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Service Binding and Parameter Specification via the DNS (SVCB and HTTPS Resource Records)
RFC 9460

Document Type RFC - Proposed Standard (November 2023)
Authors Benjamin M. Schwartz , Mike Bishop , Erik Nygren
Last updated 2024-04-16
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
IESG Responsible AD Warren "Ace" Kumari
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RFC 9460

Internet Engineering Task Force (IETF)                       B. Schwartz
Request for Comments: 9460                          Meta Platforms, Inc.
Category: Standards Track                                      M. Bishop
ISSN: 2070-1721                                                E. Nygren
                                                     Akamai Technologies
                                                           November 2023

Service Binding and Parameter Specification via the DNS (SVCB and HTTPS
                           Resource Records)


   This document specifies the "SVCB" ("Service Binding") 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 are extensible to support
   future uses (such as 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 (see
   RFC 9110, "HTTP Semantics").  By providing more information to the
   client before it attempts to establish a connection, these records
   offer potential benefits to both performance and privacy.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Goals
     1.2.  Overview of the SVCB RR
     1.3.  Terminology
   2.  The SVCB Record Type
     2.1.  Zone-File Presentation Format
     2.2.  RDATA Wire Format
     2.3.  SVCB Query Names
     2.4.  Interpretation
       2.4.1.  SvcPriority
       2.4.2.  AliasMode
       2.4.3.  ServiceMode
     2.5.  Special Handling of "." in TargetName
       2.5.1.  AliasMode
       2.5.2.  ServiceMode
   3.  Client Behavior
     3.1.  Handling Resolution Failures
     3.2.  Clients Using a Proxy
   4.  DNS Server Behavior
     4.1.  Authoritative Servers
     4.2.  Recursive Resolvers
       4.2.1.  DNS64
     4.3.  General Requirements
     4.4.  EDNS Client Subnet (ECS)
   5.  Performance Optimizations
     5.1.  Optimistic Pre-connection and Connection Reuse
     5.2.  Generating and Using Incomplete Responses
   6.  SVCB-Compatible RR Types
   7.  Initial SvcParamKeys
     7.1.  "alpn" and "no-default-alpn"
       7.1.1.  Representation
       7.1.2.  Use
     7.2.  "port"
     7.3.  "ipv4hint" and "ipv6hint"
     7.4.  "mandatory"
   8.  ServiceMode RR Compatibility and Mandatory Keys
   9.  Using Service Bindings with HTTP
     9.1.  Query Names for HTTPS RRs
     9.2.  Comparison with Alt-Svc
       9.2.1.  ALPN Usage
       9.2.2.  Untrusted Channels
       9.2.3.  Cache Lifetime
       9.2.4.  Granularity
     9.3.  Interaction with Alt-Svc
     9.4.  Requiring Server Name Indication
     9.5.  HTTP Strict Transport Security (HSTS)
     9.6.  Use of HTTPS RRs in Other Protocols
   10. Zone Structures
     10.1.  Structuring Zones for Flexibility
     10.2.  Structuring Zones for Performance
     10.3.  Operational Considerations
     10.4.  Examples
       10.4.1.  Protocol Enhancements
       10.4.2.  Apex Aliasing
       10.4.3.  Parameter Binding
       10.4.4.  Multi-CDN Configuration
       10.4.5.  Non-HTTP Uses
   11. Interaction with Other Standards
   12. Security Considerations
   13. Privacy Considerations
   14. IANA Considerations
     14.1.  SVCB RR Type
     14.2.  HTTPS RR Type
     14.3.  New Registry for Service Parameters
       14.3.1.  Procedure
       14.3.2.  Initial Contents
     14.4.  Other Registry Updates
   15. References
     15.1.  Normative References
     15.2.  Informative References
   Appendix A.  Decoding Text in Zone Files
     A.1.  Decoding a Comma-Separated List
   Appendix B.  HTTP Mapping Summary
   Appendix C.  Comparison with Alternatives
     C.1.  Differences from the SRV RR Type
     C.2.  Differences from the Proposed HTTP Record
     C.3.  Differences from the Proposed ANAME Record
     C.4.  Comparison with Separate RR Types for AliasMode and
   Appendix D.  Test Vectors
     D.1.  AliasMode
     D.2.  ServiceMode
     D.3.  Failure Cases
   Acknowledgments and Related Proposals
   Authors' Addresses

1.  Introduction

   The SVCB ("Service Binding") and HTTPS resource records (RRs) provide
   clients with complete instructions for access to a service.  This
   information enables improved performance and privacy by avoiding
   transient connections to a suboptimal 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., aliasing the origin "" to a service operator
   endpoint at "".  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 RR
   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 and 2) "ServiceMode", which binds
   together configuration information for a service endpoint.
   ServiceMode provides additional key=value parameters within each
   RDATA set.

1.1.  Goals

   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

   *  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 RRsets for the same name.  For example, if
      "" is a CNAME alias that switches between one of
      three Content Delivery Networks (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
      "") for participating protocols and generally enables
      extending operational authority for a service identified by a
      domain name to other instances with alternate names.

   Additional goals specific to HTTPS RRs and the HTTP use cases

   *  Connecting directly to HTTP/3 (QUIC transport) alternative
      endpoints [HTTP/3].

   *  Supporting non-default TCP and UDP ports.

   *  Enabling SRV-like benefits (e.g., apex aliasing, as mentioned
      above) for HTTP, where SRV [SRV] has not been widely adopted.

   *  Providing an indication signaling that the "https" scheme should
      be used instead of "http" for all HTTP requests to this host and
      port, similar to HTTP Strict Transport Security [HSTS] (see
      Section 9.5).

