Reverse DNS Lookup
Enter an IPv4 or IPv6 address to find its PTR record, hostname, and verify forward DNS — the diagnostic tool built for email admins, network engineers, and security professionals.
What is reverse DNS?
Standard DNS maps a hostname to an IP address — you type dns.google and DNS returns
8.8.8.8. Reverse DNS does the opposite: given an IP address, it returns the associated
hostname. This is done using special DNS records called PTR records, stored in
dedicated zones of the DNS tree.
For IPv4, the PTR record for 8.8.8.8 is stored at 8.8.8.8.in-addr.arpa
(the octets reversed, with .in-addr.arpa appended). For IPv6, the address is expanded
to 32 hex nibbles, reversed, and stored under .ip6.arpa.
Who controls PTR records? PTR records are controlled by the organization that owns the IP address block — typically your hosting provider, cloud provider, or ISP. Unlike A records, which you can set yourself, PTR records must be requested from the IP address owner. Most cloud providers and VPS hosts allow you to set PTR records via their control panel.
What is a PTR record?
A PTR (Pointer) record is the DNS record type used for reverse lookups. It maps an IP address to a
canonical hostname. PTR records live in the in-addr.arpa (IPv4) or ip6.arpa
(IPv6) zones, which are managed by regional internet registries and delegated to IP address owners.
IPv4 PTR construction
- Take the IPv4 address:
8.8.8.8 - Reverse the octets:
8.8.8.8 - Append zone:
8.8.8.8.in-addr.arpa - PTR record value:
dns.google
IPv6 PTR construction
- Expand address:
2606:4700:4700::1111 - Full form:
2606:4700:4700:0000:…:1111 - Expand to 32 nibbles, reverse, join with dots
- Append:
…ip6.arpa
Forward DNS vs reverse DNS
| Property | Forward DNS (A / AAAA) | Reverse DNS (PTR) |
|---|---|---|
| Direction | Hostname → IP address | IP address → Hostname |
| Record type | A (IPv4), AAAA (IPv6) | PTR |
| DNS zone | Standard domain zones | in-addr.arpa / ip6.arpa |
| Controlled by | Domain owner | IP address owner (host/ISP) |
| Primary use | Website and service routing | Mail servers, security, logging |
| Required for email? | Yes (for hostname resolution) | Yes (for sender verification) |
Forward-confirmed reverse DNS (FCrDNS)
FCrDNS is the gold standard for IP identity verification. A PTR record alone is not enough — the hostname it returns must also resolve back to the original IP via an A or AAAA record. This two-way check is used by spam filters, mail servers, and intrusion detection systems to verify that the IP and hostname are consistent and not spoofed.
Why reverse DNS matters for email
Reverse DNS is one of the most important — and most overlooked — factors in email deliverability. When your mail server sends a message, the receiving server performs a series of checks. One of the first is a reverse DNS lookup on the sending IP.
What receiving servers check
- Does the sending IP have a PTR record?
- Does the PTR hostname resolve back to the same IP?
- Does the PTR hostname match the SMTP HELO/EHLO hostname?
- Does the PTR hostname match the domain in the From: header?
- Is the PTR hostname on any IP blocklists?
Consequences of missing rDNS
- Email rejected at the SMTP connection level
- Email sent to spam / junk folder
- Reduced sender reputation score
- Higher spam filter scores from providers like Spamassassin
- Rejection by strict mail servers (some gov and enterprise)
For MSPs: When troubleshooting email delivery failures for a client, always check reverse DNS on their mail server IP before investigating SPF, DKIM, or DMARC. A missing or misconfigured PTR record can cause delivery failures that SPF and DMARC cannot fix.
