| Internet-Draft | SVCB and HTTPS RRs for DNS | October 2021 |
| Schwartz, et al. | Expires 15 April 2022 | [Page] |
This document specifies the "SVCB" and "HTTPS" DNS resource record (RR) types to facilitate the lookup of information needed to make connections to network services, such as for HTTP origins. SVCB records allow a service to be provided from multiple alternative endpoints, each with associated parameters (such as transport protocol configuration and keys for encrypting the TLS ClientHello). They also enable aliasing of apex domains, which is not possible with CNAME. The HTTPS RR is a variation of SVCB for use with HTTP [HTTP]. By providing more information to the client before it attempts to establish a connection, these records offer potential benefits to both performance and privacy.¶
TO BE REMOVED: This document is being collaborated on in Github at: https://github.com/MikeBishop/dns-alt-svc. The most recent working version of the document, open issues, etc. should all be available there. The authors (gratefully) accept pull requests.¶
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Copyright (c) 2021 IETF Trust and the persons identified as the document authors. All rights reserved.¶
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The SVCB ("Service Binding") and HTTPS RRs provide clients with complete instructions for access to a service. This information enables improved performance and privacy by avoiding transient connections to a sub-optimal default server, negotiating a preferred protocol, and providing relevant public keys.¶
For example, HTTP clients currently resolve only A and/or AAAA records for the origin hostname, learning only its IP addresses. If an HTTP client learns more about the origin before connecting, it may be able to upgrade "http" URLs to "https", enable HTTP/3 or Encrypted ClientHello [ECH], or switch to an operationally preferable endpoint. It is highly desirable to minimize the number of round-trips and lookups required to learn this additional information.¶
The SVCB and HTTPS RRs also help when the operator of a service wishes to delegate operational control to one or more other domains, e.g. delegating the origin "https://example.com" to a service operator endpoint at "svc.example.net". While this case can sometimes be handled by a CNAME, that does not cover all use-cases. CNAME is also inadequate when the service operator needs to provide a bound collection of consistent configuration parameters through the DNS (such as network location, protocol, and keying information).¶
This document first describes the SVCB RR as a general-purpose resource record that can be applied directly and efficiently to a wide range of services (Section 2). It also describes the rules for defining other SVCB-compatible RR types (Section 6), starting with the HTTPS RR type (Section 9), which provides improved efficiency and convenience with HTTP by avoiding the need for an Attrleaf label [Attrleaf] (Section 9.1).¶
The SVCB RR has two modes: 1) "AliasMode", which simply delegates operational control for a resource; 2) "ServiceMode", which binds together configuration information for a service endpoint. ServiceMode provides additional key=value parameters within each RDATA set.¶
The goal of the SVCB RR is to allow clients to resolve a single additional DNS RR in a way that:¶
Additional goals specific to HTTPS RRs and the HTTP use-cases include:¶
This subsection briefly describes the SVCB RR in a non-normative manner. (As mentioned above, this all applies equally to the HTTPS RR which shares the same encoding, format, and high-level semantics.)¶
The SVCB RR has two modes: AliasMode, which aliases a name to another name, and ServiceMode, which provides connection information bound to a service endpoint domain. Placing both forms in a single RR type allows clients to fetch the relevant information with a single query.¶
The SVCB RR has two required fields and one optional. The fields are:¶
Cooperating DNS recursive resolvers will perform subsequent record resolution (for SVCB, A, and AAAA records) and return them in the Additional Section of the response. Clients either use responses included in the additional section returned by the recursive resolver or perform necessary SVCB, A, and AAAA record resolutions. DNS authoritative servers can attach in-bailiwick SVCB, A, AAAA, and CNAME records in the Additional Section to responses for a SVCB query.¶
In ServiceMode, the SvcParams of the SVCB RR provide an extensible data model for describing alternative endpoints that are authoritative for the origin, along with parameters associated with each of these alternative endpoints.¶
For HTTP use-cases, the HTTPS RR enables many of the benefits of Alt-Svc [AltSvc] without waiting for a full HTTP connection initiation (multiple roundtrips) before learning of the preferred alternative, and without necessarily revealing the user's intended destination to all entities along the network path.¶
This document also defines a parameter for Encrypted ClientHello [ECH] keys. See Section 10.¶
Our terminology is based on the common case where the SVCB record is used to
access a resource identified by a URI whose authority field contains a DNS
hostname as the host.¶
authority and
scheme of the URI, capable of providing access to the resource. For "https"
URIs, the "service" corresponds to an "origin" [RFC6454].¶
host portion of the authority.¶
Additional DNS terminology intends to be consistent with [DNSTerm].¶
SVCB is a contraction of "service binding". The SVCB RR, HTTPS RR, and future RR types that share SVCB's formats and registry are collectively known as SVCB-compatible RR types. The contraction "SVCB" is also used to refer to this system as a whole.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
The SVCB DNS resource record (RR) type (RR type 64) is used to locate alternative endpoints for a service.¶
The algorithm for resolving SVCB records and associated address records is specified in Section 3.¶
Other SVCB-compatible resource record types can also be defined as-needed (see Section 6). In particular, the HTTPS RR (RR type 65) provides special handling for the case of "https" origins as described in Section 9.¶
SVCB RRs are extensible by a list of SvcParams, which are pairs consisting of a SvcParamKey and a SvcParamValue. Each SvcParamKey has a presentation name and a registered number. Values are in a format specific to the SvcParamKey. Their definition should specify both their presentation format and wire encoding (e.g., domain names, binary data, or numeric values). The initial SvcParamKeys and formats are defined in Section 7.¶
The presentation format of the record is:¶
Name TTL IN SVCB SvcPriority TargetName SvcParams¶
The SVCB record is defined specifically within the Internet ("IN") Class ([RFC1035]).¶
SvcPriority is a number in the range 0-65535,
TargetName is a <domain-name> ([RFC1035], Section 5.1),
and the SvcParams are a whitespace-separated list, with each SvcParam
consisting of a SvcParamKey=SvcParamValue pair or a standalone SvcParamKey.
SvcParamKeys are subject to IANA control (Section 15.3).¶
Each SvcParamKey SHALL appear at most once in the SvcParams. In presentation format, SvcParamKeys are lower-case alphanumeric strings. Key names should contain 1-63 characters from the ranges "a"-"z", "0"-"9", and "-". In ABNF [RFC5234],¶
alpha-lc = %x61-7A ; a-z SvcParamKey = 1*63(alpha-lc / DIGIT / "-") SvcParam = SvcParamKey ["=" SvcParamValue] SvcParamValue = char-string value = *OCTET¶
The SvcParamValue is parsed using the
character-string decoding algorithm (Appendix A), producing a value.
The value is then validated and converted into wire-format in a manner
specific to each key.¶
When the "=" is omitted, the value is interpreted as empty.¶
Arbitrary keys can be represented using the unknown-key presentation format
"keyNNNNN" where NNNNN is the numeric
value of the key type without leading zeros.
