6. Service Provider and TLSA Publisher Synchronization Whenever possible, the TLSA Publisher and the Service Provider should be the same entity. Otherwise, they need to coordinate changes to ensure that TLSA records published by the TLSA Publisher don't fall out of sync with the server certificate used by the Service Provider. Such coordination is difficult, and service outages will result when coordination fails. Publishing the TLSA record in the Service Provider's zone avoids the complexity of bilateral coordination of server certificate configuration and TLSA record management. Even when the TLSA RRset has to be published in the Customer Domain's DNS zone (perhaps the client application does not "chase" CNAMEs to the TLSA base domain), it is possible to employ CNAME records to delegate the content of the TLSA RRset to a domain operated by the Service Provider. Only certificate usages DANE-EE(3) and DANE-TA(2) work well with TLSA CNAMEs across organizational boundaries. With PKIX-TA(0) or PKIX-EE(1), the Service Provider would need to obtain certificates in the name of the Customer Domain from a suitable public CA (securely impersonate the customer), or the customer would need to provision the relevant private keys and certificates at the Service Provider's systems. Certificate Usage DANE-EE(3): In this case, the Service Provider can publish a single TLSA RRset that matches the server certificate or public key digest. The same RRset works for all Customer Domains because name checks do not apply with DANE-EE(3) TLSA records (see Section 5.1). A Customer Domain can create a CNAME record pointing to the TLSA RRset published by the Service Provider. Certificate Usage DANE-TA(2): When the Service Provider operates a private CA, the Service Provider is free to issue a certificate bearing any customer's domain name. Without DANE, such a certificate would not pass trust verification, but with DANE, the customer's TLSA RRset that is aliased to the provider's TLSA RRset can delegate authority to the provider's CA for the corresponding service. The Service Provider can generate appropriate
certificates for each customer and use the SNI information provided by clients to select the right certificate chain to present to each client. Below are example DNS records (assumed "secure" and shown without the associated DNSSEC information, such as record signatures) that illustrate both of the above models in the case of an HTTPS service whose clients all support DANE TLS. These examples work even with clients that don't "chase" CNAMEs when constructing the TLSA base domain (see Section 7 below). ; The hosted web service is redirected via a CNAME alias. ; The associated TLSA RRset is also redirected via a CNAME alias. ; ; Certificate usage DANE-EE(3) makes it possible to deploy ; a single provider certificate for all Customer Domains. ; www1.example.com. IN CNAME w1.example.net. _443._tcp.www1.example.com. IN CNAME _443._tcp.w1.example.net. _443._tcp.w1.example.net. IN TLSA 3 1 1 ( 8A9A70596E869BED72C69D97A8895DFA D86F300A343FECEFF19E89C27C896BC9 ) ; ; A CA at the provider can also issue certificates for each Customer ; Domain and employ the DANE-TA(2) certificate usage to ; designate the provider's CA as a TA. ; www2.example.com. IN CNAME w2.example.net. _443._tcp.www2.example.com. IN CNAME _443._tcp.w2.example.net. _443._tcp.w2.example.net. IN TLSA 2 0 1 ( C164B2C3F36D068D42A6138E446152F5 68615F28C69BD96A73E354CAC88ED00C ) With protocols that support explicit transport redirection via DNS MX records, SRV records, or other similar records, the TLSA base domain is based on the redirected transport endpoint rather than the origin domain. With SMTP, for example, when an email service is hosted by a Service Provider, the Customer Domain's MX hostnames will point at the Service Provider's SMTP hosts. When the Customer Domain's DNS zone is signed, the MX hostnames can be securely used as the base
domains for TLSA records that are published and managed by the Service Provider. For example (without the required DNSSEC information, such as record signatures): ; Hosted SMTP service. ; example.com. IN MX 0 mx1.example.net. example.com. IN MX 0 mx2.example.net. _25._tcp.mx1.example.net. IN TLSA 3 1 1 ( 8A9A70596E869BED72C69D97A8895DFA D86F300A343FECEFF19E89C27C896BC9 ) _25._tcp.mx2.example.net. IN TLSA 3 1 1 ( C164B2C3F36D068D42A6138E446152F5 68615F28C69BD96A73E354CAC88ED00C ) If redirection to the Service Provider's domain (via MX records, SRV records, or any similar mechanism) is not possible and aliasing of the TLSA record is not an option, then more complex coordination between the Customer Domain and Service Provider will be required. Either the Customer Domain periodically provides private keys and a corresponding certificate chain to the provider (after making appropriate changes in its TLSA records), or the Service Provider periodically generates the keys and certificates and needs to wait for matching TLSA records to be published by its Customer Domains before deploying newly generated keys and certificate chains. Section 7 below describes an approach that employs CNAME "chasing" to avoid the difficulties of coordinating key management across organizational boundaries. For further information about combining DANE and SRV, please see [RFC7673]. 