TLS Client Authentication via DANE TLSA records

Introduction and Motivation The Transport Layer Security (TLS) protocol optionally supports the authentication of clients using X.509 certificates or raw public keys. TLS applications that perform DANE authentication of servers using TLSA records may also desire to authenticate clients using the same mechanism, especially if the client identity is in the form of or can be represented by a DNS domain name. Some design patterns from the Internet of Things (IoT) plan to make use of this form of authentication, where large networks of physical objects identified by DNS names may authenticate themselves using TLS to centralized device management and control platforms. Other potential applications include authenticating the client side of SMTP transport security. A more detailed treatment of application architectures for DANE client authentication is provided in "An Architecture for DNS-Bound Client and Sender Identities". In this document, the term TLS is used generically to describe both the TLS and DTLS (Datagram Transport Layer Security) protocols.

Associating Client Identities in TLSA Records Different applications may have quite different conventions for naming clients via domain names. This document thus does not proscribe a single format, but mentions a few that may have wide applicability.

Format 1: Service specific client identity In this format, the owner name of the client TLSA record has the following structure: The first label identifies the application service name. The remaining labels are composed of the client domain name. Encoding the application service name into the owner name allows the same client domain name to have different authentication credentials for different application services. There is no need to encode the transport label - the same name form is usable with both TLS and DTLS. The _service label could be a custom string for an application, but more commonly is expected to be a service name registered in the IANA Service Name Registry. The RDATA or data field portion of the TLSA record is formed exactly as specified in and , and carries the same meaning.

Format 2: DevId: IOT Device Identity The DevID form of the TLSA record has the following structure: It is loosely based on the proposed PKI Certificate Identifier Format for Devices, but is simpler in form. It makes no distinction between manufacturer issued and locally issued certificates, and does away with the "serial" and "type" labels. The "_device" label that precedes the organization domain name allows all the device identities to be delegated to a subzone or to another party.

Authentication Model The authentication model assumed in this document is the following: The client is assigned an identity corresponding to a DNS domain name. This domain name doesn't necessarily have any relation to its network layer addresses. Clients often have dynamic or unpredictable addresses, and may move around the network, so tying their identity to network addresses is not feasible or wise in the general case. The client has a private and public key pair. Where client certificates are being used, the client also has a certificate binding the name to its public key. The certificate or public key has a corresponding TLSA record published in the DNS, which allows it to be authenticated directly via the DNS (using the DANE-TA or DANE-EE certificate usage modes) or via a PKIX public CA system constraint if the client's certificate was issued by a public CA (using the PKIX-TA or PKIX-EE DANE usage modes).

Client Identifiers in X.509 certificates If the TLS DANE Client Identity extension (see ) is not being used, the client certificate MUST have have the client's DNS name specified in the Subject Alternative Name extension's dNSName type. If the TLS DANE Client Identity extension is in use, then with DANE-EE(3), the subject name need not be present in the certificate.

Signaling the Client's DANE Identity in TLS The client SHOULD explicitly signal that it has a DANE identity. The most important reason is that the server may want an explicit indication from the client that it has a DANE record, so as to avoid unnecessary DNS queries in-band with the TLS handshake. The DANE Client Identity TLS extension is used for this purpose. This extension can also be used to convey the actual DANE client identity (i.e. domain name) that the TLS server should attempt to authenticate. This is required when using TLS raw public key authentication, since there is no client certificate from which to extract the client's DNS identity. It is also helpful when the client certificate contains multiple identities, and only a specific one has a DANE record. An additional case where such client signaling is helpful, is one where DANE client authentication is optional, and there is a population of buggy client software that does not react gracefully to receipt of a Certificate Request message from the TLS server. This extension allows TLS servers to deal with this situation by selectively sending a Certificate Request message only to clients that have sent this extension.

Example TLSA records for clients The following examples are provided in the textual presentation format of the TLSA record.

Format 1: Service Specific Client Identity An example TLSA record for the client "device1.example.com." and the application "smtp-client". This record specifies the SHA-256 hash of the subject public key component of the end-entity certificate corresponding to the client. The certificate usage for this record is 3 (DANE-EE) and thus is validated in accordance with section 5.1 of RFC 7671.