   *  Enabling the conveyance of Encrypted ClientHello keys [ECH]
      associated with an alternative endpoint.

1.2.  Overview of the SVCB RR

   This subsection briefly describes the SVCB RR with forward references
   to the full exposition of each component.  (As discussed in
   Section 6, this all applies equally to the HTTPS RR, which shares the
   same encoding, format, and high-level semantics.)

   The SVCB RR has two modes: 1) AliasMode (Section 2.4.2), which
   aliases a name to another name and 2) ServiceMode (Section 2.4.3),
   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 (Section 2.3).

   The SVCB RR has two required fields and one optional field.  The
   fields are:

   SvcPriority (Section 2.4.1):  The priority of this record (relative
      to others, with lower values preferred).  A value of 0 indicates

   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).  SvcParams are described in Section 2.1.

   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 (Section 4.2).  Clients either use
   responses included in the Additional section returned by the
   recursive resolver or perform necessary SVCB, A, and AAAA record
   resolutions (Section 3).  DNS authoritative servers can attach in-
   bailiwick SVCB, A, AAAA, and CNAME records in the Additional section
   to responses for a SVCB query (Section 4.1).

   In ServiceMode, the SvcParams of the SVCB RR provide an extensible
   data model for describing alternative endpoints that are
   authoritative for a service, along with parameters associated with
   each of these alternative endpoints (Section 7).

   For HTTP use cases, the HTTPS RR (Section 9) enables many of the
   benefits of Alt-Svc [AltSvc] without waiting for a full HTTP
   connection initiation (multiple round trips) before learning of the
   preferred alternative, and without necessarily revealing the user's
   intended destination to all entities along the network path.

1.3.  Terminology

   Terminology in this document 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
      "origin" [RFC6454].

   *  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 a service.

   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",
   "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.

2.  The SVCB Record Type

   The SVCB DNS 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 RR 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.  Each SvcParam has a specified
   presentation format (used in zone files) and wire encoding (e.g.,
   domain names, binary data, or numeric values).  The initial
   SvcParamKeys and their formats are defined in Section 7.

2.1.  Zone-File Presentation Format

   The presentation format <RDATA> of the record ([RFC1035],
   Section 5.1) has the form:

   SvcPriority TargetName SvcParams

   The SVCB record is defined specifically within the Internet ("IN")
   Class ([RFC1035], Section 3.2.4).

   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 registered by IANA (Section 14.3).

   Each SvcParamKey SHALL appear at most once in the SvcParams.  In
   presentation format, SvcParamKeys are lowercase alphanumeric strings.
   Key names 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 ; See Appendix A.
   value         = *OCTET ; Value before key-specific parsing

   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

   When the optional "=" and SvcParamValue are 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.

2.2.  RDATA Wire Format

   The RDATA for the SVCB RR consists of:

   *  a 2-octet field for SvcPriority as an integer in network byte

   *  the uncompressed, fully qualified TargetName, represented as a
      sequence of length-prefixed labels per Section 3.1 of [RFC1035].

   *  the SvcParams, consuming the remainder of the record (so smaller
      than 65535 octets and constrained by the RDATA and DNS message

   When the list of SvcParams is non-empty, 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 Section 14.3.2 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.

   *  an octet string of this length whose contents are the
      SvcParamValue 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 a SvcParamKey does not have the expected

   Note that the second condition implies that there are no duplicate

   If any RRs are malformed, the client MUST reject the entire RRset and
   fall back to non-SVCB connection establishment.

2.3.  SVCB Query Names

   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
   "".  This follows the Attrleaf naming
   pattern [Attrleaf], so the scheme MUST be registered appropriately
   with IANA (see Section 11).

   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.
   This document terms this behavior "Port Prefix Naming" and uses it in
   the examples throughout.

   See Section 9.1 for information regarding 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, as in ordinary CNAME
   processing ([RFC1034], Section 3.6.2).  For details, see the
   recommendations regarding aliases for clients (Section 3), servers
   (Section 4), and zones (Section 10).

   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 "" could publish this record: 7200 IN SVCB 0

   This record would indicate that "foo://" is
   aliased to "".  The owner of "", in turn,
   could publish this record:  7200  IN SVCB 3 (
       alpn="bar" port="8004" )

   This record would 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, per Section 5.1 of [RFC1035].)

2.4.  Interpretation

2.4.1.  SvcPriority

   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.  A smaller SvcPriority
   indicates that the domain owner recommends the use of this record
   over ServiceMode 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.

2.4.2.  AliasMode

   In AliasMode, the SVCB record aliases a service to a TargetName.
   SVCB RRsets SHOULD only have a single RR in AliasMode.  If multiple
   AliasMode RRs 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 (see, for example, [RFC1912],
   Section 2.4).  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.)  Unlike CNAME, AliasMode
   records do not affect the resolution of other RR types and apply only
   to a specific service, not an entire domain name.

   The AliasMode TargetName SHOULD NOT be equal to the owner name, as
   this would result in a loop.  In AliasMode, recipients MUST ignore
   any SvcParams that are present.  Zone-file parsers MAY emit a warning
   if an AliasMode record has SvcParams.  The use of SvcParams in
   AliasMode records is currently not defined, but a future
   specification could extend AliasMode records to include SvcParams.

   For example, the operator of "foo://" could point
   requests to a service operating at "" by
   publishing: 3600 IN SVCB 0

   Using AliasMode maintains a separation of concerns: the owner of
   "" can add or remove ServiceMode SVCB records
   without requiring a corresponding change to "".  Note that
   if "" promises to always publish a SVCB record,
   this AliasMode record can be replaced by a CNAME at the same owner

   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 "" could point requests
   to a server at "" by publishing this record at the
   zone apex: 3600 IN HTTPS 0

   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.