7. TLSA Base Domain and CNAMEs When the application protocol does not support service location indirection via MX, SRV, or similar DNS records, the service may be redirected via a CNAME. A CNAME is a more blunt instrument for this purpose because, unlike an MX or SRV record, it remaps the entire origin domain to the target domain for all protocols. The complexity of coordinating key management is largely eliminated when DANE TLSA records are found in the Service Provider's domain, as discussed in Section 6. Therefore, DANE TLS clients connecting to a server whose domain name is a CNAME alias SHOULD follow the CNAME "hop by hop" to its ultimate target host (noting at each step whether or not the CNAME is DNSSEC validated). If at each stage of CNAME expansion the DNSSEC validation status is "secure", the final target name SHOULD be the preferred base domain for TLSA lookups.
Implementations failing to find a TLSA record using a base name of the final target of a CNAME expansion SHOULD issue a TLSA query using the original destination name. That is, the preferred TLSA base domain SHOULD be derived from the fully expanded name and, failing that, SHOULD be the initial domain name. When the TLSA base domain is the result of "secure" CNAME expansion, the resulting domain name MUST be used as the HostName in the client's SNI extension and MUST be the primary reference identifier for peer certificate matching with certificate usages other than DANE-EE(3). Protocol-specific TLSA specifications may provide additional guidance or restrictions when following CNAME expansions. Though CNAMEs are illegal on the right-hand side of most indirection records, such as MX and SRV records, they are supported by some implementations. For example, if the MX or SRV host is a CNAME alias, some implementations may "chase" the CNAME. If they do, they SHOULD use the target hostname as the preferred TLSA base domain as described above (and, if the TLSA records are found there, also use the CNAME-expanded domain in SNI and certificate name checks). 8. TLSA Publisher Requirements This section updates [RFC6698] by specifying that the TLSA Publisher MUST ensure that each combination of certificate usage, selector, and matching type in the server's TLSA RRset includes at least one record that matches the server's current certificate chain. TLSA records that match recently retired or yet-to-be-deployed certificate chains will be present during key rollover. Such past or future records MUST NOT at any time be the only records published for any given combination of usage, selector, and matching type. The TLSA record update process described below ensures that this requirement is met. While a server is to be considered authenticated when its certificate chain is matched by any of the published TLSA records, not all clients support all combinations of TLSA record parameters. Some clients may not support some digest algorithms; others may either not support or exclusively support the PKIX certificate usages. Some clients may prefer to negotiate [RFC7250] raw public keys, which are only compatible with TLSA records whose certificate usage is DANE-EE(3) with selector SPKI(1). The only other TLSA record type that is potentially compatible with raw public keys is "DANE-EE(3) Cert(0) Full(0)", but support for raw public keys with that TLSA record type is not expected to be broadly implemented.
A consequence of the above uncertainty as to which TLSA parameters are supported by any given client is that servers need to ensure that each and every parameter combination that appears in the TLSA RRset is, on its own, sufficient to match the server's current certificate chain. In particular, when deploying new keys or new parameter combinations, some care is required to not generate parameter combinations that only match past or future certificate chains (or raw public keys). The rest of this section explains how to update the TLSA RRset in a manner that ensures that the above requirement is met. 8.1. Key Rollover with Fixed TLSA Parameters The simplest case is key rollover while retaining the same set of published parameter combinations. In this case, TLSA records matching the existing server certificate chain (or raw public keys) are first augmented with corresponding records matching the future keys, at least two Times to Live (TTLs) or longer before the new chain is deployed. This allows the obsolete RRset to age out of client caches before the new chain is used in TLS handshakes. Once sufficient time has elapsed and all clients performing DNS lookups are retrieving the updated TLSA records, the server administrator may deploy the new certificate chain, verify that it works, and then remove any obsolete records matching the chain that is no longer active: ; Initial TLSA RRset. ; _443._tcp.www.example.org. IN TLSA 3 1 1 01d09d19c2139a46... ; Transitional TLSA RRset published at least two TTLs before ; the actual key change. ; _443._tcp.www.example.org. IN TLSA 3 1 1 01d09d19c2139a46... _443._tcp.www.example.org. IN TLSA 3 1 1 7aa7a5359173d05b... ; Final TLSA RRset after the key change. ; _443._tcp.www.example.org. IN TLSA 3 1 1 7aa7a5359173d05b...
