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--- networking:reverse_dns [2026/07/22 19:33] – created - external edit 127.0.0.1
+++ networking:reverse_dns [2026/07/22 20:46] (current) – removed - external edit (Unknown date) 127.0.0.1
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-====== 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
-      │
-      ▼
-.184.216.34
-</code>
-Reverse DNS:
-<code>
-.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>
-.184.216.34
-becomes
-.216.184.93.in-addr.arpa
-</code>
-A resolver queries that name for a PTR record.
-Example:
-<code>
-.216.184.93.in-addr.arpa
-PTR
-example.com
-</code>
------
-===== IPv6 Reverse DNS =====
-IPv6 uses the **ip6.arpa** domain.
-Example:
-<code>
-: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>
--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
-↓
-.0.113.15
-</code>
-The corresponding PTR record should be:
-<code>
-.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:start|DNS Explained]]
-  * [[networking:dns:dnssec|DNSSEC]]
-  * [[email:spf|SPF]]
-  * [[email:dkim|DKIM]]
-  * [[tools:dnscheck|DNS Check]]
------
-{{tag>dns ptr reverse-dns rdns email smtp}}