How does a full circle reverse DNS check work?

Summary

A full circle reverse DNS (FCrDNS) check is a method to verify the authenticity and legitimacy of a server, particularly for email. It operates by confirming that an IP address maps to a specific hostname via a PTR record, and that the hostname's A record then resolves back to the original IP address. This bidirectional validation ensures that the server is authorized to use the IP. The process involves reverse DNS lookups using PTR records which map IP addresses to domain names. Correct FCrDNS configuration contributes to email deliverability, establishing trust with email providers and reducing the likelihood of emails being marked as spam. While some systems might also check the HELO/EHLO value, it's separate from FCrDNS, and mismatches are common. Setting up reverse DNS often requires contacting the ISP or hosting provider, as they control the PTR records for your IP address.

Key findings

  • PTR Records: PTR records map IP addresses to hostnames in reverse DNS lookups.
  • A Records: The A record of the hostname must resolve back to the original IP address for a successful FCrDNS.
  • FCrDNS Process: The FCrDNS process involves confirming that an IP address resolves to a hostname (PTR record) and the hostname resolves back to the IP address (A record).
  • Authentication: Properly configured FCrDNS improves email deliverability and establishes trust.
  • ISP Requirement: Reverse DNS is often a requirement for whitelisting at ISPs.

Key considerations

  • ISP Involvement: Setting up reverse DNS typically requires contacting the ISP or hosting provider.
  • Multiple Records: An IP address can have multiple PTR records, and a hostname can have multiple A records, requiring careful matching.
  • HELO/EHLO: HELO/EHLO mismatches are common and separate from FCrDNS.
  • Enhanced Security: Ensure that the reverse DNS is set up correctly in order to increase security and prevent phishing.
  • Accuracy: Reverse DNS lookups depend on proper configurations to accurately translate IP addresses to hostnames.

What email marketers say
9Marketer opinions

A full circle reverse DNS (FCrDNS) check is a process used to verify the authenticity of a server, particularly for email. It involves ensuring that an IP address has a corresponding PTR record that maps it to a hostname, and that the A record of that hostname resolves back to the original IP address. This bidirectional validation confirms that the server is authorized to use the IP address, enhancing email deliverability and trust. Setting up reverse DNS typically requires contacting your ISP or hosting provider to configure the PTR record. Properly configured FCrDNS improves email deliverability by helping email providers verify the sending server's identity, reducing the chances of emails being marked as spam, and is essential for email authentication protocols.

Key opinions

  • Verification: FCrDNS helps verify that a server sending email is authorized to use the associated IP address.
  • Process: FCrDNS checks that an IP address maps to a hostname (PTR record) and that the hostname resolves back to the IP address (A record).
  • Purpose: Reverse DNS resolves IP addresses to domain names, verifying the identity of a server or client.
  • Improve Deliverability: Properly configured FCrDNS improves email deliverability.
  • Trust Building: Reverse DNS helps establish trust and authenticity for email servers.

Key considerations

  • Configuration: Setting up reverse DNS requires contacting your ISP or hosting provider, as they control the PTR records.
  • Impact on Deliverability: Having a properly configured FCrDNS can significantly improve email deliverability and reduce the likelihood of being marked as spam.
  • Email Authentication: Reverse DNS is essential for email authentication protocols like SPF and DKIM.
  • Reputation: Reverse DNS ensures an IP address is legitimate and not associated with spam, increasing confidence among mail servers.
  • Essential Records: Reverse DNS uses PTR records to map IP addresses to their corresponding hostnames.
Marketer view

Email marketer from Reddit explains that reverse DNS helps ensure that an IP address is legitimate and not associated with spam activity, increasing confidence among mail servers. This reputation boost can improve deliverability rates and reduce the chances of emails ending up in the spam folder.

October 2021 - Reddit
Marketer view

Email marketer from EasyDMARC explains that to set up reverse DNS, you'll need to contact your ISP or hosting provider, as they control the PTR records for your IP address. You'll need to provide them with the hostname you want associated with your IP.

May 2023 - EasyDMARC
Marketer view

Email marketer from Stack Overflow responds that the purpose of reverse DNS is to resolve IP addresses to domain names, which is the opposite of what DNS normally does. This is used in many applications to verify the identity of a server or client.

