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

RFC 9460: Service Binding and Parameter Specification via the DNS (SVCB and HTTPS Resource Records)
RFC 9460
SVCB and HTTPS RRs for DNS
November 2023
Schwartz, et al.
Standards Track
[Page]
Stream:
Internet Engineering Task Force (IETF)
RFC:
9460
Category:
Standards Track
Published:
November 2023
ISSN:
2070-1721
Authors:
B. Schwartz
Meta Platforms, Inc.
M. Bishop
Akamai Technologies
E. Nygren
Akamai Technologies
RFC 9460
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
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with
respect to this document. Code Components extracted from this
document must include Revised BSD License text as described in
Section 4.e of the Trust Legal Provisions and are provided without
warranty as described in the Revised BSD License.
Table of Contents
1.
Introduction
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

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

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:
Let $QNAME be the service name plus appropriate prefixes for the
scheme (see
Section 2.3
).
Issue a SVCB query for $QNAME.
If an AliasMode SVCB record is returned for $QNAME (after following CNAMEs
as normal), set $QNAME to its TargetName (without
additional prefixes) and loop back to Step 2,
subject to chain length limits and loop detection heuristics (see
Section 3.1
).
If one or more "compatible" (
Section 8
) ServiceMode records are returned,
these represent the alternative endpoints. Sort the records by ascending SvcPriority.
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:
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.
If any of the resolved SVCB records are in AliasMode, choose one of them
at random, and resolve SVCB, A, and AAAA records for its
TargetName.
If any SVCB records are resolved, go to Step 1.
Otherwise, incorporate the results of A and AAAA resolution, and
terminate.
All the resolved SVCB records are in ServiceMode. Resolve A and AAAA
queries for each TargetName (or for the owner name if TargetName
is "."), incorporate all the results, and terminate.
In this procedure, "resolve" means the resolver's ordinary recursive
resolution procedure, as if processing a query for that RRset.
This includes following any aliases that the resolver would ordinarily
follow (e.g., CNAME, DNAME
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-id
s. 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-id
s 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
Let SVCB-ALPN-Intersection be the set of protocols in the SVCB ALPN set
that the client supports.
Let Intersection-Transports be the set of transports (e.g., TLS, DTLS, QUIC)
implied by the protocols in SVCB-ALPN-Intersection.
For each transport in Intersection-Transports, construct a ProtocolNameList
containing the Identification Sequences of all the client's supported ALPN
protocols for that transport, without regard to the SVCB ALPN set.
For example, if the SVCB ALPN set is ["http/1.1", "h3"] and the client
supports HTTP/1.1, HTTP/2, and HTTP/3, the client could attempt to connect using
TLS over TCP with a ProtocolNameList of ["http/1.1", "h2"] and could also
attempt a connection using QUIC with a ProtocolNameList of ["h3"].
Once the client has constructed a ClientHello, protocol negotiation in that
handshake proceeds as specified in
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
).
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:
Replace the "http" scheme with "https".
If the "http" URL explicitly specifies port 80, specify port 443.
