Google

bemasc@google.com

Akamai Technologies

mbishop@evequefou.be

Akamai Technologies

erik+ietf@nygren.org

General DNSOP Working Group Internet-Draft 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 HTTPS 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 HTTPS and HTTP origins. 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.

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, when clients need to make a connection to fetch resources associated with an HTTPS URI, they currently resolve only A and/or AAAA records for the origin hostname. This is adequate for services that use basic HTTPS (fixed port, no QUIC, no ). Going beyond basic HTTPS confers privacy, performance, and operational advantages, but it requires the client to learn additional information, and 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 (). The HTTPS RR is then defined as a special case of SVCB that improves efficiency and convenience for use with HTTPS () by avoiding the need for an Attrleaf label (). Other protocols with similar needs may follow the pattern of HTTPS and assign their own SVCB-compatible RR types. All behaviors described as applying to the SVCB RR also apply to the HTTPS RR unless explicitly stated otherwise. describes additional behaviors specific to the HTTPS RR. Apart from and introductory examples, much of this document refers only to the SVCB RR, but those references should be taken to apply to SVCB, HTTPS, and any future SVCB-compatible RR types. The SVCB RR has two modes: 1) “AliasMode” simply delegates operational control for a resource; 2) “ServiceMode” 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: Provides alternative endpoints that are authoritative for the service, along with parameters associated with each of these endpoints. Does not assume that all alternative endpoints have the same parameters or capabilities, or are even operated by the same entity. This is important as DNS does not provide any way to tie together multiple RRs for the same name. For example, if www.example.com is a CNAME alias that switches between one of three CDNs or hosting environments, successive queries for that name may return records that correspond to different environments. Enables CNAME-like functionality at a zone apex (such as “example.com”) for participating protocols, and generally enables delegation of operational authority for an origin within the DNS to an alternate name. Additional goals specific to HTTPS RRs and the HTTPS use-case include: Connect directly to HTTP3 (QUIC transport) alternative endpoints Obtain the Encrypted ClientHello keys associated with an alternative endpoint Support non-default TCP and UDP ports Enable SRV-like benefits (e.g. apex delegation, as mentioned above) for HTTP(S), where SRV has not been widely adopted Provide an HSTS-like indication signaling that the HTTPS scheme should be used instead of HTTP for this request (see ).

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 mandatory fields and one optional. The fields are: SvcPriority: The priority of this record (relative to others, with lower values preferred). A value of 0 indicates AliasMode. (Described in .) TargetName: The domain name of either the alias target (for AliasMode) or the alternative endpoint (for ServiceMode). SvcParams (optional): A list of key=value pairs describing the alternative endpoint at TargetName (only used in ServiceMode and otherwise ignored). Described in . 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 the HTTPS use-case, the HTTPS RR enables many of the benefits of Alt-Svc 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 keys. See .

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. The “service” is the information source identified by the authority and scheme of the URI, capable of providing access to the resource. For HTTPS URIs, the “service” corresponds to an HTTPS “origin” . The “service name” is the host portion of the authority. The “authority endpoint” is the authority’s hostname and a port number implied by the scheme or specified in the URI. An “alternative endpoint” is a hostname, port number, and other associated instructions to the client on how to reach an instance of service. Additional DNS terminology intends to be consistent with . SVCB is a contraction of “service binding”. The SVCB RR, HTTPS RR, and future RR types that share SVCB’s format and registry are collectively known as SVCB-compatible RR types. 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 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 . Other SVCB-compatible resource record types can also be defined as-needed. In particular, the HTTPS RR (RR type 65) provides special handling for the case of “https” origins as described in . 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 .

The presentation format of the record is:

The SVCB record is defined specifically within the Internet (“IN”) Class (). SvcPriority is a number in the range 0-65535, TargetName is a <domain-name> ( 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 (). 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 ,

The SvcParamValue is parsed using the character-string decoding algorithm (), 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. Unrecognized keys are represented in presentation format as “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 (). SvcParams in presentation format MAY appear in any order, but keys MUST NOT be repeated.