   Zones that require following multiple AliasMode records could
   encounter compatibility and performance issues.

   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 or vice

2.4.3.  ServiceMode

   In ServiceMode, the TargetName and SvcParams within each RR associate
   an alternative endpoint for the service with its connection

   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 other's requirements.  Clients MUST reject any RR
   whose recognized SvcParams are not self-consistent and MAY reject the
   entire RRset.  To help zone operators avoid this condition, zone-file
   implementations SHOULD enforce self-consistency as well.

2.5.  Special Handling of "." in TargetName

   If TargetName has the value "." (represented in the wire format as a
   zero-length label), special rules apply.

2.5.1.  AliasMode

   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.

2.5.2.  ServiceMode

   For ServiceMode SVCB RRs, if TargetName has the value ".", then the
   owner name of this record MUST be used as the effective TargetName.
   If the record has a wildcard owner name in the zone file, the
   recipient SHALL use the response's synthesized owner name as the
   effective TargetName.

   Here, for example, "" is the effective TargetName:      7200  IN HTTPS 0  7200  IN CNAME 7200  IN HTTPS 1 . port=8002 300   IN A 300   IN AAAA  2001:db8::2

3.  Client Behavior

   "SVCB resolution" is the process of enumerating and ordering the
   available endpoints for a service, as performed by the client.  SVCB
   resolution is implemented as follows:

   1.  Let $QNAME be the service name plus appropriate prefixes for the
       scheme (see Section 2.3).

   2.  Issue a SVCB query for $QNAME.

   3.  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
       Section 3.1).

   4.  If one or more "compatible" (Section 8) ServiceMode records are
       returned, these represent the alternative endpoints.  Sort the
       records by ascending SvcPriority.

   5.  Otherwise, SVCB resolution has failed, and the list of available
       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

   A client is called "SVCB-optional" if it can connect without the use
   of ServiceMode records; otherwise, it is called "SVCB-reliant".
   Clients for pre-existing protocols (e.g., HTTP) SHALL implement SVCB-
   optional behavior (except as noted in Section 3.1 or when modified by
   future specifications).

   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, if at
   least one AliasMode record was processed, SVCB-optional clients SHALL
   append to the list of endpoints 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 this 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 now attempts to use non-SVCB
   connection modes.

   Some important optimizations are discussed in Section 5 to avoid
   additional latency in comparison to ordinary AAAA/A lookups.

3.1.  Handling Resolution Failures

   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 its attempt to reach the service, 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.  Otherwise, an
   attacker could defeat the A/AAAA protection by forging SVCB responses
   that direct the client to other IP addresses.

   If DNS responses are not cryptographically protected, clients MAY
   treat SVCB resolution failure as fatal or nonfatal.

   If the client is unable to complete SVCB resolution due to its chain
   length limit, the client MUST fall back to the authority endpoint, as
   if the service's SVCB record did not exist.

3.2.  Clients Using a Proxy

   Clients using a domain-oriented transport proxy like HTTP CONNECT
   ([RFC7231], Section 4.3.6) or SOCKS5 [RFC1928] have the option of
   using named destinations, in which case the client does not perform
   any A or AAAA queries for destination domains.  If the client is
   configured to use 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
   and not just the proxy.  Clients in this configuration SHOULD arrange
   for a separate SVCB resolution procedure with appropriate privacy
   properties.  If this is not possible, SVCB-optional clients MUST
   disable SVCB resolution entirely, and SVCB-reliant clients MUST treat
   the configuration as invalid.

   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:

      -  It allows the client to use the SvcParams, if present, which
         are only usable with a specific TargetName.  The SvcParams may
         include information that enhances performance (e.g., supported
         protocols) and privacy.

      -  It allows a service on an apex domain to use aliasing.

   *  Compared to providing the proxy with an IP address:

      -  It allows the proxy to select between IPv4 and IPv6 addresses
         for the server according to its configuration.

      -  It ensures that the proxy receives addresses based on its
         network geolocation, not the client's.

      -  It enables faster fallback for TCP destinations with multiple
         addresses of the same family.

4.  DNS Server Behavior

4.1.  Authoritative Servers

   When replying to a SVCB query, authoritative DNS servers SHOULD
   return A, AAAA, and SVCB records in the Additional section for any
   TargetNames that are in the zone.  If the zone is signed, the server
   SHOULD also include DNSSEC records authenticating the existence or
   nonexistence of these records in the Additional section.

   See Section 4.4 for exceptions.

4.2.  Recursive Resolvers

   Whether the recursive resolver is aware of SVCB or not, the normal
   response construction process used for unknown RR types [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:

   1.  Incorporate the results of SVCB resolution.  If the recursive
       resolver's local chain length limit (which may be different from
       the client's limit) has been reached, terminate.

   2.  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

       *  If any SVCB records are resolved, go to Step 1.

       *  Otherwise, incorporate the results of A and AAAA resolution,
          and terminate.

   3.  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 [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.

4.2.1.  DNS64

   DNS64 resolvers synthesize responses to AAAA queries for names that
   only have an A record (Section 5.1.7 of [RFC6147]).  SVCB-aware DNS64
   resolvers SHOULD apply the same synthesis logic when resolving AAAA
   records for the TargetName for inclusion in the Additional section
   (Step 2 in Section 4.2) and MAY omit the A records from this section.