The next case to consider is adding or switching to a new combination of TLSA parameters. In this case, publish the new parameter combinations for the server's existing certificate chain first, and only then deploy new keys if desired: ; Initial TLSA RRset. ; _443._tcp.www.example.org. IN TLSA 1 1 1 01d09d19c2139a46... ; New TLSA RRset, same key re-published as DANE-EE(3). ; _443._tcp.www.example.org. IN TLSA 3 1 1 01d09d19c2139a46... 8.2. Switching to DANE-TA(2) from DANE-EE(3) This section explains how to migrate to a new certificate chain and TLSA record with usage DANE-TA(2) from a self-signed server certificate and a "DANE-EE(3) SPKI(1) SHA2-256(1)" TLSA record. This example assumes that a new private key is generated in conjunction with transitioning to a new certificate issued by the desired TA. The original "3 1 1" TLSA record supports [RFC7250] raw public keys, and clients may choose to negotiate their use. The use of raw public keys rules out the possibility of certificate chain verification. Therefore, the transitional TLSA record for the planned DANE-TA(2) certificate chain is a "3 1 1" record that works even when raw public keys are used. The TLSA RRset is updated to use DANE-TA(2) only after the new chain is deployed and the "3 1 1" record matching the original key is dropped. This process follows the requirement that each combination of parameters present in the RRset is always sufficient to validate the server. It avoids publishing a transitional TLSA RRset in which "3 1 1" matches only the current key and "2 0 1" matches only the future certificate chain, because these might not work reliably during the initial deployment of the new keys.
; Initial TLSA RRset. ; _443._tcp.www.example.org. IN TLSA 3 1 1 01d09d19c2139a46... ; Transitional TLSA RRset, published at least two TTLs before the ; actual key change. The new keys are issued by a DANE-TA(2) CA ; but are initially specified via a DANE-EE(3) association. ; _443._tcp.www.example.org. IN TLSA 3 1 1 01d09d19c2139a46... _443._tcp.www.example.org. IN TLSA 3 1 1 7aa7a5359173d05b... ; The final TLSA RRset after the key change. Now that the old ; self-signed EE key is out of the picture, publish the issuing ; TA of the new chain. ; _443._tcp.www.example.org. IN TLSA 2 0 1 c57bce38455d9e3d... 8.3. Switching to New TLSA Parameters When employing a new digest algorithm in the TLSA RRset, for compatibility with digest algorithm agility as specified in Section 9 below, administrators SHOULD publish the new digest algorithm with each combination of certificate usage and selector for each associated key or chain used with any other digest algorithm. When removing an algorithm, remove it entirely. Each digest algorithm employed SHOULD match the same set of chains (or raw public keys). ; Initial TLSA RRset with "DANE-EE(3) SHA2-256(1)" associations ; for two keys. ; _443._tcp.www.example.org. IN TLSA 3 1 1 01d09d19c2139a46... _443._tcp.www.example.org. IN TLSA 3 1 1 7aa7a5359173d05b... ; New TLSA RRset, also with SHA2-512(2) associations ; for each key. ; _443._tcp.www.example.org. IN TLSA 3 1 1 01d09d19c2139a46... _443._tcp.www.example.org. IN TLSA 3 1 2 d9947c35089310bc... _443._tcp.www.example.org. IN TLSA 3 1 1 7aa7a5359173d05b... _443._tcp.www.example.org. IN TLSA 3 1 2 89a7486a4b6ae714...
