What is a PTR Record? Reverse DNS Lookup Made Easy

PTR records (or Pointer records) are used for the Reverse DNS (Domain Name System) lookup. Using the IP address you can get the associated domain/hostname. An A record should exist for every PTR record. The usage of a reverse DNS setup for a mail server is a good solution.

The PTR record resolves an IP address to a domain/hostname. Yet, to complete this assignment successfully, the Pointer record requires to be established in a Reverse DNS zone. While in the Primary DNS zone, the hostname is pointed to an IP address, using the Reverse DNS zone allows pointing an IP address to a hostname.

The PTR records have the following look in the ClouDNS Control Panel:

After creating the PTR record, make sure that the hosts have corresponding A records. In our example, hostname1.example.com must be with A record pointed to 192.168.0.1 and hostname2.example.com with 192.168.0.2

To complete the Reverse DNS configuration, you need to change the name servers of your Reverse zone at your IP provider or ask him to assist you with the modification. In our case, there must be configured NS records at the IP provider pointed to the name servers listed at the dashboard of your control panel.

Why do you need a PTR record?

• Trust and Verification: PTR records ensure that the given hostname or domain is correctly connected to the IP address.
• Email Servers Requirement: PTR records are essential for outgoing mail servers, particularly those using SMTP (Simple Mail Transfer Protocol), because most mail providers reject or mark as spam messages from mail servers without valid Reverse DNS configuration.
• Avoiding Spam Filters: Emails sent from servers with missing PTR records or mismatched A records are often rejected or marked as spam.
• Industry Standard: This practice has become a standard as all mail providers strive to keep spam out of their clients' inboxes.

How to configure a PTR record?

In order to create a PTR record, at first need to be created a Reverse DNS zone (read more about the reverse DNS zones here). The Reverse DNS zone depends on the size of your IP network. In our example we will use an IPv4 /24 network. In the /24 network, you have 255 unique IP addresses. For example, if our network is 192.168.0.0/24, the available IPs in this network are from 192.168.0.1 to 192.168.0.255 and our Reverse zone has following format: 0.168.192.in-addr.arpa

In order to create a Reverse DNS zone go to the Control Panel, click on the "Add new zone" button and choose "Master Reverse DNS zone" and fill the reverse zone name of your IP network. In our case this is 0.168.192.in-addr.arpa

Now in our Reverse zone 0.168.192.in-addr.arpa we can add PTR records for each IP from the network. From the button "Add new record" we can add a new PTR record and here are few examples for the first two IP addresses:

For 192.168.0.1:

Type: PTR
Host: 1
Points to: hostname1.example.com

For 192.168.0.2:

Type: PTR
Host: 2
Points to: hostname2.example.com

How to add the PTR records - Step by Step video:

PTR record technical rules

• Static IPs Only: PTR records should be used with static IP addresses because static IPs don't change, ensuring accurate reverse DNS lookups.
• Match A/AAAA Records: Each PTR record must have a matching A record (for IPv4) or AAAA record (for IPv6). This links the IP address back to the correct domain name.
• One PTR per IP: Only one PTR record should exist for each IP address. Multiple PTR records for a single IP can cause confusion and errors.
• Proper Setup: The reverse DNS zone must be correctly set up. For IPv4 addresses, use the in-addr.arpa domain; for IPv6, use the ip6.arpa domain. The zone name should reflect the reversed IP address.
• Delegation: Make sure the reverse DNS zone is properly delegated to the DNS server where it is hosted. Update the NS records at your IP provider to point to the correct name servers.

How are your PTR records stored?

PTR records are stored under the IP address but reversed. 

In IPv4:

The PTR record for IPv4 addresses like 185.136.97.96 must be added in reverse mode, for example, 96.97.136.185.in-addr.arpa.

The in-addr.arpa at the end is added automatically, and it is mandatory as all IPv4 PTR records are stored within the .arpa top-level domain.

In IPv6:

IPv6 PTR records are also stored under the relevant IPv6 addresses, but instead of .arpa, they use the .ip6.arpa namespace.

How to check PTR records?

You can perform a Reverse lookup for the relevant IP address with the following Dig command:

$ dig -x 1.2.3.4

You can check the Reverse DNS with Nslookup as well. A sample is shown below:

$ nslookup 1.2.3.4

The outputs for both methods will show which hostname does this IP address belongs to.

If you prefer using an online tool to check your PTR record you can try the ClouDNS Free DNS tool.

How long does it take for a PTR record to propagate?

Propagation time depends mostly on the TTL value for the given record. But you must bear in mind, in order for your PTR record to resolve properly, the management of your IP address range must be delegated to ClouDNS. The procedure is further explained above in this article.

PTR record vs A record

PTR and A records are like the two sides of the same coin. The A records are responsible for the forward DNS resolution. They resolve domain names to the corresponding IP addresses. And PTR records work in exactly the opposite way. They resolve IP addresses to hostnames. Most importantly, you should remember that for every PTR record you create, you should have an A record (for IPv4 addresses) or an AAAA record (for IPv6 addresses).

Supported Reverse DNS zones

ClouDNS supports Reverse DNS zones both for IPv4 and IPv6 networks.

• IPv4 Networks: For IPv4, the Reverse DNS zones use the in-addr.arpa domain. Each IPv4 address in your network needs a PTR record in this zone to map it back to a hostname. For example, if your network is 192.168.0.0/24, the reverse zone format would be 0.168.192.in-addr.arpa, and each IP address would have a corresponding PTR record within this zone.
• IPv6 Networks: For IPv6, the Reverse DNS zones use the ip6.arpa domain. IPv6 addresses are longer and use hexadecimal notation, so the reverse zone setup is slightly different. For instance, an IPv6 address like 2001:0db8::1 would have a reverse DNS entry within the ip6.arpa domain.

Just write to our technical support, if you need any assistance with the configuration of the Reverse DNS and PTR records for your network and IPs.

How to start managing PTR records with ClouDNS?

1. Open 30 days free trial account from here 
2. Verify your e-mail address
3. Log into your control panel
4. Create new Reverse DNS from the [add new] button - read more here
5. Add the PTR records as it is described in this article

FAQ

Question: Why are PTR records important?

Answer: PTR records are crucial for reverse DNS lookups, helping identify the domain associated with a given IP address. This is often used for security, email verification, and network troubleshooting.

Question: Are PTR records required for all IP addresses?

Answer: While PTR records are not strictly mandatory, having them in place is highly recommended for proper email delivery and network security. Some email servers may treat emails from hosts without PTR records as suspicious, leading to potential delivery issues.

Question: Can I have multiple PTR records for one IP address?

Answer: In general, it is recommended to have only one PTR record per IP address. Having multiple PTR records for a single IP address can lead to unpredictable results and may cause issues with reverse DNS lookups.

Question: What happens if a PTR record does not match the corresponding A record?

Answer: A mismatch between PTR and A records can lead to issues with reverse DNS lookups, potentially causing authentication failures and service disruptions. It's important to keep PTR records synchronized with the associated A records to maintain a reliable and secure DNS infrastructure.