   DNS64 resolvers MUST NOT extrapolate the AAAA synthesis logic to the
   IP hints in the SvcParams (Section 7.3).  Modifying the IP hints
   would break DNSSEC validation for the SVCB record and would not
   improve performance when the above recommendation is implemented.

4.3.  General Requirements

   Recursive resolvers MUST be able to convey SVCB records with
   unrecognized SvcParamKeys.  Resolvers MAY accomplish this by treating
   the entire SvcParams portion of the record as opaque, even if the
   contents are invalid.  If a recognized SvcParamKey is followed by a
   value that is invalid according to the SvcParam's specification, a
   recursive resolver MAY report an error such as SERVFAIL instead of
   returning the record.  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.

4.4.  EDNS Client Subnet (ECS)

   The EDNS Client Subnet (ECS) option [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 set to 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 follow-up
   query that can receive a properly tailored ECS.  (This is similar to
   the usage of CNAME with the ECS option as discussed in [RFC7871],
   Section 7.2.1.)

   Authoritative servers that omit Additional records can avoid the
   added latency of a follow-up query by following the advice in
   Section 10.2.

5.  Performance Optimizations

   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 follow-up queries.  With this behavior, and
   with 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 10.2).

   After a ServiceMode RRset is received, clients MAY try more than one
   option in parallel and MAY prefetch A and AAAA records for multiple

5.1.  Optimistic Pre-connection and Connection Reuse

   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, future
   SvcParamKeys could be defined that alter the TLS ClientHello.

   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: 300 IN SVCB 1 (
       ipv6hint=2001:db8::1 port=1234 )
                                  SVCB 2 (
       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, 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.

5.2.  Generating and Using Incomplete Responses

   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 (Step 1 in Section 4.2) 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.

6.  SVCB-Compatible RR Types

   An RR type is called "SVCB-compatible" if it permits an
   implementation that is identical to SVCB in its:

   *  RDATA presentation format

   *  RDATA wire format

   *  IANA registry used for SvcParamKeys

   *  Authoritative server Additional section processing

   *  Recursive resolution process

   *  Relevant Class (i.e., Internet ("IN") [RFC1035])

   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
   follow-up 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.

7.  Initial SvcParamKeys

   A few initial SvcParamKeys are defined here.  These keys are useful
   for the "https" scheme, and most are expected to be generally
   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.

7.1.  "alpn" and "no-default-alpn"

   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.

7.1.1.  Representation

   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 in ALPN IDs 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

   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

   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
   ALPN IDs that are supported by nearly all clients and servers; this
   set MAY be empty.  To determine the SVCB ALPN set, the client starts
   with the list of alpn-ids from the "alpn" SvcParamKey, and it adds
   the default set unless the "no-default-alpn" SvcParamKey is present.

7.1.2.  Use

   To establish a connection to the endpoint, clients MUST

   1.  Let SVCB-ALPN-Intersection be the set of protocols in the SVCB
       ALPN set that the client supports.

   2.  Let Intersection-Transports be the set of transports (e.g., TLS,
       DTLS, QUIC) implied by the protocols in SVCB-ALPN-Intersection.

   3.  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 [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
   compatible RRs indicate "no-default-alpn", even if connection could
   have succeeded using a non-default ALPN protocol.

   Zone operators SHOULD ensure that at least one RR in each RRset
   supports the default transports.  This enables compatibility with the
   greatest number of clients.

7.2.  "port"

   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
   SvcParamValue 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.

7.3.  "ipv4hint" and "ipv6hint"

   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 per 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] [RFC4001].  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 (for the respective address family).  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, NAT64 clients may synthesize IPv6 addresses as
   specified in [RFC7050] or ignore the "ipv4hint" key and wait for AAAA
   resolution (Section 3).  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
   service name 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.

7.4.  "mandatory"

   See Section 8.

8.  ServiceMode RR Compatibility and Mandatory Keys

   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" by a client if the client
   recognizes all the mandatory keys and their values indicate that
   successful connection establishment is possible.  Incompatible RRs
   are ignored (see step 5 of the procedure defined in Section 3).

   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

   For example, the following is a valid list of SvcParams:

   ipv6hint=... key65333=ex1 key65444=ex2 mandatory=key65444,ipv6hint

   In wire format, the keys are represented by their numeric values in
   network byte order, concatenated in strictly increasing numeric

   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.)

9.  Using Service Bindings with HTTP

   The use of any protocol with SVCB requires a protocol-specific
   mapping specification.  This section specifies the mapping for the
   "http" and "https" URI schemes [HTTP].

   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 "HTTP Alternative
   Services" [AltSvc].  Clients and servers that implement HTTPS RRs are
   not required to implement Alt-Svc.

9.1.  Query Names for HTTPS RRs

   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.

   The procedure for following HTTPS AliasMode RRs and CNAME aliases is
   unchanged from SVCB (as described in Sections 2.4.2 and 3).

   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.

9.2.  Comparison with Alt-Svc

   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

9.2.1.  ALPN Usage

   Unlike Alt-Svc field values, HTTPS RRs can contain multiple ALPN IDs.
   The meaning and use of these IDs are discussed in Section 7.1.2.

9.2.2.  Untrusted Channels

   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

9.2.3.  Cache Lifetime

   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.

9.2.4.  Granularity

   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.