8.4. TLSA Publisher Requirements: Summary In summary, server operators updating TLSA records should make one change at a time. The individual safe changes are as follows: o Pre-publish new certificate associations that employ the same TLSA parameters (usage, selector, and matching type) as existing TLSA records, but match certificate chains that will be deployed in the near future. o Wait for stale TLSA RRsets to expire from DNS caches before configuring servers to use the new certificate chain. o Remove TLSA records matching any certificate chains that are no longer deployed. o Publish TLSA RRsets in which all parameter combinations (certificate usage, selector, and matching type) present in the RRset match the same set of current and planned certificate chains. The above steps are intended to ensure that at all times, and for each combination of usage, selector, and matching type, at least one TLSA record corresponds to the server's current certificate chain. Each combination of certificate usage, selector, and matching type in a server's TLSA RRset SHOULD NOT at any time (including unexpired RRsets in client caches) match only some combination of future or past certificate chains. As a result, no matter what combinations of usage, selector, and matching type may be supported by a given client, they will be sufficient to authenticate the server. 9. Digest Algorithm Agility While [RFC6698] specifies multiple digest algorithms, it does not specify a protocol by which the client and TLSA record publisher can agree on the strongest shared algorithm. Such a protocol would allow the client and server to avoid exposure to deprecated weaker algorithms that are published for compatibility with less capable clients but that SHOULD be avoided when possible. Such a protocol is specified below. This section defines a protocol for avoiding deprecated digest algorithms when these are published in a peer's TLSA RRset alongside stronger digest algorithms. Note that this protocol never avoids RRs with a DANE matching type of Full(0), as these do not employ a digest algorithm that might someday be weakened by cryptanalysis.
Client implementations SHOULD implement a default order of digest algorithms by strength. This order SHOULD be configurable by the administrator or user of the client software. If possible, a configurable mapping from numeric DANE TLSA matching types to underlying digest algorithms provided by the cryptographic library SHOULD be implemented to allow new matching types to be used with software that predates their introduction. Configurable ordering of digest algorithms SHOULD be extensible to any new digest algorithms. To make digest algorithm agility possible, all published DANE TLSA RRsets MUST conform to the requirements of Section 8. Clients SHOULD use digest algorithm agility when processing the peer's DANE TLSA records. Algorithm agility is to be applied after first discarding any unusable or malformed records (unsupported digest algorithm, or incorrect digest length). For each usage and selector, the client SHOULD process only any usable records with a matching type of Full(0) and the usable records whose digest algorithm is considered by the client to be the strongest among usable records with the given usage and selector. Example: a client implements digest algorithm agility and prefers SHA2-512(2) over SHA2-256(1), while the server publishes an RRset that employs both digest algorithms as well as a Full(0) record. _25._tcp.mail.example.com. IN TLSA 3 1 1 ( 3FE246A848798236DD2AB78D39F0651D 6B6E7CA8E2984012EB0A2E1AC8A87B72 ) _25._tcp.mail.example.com. IN TLSA 3 1 2 ( D4F5AF015B46C5057B841C7E7BAB759C BF029526D29520C5BE6A32C67475439E 54AB3A945D80C743347C9BD4DADC9D8D 57FAB78EAA835362F3CA07CCC19A3214 ) _25._tcp.mail.example.com. IN TLSA 3 1 0 ( 3059301306072A8648CE3D020106082A 8648CE3D0301070342000471CB1F504F 9E4B33971376C005445DACD33CD79A28 81C3DED1981F18E7AAA76609DD0E4EF2 8265C82703030AD60C5DBA6FB8A9397A C0FCF06D424C885D484887 ) In this case, the client SHOULD accept a server public key that matches either the "3 1 0" record or the "3 1 2" record, but it SHOULD NOT accept keys that match only the weaker "3 1 1" record.