November 2022 - Stack Overflow
Marketer view

Email marketer from DNSimple shares that reverse DNS helps establish trust and authenticity for email servers by matching IP addresses to domain names. This process is essential for email authentication protocols like SPF and DKIM, which help prevent spoofing and phishing attempts.

September 2021 - DNSimple
Marketer view

Email marketer from MXToolbox shares that a reverse DNS lookup is a DNS query that asks for the domain name associated with an IP address. This is the opposite of a forward DNS lookup, which asks for the IP address associated with a domain name.

February 2025 - MXToolbox
Marketer view

Email marketer from Reddit explains that a full circle reverse DNS check (FCrDNS) involves checking both the forward and reverse DNS records. First, it checks if the IP address has a PTR record pointing to a hostname. Then, it checks if the A record of that hostname resolves back to the original IP address. This confirms the hostname is valid and authorized for the IP.

September 2022 - Reddit
Marketer view

Email marketer from DigitalOcean explains that reverse DNS records, or PTR records, map IP addresses to domain names. This helps in verifying the authenticity of mail servers by ensuring the IP address is associated with a legitimate domain, which is a key factor in achieving email deliverability.

June 2022 - DigitalOcean
Marketer view

Email marketer from ReturnPath shares that having a properly configured FCrDNS record can significantly improve email deliverability. It helps establish trust with email providers and reduces the likelihood of your emails being flagged as spam, therefore your emails arrive in the recipient's inbox.

October 2022 - ReturnPath
Marketer view

Email marketer from SparkPost shares that a reverse DNS record is important because it helps email providers verify that the server sending email from a specific IP address is authorized to do so. This can improve email deliverability by reducing the likelihood of emails being marked as spam.

March 2022 - SparkPost

What the experts say
6Expert opinions

A full circle reverse DNS (FCrDNS) check verifies the legitimacy of a server by ensuring its IP address resolves to a hostname (PTR record) and that the hostname resolves back to the original IP address (A record). This process helps confirm the server's identity and is crucial for email authentication and deliverability. While some systems also check the HELO, it's separate from the FCrDNS and mismatches are common. In practice, an IP address can have multiple PTR records and a hostname can have multiple A records, requiring careful matching. FCrDNS is often a requirement for whitelisting at ISPs, improving email deliverability.

Key opinions

  • FCrDNS Process: FCrDNS involves an IP address looking up a PTR record, then looking up the A record associated with that PTR to match back to the IP.
  • Reverse Lookup: Reverse DNS uses an IP address to look up a hostname, facilitated by PTR records.
  • Whitelisting Requirement: Reverse DNS is often required for whitelisting at ISPs.
  • FCrDNS Configuration: Correct FCrDNS configuration means a domain name resolves to an IP address, and the IP address resolves back to the same domain name.
  • Complexity: An IP address can have multiple PTR records and a hostname can have multiple A records.

Key considerations

  • HELO vs. PTR: While some systems check the HELO, it's separate from FCrDNS, and mismatches between HELO and PTR are common.
  • Matching Records: When multiple PTR or A records exist, ensure they are correctly matched.
  • Email Deliverability: Ensure FCrDNS is correctly configured, as it's vital for email authentication and deliverability.
  • Multiple Records Handling: Software may handle multiple PTR records incorrectly, so careful monitoring is necessary.
  • Reverse DNS Importance: Without matching reverse DNS, you may not be whitelisted by ISPs.
Expert view

Expert from Email Geeks explains that it's slightly more complex than that in practice, as there's not really any such thing as a reverse hostname. An IP address can have multiple PTR records, and a hostname can have multiple A and AAAA records. To check it you need to get all the PTR records and get all the A or AAAA records for each, and see if one of them matches. Some software gets that wrong, so if you have multiple PTR records some checkers might randomly decide it's good or it's bad each time they check.

July 2023 - Email Geeks
Expert view

Expert from Email Geeks shares that most of the time when they’re comparing the values they do the FCrDNS lookup IP -> hostname then hostname -> IP. Folks who suggest doing that may not even actually do the check comparing hostname and EHLO / HELO value unless there is something hinky about the EHLO/HELO value.

