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)
Abstract
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
https://www.rfc-editor.org/info/rfc9460.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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
ServiceMode
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 "https://example.com" to a service operator
endpoint at "svc.example.net". While this case can sometimes be
handled by a CNAME, that does not cover all use cases. CNAME is also
inadequate when the service operator needs to provide a bound
collection of consistent configuration parameters through the DNS
(such as network location, protocol, and keying information).
This document first describes the SVCB RR as a general-purpose 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
endpoints.
* Does not assume that all alternative endpoints have the same
parameters or capabilities, or are even operated by the same
entity. This is important, as DNS does not provide any way to tie
together multiple RRsets for the same name. For example, if
"www.example.com" 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
"example.com") 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
include:
* 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
AliasMode.
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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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
key.
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
order.
* 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
sizes).
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
format.
Note that the second condition implies that there are no duplicate
SvcParamKeys.
If any RRs are malformed, the client MUST reject the entire RRset and
fall back to non-SVCB connection establishment.
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
"_examplescheme.api.example.com.". 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 "example.com" could publish this record:
_8443._foo.api.example.com. 7200 IN SVCB 0 svc4.example.net.
This record would indicate that "foo://api.example.com:8443" is
aliased to "svc4.example.net". The owner of "example.net", in turn,
could publish this record:
svc4.example.net. 7200 IN SVCB 3 svc4.example.net. (
alpn="bar" port="8004" )
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://example.com:8080" could point
requests to a service operating at "foosvc.example.net" by
publishing:
_8080._foo.example.com. 3600 IN SVCB 0 foosvc.example.net.
Using AliasMode maintains a separation of concerns: the owner of
"foosvc.example.net" can add or remove ServiceMode SVCB records
without requiring a corresponding change to "example.com". Note that
if "foosvc.example.net" promises to always publish a SVCB record,
this AliasMode record can be replaced by a CNAME at the same owner
name.
AliasMode is especially useful for SVCB-compatible RR types that do
not require an underscore prefix, such as the HTTPS RR type. For
example, the operator of "https://example.com" could point requests
to a server at "svc.example.net" by publishing this record at the
zone apex:
example.com. 3600 IN HTTPS 0 svc.example.net.
Note that the SVCB record's owner name MAY be the canonical name of a
CNAME record, and the TargetName MAY be the owner of a CNAME record.
Clients and recursive resolvers MUST follow CNAMEs as normal.
To avoid unbounded alias chains, clients and recursive resolvers MUST
impose a limit on the total number of SVCB aliases they will follow
for each resolution request. This limit MUST NOT be zero, i.e.,
implementations MUST be able to follow at least one AliasMode record.
The exact value of this limit is left to implementations.
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
versa.
2.4.3. ServiceMode
In ServiceMode, the TargetName and SvcParams within each RR associate
an alternative endpoint for the service with its connection
parameters.
Each protocol scheme that uses SVCB MUST define a protocol mapping
that explains how SvcParams are applied for connections of that
scheme. Unless specified otherwise by the protocol mapping, clients
MUST ignore any SvcParam that they do not recognize.
Some SvcParams impose requirements on other SvcParams in the RR. A
ServiceMode RR is called "self-consistent" if its SvcParams all
comply with each 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, "svc2.example.net" is the effective TargetName:
example.com. 7200 IN HTTPS 0 svc.example.net.
svc.example.net. 7200 IN CNAME svc2.example.net.
svc2.example.net. 7200 IN HTTPS 1 . port=8002
svc2.example.net. 300 IN A 192.0.2.2
svc2.example.net. 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
SVCB.
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
TargetName.
* 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
TargetNames.
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:
_1234._bar.example.com. 300 IN SVCB 1 svc1.example.net. (
ipv6hint=2001:db8::1 port=1234 )
SVCB 2 svc2.example.net. (
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
needed.
The wire-format value for "alpn" consists of at least one alpn-id
prefixed by its length as a single octet, and these length-value
pairs are concatenated to form the SvcParamValue. These pairs MUST
exactly fill the SvcParamValue; otherwise, the SvcParamValue is
malformed.
For "no-default-alpn", the presentation and wire-format values MUST
be empty. When "no-default-alpn" is specified in an RR, "alpn" must
also be specified in order for the RR to be "self-consistent"
(Section 2.4.3).
Each scheme that uses this SvcParamKey defines a "default set" of
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
sequences.
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
order.
