====== 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:

<code>
example.com
      │
      ▼
93.184.216.34
</code>

Reverse DNS:

<code>
93.184.216.34
      │
      ▼
example.com
</code>

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:

<code>
93.184.216.34

becomes

34.216.184.93.in-addr.arpa
</code>

A resolver queries that name for a PTR record.

Example:

<code>
34.216.184.93.in-addr.arpa

PTR

example.com
</code>

-----

===== IPv6 Reverse DNS =====

IPv6 uses the **ip6.arpa** domain.

Example:

<code>
2001:db8::1

↓

...ip6.arpa
</code>

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:

<code>
IP Address
     │
     ▼
PTR Record

mail.example.com

     │
     ▼

A Record

mail.example.com
     │
     ▼

Same IP Address
</code>

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:

<code>
550-5.7.25

The sending IP does not have a PTR record.
</code>

This is one of the most common email configuration problems.

-----

===== Example =====

Suppose your mail server is:

<code>
mail.example.com

A Record

mail.example.com
↓

203.0.113.15
</code>

The corresponding PTR record should be:

<code>
203.0.113.15

PTR

mail.example.com
</code>

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:

<code>
example.com
</code>

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.

-----

===== Related Topics =====

  * [[networking:dns:dns|DNS Explained]]
  * [[networking:dns:dnssec|DNSSEC]]
  * [[email:spf|SPF]]
  * [[email:dkim|DKIM]]
  * [[tools:dnscheck|DNS Check]]

-----

{{tag>dns ptr reverse-dns rdns email smtp}}