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
9 marketer 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.
23 Jul 2024 - 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.
18 Apr 2024 - EasyDMARC
What the experts say
6 expert 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.
3 Sep 2021 - 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.
18 Nov 2024 - Email Geeks
What the documentation says
5 technical 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.
8 Sep 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'.
18 Aug 2023 - ietf.org
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