A SvcParam in this form SHALL be parsed as specified above, and
the decoded value SHALL be used as its wire format encoding.¶
For some SvcParamKeys, the value corresponds to a list or set of
items. Presentation formats for such keys SHOULD use a comma-separated list
(Appendix A.1).¶
SvcParams in presentation format MAY appear in any order, but keys MUST NOT be repeated.¶
The RDATA for the SVCB RR consists of:¶
When the list of SvcParams is non-empty, it contains a series of SvcParamKey=SvcParamValue pairs, represented as:¶
SvcParamKeys SHALL appear in increasing numeric order.¶
Clients MUST consider an RR malformed if:¶
Note that the second condition implies that there are no duplicate SvcParamKeys.¶
If any RRs are malformed, the client MUST reject the entire RRSet and fall back to non-SVCB connection establishment.¶
When querying the SVCB RR, a service is translated into a QNAME by prepending the service name with a label indicating the scheme, prefixed with an underscore, resulting in a domain name like "_examplescheme.api.example.com.". This follows the Attrleaf naming pattern [Attrleaf], so the scheme MUST be registered appropriately with IANA (see Section 12).¶
Protocol mapping documents MAY specify additional underscore-prefixed labels to be prepended. For schemes that specify a port (Section 3.2.3 of [URI]), one reasonable possibility is to prepend the indicated port number if a non-default port number is specified. We term this behavior "Port Prefix Naming", and use it in the examples throughout this document.¶
See Section 9.1 for the HTTPS RR behavior.¶
When a prior CNAME or SVCB record has aliased to a SVCB record, each RR shall be returned under its own owner name.¶
Note that none of these forms alter the origin or authority for validation purposes. For example, TLS clients MUST continue to validate TLS certificates for the original service name.¶
As an example, the owner of example.com could publish this record:¶
_8443._foo.api.example.com. 7200 IN SVCB 0 svc4.example.net.¶
to indicate that "foo://api.example.com:8443" is aliased to "svc4.example.net". The owner of example.net, in turn, could publish this record:¶
svc4.example.net. 7200 IN SVCB 3 svc4.example.net. (
alpn="bar" port="8004" ech="..." )
¶
to indicate that these services are served on port number 8004, which supports the protocol "bar" and its associated transport in addition to the default transport protocol for "foo://".¶
(Parentheses are used to ignore a line break in DNS zone file presentation format ([RFC1035], Section 5.1).)¶
When SvcPriority is 0 the SVCB record is in AliasMode (Section 2.4.2). Otherwise, it is in ServiceMode (Section 2.4.3).¶
Within a SVCB RRSet, all RRs SHOULD have the same Mode. If an RRSet contains a record in AliasMode, the recipient MUST ignore any ServiceMode records in the set.¶
RRSets are explicitly unordered collections, so the SvcPriority field is used to impose an ordering on SVCB RRs. SVCB RRs with a smaller SvcPriority value SHOULD be given preference over RRs with a larger SvcPriority value.¶
When receiving an RRSet containing multiple SVCB records with the same SvcPriority value, clients SHOULD apply a random shuffle within a priority level to the records before using them, to ensure uniform load-balancing.¶
In AliasMode, the SVCB record aliases a service to a TargetName. SVCB RRSets SHOULD only have a single resource record in AliasMode. If multiple are present, clients or recursive resolvers SHOULD pick one at random.¶
The primary purpose of AliasMode is to allow aliasing at the zone apex, where CNAME is not allowed. In AliasMode, the TargetName will be the name of a domain that resolves to SVCB, AAAA, and/or A records. (See Section 6 for aliasing of SVCB-compatible RR types.) The TargetName SHOULD NOT be equal to the owner name, as this would result in a loop.¶
In AliasMode, records SHOULD NOT include any SvcParams, and recipients MUST ignore any SvcParams that are present.¶
For example, the operator of foo://example.com:8080 could point requests to a service operating at foosvc.example.net by publishing:¶
_8080._foo.example.com. 3600 IN SVCB 0 foosvc.example.net.¶
Using AliasMode maintains a separation of concerns: the owner of foosvc.example.net can add or remove ServiceMode SVCB records without requiring a corresponding change to example.com. Note that if foosvc.example.net promises to always publish a SVCB record, this AliasMode record can be replaced by a CNAME, which would likely improve performance.¶
AliasMode is especially useful for SVCB-compatible RR types that do not require an underscore prefix, such as the HTTPS RR type. For example, the operator of https://example.com could point requests to a server at svc.example.net by publishing this record at the zone apex:¶
example.com. 3600 IN HTTPS 0 svc.example.net.¶
Note that the SVCB record's owner name MAY be the canonical name of a CNAME record, and the TargetName MAY be the owner of a CNAME record. Clients and recursive resolvers MUST follow CNAMEs as normal.¶
To avoid unbounded alias chains, clients and recursive resolvers MUST impose a limit on the total number of SVCB aliases they will follow for each resolution request. This limit MUST NOT be zero, i.e. implementations MUST be able to follow at least one AliasMode record. The exact value of this limit is left to implementations.¶
For compatibility and performance, zone owners SHOULD NOT configure their zones to require following multiple AliasMode records.¶
As legacy clients will not know to use this record, service operators will likely need to retain fallback AAAA and A records alongside this SVCB record, although in a common case the target of the SVCB record might offer better performance, and therefore would be preferable for clients implementing this specification to use.¶
AliasMode records only apply to queries for the specific RR type. For example, a SVCB record cannot alias to an HTTPS record, nor vice-versa.¶
In ServiceMode, the TargetName and SvcParams within each resource record associate an alternative endpoint for the service with its connection parameters.¶
Each protocol scheme that uses SVCB MUST define a protocol mapping that explains how SvcParams are applied for connections of that scheme. Unless specified otherwise by the protocol mapping, clients MUST ignore any SvcParam that they do not recognize.¶
Some SvcParams impose requirements on other SvcParams in the RR. A ServiceMode RR is called "self-consistent" if its SvcParams all comply with each others' requirements. Zone-file implementations SHOULD enforce self-consistency. Clients MUST reject any RR whose recognized SvcParams are not self-consistent, and MAY reject the entire RRSet.¶
If TargetName has the value "." (represented in the wire format as a zero-length label), special rules apply.¶
For AliasMode SVCB RRs, a TargetName of "." indicates that the service is not available or does not exist. This indication is advisory: clients encountering this indication MAY ignore it and attempt to connect without the use of SVCB.¶
For ServiceMode SVCB RRs, if TargetName has the value ".", then the owner name of this record MUST be used as the effective TargetName.¶
For example, in the following example "svc2.example.net" is the effective TargetName:¶
example.com. 7200 IN HTTPS 0 svc.example.net. svc.example.net. 7200 IN CNAME svc2.example.net. svc2.example.net. 7200 IN HTTPS 1 . port=8002 ech="..." svc2.example.net. 300 IN A 192.0.2.2 svc2.example.net. 300 IN AAAA 2001:db8::2¶
"SVCB resolution" is the process of enumerating the priority-ordered endpoints for a service, as performed by the client. SVCB resolution is implemented as follows:¶
This procedure does not rely on any recursive or authoritative DNS server to comply with this specification or have any awareness of SVCB.¶
A client is called "SVCB-optional" if it can connect without the use of ServiceMode records, and "SVCB-reliant" otherwise. Clients for pre-existing protocols (e.g. HTTP) SHALL implement SVCB-optional behavior (except as noted in Section 3.1 and Section 10.1).¶
SVCB-optional clients SHOULD issue in parallel any other DNS queries that might be needed for connection establishment if the SVCB record is absent, in order to minimize delay in that case and enable the optimizations discussed in Section 5.¶
Once SVCB resolution has concluded, whether successful or not, SVCB-optional clients SHALL append to the priority list an endpoint consisting of the final value of $QNAME, the authority endpoint's port number, and no SvcParams. (This endpoint will be attempted before falling back to non-SVCB connection modes. This ensures that SVCB-optional clients will make use of an AliasMode record whose TargetName has A and/or AAAA records but no SVCB records.)¶
The client proceeds with connection establishment using the resolved list of endpoints. Clients SHOULD try higher-priority alternatives first, with fallback to lower-priority alternatives. Clients resolve AAAA and/or A records for the selected TargetName, and MAY choose between them using an approach such as Happy Eyeballs [HappyEyeballsV2].¶
If the client is SVCB-optional, and connecting using this list of endpoints has failed, the client SHOULD attempt non-SVCB connection modes.¶
Some important optimizations are discussed in Section 5 to avoid additional latency in comparison to ordinary AAAA/A lookups.¶
