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|Domain Name System|
DNS-based Authentication of Named Entities (DANE) is a protocol to allow X.509 certificates, commonly used for Transport Layer Security (TLS), to be bound to DNS names using Domain Name System Security Extensions (DNSSEC).
It is proposed in RFC 6698 as a way to authenticate TLS client and server entities without a certificate authority (CA). It is updated with operational and deployment guidance in RFC 7671. Application specific usage of DANE is defined in RFC 7672 for SMTP and RFC 7673 for using DANE with Service (SRV) records.
TLS/SSL encryption is currently based on certificates issued by certificate authorities (CAs). Within the last few years, a number of CA providers suffered serious security breaches, allowing the issuance of certificates for well-known domains to those who don't own those domains. Trusting a large number of CAs might be a problem because any breached CA could issue a certificate for any domain name. DANE enables the administrator of a domain name to certify the keys used in that domain's TLS clients or servers by storing them in the Domain Name System (DNS). DANE needs the DNS records to be signed with DNSSEC for its security model to work.
Additionally DANE allows a domain owner to specify which CA is allowed to issue certificates for a particular resource, which solves the problem of any CA being able to issue certificates for any domain.
DANE solves similar problems as:
However, unlike DANE, those technologies have wide support from browsers.
Until recently, there has been no widely implemented standard for encrypted email transfer. Sending an email is security agnostic; there is no URI scheme to designate secure SMTP. Consequently, most email that is delivered over TLS uses only opportunistic encryption. Since DNSSEC provides authenticated denial of existence (allows a resolver to validate that a certain domain name does not exist), DANE enables an incremental transition to verified, encrypted SMTP without any other external mechanisms, as described by RFC 7672. A DANE record indicates that the sender must use TLS.
The TLSA RR (Resource Record) for a service is located at a DNS name that specifies certificate constraints should be applied for the services at a certain TCP or UDP port. At least one of the TLSA RRs must provide a validation (path) for the certificate offered by the service at the specified address.
Not all protocols handle Common Name matching the same way. HTTP requires that the Common Name in the X.509 certificate provided by the service matches regardless of the TLSA asserting its validity. SMTP does not require the Common Name matches, if the certificate usage value is 3 (DANE-EE), but otherwise do require a Common Name match. It is important to verify if there are specific instructions for the protocol being used.
The TLSA RR for a service at
somehost.example.com will be published at
_<portnumber>._<protocol>.somehost.example.com. SMTP uses port 25/TCP, so the TLSA RR for the mail service at
somehost.example.com will be placed at
_25._tcp.somehost.example.com. Likewise for its web service it will be placed at
_443._tcp.somehost.example.com as HTTPS uses port 443/TCP.
The RR itself has 4 fields of data, describing which level of validation the domain owner provides.
_25._tcp.somehost.example.com. TLSA 3 1 1 BASE64==
The first field after the TLSA text in the DNS RR, specifies how to verify the certificate.
to a trust anchor
|RR points to an
end entity certificate,
i.e. a specific certificate
used in the TLS
|Require PKIX validation||0||1|
|PKIX path validation not required||2||3|
When connecting to the service and a certificate is received, the selector field specifies which parts of it should be checked.
The actual data to be matched given the settings of the other fields. This is a long "text string" of BASE64 data.
The HTTPS certificate for www.ietf.org specifies to check the SHA-256 hash of the public key of the certificate provided, ignoring any CA.
_443._tcp.www.ietf.org. TLSA 3 1 1 0C72AC70B745AC19998811B131D662C9AC69DBDBE7CB23E5B514B56664C5D3D6
Their mail service has the same exact certificate and TLSA.
ietf.org. MX 0 mail.ietf.org. _25._tcp.mail.ietf.org. TLSA 3 1 1 0C72AC70B745AC19998811B131D662C9AC69DBDBE7CB23E5B514B56664C5D3D6
Finally, this fake example, does the same as the others, but does the hash calculation over the entire certificate.
_25._tcp.mailserver.example.com. TLSA 3 0 1 AB9BEB9919729F3239AF08214C1EF6CCA52D2DBAE788BB5BE834C13911292ED9
... Last May, [Posteo] became the world's first email provider to adopt DNS-based Authentication of Named Entities (Dane) on its servers. ...