10. General DANE Guidelines These guidelines provide guidance for using or designing protocols for DANE. 10.1. DANE DNS Record Size Guidelines Selecting a combination of TLSA parameters to use requires careful thought. One important consideration to take into account is the size of the resulting TLSA record after its parameters are selected. 10.1.1. UDP and TCP Considerations Deployments SHOULD avoid TLSA record sizes that cause UDP fragmentation. Although DNS over TCP would provide the ability to more easily transfer larger DNS records between clients and servers, it is not universally deployed and is still prohibited by some firewalls. Clients that request DNS records via UDP typically only use TCP upon receipt of a truncated response in the DNS response message sent over UDP. Setting the Truncation (TC) bit (Section 4.1.1 of [RFC1035]) alone will be insufficient if the response containing the TC bit is itself fragmented. 10.1.2. Packet Size Considerations for TLSA Parameters Server operators SHOULD NOT publish TLSA records using both a TLSA selector of Cert(0) and a TLSA matching type of Full(0), as even a single certificate is generally too large to be reliably delivered via DNS over UDP. Furthermore, two TLSA records containing full certificates will need to be published simultaneously during a certificate rollover, as discussed in Section 8.1. While TLSA records using a TLSA selector of SPKI(1) and a TLSA matching type of Full(0) (which publish the bare public keys, i.e., without the overhead of encapsulating the keys in an X.509 certificate) are generally more compact, these are also best avoided when significantly larger than their digests. Rather, servers SHOULD publish digest-based TLSA matching types in their TLSA records, in which case the complete corresponding certificate MUST be transmitted to the client in-band during the TLS handshake. The certificate (or raw public key) can be easily verified using the digest value. In summary, the use of a TLSA matching type of Full(0) is NOT RECOMMENDED, and a digest-based matching type, such as SHA2-256(1), SHOULD be used instead.
10.2. Certificate Name Check Conventions Certificates presented by a TLS server will generally contain a subjectAltName (SAN) extension or a Common Name (CN) element within the subject Distinguished Name (DN). The TLS server's DNS domain name is normally published within these elements, ideally within the SAN extension. (The use of the CN field for this purpose is deprecated.) When a server hosts multiple domains at the same transport endpoint, the server's ability to respond with the right certificate chain is predicated on correct SNI information from the client. DANE clients MUST send the SNI extension with a HostName value of the base domain of the TLSA RRset. With the exception of TLSA certificate usage DANE-EE(3), where name checks are not applicable (see Section 5.1), DANE clients MUST verify that the client has reached the correct server by checking that the server name is listed in the server certificate's SAN or CN (when still supported). The primary server name used for this comparison MUST be the TLSA base domain; however, additional acceptable names may be specified by protocol-specific DANE standards. For example, with SMTP, both the destination domain name and the MX hostname are acceptable names to be found in the server certificate (see [RFC7672]). It is the responsibility of the service operator, in coordination with the TLSA Publisher, to ensure that at least one of the TLSA records published for the service will match the server's certificate chain (either the default chain or the certificate that was selected based on the SNI information provided by the client). Given the DNSSEC-validated DNS records below: example.com. IN MX 0 mail.example.com. mail.example.com. IN A 192.0.2.1 _25._tcp.mail.example.com. IN TLSA 2 0 1 ( E8B54E0B4BAA815B06D3462D65FBC7C0 CF556ECCF9F5303EBFBB77D022F834C0 ) The TLSA base domain is "mail.example.com" and is required to be the HostName in the client's SNI extension. The server certificate chain is required to be signed by a TA with the above certificate SHA2-256 digest. Finally, one of the DNS names in the server certificate is required to be either "mail.example.com" or "example.com" (this additional name is a concession to compatibility with prior practice; see [RFC7672] for details).
[RFC6125] specifies the semantics of wildcards in server certificates for various application protocols. DANE does not change how wildcards are treated by any given application. 10.3. Design Considerations for Protocols Using DANE When a TLS client goes to the trouble of authenticating a certificate chain presented by a TLS server, it will typically not continue to use that server in the event of authentication failure, or else authentication serves no purpose. Some clients may, at times, operate in an "audit" mode, where authentication failure is reported to the user or in logs as a potential problem, but the connection proceeds despite the failure. Nevertheless, servers publishing TLSA records MUST be configured to allow correctly configured clients to successfully authenticate their TLS certificate chains. A service with DNSSEC-validated TLSA records implicitly promises TLS support. When all the TLSA records for a service are found "unusable" due to unsupported parameter combinations or malformed certificate association data, DANE clients cannot authenticate the service certificate chain. When authenticated TLS is mandatory, the client MUST NOT connect to the associated server. If, on the other hand, the use of TLS and DANE is "opportunistic" [RFC7435], then when all TLSA records are unusable, the client SHOULD connect to the server via an unauthenticated TLS connection, and if TLS encryption cannot be established, the client MUST NOT connect to the server. Standards for opportunistic DANE TLS specific to a particular application protocol may modify the above requirements. The key consideration is whether or not mandating the use of (unauthenticated) TLS even with unusable TLSA records is asking for more security than one can realistically expect. If expecting TLS support when unusable TLSA records are published is realistic for the application in question, then the application MUST avoid cleartext. If not realistic, then mandating TLS would cause clients (even in the absence of active attacks) to run into problems with various peers that do not interoperate "securely enough". That would create strong incentives to just disable Opportunistic Security and stick with cleartext.
