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Table of Contents
Reverse DNS (PTR Records)
Unlike a normal DNS lookup, which translates a domain name into an IP address, Reverse DNS (rDNS) translates an IP address back into a hostname.
Reverse DNS is implemented using PTR (Pointer) records.
Forward DNS vs Reverse DNS
Forward DNS:
example.com
│
▼
93.184.216.34
Reverse DNS:
93.184.216.34
│
▼
example.com
Forward DNS uses A or AAAA records.
Reverse DNS uses PTR records.
Why Reverse DNS Exists
Reverse DNS helps verify that an IP address belongs to a particular hostname.
Many services use PTR records to:
- Identify servers
- Reduce spam
- Validate mail servers
- Improve logging and troubleshooting
How Reverse DNS Works
Reverse DNS uses a special namespace.
For IPv4:
93.184.216.34 becomes 34.216.184.93.in-addr.arpa
A resolver queries that name for a PTR record.
Example:
34.216.184.93.in-addr.arpa PTR example.com
IPv6 Reverse DNS
IPv6 uses the ip6.arpa domain.
Example:
2001:db8::1 ↓ ...ip6.arpa
The lookup principle is the same as IPv4.
Who Controls Reverse DNS?
Unlike normal DNS records, PTR records are not controlled by the domain owner.
PTR records are controlled by the owner of the IP address.
Usually this is:
- ISP
- VPS provider
- Cloud provider
- Hosting company
Therefore, changing a PTR record often requires contacting your hosting provider.
Forward-Confirmed Reverse DNS (FCrDNS)
Many mail servers perform an additional check called Forward-Confirmed Reverse DNS.
The process is:
IP Address
│
▼
PTR Record
mail.example.com
│
▼
A Record
mail.example.com
│
▼
Same IP Address
If the forward lookup returns the original IP address, the reverse DNS is considered consistent.
Why PTR Records Matter for Email
Many email providers, including Gmail, Outlook, and Yahoo, verify the sending server's PTR record.
If a mail server has:
- no PTR record, or
- an incorrect PTR record,
messages may be rejected or marked as spam.
Typical error:
550-5.7.25 The sending IP does not have a PTR record.
This is one of the most common email configuration problems.
Example
Suppose your mail server is:
mail.example.com A Record mail.example.com ↓ 203.0.113.15
The corresponding PTR record should be:
203.0.113.15 PTR mail.example.com
This creates a consistent forward and reverse mapping.
Common Uses of Reverse DNS
PTR records are commonly used for:
- Mail server verification
- Spam prevention
- Network diagnostics
- Security monitoring
- Server identification
Most websites themselves do not require PTR records.
Reverse DNS and DNS Check
A DNS lookup for a PTR record differs from an A record lookup.
Instead of querying a hostname:
example.com
the resolver queries the reversed IP under in-addr.arpa (IPv4) or ip6.arpa (IPv6).
Reverse DNS and Telenegar
Telenegar DNS Check supports PTR lookups.
Given an IPv4 or IPv6 address, the tool can display:
- PTR record
- Hostname
- DNS provider response
- TTL
This is useful for diagnosing:
- Email delivery problems
- Mail server configuration
- Reverse DNS consistency
Common Misconceptions
- PTR records are configured by the domain owner.
- False. They are configured by the owner of the IP address.
- Every web server needs a PTR record.
- False. PTR records are primarily important for mail servers.
- Forward DNS automatically creates Reverse DNS.
- False. Forward and reverse DNS are configured independently.