If DNS responses are cryptographically protected (e.g. using DNSSEC or TLS [DoT][DoH]), and SVCB resolution fails due to an authentication error, SERVFAIL response, transport error, or timeout, the client SHOULD abandon the connection attempt even if the client is SVCB-optional. Otherwise, an active attacker could mount a downgrade attack by denying the user access to the SvcParams.¶
A SERVFAIL error can occur if the domain is DNSSEC-signed, the recursive resolver is DNSSEC-validating, and the attacker is between the recursive resolver and the authoritative DNS server. A transport error or timeout can occur if an active attacker between the client and the recursive resolver is selectively dropping SVCB queries or responses, based on their size or other observable patterns.¶
If the client enforces DNSSEC validation on A/AAAA responses, it SHOULD apply the same validation policy to SVCB.¶
If the client is unable to complete SVCB resolution due to its chain length limit, the client SHOULD fall back to the authority endpoint, as if the origin's SVCB record did not exist.¶
Clients using a domain-oriented transport proxy like HTTP CONNECT ([RFC7231], Section 4.3.6) or SOCKS5 ([RFC1928]) have the option to use named destinations, in which case the client does not perform any A or AAAA queries for destination domains. If the client is using named destinations with a proxy that does not provide SVCB query capability (e.g. through an affiliated DNS resolver), the client would have to perform SVCB resolution separately, likely disclosing the destinations to additional parties. Clients that support such proxies SHOULD arrange for a separate SVCB resolution procedure with appropriate privacy properties, or disable SVCB resolution entirely if SVCB-optional.¶
If the client does use SVCB and named destinations, the client SHOULD follow the standard SVCB resolution process, selecting the smallest-SvcPriority option that is compatible with the client and the proxy. When connecting using a SVCB record, clients MUST provide the final TargetName and port to the proxy, which will perform any required A and AAAA lookups.¶
This arrangement has several benefits:¶
Compared to disabling SVCB:¶
Compared to providing the proxy with an IP address:¶
Whether or not the recursive resolver is aware of SVCB, the normal response construction process (i.e. unknown RR type resolution under [RFC3597]) generates the Answer section of the response. Recursive resolvers that are aware of SVCB SHOULD help the client to execute the procedure in Section 3 with minimum overall latency by incorporating additional useful information into the Additional section of the response as follows:¶
If any of the resolved SVCB records are in AliasMode, choose one of them at random, and resolve SVCB, A, and AAAA records for its TargetName.¶
In this procedure, "resolve" means the resolver's ordinary recursive resolution procedure, as if processing a query for that RRSet. This includes following any aliases that the resolver would ordinarily follow (e.g. CNAME, DNAME [DNAME]). Errors or anomalies in obtaining additional records MAY cause this process to terminate, but MUST NOT themselves cause the resolver to send a failure response.¶
See Section 2.4.2 for additional safeguards for recursive resolvers to implement to mitigate loops.¶
See Section 5.2 for possible optimizations of this procedure.¶
Recursive resolvers MUST be able to convey SVCB records with unrecognized SvcParamKeys, and MAY treat the entire SvcParams portion of the record as opaque, even if the contents are invalid. Alternatively, recursive resolvers MAY report an error such as SERVFAIL to avoid returning a SvcParamValue that is invalid according to the SvcParam's specification. For complex value types whose interpretation might differ between implementations or have additional future allowed values added (e.g. URIs or "alpn"), resolvers SHOULD limit validation to specified constraints.¶
When responding to a query that includes the DNSSEC OK bit ([RFC3225]), DNSSEC-capable recursive and authoritative DNS servers MUST accompany each RRSet in the Additional section with the same DNSSEC-related records that they would send when providing that RRSet as an Answer (e.g. RRSIG, NSEC, NSEC3).¶
According to Section 5.4.1 of [RFC2181], "Unauthenticated RRs received and cached from ... the additional data section ... should not be cached in such a way that they would ever be returned as answers to a received query. They may be returned as additional information where appropriate.". Recursive resolvers therefore MAY cache records from the Additional section for use in populating Additional section responses, and MAY cache them for general use if they are authenticated by DNSSEC.¶
The EDNS Client Subnet option (ECS, [RFC7871]) allows recursive resolvers to request IP addresses that are suitable for a particular client IP range. SVCB records may contain IP addresses (in ipv*hint SvcParams), or direct users to a subnet-specific TargetName, so recursive resolvers SHOULD include the same ECS option in SVCB queries as in A/AAAA queries.¶
According to Section 7.3.1 of [RFC7871], "Any records from [the Additional section] MUST NOT be tied to a network". Accordingly, when processing a response whose QTYPE is SVCB-compatible, resolvers SHOULD treat any records in the Additional section as having SOURCE PREFIX-LENGTH zero and SCOPE PREFIX-LENGTH as specified in the ECS option. Authoritative servers MUST omit such records if they are not suitable for use by any stub resolvers that set SOURCE PREFIX-LENGTH to zero. This will cause the resolver to perform a followup query that can receive properly tailored ECS. (This is similar to the usage of CNAME with ECS discussed in [RFC7871], Section 7.2.1.)¶
Authoritative servers that omit Additional records can avoid the added latency of a followup query by following the advice in Section 11.2.¶
For optimal performance (i.e. minimum connection setup time), clients SHOULD implement a client-side DNS cache. Responses in the Additional section of a SVCB response SHOULD be placed in cache before performing any followup queries. With this behavior, and conforming DNS servers, using SVCB does not add network latency to connection setup.¶
To improve performance when using a non-conforming recursive resolver, clients SHOULD issue speculative A and/or AAAA queries in parallel with each SVCB query, based on a predicted value of TargetName (see Section 11.2).¶
After a ServiceMode RRSet is received, clients MAY try more than one option in parallel, and MAY prefetch A and AAAA records for multiple TargetNames.¶
If an address response arrives before the corresponding SVCB response, the client MAY initiate a connection as if the SVCB query returned NODATA, but MUST NOT transmit any information that could be altered by the SVCB response until it arrives. For example, a TLS ClientHello can be altered by the "ech" value of a SVCB response (Section 7.3). Clients implementing this optimization SHOULD wait for 50 milliseconds before starting optimistic pre-connection, as per the guidance in [HappyEyeballsV2].¶
A SVCB record is consistent with a connection if the client would attempt an equivalent connection when making use of that record. If a SVCB record is consistent with an active or in-progress connection C, the client MAY prefer that record and use C as its connection. For example, suppose the client receives this SVCB RRSet for a protocol that uses TLS over TCP:¶
_1234._bar.example.com. 300 IN SVCB 1 svc1.example.net. (
ech="111..." ipv6hint=2001:db8::1 port=1234 )
SVCB 2 svc2.example.net. (
ech="222..." ipv6hint=2001:db8::2 port=1234 )
¶
If the client has an in-progress TCP connection to [2001:db8::2]:1234,
it MAY proceed with TLS on that connection using ech="222...", even
though the other record in the RRSet has higher priority.¶
If none of the SVCB records are consistent with any active or in-progress connection, clients proceed with connection establishment as described in Section 3.¶
When following the procedure in Section 4.2, recursive resolvers MAY terminate the procedure early and produce a reply that omits some of the associated RRSets. This is REQUIRED when the chain length limit is reached (Section 4.2 step 1), but might also be appropriate when the maximum response size is reached, or when responding before fully chasing dependencies would improve performance. When omitting certain RRSets, recursive resolvers SHOULD prioritize information for smaller-SvcPriority records.¶
As discussed in Section 3, clients MUST be able to fetch additional information that is required to use a SVCB record, if it is not included in the initial response. As a performance optimization, if some of the SVCB records in the response can be used without requiring additional DNS queries, the client MAY prefer those records, regardless of their priorities.¶
An RR type is called "SVCB-compatible" if it permits an implementation that is identical to SVCB in its:¶
This allows authoritative and recursive DNS servers to apply identical processing to all SVCB-compatible RR types.¶
All other behaviors described as applying to the SVCB RR also apply to all SVCB-compatible RR types unless explicitly stated otherwise. When following an AliasMode record (Section 2.4.2) of RR type $T , the followup query to the TargetName MUST also be for type $T.¶
This document defines one SVCB-compatible RR type (other than SVCB itself): the HTTPS RR type (Section 9), which avoids Attrleaf label prefixes [Attrleaf] in order to improve compatibility with wildcards and CNAMEs, which are widely used with HTTP.¶