9.3.  Interaction with Alt-Svc

   Clients that implement support for both Alt-Svc and HTTPS records and
   are making a connection based on a cached Alt-Svc response 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
   (per Section 3).  For example, suppose that ""
   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 foo=...
   alt2.example.             IN HTTPS 1 alt2b.example. alpn=h3 foo=... IN HTTPS 1 alt3.example. (
       port=9443 alpn=h2,h3 foo=... )

   Based on these inputs, the following connection attempts would always
   be allowed:

   *  HTTP/2 to alt.example:443

   *  HTTP/3 to alt3.example:9443

   *  Fallback to the client's non-Alt-Svc connection behavior

   The following connection attempts would not be allowed:

   *  HTTP/3 to alt.example:443 (not consistent with Alt-Svc)

   *  Any connection to alt2b.example (no ALPN ID consistent with both
      the HTTPS record and Alt-Svc)

   *  HTTPS over TCP to any port on alt3.example (not consistent with

   Suppose that "foo" is a SvcParamKey that renders the client SVCB-
   reliant.  The following Alt-Svc-only connection attempts would be
   allowed only if the client does not support "foo", as they rely on
   SVCB-optional fallback behavior:

   *  HTTP/2 to alt2.example:443

   *  HTTP/3 to

   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 support for privacy features that are available
   on the authority endpoint.

9.4.  Requiring Server Name Indication

   Clients MUST NOT use an HTTPS RR response unless the client supports
   the TLS Server Name Indication (SNI) extension and indicates the
   origin name in the TLS ClientHello (which might be encrypted via a
   future specification such as [ECH]).  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.

9.5.  HTTP Strict Transport Security (HSTS)

   An HTTPS RR directs the client to communicate with this host only
   over a secure transport, similar to HSTS [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:

   1.  Replace the "http" scheme with "https".

   2.  If the "http" URL explicitly specifies port 80, specify port 443.

   3.  Do not alter any other aspect of the URL.

   This construction is equivalent to Section 8.3 of [HSTS], Step 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 (Temporary Redirect) response
   over cleartext HTTP.

   Publishing an HTTPS RR can potentially lead to unexpected results or
   a loss in functionality in cases where the "http" resource neither
   redirects to the "https" resource nor references the same underlying

   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 Sections 8.4 and
   12.1 of [HSTS].

9.6.  Use of HTTPS RRs in Other Protocols

   All HTTP connections to named origins are eligible to use HTTPS RRs,
   even when HTTP is used as part of another protocol or without an
   explicit HTTP-related URI scheme (Section 4.2 of [HTTP]).  For
   example, clients that support HTTPS RRs and implement [WebSocket]
   using the altered opening handshake from [FETCH-WEBSOCKETS] SHOULD
   use HTTPS RRs for the requestURL.

   When HTTP is used in a context where URLs or redirects are not
   applicable (e.g., connections to an HTTP proxy), clients that find a
   corresponding HTTPS RR SHOULD implement security upgrade behavior
   equivalent to that specified in Section 9.5.

   Such protocols MAY define their own SVCB mappings, which MAY be
   defined to take precedence over HTTPS RRs.

10.  Zone Structures

10.1.  Structuring Zones for Flexibility

   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.,
   "" for "foo" schemes).  This will help to avoid
   confusion when another scheme needs to be added to the configuration.
   When multiple port numbers are in use, it may be helpful to repeat
   the prefix labels in the alias target name (e.g.,

10.2.  Structuring Zones for Performance

   To avoid a delay for clients using a non-conforming 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

   $ORIGIN ; Origin
   foo                  3600 IN CNAME       3600 IN CNAME
   bar                   300 IN AAAA 2001:db8::2       3600 IN SVCB 1 bar key65444=...

   $ORIGIN ; Service provider zone
   foosvc               3600 IN SVCB 1 . key65333=...
   foosvc                300 IN AAAA 2001:db8::1

           Figure 1: "foo://" Is Available at
      "", but "bar://" Is Served

   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 eight aliases
   (counting both AliasMode and CNAME records) are NOT RECOMMENDED.

10.3.  Operational Considerations

   Some authoritative DNS servers may not allow A or AAAA records on
   names starting with an underscore (e.g., [BIND-CHECK-NAMES]).  This
   could create an operational issue when the TargetName contains an
   Attrleaf label, or when using a TargetName of "." if the owner name
   contains an Attrleaf label.

10.4.  Examples

10.4.1.  Protocol Enhancements

   Consider a simple zone of the form:

   $ORIGIN simple.example. ; Simple example zone
   @ 300 IN A
            AAAA 2001:db8::1

   The domain owner could add this record:

   @ 7200 IN HTTPS 1 . alpn=h3

   This record would indicate that "https://simple.example" supports
   QUIC in addition to HTTP/1.1 over TLS over TCP (the implicit
   default).  The record could also include other information (e.g., a
   non-standard port).  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

10.4.2.  Apex Aliasing

   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
                        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.

10.4.3.  Parameter Binding

   Suppose that svc.example's primary server pool supports HTTP/3 but
   its backup server pool does not.  This can be expressed in the
   following form:

   $ORIGIN svc.example. ; A hosting provider
   pool  7200 IN HTTPS 1 . alpn=h2,h3
                 HTTPS 2 backup alpn=h2 port=8443
   pool   300 IN A
                 AAAA     2001:db8::2
   backup 300 IN A
                 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, port
   number, etc.