This SvcParamKey is always automatically mandatory and MUST NOT
appear in its own value-list. Other automatically mandatory keys
SHOULD NOT appear in the list either. (Including them wastes space
and otherwise has no effect.)
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
behavior.
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
channel.
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 "https://example.com"
sends an Alt-Svc field value of:
Alt-Svc: h2="alt.example:443", h2="alt2.example:443", h3=":8443"
The client would retrieve the following HTTPS records:
alt.example. IN HTTPS 1 . alpn=h2,h3 foo=...
alt2.example. IN HTTPS 1 alt2b.example. alpn=h3 foo=...
_8443._https.example.com. 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
Alt-Svc)
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 example.com:8443
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
resource.
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.,
"foosvc.example.net" 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.,
"_1234._foo.svc.example.net").
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
alias.
$ORIGIN example.com. ; Origin
foo 3600 IN CNAME foosvc.example.net.
_8080._foo.foo 3600 IN CNAME foosvc.example.net.
bar 300 IN AAAA 2001:db8::2
_9090._bar.bar 3600 IN SVCB 1 bar key65444=...
$ORIGIN example.net. ; Service provider zone
foosvc 3600 IN SVCB 1 . key65333=...
foosvc 300 IN AAAA 2001:db8::1
Figure 1: "foo://foo.example.com:8080" Is Available at
"foosvc.example.net", but "bar://bar.example.com:9090" Is Served
Locally
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 192.0.2.1
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
"http://simple.example:8443".
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 192.0.2.1
IN AAAA 2001:db8::1
With HTTPS RRs, the owner of aliased.example could alias the apex by
adding one additional record:
@ 7200 IN HTTPS 0 pool.svc.example.
With this record in place, HTTPS-RR-aware clients will use the same
server pool for aliased.example and www.aliased.example. (They will
also upgrade "http://aliased.example/..." to "https".) Non-HTTPS-RR-
aware clients will just ignore the new record.
Similar to CNAME, HTTPS RRs have no impact on the origin name. When
connecting, clients will continue to treat the authoritative origins
as "https://www.aliased.example" and "https://aliased.example",
respectively, and will validate TLS server certificates accordingly.
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 192.0.2.2
AAAA 2001:db8::2
backup 300 IN A 192.0.2.3
AAAA 2001:db8::3
This configuration is entirely compatible with the "apex aliasing"
example, whether the client supports HTTPS RRs or not. If the client
does support HTTPS RRs, all connections will be upgraded to HTTPS,
and clients will use HTTP/3 if they can. Parameters are "bound" to
each server pool, so each server pool can have its own protocol, 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.
A 203.0.113.82
AAAA 2001:db8:203::2
; Resolutions following the cdn1.svc1.example
; path use these records.
; This CDN uses a different alternative service for HTTP/3.
$ORIGIN svc1.example. ; domain for CDN 1
cdn1 1800 IN HTTPS 1 h3pool alpn=h3
HTTPS 2 . alpn=h2
A 192.0.2.2
AAAA 2001:db8:192::4
h3pool 300 IN A 192.0.2.3
AAAA 2001:db8:192:7::3
; Resolutions following the customer.svc2.example
; path use these records.
; Note that this CDN only supports HTTP/2.
$ORIGIN svc2.example. ; domain operated by CDN 2
customer 300 IN HTTPS 1 . alpn=h2
60 IN A 198.51.100.2
A 198.51.100.3
A 198.51.100.4
AAAA 2001:db8:198::7
AAAA 2001:db8:198::12
; Resolutions following the 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 203.0.113.8
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://api.example.com:8765", the following SVCB record would be
used to alias it to "svc4-baz.example.net.", which in turn could
return AAAA/A records and/or SVCB records in ServiceMode:
_8765._baz.api.example.com. 7200 IN SVCB 0 svc4-baz.example.net.
HTTPS RRs use similar Attrleaf labels if the origin contains a non-
default port.
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
detected.
A hostile DNS intermediary might forge AliasMode "." records
(Section 2.5.1) as a way to block clients from accessing particular
services. Such an adversary could already block entire domains by
forging erroneous responses, but this mechanism allows them to target
particular protocols or ports within a domain. Clients that might be
subject to such attacks SHOULD ignore AliasMode "." records.
A hostile DNS intermediary or 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
appropriate
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, <https://www.rfc-editor.org/info/rfc7301>.