Standards authors should consider carefully whether to use SVCB or define a new SVCB-compatible RR type, as this choice cannot easily be reversed after deployment.¶
A few initial SvcParamKeys are defined here. These keys are useful for the "https" scheme, and most are applicable to other schemes as well.¶
Each new protocol mapping document MUST specify which keys are applicable and safe to use. Protocol mappings MAY alter the interpretation of SvcParamKeys but MUST NOT alter their presentation or wire formats.¶
The "alpn" and "no-default-alpn" SvcParamKeys together indicate the set of Application Layer Protocol Negotiation (ALPN) protocol identifiers [ALPN] and associated transport protocols supported by this service endpoint (the "SVCB ALPN set").¶
As with Alt-Svc [AltSvc], each ALPN protocol identifier is used to identify the application protocol and associated suite of protocols supported by the endpoint (the "protocol suite"). The presence of an ALPN protocol identifier in the SVCB ALPN set indicates that this service endpoint, described by TargetName and the other parameters (e.g. "port") offers service with the protocol suite associated with this ALPN identifier.¶
Clients filter the set of ALPN identifiers to match the protocol suites they support, and this informs the underlying transport protocol used (such as QUIC-over-UDP or TLS-over-TCP). ALPN protocol identifiers that do not uniquely identify a protocol suite (e.g. an Identification Sequence that can be used with both TLS and DTLS) are not compatible with this SvcParamKey and MUST NOT be included in the SVCB ALPN set.¶
ALPNs are identified by their registered "Identification Sequence"
(alpn-id), which is a sequence of 1-255 octets.¶
alpn-id = 1*255OCTET¶
For "alpn", the presentation value SHALL be
a comma-separated list (Appendix A.1)
of one or more alpn-ids. Zone file implementations MAY disallow the
"," and "\" characters instead of implementing the value-list escaping
procedure, relying on the opaque key format (e.g. key1=\002h2) in the
event that these characters are needed.¶
The wire format value for "alpn" consists of at least one
alpn-id prefixed by its length as a single octet, and these length-value
pairs are concatenated to form the SvcParamValue. These pairs MUST exactly
fill the SvcParamValue; otherwise, the SvcParamValue is malformed.¶
For "no-default-alpn", the presentation and wire format values MUST be empty. When "no-default-alpn" is specified in an RR, "alpn" must also be specified in order for the RR to be "self-consistent" (Section 2.4.3).¶
Each scheme that uses this SvcParamKey defines a "default set" of ALPNs
that are supported by nearly all clients and servers, which MAY
be empty. To determine the SVCB ALPN set, the client starts with the list of
alpn-ids from the "alpn" SvcParamKey, and adds the default set unless the
"no-default-alpn" SvcParamKey is present.¶
To establish a connection to the endpoint, clients MUST¶
For example, if the SVCB ALPN set is ["http/1.1", "h3"], and the client supports HTTP/1.1, HTTP/2, and HTTP/3, the client could attempt to connect using TLS over TCP with a ProtocolNameList of ["http/1.1", "h2"], and could also attempt a connection using QUIC, with a ProtocolNameList of ["h3"].¶
Once the client has constructed a ClientHello, protocol negotiation in that handshake proceeds as specified in [ALPN], without regard to the SVCB ALPN set.¶
Clients MAY implement a fallback procedure, using a less-preferred transport if more-preferred transports fail to connect. This fallback behavior is vulnerable to manipulation by a network attacker who blocks the more-preferred transports, but it may be necessary for compatibility with existing networks.¶
With this procedure in place, an attacker who can modify DNS and network traffic can prevent a successful transport connection, but cannot otherwise interfere with ALPN protocol selection. This procedure also ensures that each ProtocolNameList includes at least one protocol from the SVCB ALPN set.¶
Clients SHOULD NOT attempt connection to a service endpoint whose SVCB ALPN set does not contain any supported protocols. To ensure consistency of behavior, clients MAY reject the entire SVCB RRSet and fall back to basic connection establishment if all of the RRs indicate "no-default-alpn", even if connection could have succeeded using a non-default alpn.¶
For compatibility with clients that require default transports, zone operators SHOULD ensure that at least one RR in each RRSet supports the default transports.¶
The "port" SvcParamKey defines the TCP or UDP port that should be used to reach this alternative endpoint. If this key is not present, clients SHALL use the authority endpoint's port number.¶
The presentation value of the SvcParamValue is a single decimal integer
between 0 and 65535 in ASCII. Any other value (e.g. an empty value)
is a syntax error. To enable simpler parsing, this SvcParam MUST NOT contain
escape sequences.¶
The wire format of the SvcParamValue is the corresponding 2 octet numeric value in network byte order.¶
If a port-restricting firewall is in place between some client and the service endpoint, changing the port number might cause that client to lose access to the service, so operators should exercise caution when using this SvcParamKey to specify a non-default port.¶
The SvcParamKey to enable Encrypted ClientHello (ECH) is "ech". Its value is defined in Section 10. It is applicable to most TLS-based protocols.¶
When publishing a record containing an "ech" parameter, the publisher MUST ensure that all IP addresses of TargetName correspond to servers that have access to the corresponding private key or are authoritative for the public name. (See Section 7.2.2 of [ECH] for more details about the public name.) This yields an anonymity set of cardinality equal to the number of ECH-enabled server domains supported by a given client-facing server. Thus, even with an encrypted ClientHello, an attacker who can enumerate the set of ECH-enabled domains supported by a client-facing server can guess the correct SNI with probability at least 1/K, where K is the size of this ECH-enabled server anonymity set. This probability may be increased via traffic analysis or other mechanisms.¶
The "ipv4hint" and "ipv6hint" keys convey IP addresses that clients MAY use to reach the service. If A and AAAA records for TargetName are locally available, the client SHOULD ignore these hints. Otherwise, clients SHOULD perform A and/or AAAA queries for TargetName as in Section 3, and clients SHOULD use the IP address in those responses for future connections. Clients MAY opt to terminate any connections using the addresses in hints and instead switch to the addresses in response to the TargetName query. Failure to use A and/or AAAA response addresses could negatively impact load balancing or other geo-aware features and thereby degrade client performance.¶
The presentation value SHALL be a comma-separated list (Appendix A.1)
of one or more IP addresses of the appropriate
family in standard textual format [RFC5952]. To enable simpler parsing,
this SvcParamValue MUST NOT contain escape sequences.¶
The wire format for each parameter is a sequence of IP addresses in network byte order. Like an A or AAAA RRSet, the list of addresses represents an unordered collection, and clients SHOULD pick addresses to use in a random order. An empty list of addresses is invalid.¶
When selecting between IPv4 and IPv6 addresses to use, clients may use an approach such as Happy Eyeballs [HappyEyeballsV2]. When only "ipv4hint" is present, IPv6-only clients may synthesize IPv6 addresses as specified in [RFC7050] or ignore the "ipv4hint" key and wait for AAAA resolution (Section 3). Recursive resolvers MUST NOT perform DNS64 ([RFC6147]) on parameters within a SVCB record. For best performance, server operators SHOULD include an "ipv6hint" parameter whenever they include an "ipv4hint" parameter.¶
These parameters are intended to minimize additional connection latency when a recursive resolver is not compliant with the requirements in Section 4, and SHOULD NOT be included if most clients are using compliant recursive resolvers. When TargetName is the origin hostname or the owner name (which can be written as "."), server operators SHOULD NOT include these hints, because they are unlikely to convey any performance benefit.¶
In a ServiceMode RR, a SvcParamKey is considered "mandatory" if the RR will not function correctly for clients that ignore this SvcParamKey. Each SVCB protocol mapping SHOULD specify a set of keys that are "automatically mandatory", i.e. mandatory if they are present in an RR. The SvcParamKey "mandatory" is used to indicate any mandatory keys for this RR, in addition to any automatically mandatory keys that are present.¶
A ServiceMode RR is considered "compatible" with a client if the client recognizes all the mandatory keys, and their values indicate that successful connection establishment is possible. If the SVCB RRSet contains no compatible RRs, the client will generally act as if the RRSet is empty.¶
The presentation value SHALL be a comma-separated list
(Appendix A.1) of one or more valid
SvcParamKeys, either by their registered name or in the unknown-key format
(Section 2.1). Keys MAY appear in any order, but MUST NOT appear more