10.4.4.  Multi-CDN Configuration

   The HTTPS RR is intended to support HTTPS services operated by
   multiple independent entities, such as different 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.
                  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
                     HTTPS 2 . alpn=h2
                     AAAA 2001:db8:192::4
    h3pool 300 IN A
               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
              60 IN A
                    AAAA 2001:db8:198::7
                    AAAA 2001:db8:198::12

    ; Resolutions following the cdn3.svc3.example
    ; path use these records.
    ; Note that this CDN has no HTTPS records.
    $ORIGIN svc3.example. ; domain operated by CDN 3
    cdn3      60 IN A
                    AAAA 2001:db8:113::8

   Note that in the above example, the different CDNs have different
   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:

   *  If CDN 1 supports a desired protocol or feature and CDN 2 does
      not, the client is vulnerable to 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, and HTTPS resolutions are independent lookups, so
      resolvers may observe and follow different CNAMEs to different
      CDNs.  Clients may thus find that the A and AAAA responses do not
      correspond to the TargetName in the HTTPS response; these clients
      will need to perform additional queries to retrieve the correct IP
      addresses.  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)
      but could receive A/AAAA records from a different CDN.  Clients
      will attempt to connect to this CDN without the benefit of its
      HTTPS records.

10.4.5.  Non-HTTP Uses

   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://", the following SVCB record would be
   used to alias it to "", which in turn could
   return AAAA/A records and/or SVCB records in ServiceMode: 7200 IN SVCB 0

   HTTPS RRs use similar Attrleaf labels if the origin contains a non-
   default port.

11.  Interaction with Other Standards

   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 Online Certificate Status
   Protocol (OCSP) Stapling (i.e., Certificate Status Request in
   Section 8 of [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 the use of SVCB with a protocol MUST have an
   entry for its scheme under the SVCB RR type in the IANA DNS
   "Underscored and Globally Scoped DNS Node Names" registry [Attrleaf].
   The scheme MUST have an entry in the "Uniform Resource Identifier
   (URI) Schemes" registry [RFC7595] and MUST have a defined
   specification for use with SVCB.

12.  Security Considerations

   SVCB/HTTPS RRs permit 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 can disable any other protections coordinated via
   SvcParams.  To prevent downgrades, Section 3.1 recommends that
   clients abandon the connection attempt when such an attack is

   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 authoritative server 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, the attacker could gain access to the
   internal service.  (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.

13.  Privacy Considerations

   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.

14.  IANA Considerations

14.1.  SVCB RR Type

   IANA has registered the following new DNS RR type in the "Resource
   Record (RR) TYPEs" registry on the "Domain Name System (DNS)
   Parameters" page:

   Type:  SVCB
   Value:  64
   Meaning:  General-purpose service binding
   Reference:  RFC 9460

14.2.  HTTPS RR Type

   IANA has registered the following new DNS RR type in the "Resource
   Record (RR) TYPEs" registry on the "Domain Name System (DNS)
   Parameters" page:

   Type:  HTTPS
   Value:  65
   Meaning:  SVCB-compatible type for use with HTTP
   Reference:  RFC 9460

14.3.  New Registry for Service Parameters

   IANA has created the "Service Parameter Keys (SvcParamKeys)" registry
   in the "Domain Name System (DNS) Parameters" category on a new page
   entitled "DNS Service Bindings (SVCB)".  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.

14.3.1.  Procedure

   A registration MUST include the following fields:

   Number:  Wire-format numeric identifier (range 0-65535)
   Name:  Unique presentation name
   Meaning:  A short description
   Reference:  Location of specification or registration source
   Change Controller:  Person or entity, with contact information if

   The characters in the registered Name field entry MUST be lowercase
   alphanumeric or "-" (Section 2.1).  The name MUST NOT start with
   "key" or "invalid".

   The registration policy for new entries is Expert Review ([RFC8126],
   Section 4.5).  The designated expert MUST ensure that the reference
   is stable and publicly available and that it specifies how to convert
   the SvcParamValue's presentation format to wire format.  The
   reference MAY be any individual's Internet-Draft or a document from
   any other source with similar assurances of stability and
   availability.  An entry MAY specify a 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.

14.3.2.  Initial Contents

   The "Service Parameter Keys (SvcParamKeys)" registry has been
   populated with the following initial registrations:

   |   Number  | Name            | Meaning        |Reference|Change    |
   |           |                 |                |         |Controller|
   |     0     | mandatory       | Mandatory      |RFC 9460,|IETF      |
   |           |                 | keys in this   |Section 8|          |
   |           |                 | RR             |         |          |
   |     1     | alpn            | Additional     |RFC 9460,|IETF      |
   |           |                 | supported      |Section  |          |
   |           |                 | protocols      |7.1      |          |
   |     2     | no-default-alpn | No support     |RFC 9460,|IETF      |
   |           |                 | for default    |Section  |          |
   |           |                 | protocol       |7.1      |          |
   |     3     | port            | Port for       |RFC 9460,|IETF      |
   |           |                 | alternative    |Section  |          |
   |           |                 | endpoint       |7.2      |          |
   |     4     | ipv4hint        | IPv4 address   |RFC 9460,|IETF      |
   |           |                 | hints          |Section  |          |
   |           |                 |                |7.3      |          |
   |     5     | ech             | RESERVED       |N/A      |IETF      |
   |           |                 | (held for      |         |          |
   |           |                 | Encrypted      |         |          |
   |           |                 | ClientHello)   |         |          |
   |     6     | ipv6hint        | IPv6 address   |RFC 9460,|IETF      |
   |           |                 | hints          |Section  |          |
   |           |                 |                |7.3      |          |
   |65280-65534| N/A             | Reserved for   |RFC 9460 |IETF      |
   |           |                 | Private Use    |         |          |
   |   65535   | N/A             | Reserved       |RFC 9460 |IETF      |
   |           |                 | ("Invalid      |         |          |
   |           |                 | key")          |         |          |

                                  Table 1

14.4.  Other Registry Updates

   Per [Attrleaf], the following entry has been added to the DNS
   "Underscored and Globally Scoped DNS Node Names" registry:

                   | RR Type | _NODE NAME | Reference |
                   | HTTPS   | _https     | RFC 9460  |

                                 Table 2

15.  References

15.1.  Normative References

   [ALPN]     Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
              July 2014, <>.

