Forward-confirmed reverse DNS (FCrDNS), also known as full-circle reverse DNS, double-reverse DNS, or iprev, is a networking parameter configuration in which a given IP address has both forward (name-to-address) and reverse (address-to-name) Domain Name System (DNS) entries that match each other. This is the standard configuration expected by the Internet standards supporting many DNS-reliant protocols. David Barr published an opinion i
RFC 1912
(Informational) recommending it as best practice for DNS administrators, but there are no formal requirements for it codified within the DNS standard itself. A FCrDNS verification can create a weak form of authentication that there is a valid relationship between the owner of a domain name and the owner of the network that has been given an IP address. While weak, this authentication is strong enough that it can be used for whitelisting purposes because spammers and phishers cannot usually by-pass this verification when they use zombie computers for email spoofing. That is, the reverse DNS might verify, but it will usually be part of another domain than the claimed domain name. Using an ISP's mail server as a relay may solve the reverse DNS problem, because the requirement is the forward and reverse lookup for the sending relay have to match, it does not have to be related to the from-field or sending domain of messages it relays. Other methods for establishing a relation between an IP address and a domain in email are the Sender Policy Framework (SPF) and the

MX record A mail exchanger record (MX record) specifies the mail server responsible for accepting email messages on behalf of a domain name. It is a resource record in the Domain Name System (DNS). It is possible to configure several MX records, typically ...

. ISPs that will not or cannot configure reverse DNS will generate problems for hosts on their networks, by virtue of being unable to support applications or protocols that require reverse DNS agree with the corresponding A (or AAAA) record. ISPs that cannot or will not provide reverse DNS ultimately will be limiting the ability of their client base to use Internet services they provide effectively and securely.

Applications

* Most e-mail

mail transfer agent The mail or post is a system for physically transporting postcards, letters, and parcels. A postal service can be private or public, though many governments place restrictions on private systems. Since the mid-19th century, national postal syst ...

s (server software) use a FCrDNS verification and if there is a valid domain name, put it into the "Received:" trace header field. * Some e-mail mail transfer agents will perform FCrDNS verification on the domain name given on the SMTP HELO and EHLO commands. This can violate RFC 2821 and so e-mail is usually not rejected by default. * The

Sender Policy Framework Sender Policy Framework (SPF) is an email authentication method designed to detect forging sender addresses during the delivery of the email. SPF alone, though, is limited to detecting a forged sender claim in the envelope of the email, which is ...

email anti-forgery system uses a FCrDNS check in its "ptr:" mechanism. However, the use of this "ptr:" mechanism is discouraged since the first standardization of SPF in 2006 (i
RFC 4408
. * Some

e-mail spam Email spam, also referred to as junk email, spam mail, or simply spam, is unsolicited messages sent in bulk by email ( spamming). The name comes from a Monty Python sketch in which the name of the canned pork product Spam is ubiquitous, unavoi ...

filters use FCrDNS checks as an authentication method for domain names or for

whitelist A whitelist, allowlist, or passlist is a mechanism which explicitly allows some identified entities to access a particular privilege, service, mobility, or recognition i.e. it is a list of things allowed when everything is denied by default. It is ...

ing purposes, according t
RFC 8601
for example. *

SpamCop SpamCop is an email spam reporting service, allowing recipients of unsolicited bulk or commercial email to report IP addresses found by SpamCop's analysis to be senders of the spam to the abuse reporting addresses of those IP addresses. SpamCop u ...

uses the FCrDNS check, which sometimes causes problems for SpamCop users who are also customers of

Internet service provider An Internet service provider (ISP) is an organization that provides services for accessing, using, or participating in the Internet. ISPs can be organized in various forms, such as commercial, community-owned, non-profit, or otherwise privat ...

s who do not provide properly matching DNS and rDNS records for mail servers

* Some

FTP The File Transfer Protocol (FTP) is a standard communication protocol used for the transfer of computer files from a server to a client on a computer network. FTP is built on a client–server model architecture using separate control and data ...

Telnet Telnet is an application protocol used on the Internet or local area network to provide a bidirectional interactive text-oriented communication facility using a virtual terminal connection. User data is interspersed in-band with Telnet control ...

and TCP Wrapper servers perform FCrDNS check

* Some IRC Servers perform FCrDNS checks to prevent abuse.

References

{{Reflist Domain Name System Email authentication Internet protocols Network protocols