than once. For self-consistency (Section 2.4.3), listed keys MUST also
appear in the SvcParams.¶
To enable simpler parsing, this SvcParamValue MUST NOT contain escape sequences.¶
For example, the following is a valid list of SvcParams:¶
ech=... key65333=ex1 key65444=ex2 mandatory=key65444,ech¶
In wire format, the keys are represented by their numeric values in network byte order, concatenated in ascending order.¶
This SvcParamKey is always automatically mandatory, and MUST NOT appear in its own value-list. Other automatically mandatory keys SHOULD NOT appear in the list either. (Including them wastes space and otherwise has no effect.)¶
Use of any protocol with SVCB requires a protocol-specific mapping specification. This section specifies the mapping for the "http" and "https" URI schemes [I-D.draft-ietf-httpbis-semantics].¶
To enable special handling for HTTP use-cases, the HTTPS RR type is defined as a SVCB-compatible RR type, specific to the "https" and "http" schemes. Clients MUST NOT perform SVCB queries or accept SVCB responses for "https" or "http" schemes.¶
The presentation format of the record is:¶
Name TTL IN HTTPS SvcPriority TargetName SvcParams¶
All the SvcParamKeys defined in Section 7 are permitted for use in HTTPS RRs. The default set of ALPN IDs is the single value "http/1.1". The "automatically mandatory" keys (Section 8) are "port" and "no-default-alpn". (As described in Section 8, clients must either implement these keys or ignore any RR in which they appear.) Clients that restrict the destination port in "https" URIs (e.g. using the "bad ports" list from [FETCH]) SHOULD apply the same restriction to the "port" SvcParam.¶
The presence of an HTTPS RR for an origin also indicates that clients should connect securely and use the "https" scheme, as discussed in Section 9.5. This allows HTTPS RRs to apply to pre-existing "http" scheme URLs, while ensuring that the client uses a secure and authenticated connection.¶
The HTTPS RR parallels the concepts introduced in the HTTP Alternative Services proposed standard [AltSvc]. Clients and servers that implement HTTPS RRs are not required to implement Alt-Svc.¶
The HTTPS RR uses Port Prefix Naming (Section 2.3), with one modification: if the scheme is "https" and the port is 443, then the client's original QNAME is equal to the service name (i.e. the origin's hostname), without any prefix labels.¶
By removing the Attrleaf labels [Attrleaf] used in SVCB, this construction enables offline DNSSEC signing of wildcard domains, which are commonly used with HTTP. Using the service name as the owner name of the HTTPS record, without prefixes, also allows the targets of existing CNAME chains (e.g. CDN hosts) to start returning HTTPS RR responses without requiring origin domains to configure and maintain an additional delegation.¶
Following of HTTPS AliasMode RRs and CNAME aliases is unchanged from SVCB.¶
Clients always convert "http" URLs to "https" before performing an HTTPS RR query using the process described in Section 9.5, so domain owners MUST NOT publish HTTPS RRs with a prefix of "_http".¶
Note that none of these forms alter the HTTPS origin or authority. For example, clients MUST continue to validate TLS certificate hostnames based on the origin.¶
Publishing a ServiceMode HTTPS RR in DNS is intended to be similar to transmitting an Alt-Svc field value over HTTP, and receiving an HTTPS RR is intended to be similar to receiving that field value over HTTP. However, there are some differences in the intended client and server behavior.¶
Unlike Alt-Svc Field Values, HTTPS RRs can contain multiple ALPN IDs. The meaning and use of these IDs is discussed in Section 7.1.2.¶
HTTPS records do not require or provide any assurance of authenticity. (DNSSEC signing and verification, which would provide such assurance, are OPTIONAL.) The DNS resolution process is modeled as an untrusted channel that might be controlled by an attacker, so Alt-Svc parameters that cannot be safely received in this model MUST NOT have a corresponding defined SvcParamKey. For example, there is no SvcParamKey corresponding to the Alt-Svc "persist" parameter, because this parameter is not safe to accept over an untrusted channel.¶
There is no SvcParamKey corresponding to the Alt-Svc "ma" (max age) parameter. Instead, server operators encode the expiration time in the DNS TTL.¶
The appropriate TTL value might be different from the "ma" value used for Alt-Svc, depending on the desired efficiency and agility. Some DNS caches incorrectly extend the lifetime of DNS records beyond the stated TTL, so server operators cannot rely on HTTPS RRs expiring on time. Shortening the TTL to compensate for incorrect caching is NOT RECOMMENDED, as this practice impairs the performance of correctly functioning caches and does not guarantee faster expiration from incorrect caches. Instead, server operators SHOULD maintain compatibility with expired records until they observe that nearly all connections have migrated to the new configuration.¶
Sending Alt-Svc over HTTP allows the server to tailor the Alt-Svc Field Value specifically to the client. When using an HTTPS RR, groups of clients will necessarily receive the same SvcParams. Therefore, HTTPS RRs are not suitable for uses that require single-client granularity.¶
Clients that implement support for both Alt-Svc and HTTPS records SHOULD retrieve any HTTPS records for the Alt-Svc alt-authority, and ensure that their connection attempts are consistent with both the Alt-Svc parameters and any received HTTPS SvcParams. If present, the HTTPS record's TargetName and port are used for connection establishment (as in Section 3). For example, suppose that "https://example.com" sends an Alt-Svc field value of:¶
Alt-Svc: h2="alt.example:443", h2="alt2.example:443", h3=":8443"¶
The client would retrieve the following HTTPS records:¶
alt.example. IN HTTPS 1 . alpn=h2,h3 ech=...
alt2.example. IN HTTPS 1 alt2b.example. alpn=h3 ech=...
_8443._https.example.com. IN HTTPS 1 alt3.example. (
port=9443 alpn=h2,h3 ech=... )
¶
Based on these inputs, the following connection attempts would always be allowed:¶
alt.example:443¶
alt3.example:9443¶
ECH-capable clients would use ECH when establishing any of these connections.¶
The following connection attempts would not be allowed:¶
alt.example:443 (not consistent with Alt-Svc)¶
alt2b.example (no ALPN consistent with both the HTTPS
record and Alt-Svc)¶
alt3.example (not consistent with Alt-Svc)¶
The following Alt-Svc-only connection attempts would be allowed only if the client does not support ECH, as they rely on SVCB-optional fallback behavior that the client will disable if it implements support for ECH and the "ech" SvcParam is present (Section 10.1):¶
Origins that publish an "ech" SvcParam in their HTTPS record SHOULD also publish an "ech" SvcParam for any alt-authorities. Otherwise, clients might reveal the name of the server in an unencrypted ClientHello. Similar consistency considerations could apply to future SvcParamKeys, so alt-authorities SHOULD carry the same SvcParams as the origin unless a deviation is specifically known to be safe.¶
As noted in Section 2.4 of [AltSvc], clients MAY disallow any Alt-Svc connection according to their own criteria, e.g. disallowing Alt-Svc connections that lack ECH support when there is an active ECH-protected connection for this origin.¶
Clients MUST NOT use an HTTPS RR response unless the client supports TLS Server Name Indication (SNI) and indicates the origin name when negotiating TLS. This supports the conservation of IP addresses.¶
Note that the TLS SNI (and also the HTTP "Host" or ":authority") will indicate the origin, not the TargetName.¶
An HTTPS RR directs the client to communicate with this host only over a secure transport, similar to HTTP Strict Transport Security [HSTS]. Prior to making an "http" scheme request, the client SHOULD perform a lookup to determine if any HTTPS RRs exist for that origin. To do so, the client SHOULD construct a corresponding "https" URL as follows:¶
This construction is equivalent to Section 8.3 of [HSTS], point 5.¶
If an HTTPS RR query for this "https" URL returns any AliasMode HTTPS RRs, or any compatible ServiceMode HTTPS RRs (see Section 8), the client SHOULD behave as if it has received an HTTP 307 (Temporary Redirect) status code with this "https" URL in the "Location" field. (Receipt of an incompatible ServiceMode RR does not trigger the redirect behavior.) Because HTTPS RRs are received over an often insecure channel (DNS), clients MUST NOT place any more trust in this signal than if they had received a 307 redirect over cleartext HTTP. If this redirection would result in a loss of functionality (e.g. important resources that are only available on the "http" origin), the operator MUST NOT publish an HTTPS RR.¶
When an "https" connection fails due to an error in the underlying secure transport, such as an error in certificate validation, some clients currently offer a "user recourse" that allows the user to bypass the security error and connect anyway. When making an "https" scheme request to an origin with an HTTPS RR, either directly or via the above redirect, such a client MAY remove the user recourse option. Origins that publish HTTPS RRs therefore MUST NOT rely on user recourse for access. For more information, see Section 8.4 and Section 12.1 of [HSTS].¶
All protocols employing "http://" or "https://" URLs SHOULD respect HTTPS RRs.