Common reverse DNS problems
| Problem | Cause | Fix |
|---|---|---|
| No PTR record | IP owner has not configured one | Request PTR from hosting provider or ISP |
| PTR exists, forward check fails | A record missing or points to different IP | Add A record that matches the PTR hostname and IP |
| Generic PTR hostname | ISP-assigned default (e.g. 123-45-67-89.example.isp.net) | Request a custom PTR from your provider |
| PTR does not match HELO | Mail server HELO uses a different hostname | Align SMTP HELO hostname with PTR hostname |
| PTR points to wrong domain | Stale record from previous IP owner | Contact IP owner (via WHOIS/ARIN) or request ISP to update |
| IPv6 PTR missing | Provider only configured IPv4 PTR | Request both IPv4 and IPv6 PTR from provider |
Common use cases
Email troubleshooting
Verify that your mail server IP has a valid PTR record and passes forward verification before debugging SPF, DKIM, or DMARC. Many delivery failures start here.
Security investigations
Identify the organization and infrastructure behind an IP. Map IPs from access logs, firewall alerts, or intrusion attempts to known providers and hostnames.
Network troubleshooting
Validate that server naming and DNS configuration are consistent. Identify misconfigured or stale PTR records that may cause authentication failures.
Asset discovery
Map IP addresses to hostnames to identify services, infrastructure owners, and cloud providers during security assessments or network audits.
Frequently asked questions
What is the difference between reverse DNS and reverse IP lookup?
They are the same thing — different terminology for the same operation. Reverse DNS lookup, PTR lookup, PTR record lookup, reverse IP lookup, and IP to hostname lookup all refer to querying a DNS PTR record to find the hostname associated with an IP address. This tool is technically a reverse DNS (PTR) lookup, but is often searched for using all of those terms.
What is a PTR record?
A PTR (Pointer) record is a DNS record type that maps an IP address to a hostname. It lives in special DNS zones — in-addr.arpa for IPv4 and ip6.arpa for IPv6 — that are managed by regional internet registries and delegated to IP address owners. PTR records are the reverse equivalent of A and AAAA records, which map hostnames to IP addresses.
Why does my reverse DNS lookup return no result?
If no PTR record is found, it means the IP address owner has not configured one, or the IP is in a range not delegated for PTR records. This is common for residential IPs, NAT addresses, and IPs behind shared hosting. If you expect a PTR record to exist, contact your hosting provider — they control the PTR configuration for the IP block.
What does forward verification FAIL mean?
A FAIL means the PTR record resolves to a hostname, but when that hostname is looked up via A or AAAA, the original IP address does not appear in the results. This breaks forward-confirmed reverse DNS (FCrDNS). It usually means either the A record is missing, points to a different IP, or the PTR record is stale. To fix it: ensure the PTR hostname has an A or AAAA record that resolves to the same IP.
Does reverse DNS affect email deliverability?
Yes — significantly. Many receiving mail servers check reverse DNS before accepting a connection. Without a valid PTR record, or if forward verification fails, your email may be rejected or sent to spam. This is separate from SPF, DKIM, and DMARC — all four should be configured correctly for optimal deliverability. Missing reverse DNS is one of the most common and easiest-to-fix delivery problems for self-hosted mail servers and VPS-hosted email.
How do I set reverse DNS for my server?
Log in to your hosting provider's control panel and look for a "Reverse DNS" or "PTR record" setting — most VPS and dedicated server providers offer this. Set the PTR to the fully qualified domain name (FQDN) of your server (e.g. mail.yourdomain.com). Then create an A record for that hostname pointing to the server's IP. If your provider does not offer PTR control, you will need to submit a support request or switch providers. Shared hosting providers rarely allow custom PTR records.
Does this tool support IPv6 reverse lookups?
Yes. Enter a full or abbreviated IPv6 address (e.g. 2606:4700:4700::1111) and the tool will automatically expand it, build the correct ip6.arpa PTR query name, and look up the PTR record. Forward verification for IPv6 uses an AAAA lookup instead of an A lookup.
Can I look up a private IP address?
You can enter a private IP address (10.x.x.x, 192.168.x.x, 172.16–31.x.x), but public DNS resolvers will not have PTR records for private ranges — those are handled by your internal DNS server. Results from this tool for private IPs may return no PTR record even if your internal DNS has one configured. Use your internal DNS tools to look up PTR records for private IP addresses.