[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,
<https://www.rfc-editor.org/info/rfc8552>.
[DoH] Hoffman, P. and P. McManus, "DNS Queries over HTTPS
(DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
<https://www.rfc-editor.org/info/rfc8484>.
[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, <https://www.rfc-editor.org/info/rfc7858>.
[HappyEyeballsV2]
Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
Better Connectivity Using Concurrency", RFC 8305,
DOI 10.17487/RFC8305, December 2017,
<https://www.rfc-editor.org/info/rfc8305>.
[HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022,
<https://www.rfc-editor.org/info/rfc9110>.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<https://www.rfc-editor.org/info/rfc1034>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[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,
<https://www.rfc-editor.org/info/rfc1928>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,
<https://www.rfc-editor.org/info/rfc2181>.
[RFC3225] Conrad, D., "Indicating Resolver Support of DNSSEC",
RFC 3225, DOI 10.17487/RFC3225, December 2001,
<https://www.rfc-editor.org/info/rfc3225>.
[RFC3597] Gustafsson, A., "Handling of Unknown DNS Resource Record
(RR) Types", RFC 3597, DOI 10.17487/RFC3597, September
2003, <https://www.rfc-editor.org/info/rfc3597>.
[RFC4001] Daniele, M., Haberman, B., Routhier, S., and J.
Schoenwaelder, "Textual Conventions for Internet Network
Addresses", RFC 4001, DOI 10.17487/RFC4001, February 2005,
<https://www.rfc-editor.org/info/rfc4001>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
Address Text Representation", RFC 5952,
DOI 10.17487/RFC5952, August 2010,
<https://www.rfc-editor.org/info/rfc5952>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
<https://www.rfc-editor.org/info/rfc6066>.
[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,
<https://www.rfc-editor.org/info/rfc6147>.
[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,
<https://www.rfc-editor.org/info/rfc7050>.
[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,
<https://www.rfc-editor.org/info/rfc7231>.
[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,
<https://www.rfc-editor.org/info/rfc7595>.
[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,
<https://www.rfc-editor.org/info/rfc7871>.
[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,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[WebSocket]
Fette, I. and A. Melnikov, "The WebSocket Protocol",
RFC 6455, DOI 10.17487/RFC6455, December 2011,
<https://www.rfc-editor.org/info/rfc6455>.
15.2. Informative References
[AltSvc] Nottingham, M., McManus, P., and J. Reschke, "HTTP
Alternative Services", RFC 7838, DOI 10.17487/RFC7838,
April 2016, <https://www.rfc-editor.org/info/rfc7838>.
[ANAME-DNS-RR]
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, <https://datatracker.ietf.org/doc/html/draft-
ietf-dnsop-aname-04>.
[BIND-CHECK-NAMES]
Internet Systems Consortium, "BIND v9.19.11 Configuration
Reference: "check-names"", September 2023,
<https://bind9.readthedocs.io/en/v9.19.11/
reference.html#namedconf-statement-check-names>.
[DNAME] Rose, S. and W. Wijngaards, "DNAME Redirection in the
DNS", RFC 6672, DOI 10.17487/RFC6672, June 2012,
<https://www.rfc-editor.org/info/rfc6672>.
[DNSTerm] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
January 2019, <https://www.rfc-editor.org/info/rfc8499>.
[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,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
esni-17>.
[FETCH] WHATWG, "Fetch Living Standard", October 2023,
<https://fetch.spec.whatwg.org/>.
[FETCH-WEBSOCKETS]
WHATWG, "WebSockets Living Standard", September 2023,
<https://websockets.spec.whatwg.org/>.
[HSTS] Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
Transport Security (HSTS)", RFC 6797,
DOI 10.17487/RFC6797, November 2012,
<https://www.rfc-editor.org/info/rfc6797>.
[HTTP-DNS-RR]
Bellis, R., "A DNS Resource Record for HTTP", Work in
Progress, Internet-Draft, draft-bellis-dnsop-http-record-
00, 3 November 2018,
<https://datatracker.ietf.org/doc/html/draft-bellis-dnsop-
http-record-00>.
[HTTP/3] Bishop, M., Ed., "HTTP/3", RFC 9114, DOI 10.17487/RFC9114,
June 2022, <https://www.rfc-editor.org/info/rfc9114>.