The RDATA for the SVCB RR consists of: a 2 octet field for SvcPriority as an integer in network byte order. the uncompressed, fully-qualified TargetName, represented as a sequence of length-prefixed labels as in Section 3.1 of . the SvcParams, consuming the remainder of the record (so smaller than 65535 octets and constrained by the RDATA and DNS message sizes). When the list of SvcParams is non-empty (ServiceMode), it contains a series of SvcParamKey=SvcParamValue pairs, represented as: a 2 octet field containing the SvcParamKey as an integer in network byte order. (See for the defined values.) a 2 octet field containing the length of the SvcParamValue as an integer between 0 and 65535 in network byte order (but constrained by the RDATA and DNS message sizes). an octet string of this length whose contents are in a format determined by the SvcParamKey. SvcParamKeys SHALL appear in increasing numeric order. Clients MUST consider an RR malformed if: the end of the RDATA occurs within a SvcParam. SvcParamKeys are not in strictly increasing numeric order. the SvcParamValue for an SvcParamKey does not have the expected format. 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 , so the scheme MUST be registered appropriately with IANA (see ). Protocol mapping documents MAY specify additional underscore-prefixed labels to be prepended. For schemes that specify a port (Section 3.2.3 of ), 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 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:

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:

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 ( Section 5.1).)

When SvcPriority is 0 the SVCB record is in AliasMode (). Otherwise, it is in ServiceMode (). 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 (or other SVCB-compatible record such as HTTPS), AAAA, and/or A records. 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:

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:

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:

“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: Let $QNAME be the service name plus appropriate prefixes for the scheme (see ). Issue a SVCB query for $QNAME. If an AliasMode SVCB record is returned for $QNAME (after following CNAMEs as normal), set $QNAME to its TargetName (without additional prefixes) and loop back to step 2, subject to chain length limits and loop detection heuristics (see ). If one or more “compatible” () ServiceMode records are returned, these represent the alternative endpoints. Otherwise, SVCB resolution has failed, and the list of known endpoints is empty. This procedure does not rely on any recursive or authoritative DNS server to comply with this specification or have any awareness of SVCB. Once SVCB resolution has concluded, the client proceeds with connection establishment. Clients SHOULD try higher-priority alternatives first, with fallback to lower-priority alternatives. Clients issue AAAA and/or A queries for the selected TargetName, and MAY choose between them using an approach such as Happy Eyeballs . 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. HTTPS) SHALL implement SVCB-optional behavior (except as noted in and ). SVCB-optional clients SHOULD issue in parallel any other DNS queries that might be needed for connection establishment. SVCB-optional clients SHALL append an alternative endpoint consisting of the final value of $QNAME, the authority endpoint’s port number, and no SvcParams, to the list of alternative endpoints, which is 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. Some important optimizations are discussed in to avoid additional latency in comparison to ordinary AAAA/A lookups.

If SVCB resolution fails due to a SERVFAIL error, transport error, or timeout, and DNS exchanges between the client and the recursive resolver are cryptographically protected (e.g. using TLS or HTTPS ), a SVCB-optional client SHOULD abandon the connection attempt like a SVCB-reliant client would. 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. Similarly, if the client enforces DNSSEC validation on A/AAAA responses, it SHOULD terminate the connection if a SVCB response fails to validate. 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 ( Section 4.3.6) or SOCKS5 () 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. Providing the proxy with the final TargetName has several benefits: It allows the client to use the SvcParams, if present, which is only usable with a specific TargetName. The SvcParams may include information that enhances performance (e.g. alpn) and privacy (e.g. echconfig). It allows the service to delegate the apex domain. It allows the proxy to select between IPv4 and IPv6 addresses for the server according to its configuration, and receive addresses based on its network geolocation.

When replying to a SVCB query, authoritative DNS servers SHOULD return A, AAAA, and SVCB records in the Additional Section for any in-bailiwick TargetNames. If the zone is signed, the server SHOULD also include positive or negative DNSSEC responses for these records in the Additional section.

Recursive resolvers that are aware of SVCB SHOULD help the client to execute the procedure in with minimum overall latency, by incorporating additional useful information into the response. For the initial SVCB record query, this is just the normal response construction process (i.e. unknown RR type resolution under ). For followup resolutions performed during this procedure, we define incorporation as adding all useful RRs from the response to the Additional section without altering the response code. Upon receiving a SVCB query, recursive resolvers SHOULD start with the standard resolution procedure, and then follow this procedure to construct the full response to the stub resolver: Incorporate the results of SVCB resolution. If the chain length limit has been reached, terminate successfully (i.e. a NOERROR response). 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. If any SVCB records are resolved, go to step 1. Otherwise, incorporate the results of A and AAAA resolution, and terminate. All the resolved SVCB records are in ServiceMode. Resolve A and AAAA queries for each TargetName (or for the owner name if TargetName is “.”), incorporate all the results, and terminate. 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 ). See for additional safeguards for recursive resolvers to implement to mitigate loops. See for possible optimizations of this procedure.