Format 2: DevID An example TLSA record for the device named "sensor7" managed by the organization "example.com" This record specifies the SHA-512 hash of the subject public key component of an EE certificate corresponding to the client. The example below shows a wildcard TLSA record mapped to a TLSA record with a DANE-TA usage mode. This allows all client identifiers matching the wildcard to be authenticated by client certificates issued by an organization managed Certification Authority.

Changes to Client and Server behavior A TLS Client conforming to this specification MUST have a signed DNS TLSA record published corresponding to its DNS name and X.509 certificate or public key. The client presents this certificate or public key in the TLS handshake with the server. The client should not offer ciphersuites that are incompatible with its certificate or public key. If the client's certificate has a DANE record with a certificate usage other than DANE-EE, then the presented client certificate MUST have have the client's DNS name specified in the Subject Alternative Name extension's dNSName type. Additionally, when using raw public key authentication, the client MUST send the TLS DANE Client Identity extension in its Client Hello message. When using X.509 certificate authentication, it SHOULD send this extension. A TLS Server implementing this specification performs the following steps:

Request a client certificate in the TLS handshake (the "Client Certificate Request" message). This could be done unconditionally, or only when it receives the TLS DANE Client Identity extension from the client.
If the client has sent a non-empty DANE Client Identity extension, then extract the client's domain name from the extension. Otherwise, extract the client identity from the Subject Alternative Name extension's dNSName type.
Construct the DNS query name for the corresponding TLSA record. If the TLS DANE client identity extension was present, then this name should be used. Otherwise, identities from the client certificate are used.
Look up the TLSA record set in the DNS. The response MUST be cryptographically validated using DNSSEC. The server could perform the DNSSEC validation itself. It could also be configured to trust responses obtained via a validating resolver to which it has a secure connection.
Extract the RDATA of the TLSA records and match them to the presented client certificate according to the rules specified in the DANE TLS protocol . If successfully matched, the client is authenticated and the TLS session proceeds. If unsuccessful, the server MUST treat the client as unauthenticated (e.g. it could terminate the session, or proceed with the session giving the client access to resources as a generic unauthenticated user).
If there are multiple records in the TLSA record set, then the client is authenticated as long as at least one of the TLSA records matches, subject to RFC7671 digest agility, which SHOULD be implemented.

If the DANE Client Identity extension is not present, and the presented client certificate has multiple distinct reference identifier types (e.g. a dNSName, and an rfc822Name) then TLS servers configured to perform DANE authentication according to this specification should only examine and authenticate the dNSName. If the presented client certificate has multiple dNSName identities, then the client MUST use the TLS DANE client identity extension to unambiguously indicate its intended name to the server. Specific applications may be designed to require additional validation steps. For example, a server might want to verify the client's IP address is associated with the certificate in some manner, e.g. by confirming that a secure reverse DNS lookup of that address ties it back to the same domain name, or by requiring an iPAddress component to be included in the certificate. Such details are outside the scope of this document, and should be outlined in other documents specific to the applications that require this behavior. Servers may have their own whitelisting and authorization rules for which certificates they accept. For example a TLS server may be configured to only allow TLS sessions from clients with certificate identities within a specific domain or set of domains.

Raw Public Keys When using raw public keys in TLS, this specification requires the use of the TLS DANE Client Identity extension. The associated DANE TLSA records employ only certificate usage 3 (DANE-EE) and a selector value of 1 (SPKI), as described in .

Acknowledgements The authors thank the many members of the IETF DANCE working group for helpful comments and input.

IANA Considerations This document includes no request to IANA.

Security Considerations This document updates RFC 6698 by defining the use of the TLSA record for client TLS certificates. Placing client identities in the DNS may pose privacy issues for certain applications, depending on the nature of the clients, and the structure and content of the client names. Applications employing this protocol should carefully assess those potential issues. There are no other security considerations for this document beyond those described in RFC 6698 and RFC 7671 and in the specifications for TLS and DTLS [RFC8446], [RFC5246], [RFC6347].