Do not alter any other aspect of the URL.
This construction is equivalent to
Section 8.3
of [
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:
Table 1
Number
Name
Meaning
Reference
Change Controller
mandatory
Mandatory keys in this RR
RFC 9460,
Section 8
IETF
alpn
Additional supported protocols
RFC 9460,
Section 7.1
IETF
no-default-alpn
No support for default protocol
RFC 9460,
Section 7.1
IETF
port
Port for alternative endpoint
RFC 9460,
Section 7.2
IETF
ipv4hint
IPv4 address hints
RFC 9460,
Section 7.3
IETF
ech
RESERVED (held for Encrypted ClientHello)
N/A
IETF
ipv6hint
IPv6 address hints
RFC 9460,
Section 7.3
IETF
65280-65534
N/A
Reserved for Private Use
RFC 9460
IETF
65535
N/A
Reserved ("Invalid key")
RFC 9460
IETF
14.4.
Other Registry Updates
Per
Attrleaf
, the following entry has been added to the DNS "Underscored and Globally Scoped DNS Node Names" registry:
Table 2
RR Type
_NODE NAME
Reference
HTTPS
_https
RFC 9460
15.
References
15.1.
Normative References
[ALPN]
Friedl, S.
Popov, A.
Langley, A.
, and
E. Stephan
"Transport Layer Security (TLS) Application-Layer Protocol Negotiation Extension"
RFC 7301
DOI 10.17487/RFC7301
July 2014
[Attrleaf]
Crocker, D.
"Scoped Interpretation of DNS Resource Records through "Underscored" Naming of Attribute Leaves"
BCP 222
RFC 8552
DOI 10.17487/RFC8552
March 2019
[DoH]
Hoffman, P.
and
P. McManus
"DNS Queries over HTTPS (DoH)"
RFC 8484
DOI 10.17487/RFC8484
October 2018
[DoT]
Hu, Z.
Zhu, L.
Heidemann, J.
Mankin, A.
Wessels, D.
, and
P. Hoffman
"Specification for DNS over Transport Layer Security (TLS)"
RFC 7858
DOI 10.17487/RFC7858
May 2016
[HappyEyeballsV2]
Schinazi, D.
and
T. Pauly
"Happy Eyeballs Version 2: Better Connectivity Using Concurrency"
RFC 8305
DOI 10.17487/RFC8305
December 2017
[HTTP]
Fielding, R., Ed.
Nottingham, M., Ed.
, and
J. Reschke, Ed.
"HTTP Semantics"
STD 97
RFC 9110
DOI 10.17487/RFC9110
June 2022
[RFC1034]
Mockapetris, P.
"Domain names - concepts and facilities"
STD 13
RFC 1034
DOI 10.17487/RFC1034
November 1987
[RFC1035]
Mockapetris, P.
"Domain names - implementation and specification"
STD 13
RFC 1035
DOI 10.17487/RFC1035
November 1987
[RFC1928]
Leech, M.
Ganis, M.
Lee, Y.
Kuris, R.
Koblas, D.
, and
L. Jones
"SOCKS Protocol Version 5"
RFC 1928
DOI 10.17487/RFC1928
March 1996
[RFC2119]
Bradner, S.
"Key words for use in RFCs to Indicate Requirement Levels"
BCP 14
RFC 2119
DOI 10.17487/RFC2119
March 1997
[RFC2181]
Elz, R.
and
R. Bush
"Clarifications to the DNS Specification"
RFC 2181
DOI 10.17487/RFC2181
July 1997
[RFC3225]
Conrad, D.
"Indicating Resolver Support of DNSSEC"
RFC 3225
DOI 10.17487/RFC3225
December 2001
[RFC3597]
Gustafsson, A.
"Handling of Unknown DNS Resource Record (RR) Types"
RFC 3597
DOI 10.17487/RFC3597
September 2003
[RFC4001]
Daniele, M.
Haberman, B.
Routhier, S.
, and
J. Schoenwaelder
"Textual Conventions for Internet Network Addresses"
RFC 4001
DOI 10.17487/RFC4001
February 2005
[RFC5234]
Crocker, D., Ed.
and
P. Overell
"Augmented BNF for Syntax Specifications: ABNF"
STD 68
RFC 5234
DOI 10.17487/RFC5234
January 2008
[RFC5952]
Kawamura, S.
and
M. Kawashima
"A Recommendation for IPv6 Address Text Representation"
RFC 5952
DOI 10.17487/RFC5952
August 2010
[RFC6066]
Eastlake 3rd, D.
"Transport Layer Security (TLS) Extensions: Extension Definitions"
RFC 6066
DOI 10.17487/RFC6066
January 2011
[RFC6147]
Bagnulo, M.
Sullivan, A.
Matthews, P.
, and
I. van Beijnum
"DNS64: DNS Extensions for Network Address Translation from IPv6 Clients to IPv4 Servers"
RFC 6147
DOI 10.17487/RFC6147
April 2011
[RFC7050]
Savolainen, T.
Korhonen, J.
, and
D. Wing