Recursive resolvers SHOULD treat the SvcParams portion of the SVCB RR as opaque and SHOULD NOT try to alter their behavior based on its contents. When responding to a query that includes the DNSSEC OK bit (), 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).

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 ). 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 “echconfig” value of a SVCB response (). Clients implementing this optimization SHOULD wait for 50 milliseconds before starting optimistic pre-connection, as per the guidance in . 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:

If the client has an in-progress TCP connection to [2001:db8::2]:1234, it MAY proceed with TLS on that connection using echconfig="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 .

When following the procedure in , 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 ( 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 , 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.

A few initial SvcParamKeys are defined here. These keys are useful for HTTPS, and most are applicable to other protocols 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 and associated transport protocols supported by this service endpoint. As with Alt-Svc , the ALPN protocol identifier is used to identify the application protocol and associated suite of protocols supported by the endpoint (the “protocol suite”). 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). ALPNs are identified by their registered “Identification Sequence” (alpn-id), which is a sequence of 1-255 octets.

The presentation value SHALL be a comma-separated list () of one or more alpn-ids. 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” (). Each scheme that uses this SvcParamKey defines a “default set” of supported ALPNs, which SHOULD NOT be empty. To determine the set of protocol suites supported by an endpoint (the “SVCB ALPN set”), the client adds the default set to the list of alpn-ids unless the “no-default-alpn” SvcParamKey is present. The presence of an ALPN protocol 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 protocol. ALPN protocol names 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. To establish a connection to the endpoint, clients MUST Let SVCB-ALPN-Intersection be the set of protocols in the SVCB ALPN set that the client supports. Let Intersection-Transports be the set of transports (e.g. TLS, DTLS, QUIC) implied by the protocols in SVCB-ALPN-Intersection. For each transport in Intersection-Transports, construct a ProtocolNameList containing the Identification Sequences of all the client’s supported ALPN protocols for that transport, without regard to the SVCB ALPN set. 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 , without regard to the SVCB ALPN set. 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 “echconfig”. Its value is defined in . It is applicable to most TLS-based protocols. When publishing a record containing an “echconfig” 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 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 , 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 () of one or more IP addresses of the appropriate family in standard textual format . 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 . When only “ipv4hint” is present, IPv6-only clients may synthesize IPv6 addresses as specified in or ignore the “ipv4hint” key and wait for AAAA resolution (). Recursive resolvers MUST NOT perform DNS64 () 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 , 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 () of one or more valid SvcParamKeys, either by their registered name or in the unknown-key format (). Keys MAY appear in any order, but MUST NOT appear more than once. For self-consistency (), 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:

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 HTTPS and HTTP. To enable special handling for the HTTPS and 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 HTTPS RR wire format and presentation format are identical to SVCB, and both share the SvcParamKey registry. SVCB semantics apply equally to HTTPS RRs unless specified otherwise. The presentation format of the record is:

As with SVCB, the record is defined specifically within the Internet (“IN”) Class . All the SvcParamKeys defined in are permitted for use in HTTPS RRs. The default set of ALPN IDs is the single value “http/1.1”. The “automatically mandatory” keys () are “port” and “no-default-alpn”. (As described in , clients must either implement these keys or ignore any RR in which they appear.) Clients that restrict the HTTPS destination port (e.g. using the “bad ports” list from ) SHOULD apply the same restriction to the “port” SvcParam. The presence of an HTTPS RR for an origin also indicates that all HTTP resources are available over HTTPS, as discussed in . This allows HTTPS RRs to apply to pre-existing “http” scheme URLs, while ensuring that the client uses a secure and authenticated HTTPS connection. The HTTPS RR parallels the concepts introduced in the HTTP Alternative Services proposed standard . Clients and servers that implement HTTPS RRs are not required to implement Alt-Svc.

The HTTPS RR uses Port Prefix Naming (), 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 used in SVCB, this construction enables offline DNSSEC signing of wildcard domains, which are commonly used with HTTPS. Reusing the service name 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 , 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 HTTPS, and receiving an HTTPS RR is intended to be similar to receiving that field value over HTTPS. However, there are some differences in the intended client and server behavior.