June 2022 - Email Geeks
Expert view

Expert from Spamresource explains that reverse DNS is a requirement for whitelisting at many ISPs. If you don’t have rDNS, or if it doesn’t match, you won’t get whitelisted, period.

September 2023 - Spamresource
Expert view

Expert from Email Geeks explains that full circle DNS starts with the IP then looks up the PTR then looks up the A record associated with that PTR. Some will look up the HELO as well, but that’s separate from the FCrDNS and a lot of places have some level of mismatch between HELO and PTR for operational reasons, so the fact that they don’t exactly match shouldn’t hurt anything and where I say “a lot of places” I mean both ESPs and ISPs.

October 2021 - Email Geeks
Expert view

Expert from Word to the Wise shares that “forward-confirmed reverse DNS” (FCrDNS) is the correct configuration where a domain name resolves to an IP address and the IP address resolves back to the same domain name. This is a basic test that mail servers often use to help verify the identity of a sending server and helps in email authentication.

April 2022 - Word to the Wise
Expert view

Expert from Spamresource explains that Reverse DNS (rDNS) is the opposite of forward DNS. Rather than using a hostname to look up an IP address, rDNS uses an IP address to look up a hostname. The system that makes this work is called a PTR record, which you generally get from whoever is providing the internet connection for the server.

October 2023 - Spamresource

What the documentation says
5Technical articles

A full circle reverse DNS check (FCrDNS) involves using reverse lookup zones to map an IP address to a hostname, the opposite of a forward lookup. This is achieved using PTR records, which are configured within the reverse DNS zone file. The query name is constructed by reversing the octets of the IP address and appending 'in-addr.arpa'. The purpose is to verify the sending server's identity, contributing to email deliverability and trust by confirming the server is authorized to send emails for the domain. Proper configuration of forward and reverse DNS records is essential for FCrDNS; the PTR record should point to the hostname, and the A record of the hostname should point back to the original IP address.

Key findings

  • PTR Records: PTR records map IP addresses to hostnames in reverse DNS lookups.
  • Reverse Lookup Zones: Reverse lookup zones enable discovering a hostname from an IP address.
  • Verification: rDNS records verify the sending server's identity and authorization.
  • FCrDNS Configuration: Proper FCrDNS requires forward and reverse DNS records to be correctly configured.
  • Zone File: Reverse DNS zone files contain PTR records for mapping IP addresses to hostnames.

Key considerations

  • Octet Reversal: IP addresses are reversed and 'in-addr.arpa' is appended when querying for PTR records.
  • Forward vs. Reverse: Reverse DNS is the opposite of forward DNS, where you use a name to find an IP address.
  • Trust & Deliverability: Properly configured rDNS improves email deliverability and trust.
  • Record Alignment: The PTR record should point to the hostname, and the A record should point back to the IP.
  • Accurate Translation: PTR records must be properly configured for accurate translation of IP addresses to hostnames.
Technical article

Documentation from Microsoft Learn explains that reverse lookup zones in DNS allow you to use the IP address of a host to discover its name. This is the reverse of a forward lookup, where you use the name to find the IP address.

February 2024 - Microsoft Learn
Technical article

Documentation from ietf.org explains that PTR records map an IP address to a hostname. The query name is constructed by reversing the octets of the IP address and appending the domain 'in-addr.arpa'.

October 2022 - ietf.org
Technical article

Documentation from IBM explains that within the reverse DNS zone file, PTR records are used to map IP addresses to their corresponding hostnames, enabling the reverse lookup process. In order for the reverse lookup to work, the PTR records must be properly configured to ensure accurate translation of IP addresses into hostnames.

September 2023 - IBM
Technical article

Documentation from Cloudflare explains that rDNS records help verify the sending server's identity. They contribute to email deliverability and trust by confirming the server sending the email is authorized to do so for the domain in question, and it maps the IP address to a domain name.

October 2022 - Cloudflare
Technical article

Documentation from DNS Records explains that to configure a full circle reverse DNS check (FCrDNS), you need to ensure that both your forward and reverse DNS records are properly configured. The PTR record for your IP address should point to your hostname, and the A record for your hostname should point back to the same IP address.

June 2022 - DNS Records