"Discovery of the IPv6 Prefix Used for IPv6 Address Synthesis"
RFC 7050
DOI 10.17487/RFC7050
November 2013
[RFC7231]
Fielding, R., Ed.
and
J. Reschke, Ed.
"Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content"
RFC 7231
DOI 10.17487/RFC7231
June 2014
[RFC7595]
Thaler, D., Ed.
Hansen, T.
, and
T. Hardie
"Guidelines and Registration Procedures for URI Schemes"
BCP 35
RFC 7595
DOI 10.17487/RFC7595
June 2015
[RFC7871]
Contavalli, C.
van der Gaast, W.
Lawrence, D.
, and
W. Kumari
"Client Subnet in DNS Queries"
RFC 7871
DOI 10.17487/RFC7871
May 2016
[RFC8126]
Cotton, M.
Leiba, B.
, and
T. Narten
"Guidelines for Writing an IANA Considerations Section in RFCs"
BCP 26
RFC 8126
DOI 10.17487/RFC8126
June 2017
[RFC8174]
Leiba, B.
"Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words"
BCP 14
RFC 8174
DOI 10.17487/RFC8174
May 2017
[RFC8446]
Rescorla, E.
"The Transport Layer Security (TLS) Protocol Version 1.3"
RFC 8446
DOI 10.17487/RFC8446
August 2018
[WebSocket]
Fette, I.
and
A. Melnikov
"The WebSocket Protocol"
RFC 6455
DOI 10.17487/RFC6455
December 2011
15.2.
Informative References
[AltSvc]
Nottingham, M.
McManus, P.
, and
J. Reschke
"HTTP Alternative Services"
RFC 7838
DOI 10.17487/RFC7838
April 2016
[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
[BIND-CHECK-NAMES]
Internet Systems Consortium
"BIND v9.19.11 Configuration Reference: "check-names""
September 2023
[DNAME]
Rose, S.
and
W. Wijngaards
"DNAME Redirection in the DNS"
RFC 6672
DOI 10.17487/RFC6672
June 2012
[DNSTerm]
Hoffman, P.
Sullivan, A.
, and
K. Fujiwara
"DNS Terminology"
BCP 219
RFC 8499
DOI 10.17487/RFC8499
January 2019
[ECH]
Rescorla, E.
Oku, K.
Sullivan, N.
, and
C. A. Wood
"TLS Encrypted Client Hello"
Work in Progress
Internet-Draft, draft-ietf-tls-esni-17
9 October 2023
[FETCH]
WHATWG
"Fetch Living Standard"
October 2023
[FETCH-WEBSOCKETS]
WHATWG
"WebSockets Living Standard"
September 2023
[HSTS]
Hodges, J.
Jackson, C.
, and
A. Barth
"HTTP Strict Transport Security (HSTS)"
RFC 6797
DOI 10.17487/RFC6797
November 2012
[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
[HTTP/3]
Bishop, M., Ed.
"HTTP/3"
RFC 9114
DOI 10.17487/RFC9114
June 2022
[RFC1912]
Barr, D.
"Common DNS Operational and Configuration Errors"
RFC 1912
DOI 10.17487/RFC1912
February 1996
[RFC6454]
Barth, A.
"The Web Origin Concept"
RFC 6454
DOI 10.17487/RFC6454
December 2011
[SRV]
Gulbrandsen, A.
Vixie, P.
, and
L. Esibov
"A DNS RR for specifying the location of services (DNS SRV)"
RFC 2782
DOI 10.17487/RFC2782
February 2000
[URI]
Berners-Lee, T.
Fielding, R.
, and
L. Masinter
"Uniform Resource Identifier (URI): Generic Syntax"
STD 66
RFC 3986
DOI 10.17487/RFC3986
January 2005
Appendix A.
Decoding Text in Zone Files
DNS zone files are capable of representing arbitrary octet sequences in
basic ASCII text, using various delimiters and encodings, according to an algorithm
defined in
Section 5.1
of [
RFC1035
The following summarizes some allowed inputs to that algorithm, using 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

. Note that while
the length of a

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
item
s:
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.
Table 3
Mapped scheme
"https"
Other affected schemes
"http", "wss", "ws", (other HTTP-based)
RR type
HTTPS (65)
Name prefix
None for port 443, else
_$PORT._https
Automatically mandatory keys
port
no-default-alpn
SvcParam defaults
alpn
: ["http/1.1"]
Special behaviors
Upgrade from HTTP to HTTPS
Keys that records must include
None
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
Erik Nygren
Akamai Technologies
Email:
erik+ietf@nygren.org