Unlike Alt-Svc Field Values, HTTPS RRs can contain multiple ALPN IDs, and clients are encouraged to offer additional ALPNs that they support.

SVCB does not require or provide any assurance of authenticity. (DNSSEC signing and verification, which would provide such assurance, are OPTIONAL.) The DNS resolution process is treated as an untrusted channel that learns only the QNAME, and is prevented from mounting any attack beyond denial of service. 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 do not implement support for Encrypted ClientHello MAY skip the HTTPS RR query if a usable Alt-Svc value is available in the local cache. If Alt-Svc connection fails, these clients SHOULD fall back to the HTTPS RR client connection procedure (). Clients that implement support for ECH MUST perform the HTTPS RR query first, and MUST only make use of Alt-Svc when operating in SVCB-optional mode (see ). This specification does not alter the DNS records used when connecting to an Alt-Svc hostname (typically A and/or AAAA only).

Clients MUST NOT use an HTTPS RR response unless the client supports TLS Server Name Indication (SNI) and indicate 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.

By publishing a usable HTTPS RR, the server operator indicates that all useful HTTP resources on that origin are reachable over HTTPS, similar to HTTP Strict Transport Security . 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: Replace the “http” scheme with “https”. If the “http” URL explicitly specifies port 80, specify port 443. Do not alter any other aspect of the URL. This construction is equivalent to Section 8.3 of , point 5. If an HTTPS RR query for this “https” URL returns any AliasMode HTTPS RRs, or any compatible ServiceMode HTTPS RRs (see ), the client SHOULD act as if it has received an HTTP “307 Temporary Redirect” redirect to this “https” URL. (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. 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 .

All protocols employing “http://” or “https://” URLs SHOULD respect HTTPS RRs. For example, clients that support HTTPS RRs and implement the altered WebSocket opening handshake from the W3C Fetch specification SHOULD use HTTPS RRs for the requestURL. An HTTP-based protocol MAY define its own SVCB mapping. Such mappings MAY be defined to take precedence over HTTPS RRs.

The SVCB “echconfig” parameter is defined for conveying the ECH configuration of an alternative endpoint. In wire format, the value of the parameter is an ECHConfigList , including the redundant length prefix. In presentation format, the value is a single ECHConfigList encoded in 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 . 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 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 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 ) and all alternative endpoints have an “echconfig” 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 “echconfig” 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 “echconfig” 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 ServiceForm 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 () 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 ). 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:

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:

The domain owner could add this record:

to indicate that simple.example uses HTTPS, and supports QUIC in addition to HTTPS over TCP (an implicit default). The record could also include other information (e.g. non-standard port, ECH configuration).

Consider a zone that is using CNAME aliasing:

With HTTPS RRs, the owner of aliased.example could alias the apex by adding one additional record:

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 to HTTPS on aliased.example.) 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:

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:

Note that in the above example, the different CDNs have different echconfig 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: If CDN 1 supports ECH, and CDN 2 does not, the client is vulnerable to ECH downgrade by a network adversary who forces clients to get CDN 2 records. Aliasing the apex to its subdomain simplifies the zone file but likely increases resolution latency, especially when using a non-HTTPS-aware recursive resolver. An alternative would be to alias the zone apex directly to a name managed by a CDN. The A, AAAA, HTTPS resolutions are independent lookups so clients may observe and follow different CNAMEs to different CDNs. Clients may thus find a SvcDomainName pointing to a name other than the one which returned along with the A and AAAA lookups and will need to do an additional resolution for them. Including ipv6hint and ipv4hint will reduce the performance impact of this case. If not all CDNs publish HTTPS records, clients will sometimes receive NODATA for HTTPS queries (as with cdn3.svc3.example above), and thus no echconfig, but could receive A/AAAA records from a different CDN which does support ECH. Clients will be unable to use ECH in this case.

For services other than HTTPS, the SVCB RR and an Attrleaf label 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:

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 and OCSP Stapling , 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 or DNS over HTTPS ). 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 . The scheme SHOULD have an entry in the IANA URI Schemes Registry . 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 ). 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 (), and disable the encryption enabled by the echconfig SvcParamKey (). To prevent downgrades, recommends that clients abandon the connection attempt when such an attack is detected. A hostile DNS intermediary might forge AliasForm “.” records () 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 AliasForm “.” 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: Type: SVCB Value: 64 Meaning: General Purpose Service Endpoints Reference: This document

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: Type: HTTPS Value: 65 Meaning: HTTPS Specific Service Endpoints Reference: This document

The “Service Binding (SVCB) Parameter 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 and include a reference to this registry.