[RFC1912] Barr, D., "Common DNS Operational and Configuration
Errors", RFC 1912, DOI 10.17487/RFC1912, February 1996,
<https://www.rfc-editor.org/info/rfc1912>.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
DOI 10.17487/RFC6454, December 2011,
<https://www.rfc-editor.org/info/rfc6454>.
[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,
<https://www.rfc-editor.org/info/rfc2782>.
[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,
<https://www.rfc-editor.org/info/rfc3986>.
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
ABNF:
; non-special is VCHAR minus DQUOTE, ";", "(", ")", and "\".
non-special = %x21 / %x23-27 / %x2A-3A / %x3C-5B / %x5D-7E
; non-digit is VCHAR minus DIGIT.
non-digit = %x21-2F / %x3A-7E
; dec-octet is a number 0-255 as a three-digit decimal number.
dec-octet = ( "0" / "1" ) 2DIGIT /
"2" ( ( %x30-34 DIGIT ) / ( "5" %x30-35 ) )
escaped = "\" ( non-digit / dec-octet )
contiguous = 1*( non-special / escaped )
quoted = DQUOTE *( contiguous / ( ["\"] WSP ) ) DQUOTE
char-string = contiguous / quoted
The decoding algorithm allows char-string to represent any *OCTET,
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:
"part1,part2,part3\\,part4\\\\"
part1\,\p\a\r\t2\044part3\092,part4\092\\
These inputs are equivalent: character-string decoding either of them
would produce the same value:
part1,part2,part3\,part4\\
Applying comma-separated list decoding to this value would produce a
list of three items:
part1
part2
part3,part4\
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
HTTP/2).
* 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
records.
Like that proposal, this addresses the zone-apex CNAME challenge for
clients that implement this.
However, with this SVCB proposal, it remains necessary to continue to
include address records at the zone apex for legacy clients. If
deployment of this standard is successful, the number of legacy
clients will fall over time. As the number of legacy clients
declines, the operational effort required to serve these users
without the benefit of SVCB indirection should fall. Server
operators can easily observe how much traffic reaches this legacy
endpoint and may remove the apex's address records if the observed
legacy traffic has fallen to negligible levels.
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
complexity.
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
example.com. HTTPS 0 foo.example.com.
\# 19 (
00 00 ; priority
03 66 6f 6f 07 65 78 61 6d 70 6c 65 03 63 6f 6d 00 ; target
)
\x00\x00 # priority
\x03foo\x07example\x03com\x00 # target
Figure 2: AliasMode
D.2. ServiceMode
example.com. SVCB 1 .
\# 3 (
00 01 ; priority
00 ; target (root label)
)
\x00\x01 # priority
\x00 # target (root label)
Figure 3: TargetName Is "."
example.com. SVCB 16 foo.example.com. port=53
\# 25 (
00 10 ; priority
03 66 6f 6f 07 65 78 61 6d 70 6c 65 03 63 6f 6d 00 ; target
00 03 ; key 3
00 02 ; length 2
00 35 ; value
)
\x00\x10 # priority
\x03foo\x07example\x03com\x00 # target
\x00\x03 # key 3
\x00\x02 # length 2
\x00\x35 # value
Figure 4: Specifies a Port
example.com. SVCB 1 foo.example.com. key667=hello
\# 28 (
00 01 ; priority
03 66 6f 6f 07 65 78 61 6d 70 6c 65 03 63 6f 6d 00 ; target
02 9b ; key 667
00 05 ; length 5
68 65 6c 6c 6f ; value
)
\x00\x01 # priority
\x03foo\x07example\x03com\x00 # target
\x02\x9b # key 667
\x00\x05 # length 5
hello # value
Figure 5: A Generic Key and Unquoted Value
example.com. SVCB 1 foo.example.com. key667="hello\210qoo"
\# 32 (
00 01 ; priority
03 66 6f 6f 07 65 78 61 6d 70 6c 65 03 63 6f 6d 00 ; target
02 9b ; key 667
