| Internet-Draft | TLS DANE ClientID Extension | October 2020 |
| Huque & Dukhovni | Expires 11 April 2021 | [Page] |
This document specifies a TLS and DTLS extension to convey a DNS-Based Authentication of Named Entities (DANE) Client Identity to a TLS or DTLS server. This is useful for applications that perform TLS client authentication via DANE TLSA records.¶
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This document specifies a Transport Layer Security (TLS) extension [RFC6066] to convey a DANE [RFC6698] Client Identity to the TLS server. This is useful for applications that perform TLS client authentication via DANE TLSA records, as described in [DANECLIENT]. The extension could be empty to indicate to the server that the client has a DANE record and that the server can perform DANE authentication of the client with the identity extracted from the client certificate. Or the extension can contain the full client identity, in the form of the DNS domain name that is expected to have a DANE TLSA record published for it.¶
This extension supports both TLS [RFC5246] [RFC8446] and DTLS [RFC6347], and the term TLS in this document is used generically to describe both protocols.¶
When TLS clients use X.509 client certificates or raw public keys that are authenticated via DANE TLSA records, it is useful for them to convey their intent to be authenticated via DANE, or even to convey their complete DANE identity to the server. The TLS extension defined in this document is used to accomplish this.¶
In the case of X.509 client certificates, a TLS server can learn the client's identity by examining subject alternative names included in the certificate itself. However, without a mechanism such as the one defined in this extension, the TLS server cannot know apriori that the client has a published TLSA record, and thus may unnecessarily issue DNS queries for DANE TLSA records in-band with the TLS handshake even in cases where the client has no TLSA record associated with it. When multiple identities are present in the certificate, a client can use this extension to specify exactly which one the server should use. An additional situation in which this extension helps is where some TLS servers may need to selectively prompt for client certificate credentials only for clients that are equipped to provide certificates.¶
When TLS raw public keys [RFC7250] are being used to authenticate the client, the client uses this extension to explicitly indicate to the server what its domain name identity is (since there is no X.509 certificate from which the identity can be extracted).¶
Detailed protocol behavior of TLS clients and servers is described in [DANECLIENT].¶
The DANE Client Identity Extension type, "dane_clientid", will have a value assigned and registered in the IANA TLS Extensions registry. Its format is similar to the TLS Server Name Indication extension.¶
A TLS client implementing this specification SHOULD send an extension of type "dane_clientid" in the ClientHello handshake message to TLS servers it intends to perform DANE client authentication with.¶
If the client only needs to indicate that it has a DANE record and that the client's domain name identity can be obtained from its certificate, then the extension sent can be empty.¶
If the client additionally needs to send its domain name identity, then the "extension_data" field of the extension MUST contain a "ClientNameList" data structure defined in the following format:¶
struct {
NameType name_type;
select (name_type) {
case host_name: HostName;
} name;
} ClientName;
enum {
host_name(0), (255)
} NameType;
opaque HostName<1..2^16-1>;
struct {
ClientName client_name_list<1..2^16-1>
} ClientNameList;
¶
The opaque HostName field contains the domain name of the client in textual presentation format, as described in RFC 1035 [RFC1035]. The ClientNameList MUST NOT contain more than one name of the same name_type. Currently only one "host_name" type is defined.¶
A TLS server implementing this specification MAY send back an empty extension of type "dane_clientid" in its SeverHello handshake message to indicate that it understands the extension and intends to perform DANE client authentication. (Is there a compelling reason to do this?)¶
This extension is sent in the first flight of the TLS client's network data (Client Hello), which is in clear text.¶
To prevent unnecessary privacy leakage of the client's name in cleartext, a TLS client implementing this specification should be configured to only send this extension to TLS servers it intends to perform client authentication with.¶
Ideally, this extension should be used with the proposed TLS Encrypted Client Hello extension [ECH], which encrypts the entire Client Hello message. This will prevent leakage of the hostname, if included in the extension, in clear text.¶
This extension requires the registration of a new value in the TLS ExtensionsType registry.¶