A registration MUST include the following fields: Number: wire format numeric identifier (range 0-65535) Name: unique presentation name Meaning: a short description Format Reference: pointer to specification text The characters in the registered Name MUST be lower-case alphanumeric or “-“ (). The name MUST NOT start with “key” or “invalid”. Entries in this registry are subject to a First Come First Served registration policy (, Section 4.6). 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) 1 alpn Additional supported protocols (This document) 2 no-default-alpn No support for default protocol (This document) 3 port Port for alternative endpoint (This document) 4 ipv4hint IPv4 address hints (This document) 5 echconfig Encrypted ClientHello info (This document) 6 ipv6hint IPv6 address hints (This document) 65280-65534 N/A Private Use (This document) 65535 N/A Reserved (“Invalid key”) (This document)

Per , please add the following entries to the data type range of the Resource Record (RR) TYPEs registry: TYPE Meaning Reference SVCB Service Location and Parameter Binding (This document) HTTPS HTTPS Service Location and Parameter Binding (This document) Per , 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)

There have been a wide range of proposed solutions over the years to the “CNAME at the Zone Apex” challenge proposed. These include , , and others. Thank you to Ian Swett, Ralf Weber, Jon Reed, Martin Thomson, Lucas Pardue, Ilari Liusvaara, Tim Wicinski, Tommy Pauly, Chris Wood, David Benjamin, Mark Andrews, Emily Stark, Eric Orth, Kyle Rose, Craig Taylor, Dan McArdle, Brian Dickson, and others for their feedback and suggestions on this draft.