For example, clients that
support HTTPS RRs and implement the altered WebSocket [WebSocket]
opening handshake from the W3C Fetch specification [FETCH] SHOULD use HTTPS RRs
for the requestURL.¶
Such protocols MAY define their own SVCB mappings, which MAY be defined to take precedence over HTTPS RRs.¶
The SVCB "ech" parameter is defined for conveying the ECH configuration of an alternative endpoint. In wire format, the value of the parameter is an ECHConfigList [ECH], including the redundant length prefix. In presentation format, the value is a single ECHConfigList encoded in Base64 [base64]. Base64 is used here to simplify integration with TLS server software. To enable simpler parsing, this SvcParam MUST NOT contain escape sequences.¶
When ECH is in use, the TLS ClientHello is divided into an unencrypted "outer" and an encrypted "inner" ClientHello. The outer ClientHello is an implementation detail of ECH, and its contents are controlled by the ECHConfig in accordance with [ECH]. The inner ClientHello is used for establishing a connection to the service, so its contents may be influenced by other SVCB parameters. For example, the requirements on the ProtocolNameList in Section 7.1 apply only to the inner ClientHello. Similarly, it is the inner ClientHello whose Server Name Indication identifies the desired service.¶
The SVCB-optional client behavior specified in Section 3 permits clients to fall back to a direct connection if all SVCB options fail. This behavior is not suitable for ECH, because fallback would negate the privacy benefits of ECH. Accordingly, ECH-capable SVCB-optional clients MUST switch to SVCB-reliant connection establishment if SVCB resolution succeeded (following Section 3) and all alternative endpoints have an "ech" key.¶
As a latency optimization, clients MAY prefetch DNS records that will only be used in SVCB-optional mode.¶
An HTTPS RRSet containing some RRs with "ech" and some without is vulnerable to a downgrade attack. This configuration is NOT RECOMMENDED. Zone owners who do use such a mixed configuration SHOULD mark the RRs with "ech" as more preferred (i.e. smaller SvcPriority) than those without, in order to maximize the likelihood that ECH will be used in the absence of an active adversary.¶
Each ServiceMode RRSet can only serve a single scheme. The scheme is indicated by the owner name and the RR type. For the generic SVCB RR type, this means that each owner name can only be used for a single scheme. The underscore prefixing requirement (Section 2.3) ensures that this is true for the initial query, but it is the responsibility of zone owners to choose names that satisfy this constraint when using aliases, including CNAME and AliasMode records.¶
When using the generic SVCB RR type with aliasing, zone owners SHOULD choose alias
target names that indicate the scheme in use (e.g. foosvc.example.net for
foo:// schemes). This will help to avoid confusion when another scheme needs to
be added to the configuration.¶
To avoid a delay for clients using a nonconforming recursive resolver, domain owners SHOULD minimize the use of AliasMode records, and SHOULD choose TargetName according to a predictable convention that is known to the client, so that clients can issue A and/or AAAA queries for TargetName in advance (see Section 5). Unless otherwise specified, the convention is to set TargetName to the service name for an initial ServiceMode record, or to "." if it is reached via an alias. For foo://foo.example.com:8080, this might look like:¶
$ORIGIN example.com. ; Origin foo 3600 IN CNAME foosvc.example.net. _8080._foo.foo 3600 IN CNAME foosvc.example.net. $ORIGIN example.net. ; Service provider zone foosvc 3600 IN SVCB 1 . key65333=... foosvc 300 IN AAAA 2001:db8::1¶
Domain owners SHOULD avoid using a TargetName that is below a DNAME, as this is likely unnecessary and makes responses slower and larger. Also, zone structures that require following more than 8 aliases (counting both AliasMode and CNAME records) are NOT RECOMMENDED.¶
Consider a simple zone of the form:¶
$ORIGIN simple.example. ; Simple example zone
@ 300 IN A 192.0.2.1
AAAA 2001:db8::1
¶
The domain owner could add this record:¶
@ 7200 IN HTTPS 1 . alpn=h3¶
to indicate that https://simple.example supports QUIC in addition to TLS over TCP (the implicit default). The record could also include other information (e.g. non-standard port, ECH configuration). For https://simple.example:8443, the record would be:¶
_8443._https 7200 IN HTTPS 1 . alpn=h3¶
These records also respectively tell clients to replace the scheme with "https" when loading http://simple.example or http://simple.example:8443.¶
Consider a zone that is using CNAME aliasing:¶
$ORIGIN aliased.example. ; A zone that is using a hosting service
; Subdomain aliased to a high-performance server pool
www 7200 IN CNAME pool.svc.example.
; Apex domain on fixed IPs because CNAME is not allowed at the apex
@ 300 IN A 192.0.2.1
IN AAAA 2001:db8::1
¶
With HTTPS RRs, the owner of aliased.example could alias the apex by adding one additional record:¶
@ 7200 IN HTTPS 0 pool.svc.example.¶
With this record in place, HTTPS-RR-aware clients will use the same server pool for aliased.example and www.aliased.example. (They will also upgrade "http://aliased.example/..." to "https".) Non-HTTPS-RR-aware clients will just ignore the new record.¶
Similar to CNAME, HTTPS RRs have no impact on the origin name. When connecting, clients will continue to treat the authoritative origins as "https://www.aliased.example" and "https://aliased.example", respectively, and will validate TLS server certificates accordingly.¶
Suppose that svc.example's default server pool supports HTTP/2, and it has deployed HTTP/3 on a new server pool with a different configuration. This can be expressed in the following form:¶
$ORIGIN svc.example. ; A hosting provider.
pool 7200 IN HTTPS 1 h3pool alpn=h2,h3 ech="123..."
HTTPS 2 . alpn=h2 ech="abc..."
pool 300 IN A 192.0.2.2
AAAA 2001:db8::2
h3pool 300 IN A 192.0.2.3
AAAA 2001:db8::3
¶
This configuration is entirely compatible with the "Apex aliasing" example, whether the client supports HTTPS RRs or not. If the client does support HTTPS RRs, all connections will be upgraded to HTTPS, and clients will use HTTP/3 if they can. Parameters are "bound" to each server pool, so each server pool can have its own protocol, ECH configuration, etc.¶
The HTTPS RR is intended to support HTTPS services operated by multiple independent entities, such as different Content Delivery Networks (CDNs) or different hosting providers. This includes the case where a service is migrated from one operator to another, as well as the case where the service is multiplexed between multiple operators for performance, redundancy, etc.¶
This example shows such a configuration, with www.customer.example having different DNS responses to different queries, either over time or due to logic within the authoritative DNS server:¶
; This zone contains/returns different CNAME records
; at different points-in-time. The RRset for "www" can
; only ever contain a single CNAME.
; Sometimes the zone has:
$ORIGIN customer.example. ; A Multi-CDN customer domain
www 900 IN CNAME cdn1.svc1.example.
; and other times it contains:
$ORIGIN customer.example.
www 900 IN CNAME customer.svc2.example.
; and yet other times it contains:
$ORIGIN customer.example.
www 900 IN CNAME cdn3.svc3.example.
; With the following remaining constant and always included:
$ORIGIN customer.example. ; A Multi-CDN customer domain
; The apex is also aliased to www to match its configuration
@ 7200 IN HTTPS 0 www
; Non-HTTPS-aware clients use non-CDN IPs
A 203.0.113.82
AAAA 2001:db8:203::2
; Resolutions following the cdn1.svc1.example
; path use these records.
; This CDN uses a different alternative service for HTTP/3.
$ORIGIN svc1.example. ; domain for CDN 1
cdn1 1800 IN HTTPS 1 h3pool alpn=h3 ech="123..."
HTTPS 2 . alpn=h2 ech="123..."
A 192.0.2.2
AAAA 2001:db8:192::4
h3pool 300 IN A 192.0.2.3
AAAA 2001:db8:192:7::3
; Resolutions following the customer.svc2.example
; path use these records.
; Note that this CDN only supports HTTP/2.
$ORIGIN svc2.example. ; domain operated by CDN 2
customer 300 IN HTTPS 1 . alpn=h2 ech="xyz..."
60 IN A 198.51.100.2
A 198.51.100.3
A 198.51.100.4
AAAA 2001:db8:198::7
AAAA 2001:db8:198::12
; Resolutions following the customer.svc2.example
; path use these records.
; Note that this CDN has no HTTPS records
; and thus no ECH support.