11. Note on DNSSEC Security Clearly, the security of the DANE TLSA PKI rests on the security of the underlying DNSSEC infrastructure. While this document is not a guide to DNSSEC security, a few comments may be helpful to TLSA implementers. With the existing public CA Web PKI, name constraints are rarely used, and a public root CA can issue certificates for any domain of its choice. With DNSSEC, under the Registry/Registrar/Registrant model, the situation is different: only the registrar of record can update a domain's DS record [RFC4034] in the registry parent zone (in some cases, however, the registry is the sole registrar). With many Generic Top-Level Domains (gTLDs) for which multiple registrars compete to provide domains in a single registry, it is important to make sure that rogue registrars cannot easily initiate an unauthorized domain transfer and thus take over DNSSEC for the domain. DNS operators are advised to set a registrar lock on their domains to offer some protection against this possibility. When the registrar is also the DNS operator for the domain, one needs to consider whether or not the registrar will allow orderly migration of the domain to another registrar or DNS operator in a way that will maintain DNSSEC integrity. TLSA Publishers are advised to seek out a DNS hosting registrar that makes it possible to transfer domains to another hosting provider without disabling DNSSEC. DNSSEC-signed RRsets cannot be securely revoked before they expire. Operators need to plan accordingly and not generate signatures of excessively long duration. For domains publishing high-value keys, a signature lifetime (length of the "signature validity period" as described in Section 8.1 of [RFC4033]) of a few days is reasonable, and the zone can be re-signed daily. For domains with less critical data, a reasonable signature lifetime is a couple of weeks to a month, and the zone can be re-signed weekly. Short signature lifetimes require tighter synchronization of primary and secondary nameservers, to make sure that secondary servers never serve records with expired signatures. They also limit the maximum time for which a primary server that signs the zone can be down. Therefore, short signature lifetimes are more appropriate for sites with dedicated operations staff, who can restore service quickly in case of a problem. Monitoring is important. If a DNS zone is not re-signed in a timely manner, a major outage is likely, as the entire domain and all its sub-domains become "bogus".
12. Summary of Updates to RFC 6698 o Section 3 updates [RFC6698] to specify a requirement for clients to support at least TLS 1.0 and to support SNI. o Section 4 explains that application support for all four certificate usages is NOT RECOMMENDED. The recommended design is to support just DANE-EE(3) and DANE-TA(2). o Section 5.1 updates [RFC6698] to specify that peer identity matching and validity period enforcement are based solely on the TLSA RRset properties. It also specifies DANE authentication of raw public keys [RFC7250] via TLSA records with certificate usage DANE-EE(3) and selector SPKI(1). o Section 5.2 updates [RFC6698] to require that servers publishing digest TLSA records with a usage of DANE-TA(2) MUST include the TA certificate in their TLS server certificate message. This extends to the case of "2 1 0" TLSA records that publish a full public key. o Section 5.4 observes that with usage PKIX-TA(0), clients may need to process extended trust chains beyond the first trusted issuer when that issuer is not self-signed. o Section 7 recommends that DANE application protocols specify that, when possible, securely CNAME-expanded names be used to derive the TLSA base domain. o Section 8 specifies a strategy for managing TLSA records that interoperates with DANE clients regardless of what subset of the possible TLSA record types (combinations of TLSA parameters) is supported by the client. o Section 9 specifies a digest algorithm agility protocol. o Section 10.1 recommends against the use of Full(0) TLSA records, as digest records are generally much more compact. 13. Operational Considerations The DNS TTL of TLSA records needs to be chosen with care. When an unplanned change in the server's certificate chain and TLSA RRset is required, such as when keys are compromised or lost, clients that cache stale TLSA records will fail to validate the certificate chain of the updated server. Publish TLSA RRsets with TTLs that are short enough to limit unplanned service disruption to an acceptable duration.