   [Attrleaf] Crocker, D., "Scoped Interpretation of DNS Resource
              Records through "Underscored" Naming of Attribute Leaves",
              BCP 222, RFC 8552, DOI 10.17487/RFC8552, March 2019,

   [DoH]      Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,

   [DoT]      Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
              and P. Hoffman, "Specification for DNS over Transport
              Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
              2016, <>.

              Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
              Better Connectivity Using Concurrency", RFC 8305,
              DOI 10.17487/RFC8305, December 2017,

   [HTTP]     Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <>.

   [RFC1928]  Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D., and
              L. Jones, "SOCKS Protocol Version 5", RFC 1928,
              DOI 10.17487/RFC1928, March 1996,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,

   [RFC3225]  Conrad, D., "Indicating Resolver Support of DNSSEC",
              RFC 3225, DOI 10.17487/RFC3225, December 2001,

   [RFC3597]  Gustafsson, A., "Handling of Unknown DNS Resource Record
              (RR) Types", RFC 3597, DOI 10.17487/RFC3597, September
              2003, <>.

   [RFC4001]  Daniele, M., Haberman, B., Routhier, S., and J.
              Schoenwaelder, "Textual Conventions for Internet Network
              Addresses", RFC 4001, DOI 10.17487/RFC4001, February 2005,

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,

   [RFC5952]  Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
              Address Text Representation", RFC 5952,
              DOI 10.17487/RFC5952, August 2010,

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,

   [RFC6147]  Bagnulo, M., Sullivan, A., Matthews, P., and I. van
              Beijnum, "DNS64: DNS Extensions for Network Address
              Translation from IPv6 Clients to IPv4 Servers", RFC 6147,
              DOI 10.17487/RFC6147, April 2011,

   [RFC7050]  Savolainen, T., Korhonen, J., and D. Wing, "Discovery of
              the IPv6 Prefix Used for IPv6 Address Synthesis",
              RFC 7050, DOI 10.17487/RFC7050, November 2013,

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,

   [RFC7595]  Thaler, D., Ed., Hansen, T., and T. Hardie, "Guidelines
              and Registration Procedures for URI Schemes", BCP 35,
              RFC 7595, DOI 10.17487/RFC7595, June 2015,

   [RFC7871]  Contavalli, C., van der Gaast, W., Lawrence, D., and W.
              Kumari, "Client Subnet in DNS Queries", RFC 7871,
              DOI 10.17487/RFC7871, May 2016,

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

              Fette, I. and A. Melnikov, "The WebSocket Protocol",
              RFC 6455, DOI 10.17487/RFC6455, December 2011,

15.2.  Informative References

   [AltSvc]   Nottingham, M., McManus, P., and J. Reschke, "HTTP
              Alternative Services", RFC 7838, DOI 10.17487/RFC7838,
              April 2016, <>.

              Finch, T., Hunt, E., van Dijk, P., Eden, A., and W.
              Mekking, "Address-specific DNS aliases (ANAME)", Work in
              Progress, Internet-Draft, draft-ietf-dnsop-aname-04, 8
              July 2019, <

              Internet Systems Consortium, "BIND v9.19.11 Configuration
              Reference: "check-names"", September 2023,

   [DNAME]    Rose, S. and W. Wijngaards, "DNAME Redirection in the
              DNS", RFC 6672, DOI 10.17487/RFC6672, June 2012,

   [DNSTerm]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
              January 2019, <>.

   [ECH]      Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS
              Encrypted Client Hello", Work in Progress, Internet-Draft,
              draft-ietf-tls-esni-17, 9 October 2023,

   [FETCH]    WHATWG, "Fetch Living Standard", October 2023,

              WHATWG, "WebSockets Living Standard", September 2023,

   [HSTS]     Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
              Transport Security (HSTS)", RFC 6797,
              DOI 10.17487/RFC6797, November 2012,

              Bellis, R., "A DNS Resource Record for HTTP", Work in
              Progress, Internet-Draft, draft-bellis-dnsop-http-record-
              00, 3 November 2018,

   [HTTP/3]   Bishop, M., Ed., "HTTP/3", RFC 9114, DOI 10.17487/RFC9114,
              June 2022, <>.

   [RFC1912]  Barr, D., "Common DNS Operational and Configuration
              Errors", RFC 1912, DOI 10.17487/RFC1912, February 1996,

   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
              DOI 10.17487/RFC6454, December 2011,

   [SRV]      Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              DOI 10.17487/RFC2782, February 2000,

   [URI]      Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,

Appendix A.  Decoding Text in Zone Files

   DNS zone files are capable of representing arbitrary octet sequences
   in basic ASCII text, using various delimiters and encodings,
   according to an algorithm defined in Section 5.1 of [RFC1035].  The
   following summarizes some allowed inputs to that algorithm, using

   ; 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,
   using quoting to group values (e.g., those with internal whitespace),
   and escaping to represent each non-printable octet as a single
   escaped sequence.  In this document, this algorithm is referred to as
   "character-string decoding", because Section 5.1 of [RFC1035] uses
   this algorithm to produce a <character-string>.  Note that while the
   length of a <character-string> is limited to 255 octets, the
   character-string decoding algorithm can produce output of any length.