This document describes a mechanism in Transport Layer Security (TLS) for encrypting a ClientHello message under a server public key. Formally, any DNS Resource Record (RR) may occur under any domain name. However, some services use an operational convention for defining specific interpretations of an RRset by locating the records in a DNS branch under the parent domain to which the RRset actually applies. The top of this subordinate branch is defined by a naming convention that uses a reserved node name, which begins with the underscore character (e.g., "_name"). The underscored naming construct defines a semantic scope for DNS record types that are associated with the parent domain above the underscored branch. This specification explores the nature of this DNS usage and defines the "Underscored and Globally Scoped DNS Node Names" registry with IANA. The purpose of this registry is to avoid collisions resulting from the use of the same underscored name for different services. The QUIC transport protocol has several features that are desirable in a transport for HTTP, such as stream multiplexing, per-stream flow control, and low-latency connection establishment. This document describes a mapping of HTTP semantics over QUIC. This document also identifies HTTP/2 features that are subsumed by QUIC, and describes how HTTP/2 extensions can be ported to HTTP/3. DO NOT DEPLOY THIS VERSION OF HTTP DO NOT DEPLOY THIS VERSION OF HTTP/3 UNTIL IT IS IN AN RFC. This version is still a work in progress. For trial deployments, please use earlier versions. Note to Readers Discussion of this draft takes place on the QUIC working group mailing list (quic@ietf.org), which is archived at https://mailarchive.ietf.org/arch/search/?email_list=quic. Working Group information can be found at https://github.com/quicwg; source code and issues list for this draft can be found at https://github.com/quicwg/base-drafts/labels/-http. This specification defines a mechanism enabling web sites to declare themselves accessible only via secure connections and/or for users to be able to direct their user agent(s) to interact with given sites only over secure connections. This overall policy is referred to as HTTP Strict Transport Security (HSTS). The policy is declared by web sites via the Strict-Transport-Security HTTP response header field and/or by other means, such as user agent configuration, for example. [STANDARDS-TRACK] In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. This RFC is the revised specification of the protocol and format used in the implementation of the Domain Name System. It obsoletes RFC-883. This memo documents the details of the domain name client - server communication. Internet technical specifications often need to define a formal syntax. Over the years, a modified version of Backus-Naur Form (BNF), called Augmented BNF (ABNF), has been popular among many Internet specifications. The current specification documents ABNF. It balances compactness and simplicity with reasonable representational power. The differences between standard BNF and ABNF involve naming rules, repetition, alternatives, order-independence, and value ranges. This specification also supplies additional rule definitions and encoding for a core lexical analyzer of the type common to several Internet specifications. [STANDARDS-TRACK] Many communication protocols operating over the modern Internet use hostnames. These often resolve to multiple IP addresses, each of which may have different performance and connectivity characteristics. Since specific addresses or address families (IPv4 or IPv6) may be blocked, broken, or sub-optimal on a network, clients that attempt multiple connections in parallel have a chance of establishing a connection more quickly. This document specifies requirements for algorithms that reduce this user-visible delay and provides an example algorithm, referred to as "Happy Eyeballs". This document obsoletes the original algorithm description in RFC 6555. This document describes the use of Transport Layer Security (TLS) to provide privacy for DNS. Encryption provided by TLS eliminates opportunities for eavesdropping and on-path tampering with DNS queries in the network, such as discussed in RFC 7626. In addition, this document specifies two usage profiles for DNS over TLS and provides advice on performance considerations to minimize overhead from using TCP and TLS with DNS.This document focuses on securing stub-to-recursive traffic, as per the charter of the DPRIVE Working Group. It does not prevent future applications of the protocol to recursive-to-authoritative traffic. This document defines a protocol for sending DNS queries and getting DNS responses over HTTPS. Each DNS query-response pair is mapped into an HTTP exchange. The Hypertext Transfer Protocol (HTTP) is a stateless \%application- level protocol for distributed, collaborative, hypertext information systems. This document defines the semantics of HTTP/1.1 messages, as expressed by request methods, request header fields, response status codes, and response header fields, along with the payload of messages (metadata and body content) and mechanisms for content negotiation. This memo describes a protocol that is an evolution of the previous version of the protocol, version 4 [1]. This new protocol stems from active discussions and prototype implementations. [STANDARDS-TRACK] Extending the Domain Name System (DNS) with new Resource Record (RR) types currently requires changes to name server software. This document specifies the changes necessary to allow future DNS implementations to handle new RR types transparently. [STANDARDS-TRACK] The DNAME record provides redirection for a subtree of the domain name tree in the DNS. That is, all names that end with a particular suffix are redirected to another part of the DNS. This document obsoletes the original specification in RFC 2672 as well as updates the document on representing IPv6 addresses in DNS (RFC 3363). [STANDARDS-TRACK] In order to deploy DNSSEC (Domain Name System Security Extensions) operationally, DNSSEC aware servers should only perform automatic inclusion of DNSSEC RRs when there is an explicit indication that the resolver can understand those RRs. This document proposes the use of a bit in the EDNS0 header to provide that explicit indication and describes the necessary protocol changes to implement that notification. [STANDARDS-TRACK] This document describes a Transport Layer Security (TLS) extension for application-layer protocol negotiation within the TLS handshake. For instances in which multiple application protocols are supported on the same TCP or UDP port, this extension allows the application layer to negotiate which protocol will be used within the TLS connection. As IPv6 deployment increases, there will be a dramatic increase in the need to use IPv6 addresses in text. While the IPv6 address architecture in Section 2.2 of RFC 4291 describes a flexible model for text representation of an