The signature lifetime (length of the signature validity period) for TLSA records SHOULD NOT be too long. Signed DNSSEC records can be replayed by an MITM attacker, provided the signatures have not yet expired. Shorter signature validity periods allow for faster invalidation of compromised keys. Zone refresh and expiration times for secondary nameservers often imply a lower bound on the signature validity period (Section 11). See Section 4.4.1 of [RFC6781]. 14. Security Considerations Application protocols that cannot use the existing public CA Web PKI may choose to not implement certain TLSA record types defined in [RFC6698]. If such records are published despite not being supported by the application protocol, they are treated as "unusable". When TLS is opportunistic, the client MAY proceed to use the server with mandatory unauthenticated TLS. This is stronger than opportunistic TLS without DANE, since in that case the client may also proceed with a cleartext connection. When TLS is not opportunistic, the client MUST NOT connect to the server. Thus, when TLSA records are used with opportunistic protocols where PKIX-TA(0) and PKIX-EE(1) do not apply, the recommended protocol design is for servers to not publish such TLSA records, and for opportunistic TLS clients to use them to only enforce the use of (albeit unauthenticated) TLS but otherwise treat them as unusable. Of course, when PKIX-TA(0) and PKIX-EE(1) are supported by the application protocol, clients MUST implement these certificate usages as described in [RFC6698] and this document. 15. References 15.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <http://www.rfc-editor.org/info/rfc2119>. [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "DNS Security Introduction and Requirements", RFC 4033, DOI 10.17487/RFC4033, March 2005, <http://www.rfc-editor.org/info/rfc4033>. [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "Resource Records for the DNS Security Extensions", RFC 4034, DOI 10.17487/RFC4034, March 2005, <http://www.rfc-editor.org/info/rfc4034>.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "Protocol Modifications for the DNS Security Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005, <http://www.rfc-editor.org/info/rfc4035>. [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, August 2008, <http://www.rfc-editor.org/info/rfc5246>. [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, <http://www.rfc-editor.org/info/rfc5280>. [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) Extensions: Extension Definitions", RFC 6066, DOI 10.17487/RFC6066, January 2011, <http://www.rfc-editor.org/info/rfc6066>. [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March 2011, <http://www.rfc-editor.org/info/rfc6125>. [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, January 2012, <http://www.rfc-editor.org/info/rfc6347>. [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication of Named Entities (DANE) Transport Layer Security (TLS) Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August 2012, <http://www.rfc-editor.org/info/rfc6698>. [RFC7218] Gudmundsson, O., "Adding Acronyms to Simplify Conversations about DNS-Based Authentication of Named Entities (DANE)", RFC 7218, DOI 10.17487/RFC7218, April 2014, <http://www.rfc-editor.org/info/rfc7218>. [RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J., Weiler, S., and T. Kivinen, "Using Raw Public Keys in Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250, June 2014, <http://www.rfc-editor.org/info/rfc7250>.
15.2. Informative References [RFC1035] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, November 1987, <http://www.rfc-editor.org/info/rfc1035>. [RFC6781] Kolkman, O., Mekking, W., and R. Gieben, "DNSSEC Operational Practices, Version 2", RFC 6781, DOI 10.17487/RFC6781, December 2012, <http://www.rfc-editor.org/info/rfc6781>. [RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013, <http://www.rfc-editor.org/info/rfc6962>. [RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection Most of the Time", RFC 7435, DOI 10.17487/RFC7435, December 2014, <http://www.rfc-editor.org/info/rfc7435>. [RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via Opportunistic DNS-Based Authentication of Named Entities (DANE) Transport Layer Security (TLS)", RFC 7672, DOI 10.17487/RFC7672, October 2015, <http://www.rfc-editor.org/info/rfc7672>. [RFC7673] Finch, T., Miller, M., and P. Saint-Andre, "Using DNS-Based Authentication of Named Entities (DANE) TLSA Records with SRV Records", RFC 7673, DOI 10.17487/RFC7673, October 2015, <http://www.rfc-editor.org/info/rfc7673>.
Acknowledgements The authors would like to thank Phil Pennock for his comments and advice on this document. Acknowledgements from Viktor: Thanks to Tony Finch, who finally prodded me into participating in DANE working group discussions. Thanks to Paul Hoffman, who motivated me to produce this document and provided feedback on early draft versions of it. Thanks also to Samuel Dukhovni for editorial assistance. Authors' Addresses Viktor Dukhovni Two Sigma Email: firstname.lastname@example.org Wes Hardaker Parsons P.O. Box 382 Davis, CA 95617 United States Email: email@example.com