$ORIGIN svc3.example. ; domain operated by CDN 3
cdn3 60 IN A 203.0.113.8
AAAA 2001:db8:113::8
¶
Note that in the above example, the different CDNs have different ECH configurations and different capabilities, but clients will use HTTPS RRs as a bound-together unit.¶
Domain owners should be cautious when using a multi-CDN configuration, as it introduces a number of complexities highlighted by this example:¶
For protocols other than HTTP, the SVCB RR and an Attrleaf label [Attrleaf] will be used. For example, to reach an example resource of "baz://api.example.com:8765", the following SVCB record would be used to alias it to "svc4-baz.example.net." which in-turn could return AAAA/A records and/or SVCB records in ServiceMode:¶
_8765._baz.api.example.com. 7200 IN SVCB 0 svc4-baz.example.net.¶
HTTPS RRs use similar Attrleaf labels if the origin contains a non-default port.¶
This standard is intended to reduce connection latency and improve user privacy. Server operators implementing this standard SHOULD also implement TLS 1.3 [RFC8446] and OCSP Stapling [RFC6066], both of which confer substantial performance and privacy benefits when used in combination with SVCB records.¶
To realize the greatest privacy benefits, this proposal is intended for use over a privacy-preserving DNS transport (like DNS over TLS [DoT] or DNS over HTTPS [DoH]). However, performance improvements, and some modest privacy improvements, are possible without the use of those standards.¶
Any specification for use of SVCB with a protocol MUST have an entry for its scheme under the SVCB RR type in the IANA DNS Underscore Global Scoped Entry Registry [Attrleaf]. The scheme SHOULD have an entry in the IANA URI Schemes Registry [RFC7595]. The scheme SHOULD have a defined specification for use with SVCB.¶
SVCB/HTTPS RRs are intended for distribution over untrusted channels, and clients are REQUIRED to verify that the alternative endpoint is authoritative for the service (similar to Section 2.1 of [AltSvc]). Therefore, DNSSEC signing and validation are OPTIONAL for publishing and using SVCB and HTTPS RRs.¶
Clients MUST ensure that their DNS cache is partitioned for each local network, or flushed on network changes, to prevent a local adversary in one network from implanting a forged DNS record that allows them to track users or hinder their connections after they leave that network.¶
An attacker who can prevent SVCB resolution can deny clients any associated security benefits. A hostile recursive resolver can always deny service to SVCB queries, but network intermediaries can often prevent resolution as well, even when the client and recursive resolver validate DNSSEC and use a secure transport. These downgrade attacks can prevent the "https" upgrade provided by the HTTPS RR (Section 9.5), and disable the encryption enabled by the "ech" SvcParamKey (Section 10). To prevent downgrades, Section 3.1 recommends that clients abandon the connection attempt when such an attack is detected.¶
A hostile DNS intermediary might forge AliasMode "." records (Section 2.5.1) as a way to block clients from accessing particular services. Such an adversary could already block entire domains by forging erroneous responses, but this mechanism allows them to target particular protocols or ports within a domain. Clients that might be subject to such attacks SHOULD ignore AliasMode "." records.¶
A hostile DNS intermediary or origin can return SVCB records indicating any IP address and port number, including IP addresses inside the local network and port numbers assigned to internal services. If the attacker can influence the client's payload (e.g. TLS session ticket contents), and an internal service has a sufficiently lax parser, it's possible that the attacker could gain unintended access. (The same concerns apply to SRV records, HTTP Alt-Svc, and HTTP redirects.) As a mitigation, SVCB mapping documents SHOULD indicate any port number restrictions that are appropriate for the supported transports.¶
Standard address queries reveal the user's intent to access a particular domain. This information is visible to the recursive resolver, and to many other parties when plaintext DNS transport is used. SVCB queries, like queries for SRV records and other specific RR types, additionally reveal the user's intent to use a particular protocol. This is not normally sensitive information, but it should be considered when adding SVCB support in a new context.¶
This document defines a new DNS RR type, SVCB, whose value 64 has been allocated by IANA from the "Resource Record (RR) TYPEs" subregistry of the "Domain Name System (DNS) Parameters" registry:¶
This document defines a new DNS RR type, "HTTPS", whose value 65 has been allocated by IANA from the "Resource Record (RR) TYPEs" subregistry of the "Domain Name System (DNS) Parameters" registry:¶
IANA is requested to create a new registry, entitled "Service Parameter Keys (SvcParamKeys)". This registry defines the namespace for parameters, including string representations and numeric SvcParamKey values. This registry is shared with other SVCB-compatible RR types, such as the HTTPS RR.¶
ACTION: create this registry, on a new page entitled "DNS Service Bindings (SVCB)" under the "Domain Name System (DNS) Parameters" category.¶
A registration MUST include the following fields:¶
The characters in the registered Name MUST be lower-case alphanumeric or "-" (Section 2.1). The name MUST NOT start with "key" or "invalid".¶
Entries in this registry are subject to a First Come First Served registration policy ([RFC8126], Section 4.4). The Format Reference MUST specify how to convert the SvcParamValue's presentation format to wire format and MAY detail its intended meaning and use. An entry MAY specify a Format Reference of the form "Same as (other key Name)" if it uses the same presentation and wire formats as an existing key.¶
This arrangement supports the development of new parameters while ensuring that zone files can be made interoperable.¶
The "Service Binding (SVCB) Parameter Registry" shall initially be populated with the registrations below:¶
| Number | Name | Meaning | Format Reference |
|---|---|---|---|
| 0 | mandatory | Mandatory keys in this RR | (This document) Section 8 |
| 1 | alpn | Additional supported protocols | (This document) Section 7.1 |
| 2 | no-default-alpn | No support for default protocol | (This document) Section 7.1 |
| 3 | port | Port for alternative endpoint | (This document) Section 7.2 |
| 4 | ipv4hint | IPv4 address hints | (This document) Section 7.4 |
| 5 | ech | Encrypted ClientHello info | (This document) Section 7.3 |
| 6 | ipv6hint | IPv6 address hints | (This document) Section 7.4 |
| 65280-65534 | N/A | Private Use | (This document) |
| 65535 | N/A | Reserved ("Invalid key") | (This document) |
Per [Attrleaf], please add the following entry to the DNS Underscore Global Scoped Entry Registry:¶
| RR TYPE | _NODE NAME | Meaning | Reference |
|---|---|---|---|
| HTTPS | _https | HTTPS SVCB info | (This document) |
DNS zone files are capable of representing arbitrary octet sequences in basic ASCII text, using various delimiters and encodings. The algorithm for decoding these character-strings is defined in Section 5.1 of [RFC1035]. Here we summarize the allowed input to that algorithm, using ABNF:¶
; non-special is VCHAR minus DQUOTE, ";", "(", ")", and "\".
non-special = %x21 / %x23-27 / %x2A-3A / %x3C-5B / %x5D-7E
; non-digit is VCHAR minus DIGIT
non-digit = %x21-2F / %x3A-7E
; dec-octet is a number 0-255 as a three-digit decimal number.
dec-octet = ( "0" / "1" ) 2DIGIT /
"2" ( ( %x30-34 DIGIT ) / ( "5" %x30-35 ) )
escaped = "\" ( non-digit / dec-octet )
contiguous = 1*( non-special / escaped )
quoted = DQUOTE *( contiguous / ( ["\"] WSP ) ) DQUOTE
char-string = contiguous / quoted
¶
The decoding algorithm allows char-string to represent any *OCTET.
In this document, this algorithm is referred to as "character-string decoding".
The algorithm is the same as used by <character-string> in RFC 1035,
although the output length in this document is not limited to 255 octets.¶
In order to represent lists of items in zone files, this specification uses comma-separated lists. When the allowed items in the list cannot contain "," or "\", this is trivial. (For simplicity, empty items are not allowed.) A value-list parser that splits on "," and prohibits items containing "\" is sufficient to comply with all requirements in this document.¶
For implementations that allow "," and "\" in item values, the following escaping syntax applies:¶
item = 1*OCTET
; item-allowed is OCTET minus "," and "\".
item-allowed = %x00-2B / %x2D-5B / %x5D-FF
escaped-item = 1*(item-allowed / "\," / "\\")
comma-separated = [escaped-item *("," escaped-item)]
¶
Decoding of value-lists happens after character-string decoding.