IPv6 address, this flexibility has been causing problems for operators, system engineers, and users. This document defines a canonical textual representation format. It does not define a format for internal storage, such as within an application or database. It is expected that the canonical format will be followed by humans and systems when representing IPv6 addresses as text, but all implementations must accept and be able to handle any legitimate RFC 4291 format. [STANDARDS-TRACK] This document describes a method for detecting the presence of DNS64 and for learning the IPv6 prefix used for protocol translation on an access network. The method depends on the existence of a well-known IPv4-only fully qualified domain name "ipv4only.arpa.". The information learned enables nodes to perform local IPv6 address synthesis and to potentially avoid NAT64 on dual-stack and multi-interface deployments. DNS64 is a mechanism for synthesizing AAAA records from A records. DNS64 is used with an IPv6/IPv4 translator to enable client-server communication between an IPv6-only client and an IPv4-only server, without requiring any changes to either the IPv6 or the IPv4 node, for the class of applications that work through NATs. This document specifies DNS64, and provides suggestions on how it should be deployed in conjunction with IPv6/IPv4 translators. [STANDARDS-TRACK] The WebSocket Protocol enables two-way communication between a client running untrusted code in a controlled environment to a remote host that has opted-in to communications from that code. The security model used for this is the origin-based security model commonly used by web browsers. The protocol consists of an opening handshake followed by basic message framing, layered over TCP. The goal of this technology is to provide a mechanism for browser-based applications that need two-way communication with servers that does not rely on opening multiple HTTP connections (e.g., using XMLHttpRequest or <iframe>s and long polling). [STANDARDS-TRACK] This document describes the commonly used base 64, base 32, and base 16 encoding schemes. It also discusses the use of line-feeds in encoded data, use of padding in encoded data, use of non-alphabet characters in encoded data, use of different encoding alphabets, and canonical encodings. [STANDARDS-TRACK] This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery.This document updates RFCs 5705 and 6066, and obsoletes RFCs 5077, 5246, and 6961. This document also specifies new requirements for TLS 1.2 implementations. This document provides specifications for existing TLS extensions. It is a companion document for RFC 5246, "The Transport Layer Security (TLS) Protocol Version 1.2". The extensions specified are server_name, max_fragment_length, client_certificate_url, trusted_ca_keys, truncated_hmac, and status_request. [STANDARDS-TRACK] This document updates the guidelines and recommendations, as well as the IANA registration processes, for the definition of Uniform Resource Identifier (URI) schemes. It obsoletes RFC 4395. Many protocols make use of points of extensibility that use constants to identify various protocol parameters. To ensure that the values in these fields do not have conflicting uses and to promote interoperability, their allocations are often coordinated by a central record keeper. For IETF protocols, that role is filled by the Internet Assigned Numbers Authority (IANA).To make assignments in a given registry prudently, guidance describing the conditions under which new values should be assigned, as well as when and how modifications to existing values can be made, is needed. This document defines a framework for the documentation of these guidelines by specification authors, in order to assure that the provided guidance for the IANA Considerations is clear and addresses the various issues that are likely in the operation of a registry.This is the third edition of this document; it obsoletes RFC 5226. This document describes a DNS RR which specifies the location of the server(s) for a specific protocol and domain. [STANDARDS-TRACK] This document specifies "Alternative Services" for HTTP, which allow an origin's resources to be authoritatively available at a separate network location, possibly accessed with a different protocol configuration. This document defines the concept of an "origin", which is often used as the scope of authority or privilege by user agents. Typically, user agents isolate content retrieved from different origins to prevent malicious web site operators from interfering with the operation of benign web sites. In addition to outlining the principles that underlie the concept of origin, this document details how to determine the origin of a URI and how to serialize an origin into a string. It also defines an HTTP header field, named "Origin", that indicates which origins are associated with an HTTP request. [STANDARDS-TRACK] The Domain Name System (DNS) is defined in literally dozens of different RFCs. The terminology used by implementers and developers of DNS protocols, and by operators of DNS systems, has sometimes changed in the decades since the DNS was first defined. This document gives current definitions for many of the terms used in the DNS in a single document.This document obsoletes RFC 7719 and updates RFC 2308. A Uniform Resource Identifier (URI) is a compact sequence of characters that identifies an abstract or physical resource. This specification defines the generic URI syntax and a process for resolving URI references that might be in relative form, along with guidelines and security considerations for the use of URIs on the Internet. The URI syntax defines a grammar that is a superset of all valid URIs, allowing an implementation to parse the common components of a URI reference without knowing the scheme-specific requirements of every possible identifier. This specification does not define a generative grammar for URIs; that task is performed by the individual specifications of each URI scheme. [STANDARDS-TRACK] This document specifies Internet Assigned Numbers Authority (IANA) parameter assignment considerations for the allocation of Domain Name System (DNS) resource record types, CLASSes, operation codes, error codes, DNS protocol message header bits, and AFSDB resource record subtypes. It obsoletes RFC 6195 and updates RFCs 1183, 2845, 2930, and 3597. This document specifies an "HTTP" resource record type for the DNS to facilitate the lookup of the server hostname of HTTP(s) URIs. It is intended to replace the use of CNAME records for this purpose, and in the process provides a solution for the inability of the DNS to allow a CNAME to be placed at the apex of a domain name. This document defines the "ANAME" DNS RR type, to provide similar functionality to CNAME, but only for address queries. Unlike CNAME, an ANAME can coexist with other record types. The ANAME RR allows zone owners to make an apex domain name into an alias in a standards compliant manner.