A.1.  Decoding a Comma-Separated List

   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.  This corresponds to
   the simple-comma-separated syntax:

   ; item-allowed is OCTET minus "," and "\".
   item-allowed           = %x00-2B / %x2D-5B / %x5D-FF
   simple-item            = 1*item-allowed
   simple-comma-separated = [simple-item *("," simple-item)]

   For implementations that allow "," and "\" in item values, the
   following escaping syntax applies:

   item            = 1*OCTET
   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:


   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:


Appendix B.  HTTP Mapping Summary

   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 | port, no-default-    |
            | keys*                    | alpn                 |
            | *SvcParam defaults*      | alpn: ["http/1.1"]   |
            | *Special behaviors*      | Upgrade from HTTP to |
            |                          | HTTPS                |
            | *Keys that records must  | None                 |
            | include*                 |                      |

                                  Table 3

Appendix C.  Comparison with Alternatives

   The SVCB and HTTPS RR types closely resemble, and are inspired by,
   some existing record types and proposals.  One complaint regarding
   all of the alternatives is that web clients have seemed
   unenthusiastic about implementing them.  The hope here is that an
   extensible solution that solves multiple problems will overcome this
   inertia and have a path to achieve client implementation.

C.1.  Differences from the SRV RR Type

   An SRV record [SRV] can perform a function similar to that of 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 SVCB is intended to
      be optional when used with pre-existing protocols.

   *  SRV records cannot instruct the client to switch or upgrade
      protocols, whereas SVCB can signal such an upgrade (e.g., to

   *  SRV records are not extensible, whereas SVCB and HTTPS RRs can be
      extended with new parameters.

   *  SRV records specify a "weight" for unbalanced randomized load
      balancing.  SVCB only supports balanced randomized load balancing,
      although weights could be added via a future SvcParam.

C.2.  Differences from the Proposed HTTP Record

   Unlike [HTTP-DNS-RR], 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.

C.3.  Differences from the Proposed ANAME Record

   Unlike [ANAME-DNS-RR], this approach is extensible to cover Alt-Svc
   and Encrypted ClientHello 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

   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.

C.4.  Comparison with Separate RR Types for AliasMode and ServiceMode

   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 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 query
   and an AliasMode query to the authoritative DNS server.  Thus,
   splitting the RR type would double, or in some cases triple, the load
   on clients and servers, and would not reduce implementation

Appendix D.  Test Vectors

   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 wire format is long, it is broken into several lines.

D.1.  AliasMode   HTTPS   0

   \# 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

                            Figure 2: AliasMode

D.2.  ServiceMode   SVCB   1 .

   \# 3 (
   00 01      ; priority
   00         ; target (root label)

   \x00\x01   # priority
   \x00       # target (root label)

                        Figure 3: TargetName Is "."   SVCB   16 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
   \x00\x35                                           # value

                         Figure 4: Specifies a Port   SVCB   1 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

                 Figure 5: A Generic Key and Unquoted Value   SVCB   1 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

       Figure 6: A Generic Key and Quoted Value with a Decimal Escape   SVCB   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
        \x00\x00\x00\x00\x00\x00\x00\x01              # first address
        \x00\x00\x00\x00\x00\x53\x00\x01              # second address

                      Figure 7: Two Quoted IPv6 Hints   SVCB   1 (
   \# 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
   20 01 0d b8 01 22 03 44 00 00 00 00 c0 00 02 21    ; address

   \x00\x01                                           # priority
   \x07example\x03com\x00                             # target
   \x00\x06                                           # key 6
   \x00\x10                                           # length 16
        \x00\x00\x00\x00\xc0\x00\x02\x21              # address

           Figure 8: An IPv6 Hint Using the Embedded IPv4 Syntax   SVCB   16 (
                         alpn=h2,h3-19 mandatory=ipv4hint,alpn

   \# 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

        Figure 9: SvcParamKey Ordering Is Arbitrary in Presentation
                      Format but Sorted in Wire Format   SVCB   16 alpn="f\\\\oo\\,bar,h2"   SVCB   16 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

      Figure 10: An "alpn" Value with an Escaped Comma and an Escaped
                   Backslash in Two Presentation Formats

D.3.  Failure Cases

   This subsection contains test vectors that are not compliant with
   this document.  The various reasons for non-compliance are explained
   with each example.   SVCB   1 (
                          key123=abc key123=def

           Figure 11: Multiple Instances of the Same SvcParamKey   SVCB   1 mandatory   SVCB   1 alpn   SVCB   1 port   SVCB   1 ipv4hint   SVCB   1 ipv6hint

          Figure 12: Missing SvcParamValues That Must Be Non-Empty   SVCB   1 no-default-alpn=abc

      Figure 13: The "no-default-alpn" SvcParamKey Value Must Be Empty   SVCB   1 mandatory=key123

                 Figure 14: A Mandatory SvcParam Is Missing   SVCB   1 mandatory=mandatory

       Figure 15: The "mandatory" SvcParamKey Must Not Be Included in
                             the Mandatory List   SVCB   1 (
                         mandatory=key123,key123 key123=abc

        Figure 16: Multiple Instances of the Same SvcParamKey in the
                               Mandatory List

Acknowledgments and Related Proposals

   Over the years, IETF participants have proposed a wide range of
   solutions to the "CNAME at the zone apex" challenge, including
   [HTTP-DNS-RR], [ANAME-DNS-RR], and others.  The authors are grateful
   for their work to elucidate the problem and identify promising
   strategies to address it, some of which are reflected in this

   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, Willem Toorop, Pieter Lexis,
   Puneet Sood, Olivier Poitrey, Mashooq Muhaimen, Tom Carpay, and many
   others for their feedback and suggestions on this document.

Authors' Addresses

   Ben Schwartz
   Meta Platforms, Inc.

   Mike Bishop
   Akamai Technologies

   Erik Nygren
   Akamai Technologies