What is a PTR Record (Reverse DNS)? Why is it Important for Mail Servers?

Today, email communication, one of the cornerstones of the internet, has become more complex with the proliferation of spam and malware. The security and proper functioning of email servers are critical for users to have a reliable communication experience. This is where the PTR record (Reverse DNS) comes into play as a mechanism that verifies the identity of email servers and plays an important role in spam filtering processes.

What is a PTR Record? Basic Concepts

PTR record is an abbreviation for "Pointer Record" and is also known as Reverse DNS. DNS (Domain Name System) is a system that translates domain names (e.g., example.com) into IP addresses (e.g., 192.168.1.1) on the internet. The PTR record does the exact opposite of this process; that is, it translates an IP address to the domain name corresponding to that IP address. This is used to verify which domain name an IP address belongs to.

In a traditional DNS query, the IP address of a domain name is found. For example, a DNS query is made to find the IP address of "example.com" and an IP address (e.g. 192.168.1.1) is received as a response. In a PTR record query, an IP address (e.g. 192.168.1.1) is used and a domain name (e.g. example.com) is received as a response. This process is performed in special DNS zones such as "in-addr.arpa" or "ip6.arpa". "in-addr.arpa" is used for IPv4 addresses, while "ip6.arpa" is used for IPv6 addresses.

How Does a PTR Record Work?

The working principle of a PTR record is quite simple. When an email server sends an email, the receiving server first obtains the IP address of the sending server. It then performs a PTR record query for this IP address. If a PTR record is found and this record matches the domain name of the sending server, this helps to verify the identity of the sender. This makes it difficult for spammers to impersonate IP addresses.

For example, when an email server receives an email from the IP address 192.0.2.1, it performs a PTR record query for "1.2.0.192.in-addr.arpa". If the answer to this query is "mail.example.com", this indicates that the IP address 192.0.2.1 belongs to the domain name "mail.example.com". The receiving server can then use this information to verify the identity of the sender.

Importance of PTR Record for Mail Servers

The PTR record is critical for email servers in many ways. Here are the main reasons for this importance:

Spam Filtering: The PTR record helps email servers filter spam emails. Many spam filters mark emails from servers without a PTR record or with a misconfigured PTR record as spam. This prevents users' inboxes from being filled with spam emails.
Deliverability: Getting emails to recipients is vital for email marketing and general communication. The PTR record increases deliverability rates by reducing the likelihood of emails being marked as spam.
Authentication: The PTR record is a mechanism used to authenticate email servers. This makes it difficult for malicious people to impersonate email servers and provides protection against phishing attacks.
Reliability: The PTR record increases the reliability of an email server. Having email servers with properly configured PTR records creates a professional and reliable image.

Problems Faced by Mail Servers Without a PTR Record

Email servers without a PTR record or with a misconfigured PTR record may encounter various problems:

Emails Being Marked as Spam: The most common problem is that emails are marked as spam. Many email servers automatically direct emails from servers without a PTR record to the spam folder.
Emails Being Rejected: Some email servers completely reject emails from servers without a PTR record. This prevents emails from reaching recipients.
Loss of Reputation: The IP address of the email server being blacklisted can lead to loss of reputation. This can lead to future emails being marked as spam as well.
Communication Problems: Lack of a PTR record can cause communication problems with business partners, customers, and other stakeholders. Loss or delay of important emails can negatively affect business processes.

How to Create and Configure a PTR Record?

Creating and configuring a PTR record is usually done through your hosting provider or internet service provider (ISP). The PTR record is managed by the organization that owns your IP address. Here are the general steps:

Contact Your ISP or Hosting Provider: First, you need to contact your ISP or hosting provider to create the PTR record corresponding to your IP address. Tell them your IP address and the domain name you want to assign to this IP address (e.g., mail.example.com).
Check Your DNS Settings: After your ISP or hosting provider creates the PTR record, check your DNS settings. Make sure the PTR record is configured correctly.
Use Verification Tools: You can use various online tools to verify that the PTR record is configured correctly. For example, you can query the PTR record with the "dig" command or online DNS query tools.

Example: Querying PTR Record with Dig Command

You can query the PTR record using the "dig" command on Linux or macOS. For example, use the following command to query the PTR record for the IP address 192.0.2.1:

dig -x 192.0.2.1

In the output of this command, the PTR record information will be in the "ANSWER SECTION" section. For example:

; <<>> DiG 9.16.1 <<>> -x 192.0.2.1
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 53
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 512
;; QUESTION SECTION:
;1.2.0.192.in-addr.arpa.  IN PTR

;; ANSWER SECTION:
1.2.0.192.in-addr.arpa. 86400 IN PTR mail.example.com.

;; Query time: 0 msec
;; SERVER: 192.168.1.1#53(192.168.1.1)
;; WHEN: Tue Oct 27 10:00:00 2023
;; MSG SIZE  rcvd: 77

In this output, it is seen that the PTR record for "1.2.0.192.in-addr.arpa" is "mail.example.com".

Conclusion and Summary

The PTR record (Reverse DNS) is critical to increasing the security, deliverability, and reputation of email servers. The PTR record, which plays an important role in spam filtering processes, ensures that emails are delivered correctly and makes it difficult for malicious people to impersonate email servers. If you are running an email server, making sure your PTR record is configured correctly is vital for your email communication to run smoothly. You can create your PTR record by contacting your hosting provider or ISP and confirm that it is configured correctly using verification tools. Remember, a properly configured PTR record increases the reliability of your email server and improves the quality of your communication.