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 . Here we summarize the allowed input to that algorithm, using ABNF:

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 “,” is not escaped (by a preceding “\”), it separates items in the list. (For simplicity, empty items are not allowed.)

Decoding of value-lists happens after character-string decoding. For example, consider these char-string SvcParamValues:

These inputs are equivalent: character-string decoding either of them would produce the same value:

Applying comma-separated list decoding to this value would produce a list of three items:

This table serves as a non-normative summary of the HTTP mapping for SVCB (). 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 This table does not indicate any SvcParamKeys that servers are required to publish, or that clients are required to implement, because there are none in this mapping.

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 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: SRV records are typically mandatory, whereas clients will always continue to function correctly without making use of SVCB. SRV records cannot instruct the client to switch or upgrade protocols, whereas SVCB can signal such an upgrade (e.g. to HTTP/2). SRV records are not extensible, whereas SVCB and HTTPS RRs can be extended with new parameters. SVCB records use 16 bit for SvcPriority for consistency with SRV and other RR types that also use 16 bit priorities.

Unlike , 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 , 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.

draft-ietf-dnsop-svcb-https-04 Simplify the IANA instructions (pure First Come First Served) Recommend against publishing chains of >8 aliases Clarify requirements for using SVCB with a transport proxy Adjust guidance for Port Prefix Naming Minor editorial updates draft-ietf-dnsop-svcb-https-03 Simplified escaping of comma-separated values Reorganized client requirements Added a warning about port filtering for cross-protocol attacks Clarified self-consistency rules for SvcParams Added a non-normative mapping summary table for HTTPS draft-ietf-dnsop-svcb-https-02 Added a Privacy Considerations section Adjusted resolution fallback description Clarified status of SvcParams in AliasMode Improved advice on zone structuring and use with Alt-Svc Improved examples, including a new Multi-CDN example Reorganized text on value-list parsing and SvcPriority Improved phrasing and other editorial improvements throughout draft-ietf-dnsop-svcb-https-01 Added a “mandatory” SvcParamKey Added the ability to indicate that a service does not exist Adjusted resolution and ALPN algorithms Major terminology revisions for “origin” and CamelCase names Revised ABNF Include allocated RR type numbers Various corrections, explanations, and recommendations draft-ietf-dnsop-svcb-https-00 Rename HTTPSSVC RR to HTTPS RR Rename “an SVCB” to “a SVCB” Removed “design considerations and open issues” section and some other “to be removed” text draft-ietf-dnsop-svcb-httpssvc-03 Revised chain length limit requirements Revised IANA registry rules for SvcParamKeys Require HTTPS clients to implement SNI Update terminology for Encrypted ClientHello Clarifications: non-default ports, transport proxies, HSTS procedure, WebSocket behavior, wire format, IP hints, inner/outer ClientHello with ECH Various textual and ABNF corrections draft-ietf-dnsop-svcb-httpssvc-02 All changes to Alt-Svc have been removed Expanded and reorganized examples Priority zero is now the definition of AliasForm Repeated SvcParamKeys are no longer allowed The “=” sign may be omitted in a key=value pair if the value is also empty In the wire format, SvcParamKeys must be in sorted order New text regarding how to handle resolution timeouts Expanded description of recursive resolver behavior Much more precise description of the intended ALPN behavior Match the HSTS specification’s language on HTTPS enforcement Removed ‘esniconfig=””’ mechanism and simplified ESNI connection logic draft-ietf-dnsop-svcb-httpssvc-01 Reduce the emphasis on conversion between HTTPSSVC and Alt-Svc Make the “untrusted channel” concept more precise. Make SvcFieldPriority = 0 the definition of AliasForm, instead of a requirement. draft-ietf-dnsop-svcb-httpssvc-00 Document an optimization for optimistic pre-connection. (Chris Wood) Relax IP hint handling requirements. (Eric Rescorla) draft-nygren-dnsop-svcb-httpssvc-00 Generalize to an SVCB record, with special-case handling for Alt-Svc and HTTPS separated out to dedicated sections. Split out a separate HTTPSSVC record for the HTTPS use-case. Remove the explicit SvcRecordType=0/1 and instead make the AliasForm vs ServiceForm be implicit. This was based on feedback recommending against subtyping RR type. Remove one optimization. draft-nygren-httpbis-httpssvc-03 Change redirect type for HSTS-style behavior from 302 to 307 to reduce ambiguities. draft-nygren-httpbis-httpssvc-02 Remove the redundant length fields from the wire format. Define a SvcDomainName of “.” for SvcRecordType=1 as being the HTTPSSVC RRNAME. Replace “hq” with “h3”. draft-nygren-httpbis-httpssvc-01 Fixes of record name. Replace references to “HTTPSVC” with “HTTPSSVC”. draft-nygren-httpbis-httpssvc-00 Initial version