For example, consider these char-string SvcParamValues:¶
"part1,part2,part3\\,part4\\\\" part1\,\p\a\r\t2\044part3\092,part4\092\\¶
These inputs are equivalent: character-string decoding either of them would
produce the same value:¶
part1,part2,part3\,part4\\¶
Applying comma-separated list decoding to this value would produce a list
of three items:¶
part1 part2 part3,part4\¶
This table serves as a non-normative summary of the HTTP mapping for SVCB (Section 9). Future protocol mappings may provide a similar summary table.¶
| Mapped scheme | "https" |
| Other affected schemes | "http", "wss", "ws", (other HTTP-based) |
| RR type | HTTPS (65) |
| Name prefix | None for port 443, else _$PORT._https
|
| Automatically Mandatory Keys |
port, no-default-alpn
|
| SvcParam defaults |
alpn: ["http/1.1"] |
| Special behaviors | HTTP to HTTPS upgrade |
| Keys that records must include | None |
The SVCB and HTTPS RR types closely resemble, and are inspired by, some existing record types and proposals. A complaint with all of the alternatives is that web clients have seemed unenthusiastic about implementing them. The hope here is that by providing an extensible solution that solves multiple problems we will overcome the inertia and have a path to achieve client implementation.¶
An SRV record [SRV] can perform a similar function to the SVCB record, informing a client to look in a different location for a service. However, there are several differences:¶
Unlike [I-D.bellis-dnsop-http-record], this approach is extensible to cover Alt-Svc and Encrypted ClientHello use-cases. Like that proposal, this addresses the zone apex CNAME challenge.¶
Like that proposal, it remains necessary to continue to include address records at the zone apex for legacy clients.¶
Unlike [I-D.ietf-dnsop-aname], this approach is extensible to cover Alt-Svc and ECH use-cases. This approach also does not require any changes or special handling on either authoritative or primary servers, beyond optionally returning in-bailiwick additional records.¶
Like that proposal, this addresses the zone apex CNAME challenge for clients that implement this.¶
However, with this SVCB proposal, it remains necessary to continue to include address records at the zone apex for legacy clients. If deployment of this standard is successful, the number of legacy clients will fall over time. As the number of legacy clients declines, the operational effort required to serve these users without the benefit of SVCB indirection should fall. Server operators can easily observe how much traffic reaches this legacy endpoint, and may remove the apex's address records if the observed legacy traffic has fallen to negligible levels.¶
Abstractly, functions of AliasMode and ServiceMode are independent, so it might be tempting to specify them as separate RR types. However, this would result in a serious performance impairment, because clients cannot rely on their recursive resolver to follow SVCB aliases (unlike CNAME). Thus, clients would have to issue queries for both RR types in parallel, potentially at each step of the alias chain. Recursive resolvers that implement the specification would, upon receipt of a ServiceMode query, emit both a ServiceMode and an AliasMode query to the authoritative. Thus, splitting the RR type would double, or in some cases triple, the load on clients and servers, and would not reduce implementation complexity.¶
These test vectors only contain the RDATA portion of SVCB/HTTPS records in presentation format, generic format ([RFC3597]) and wire format. The wire format uses hexadecimal (\xNN) for each non-ascii byte. As the wireformat is long, it is broken into several lines.¶
example.com. HTTPS 0 foo.example.com. \# 19 ( 00 00 ; priority 03 66 6f 6f 07 65 78 61 6d 70 6c 65 03 63 6f 6d 00 ; target ) \x00\x00 # priority \x03foo\x07example\x03com\x00 # target
example.com. SVCB 1 . \# 3 ( 00 01 ; priority 00 ; target (root label) ) \x00\x01 # priority \x00 # target, root label
example.com. SVCB 16 foo.example.com. port=53 \# 25 ( 00 10 ; priority 03 66 6f 6f 07 65 78 61 6d 70 6c 65 03 63 6f 6d 00 ; target 00 03 ; key 3 00 02 ; length 2 00 35 ; value ) \x00\x10 # priority \x03foo\x07example\x03com\x00 # target \x00\x03 # key 3 \x00\x02 # length: 2 bytes \x00\x35 # value
example.com. SVCB 1 foo.example.com. key667=hello \# 28 ( 00 01 ; priority 03 66 6f 6f 07 65 78 61 6d 70 6c 65 03 63 6f 6d 00 ; target 02 9b ; key 667 00 05 ; length 5 68 65 6c 6c 6f ; value ) \x00\x01 # priority \x03foo\x07example\x03com\x00 # target \x02\x9b # key 667 \x00\x05 # length 5 hello # value
example.com. SVCB 1 foo.example.com. key667="hello\210qoo" \# 32 ( 00 01 ; priority 03 66 6f 6f 07 65 78 61 6d 70 6c 65 03 63 6f 6d 00 ; target 02 9b ; key 667 00 09 ; length 9 68 65 6c 6c 6f d2 71 6f 6f ; value ) \x00\x01 # priority \x03foo\x07example\x03com\x00 # target \x02\x9b # key 667 \x00\x09 # length 9 hello\xd2qoo # value
example.com. SVCB 1 foo.example.com. (
ipv6hint="2001:db8::1,2001:db8::53:1"
)
\# 55 (
00 01 ; priority
03 66 6f 6f 07 65 78 61 6d 70 6c 65 03 63 6f 6d 00 ; target
00 06 ; key 6
00 20 ; length 32
20 01 0d b8 00 00 00 00 00 00 00 00 00 00 00 01 ; first address
20 01 0d b8 00 00 00 00 00 00 00 00 00 53 00 01 ; second address
)
\x00\x01 # priority
\x03foo\x07example\x03com\x00 # target
\x00\x06 # key 6
\x00\x20 # length 32
\x20\x01\x0d\xb8\x00\x00\x00\x00
\x00\x00\x00\x00\x00\x00\x00\x01 # first address
\x20\x01\x0d\xb8\x00\x00\x00\x00
\x00\x00\x00\x00\x00\x53\x00\x01 # second address
example.com. SVCB 1 example.com. ipv6hint="::ffff:198.51.100.100"
\# 35 (
00 01 ; priority
07 65 78 61 6d 70 6c 65 03 63 6f 6d 00 ; target
00 06 ; key 6
00 10 ; length 16
00 00 00 00 00 00 00 00 00 00 ff ff c6 33 64 64 ; address
)
\x00\x01 # priority
\x07example\x03com\x00 # target
\x00\x06 # key 6
\x00\x10 # length 16
\x00\x00\x00\x00\x00\x00\x00\x00
\x00\x00\xff\xff\xc6\x33\x64\x64 # address
example.com. SVCB 16 foo.example.org. (
alpn=h2,h3-19 mandatory=ipv4hint,alpn
ipv4hint=192.0.2.1
)
\# 48 (
00 10 ; priority
03 66 6f 6f 07 65 78 61 6d 70 6c 65 03 6f 72 67 00 ; target
00 00 ; key 0
00 04 ; param length 4
00 01 ; value: key 1
00 04 ; value: key 4
00 01 ; key 1
00 09 ; param length 9
02 ; alpn length 2
68 32 ; alpn value
05 ; alpn length 5
68 33 2d 31 39 ; alpn value
00 04 ; key 4
00 04 ; param length 4
c0 00 02 01 ; param value
)
\x00\x10 # priority
\x03foo\x07example\x03org\x00 # target
\x00\x00 # key 0
\x00\x04 # param length 4
\x00\x01 # value: key 1
\x00\x04 # value: key 4
\x00\x01 # key 1
\x00\x09 # param length 9
\x02 # alpn length 2
h2 # alpn value
\x05 # alpn length 5
h3-19 # alpn value
\x00\x04 # key 4
\x00\x04 # param length 4
\xc0\x00\x02\x01 # param value
example.com. SVCB 16 foo.example.org. alpn="f\\\\oo\\,bar,h2" example.com. SVCB 16 foo.example.org. alpn=f\\\092oo\092,bar,h2 \# 35 ( 00 10 ; priority 03 66 6f 6f 07 65 78 61 6d 70 6c 65 03 6f 72 67 00 ; target 00 01 ; key 1 00 0c ; param length 12 08 ; alpn length 8 66 5c 6f 6f 2c 62 61 72 ; alpn value 02 ; alpn length 2 68 32 ; alpn value ) \x00\x10 # priority \x03foo\x07example\x03org\x00 # target \x00\x01 # key 1 \x00\x0c # param length 12 \x08 # alpn length 8 f\oo,bar # alpn value \x02 # alpn length 2 h2 # alpn value
This subsection contains test vectors which are not compliant with this document. The various reasons for non-compliance are explained with each example.¶
example.com. SVCB 1 foo.example.com. (
key123=abc key123=def
)
example.com. SVCB 1 foo.example.com. mandatory example.com. SVCB 1 foo.example.com. alpn example.com. SVCB 1 foo.example.com. port example.com. SVCB 1 foo.example.com. ipv4hint example.com. SVCB 1 foo.example.com. ipv6hint
example.com. SVCB 1 foo.example.com. no-default-alpn=abc
example.com. SVCB 1 foo.example.com. mandatory=key123
example.com. SVCB 1 foo.example.com. mandatory=mandatory
example.com. SVCB 1 foo.example.com. (
mandatory=key123,key123 key123=abc
)
(This section to be removed by the RFC editor.)¶
draft-ietf-dnsop-svcb-https-08¶
Extensive structural and editorial adjustments based on area reviews, including:¶
draft-ietf-dnsop-svcb-https-07¶
draft-ietf-dnsop-svcb-https-06¶
draft-ietf-dnsop-svcb-https-05¶
draft-ietf-dnsop-svcb-https-04¶
draft-ietf-dnsop-svcb-https-03¶
draft-ietf-dnsop-svcb-https-02¶
draft-ietf-dnsop-svcb-https-01¶
draft-ietf-dnsop-svcb-https-00¶
draft-ietf-dnsop-svcb-httpssvc-03¶
draft-ietf-dnsop-svcb-httpssvc-02¶
draft-ietf-dnsop-svcb-httpssvc-01¶
draft-ietf-dnsop-svcb-httpssvc-00¶
draft-nygren-dnsop-svcb-httpssvc-00¶
draft-nygren-httpbis-httpssvc-03¶
draft-nygren-httpbis-httpssvc-02¶
draft-nygren-httpbis-httpssvc-01¶
draft-nygren-httpbis-httpssvc-00¶