00 09 ; length 9
68 65 6c 6c 6f d2 71 6f 6f ; value
)
\x00\x01 # priority
\x03foo\x07example\x03com\x00 # target
\x02\x9b # key 667
\x00\x09 # length 9
hello\xd2qoo # value
Figure 6: A Generic Key and Quoted Value with a Decimal Escape
example.com. SVCB 1 foo.example.com. (
ipv6hint="2001:db8::1,2001:db8::53:1"
)
\# 55 (
00 01 ; priority
03 66 6f 6f 07 65 78 61 6d 70 6c 65 03 63 6f 6d 00 ; target
00 06 ; key 6
00 20 ; length 32
20 01 0d b8 00 00 00 00 00 00 00 00 00 00 00 01 ; first address
20 01 0d b8 00 00 00 00 00 00 00 00 00 53 00 01 ; second address
)
\x00\x01 # priority
\x03foo\x07example\x03com\x00 # target
\x00\x06 # key 6
\x00\x20 # length 32
\x20\x01\x0d\xb8\x00\x00\x00\x00
\x00\x00\x00\x00\x00\x00\x00\x01 # first address
\x20\x01\x0d\xb8\x00\x00\x00\x00
\x00\x00\x00\x00\x00\x53\x00\x01 # second address
Figure 7: Two Quoted IPv6 Hints
example.com. SVCB 1 example.com. (
ipv6hint="2001:db8:122:344::192.0.2.33"
)
\# 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
\x20\x01\x0d\xb8\x01\x22\x03\x44
\x00\x00\x00\x00\xc0\x00\x02\x21 # address
Figure 8: An IPv6 Hint Using the Embedded IPv4 Syntax
example.com. SVCB 16 foo.example.org. (
alpn=h2,h3-19 mandatory=ipv4hint,alpn
ipv4hint=192.0.2.1
)
\# 48 (
00 10 ; priority
03 66 6f 6f 07 65 78 61 6d 70 6c 65 03 6f 72 67 00 ; target
00 00 ; key 0
00 04 ; param length 4
00 01 ; value: key 1
00 04 ; value: key 4
00 01 ; key 1
00 09 ; param length 9
02 ; alpn length 2
68 32 ; alpn value
05 ; alpn length 5
68 33 2d 31 39 ; alpn value
00 04 ; key 4
00 04 ; param length 4
c0 00 02 01 ; param value
)
\x00\x10 # priority
\x03foo\x07example\x03org\x00 # target
\x00\x00 # key 0
\x00\x04 # param length 4
\x00\x01 # value: key 1
\x00\x04 # value: key 4
\x00\x01 # key 1
\x00\x09 # param length 9
\x02 # alpn length 2
h2 # alpn value
\x05 # alpn length 5
h3-19 # alpn value
\x00\x04 # key 4
\x00\x04 # param length 4
\xc0\x00\x02\x01 # param value
Figure 9: SvcParamKey Ordering Is Arbitrary in Presentation
Format but Sorted in Wire Format
example.com. SVCB 16 foo.example.org. alpn="f\\\\oo\\,bar,h2"
example.com. SVCB 16 foo.example.org. alpn=f\\\092oo\092,bar,h2
\# 35 (
00 10 ; priority
03 66 6f 6f 07 65 78 61 6d 70 6c 65 03 6f 72 67 00 ; target
00 01 ; key 1
00 0c ; param length 12
08 ; alpn length 8
66 5c 6f 6f 2c 62 61 72 ; alpn value
02 ; alpn length 2
68 32 ; alpn value
)
\x00\x10 # priority
\x03foo\x07example\x03org\x00 # target
\x00\x01 # key 1
\x00\x0c # param length 12
\x08 # alpn length 8
f\oo,bar # alpn value
\x02 # alpn length 2
h2 # alpn value
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.
example.com. SVCB 1 foo.example.com. (
key123=abc key123=def
)
Figure 11: Multiple Instances of the Same SvcParamKey
example.com. SVCB 1 foo.example.com. mandatory
example.com. SVCB 1 foo.example.com. alpn
example.com. SVCB 1 foo.example.com. port
example.com. SVCB 1 foo.example.com. ipv4hint
example.com. SVCB 1 foo.example.com. ipv6hint
Figure 12: Missing SvcParamValues That Must Be Non-Empty
example.com. SVCB 1 foo.example.com. no-default-alpn=abc
Figure 13: The "no-default-alpn" SvcParamKey Value Must Be Empty
example.com. SVCB 1 foo.example.com. mandatory=key123
Figure 14: A Mandatory SvcParam Is Missing
example.com. SVCB 1 foo.example.com. mandatory=mandatory
Figure 15: The "mandatory" SvcParamKey Must Not Be Included in
the Mandatory List
example.com. SVCB 1 foo.example.com. (
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
document.
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.
Email: ietf@bemasc.net
Mike Bishop
Akamai Technologies
Email: mbishop@evequefou.be