with the in-addr.arpa or ip6.arpa query name.
- 4
Check forward confirmation
Each PTR hostname is resolved back to A/AAAA; a green badge means the original IP was found (FCrDNS pass).
Frequently asked questions
What is a PTR record?
A PTR (pointer) record maps an IP address to a hostname. It lives in the reverse zone in-addr.arpa (IPv4) or ip6.arpa (IPv6) and is controlled by whoever owns the IP block.
How do I change the reverse DNS for my server?
Ask the owner of the IP — usually your hosting or cloud provider. Most VPS dashboards have a reverse DNS field; otherwise open a support ticket. You can't set it from your domain's DNS zone.
What does an FCrDNS pass mean?
The PTR hostname resolves back (via A or AAAA) to the same IP. Mail servers and bot-verification checks use it because only the domain owner can create that forward record.
Why is there no PTR for my private IP?
Private ranges like 10.x.x.x and 192.168.x.x aren't delegated on the public internet, so public resolvers have no reverse data for them. Only your local network's DNS server can answer.
Can one IP have several PTR records?
It's allowed but discouraged — many systems only read the first one. The tool lists all of them and forward-confirms each.
Related on TechLogHub
Related Network Tools
cURL to Fetch Converter
Convert cURL commands to JavaScript fetch() calls instantly
cURL to Axios Converter
Convert cURL commands to Axios request code in seconds
Test API Online: REST Client with cURL Export
Send HTTP requests to CORS-enabled APIs from your browser and copy them as cURL
Mock REST API with Fake Users, Posts and Todos
Free fake REST API with users, posts, todos, comments and products for testing
HTTP Headers Viewer
View HTTP response headers for any URL
User-Agent Parser
Parse and decode user-agent strings
Get the next tool before you need it
One email a week: new and trending developer tools, fresh comparisons, and what shipped. Unsubscribe in one click.