DNSOP Working Group B. Schwartz
Internet-Draft Google
Intended status: Standards Track M. Bishop
Expires: September 18, 2021 E. Nygren
Akamai Technologies
March 17, 2021
Service binding and parameter specification via the DNS (DNS SVCB and
HTTPS RRs)
draft-ietf-dnsop-svcb-https-04
Abstract
This document specifies the "SVCB" and "HTTPS" DNS resource record
(RR) types to facilitate the lookup of information needed to make
connections to network services, such as for HTTPS origins. SVCB
records allow a service to be provided from multiple alternative
endpoints, each with associated parameters (such as transport
protocol configuration and keys for encrypting the TLS ClientHello).
They also enable aliasing of apex domains, which is not possible with
CNAME. The HTTPS RR is a variation of SVCB for HTTPS and HTTP
origins. By providing more information to the client before it
attempts to establish a connection, these records offer potential
benefits to both performance and privacy.
TO BE REMOVED: This document is being collaborated on in Github at:
https://github.com/MikeBishop/dns-alt-svc [1]. The most recent
working version of the document, open issues, etc. should all be
available there. The authors (gratefully) accept pull requests.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 18, 2021.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Goals of the SVCB RR . . . . . . . . . . . . . . . . . . 5
1.2. Overview of the SVCB RR . . . . . . . . . . . . . . . . . 5
1.3. Parameter for Encrypted ClientHello . . . . . . . . . . . 6
1.4. Terminology . . . . . . . . . . . . . . . . . . . . . . . 7
2. The SVCB record type . . . . . . . . . . . . . . . . . . . . 7
2.1. Zone file presentation format . . . . . . . . . . . . . . 8
2.2. RDATA wire format . . . . . . . . . . . . . . . . . . . . 9
2.3. SVCB query names . . . . . . . . . . . . . . . . . . . . 10
2.4. Interpretation . . . . . . . . . . . . . . . . . . . . . 10
2.4.1. SvcPriority . . . . . . . . . . . . . . . . . . . . . 11
2.4.2. AliasMode . . . . . . . . . . . . . . . . . . . . . . 11
2.4.3. ServiceMode . . . . . . . . . . . . . . . . . . . . . 12
2.5. Special handling of "." in TargetName . . . . . . . . . . 13
2.5.1. AliasMode . . . . . . . . . . . . . . . . . . . . . . 13
2.5.2. ServiceMode . . . . . . . . . . . . . . . . . . . . . 13
3. Client behavior . . . . . . . . . . . . . . . . . . . . . . . 13
3.1. Handling resolution failures . . . . . . . . . . . . . . 14
3.2. Clients using a Proxy . . . . . . . . . . . . . . . . . . 15
4. DNS Server Behavior . . . . . . . . . . . . . . . . . . . . . 15
4.1. Authoritative servers . . . . . . . . . . . . . . . . . . 15
4.2. Recursive resolvers . . . . . . . . . . . . . . . . . . . 16
4.3. General requirements . . . . . . . . . . . . . . . . . . 16
5. Performance optimizations . . . . . . . . . . . . . . . . . . 17
5.1. Optimistic pre-connection and connection reuse . . . . . 17
5.2. Generating and using incomplete responses . . . . . . . . 18
6. Initial SvcParamKeys . . . . . . . . . . . . . . . . . . . . 18
6.1. "alpn" and "no-default-alpn" . . . . . . . . . . . . . . 18
6.2. "port" . . . . . . . . . . . . . . . . . . . . . . . . . 20
6.3. "echconfig" . . . . . . . . . . . . . . . . . . . . . . . 21
6.4. "ipv4hint" and "ipv6hint" . . . . . . . . . . . . . . . . 21
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7. ServiceMode RR compatibility and mandatory keys . . . . . . . 22
8. Using SVCB with HTTPS and HTTP . . . . . . . . . . . . . . . 23
8.1. Query names for HTTPS RRs . . . . . . . . . . . . . . . . 23
8.2. Relationship to Alt-Svc . . . . . . . . . . . . . . . . . 24
8.2.1. ALPN usage . . . . . . . . . . . . . . . . . . . . . 24
8.2.2. Untrusted channel . . . . . . . . . . . . . . . . . . 24
8.2.3. Cache lifetime . . . . . . . . . . . . . . . . . . . 25
8.2.4. Granularity . . . . . . . . . . . . . . . . . . . . . 25
8.3. Interaction with Alt-Svc . . . . . . . . . . . . . . . . 25
8.4. Requiring Server Name Indication . . . . . . . . . . . . 25
8.5. HTTP Strict Transport Security . . . . . . . . . . . . . 26
8.6. HTTP-based protocols . . . . . . . . . . . . . . . . . . 26
9. SVCB/HTTPS RR parameter for ECH configuration . . . . . . . . 27
9.1. Client behavior . . . . . . . . . . . . . . . . . . . . . 27
9.2. Deployment considerations . . . . . . . . . . . . . . . . 27
10. Zone Structures . . . . . . . . . . . . . . . . . . . . . . . 27
10.1. Structuring zones for flexibility . . . . . . . . . . . 28
10.2. Structuring zones for performance . . . . . . . . . . . 28
10.3. Examples . . . . . . . . . . . . . . . . . . . . . . . . 28
10.3.1. Protocol enhancements . . . . . . . . . . . . . . . 28
10.3.2. Apex aliasing . . . . . . . . . . . . . . . . . . . 29
10.3.3. Parameter binding . . . . . . . . . . . . . . . . . 29
10.3.4. Multi-CDN . . . . . . . . . . . . . . . . . . . . . 30
10.3.5. Non-HTTPS uses . . . . . . . . . . . . . . . . . . . 32
11. Interaction with other standards . . . . . . . . . . . . . . 32
12. Security Considerations . . . . . . . . . . . . . . . . . . . 32
13. Privacy Considerations . . . . . . . . . . . . . . . . . . . 33
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 34
14.1. SVCB RRType . . . . . . . . . . . . . . . . . . . . . . 34
14.2. HTTPS RRType . . . . . . . . . . . . . . . . . . . . . . 34
14.3. New registry for Service Parameters . . . . . . . . . . 34
14.3.1. Procedure . . . . . . . . . . . . . . . . . . . . . 34
14.3.2. Initial contents . . . . . . . . . . . . . . . . . . 35
14.4. Registry updates . . . . . . . . . . . . . . . . . . . . 36
15. Acknowledgments and Related Proposals . . . . . . . . . . . . 37
16. References . . . . . . . . . . . . . . . . . . . . . . . . . 37
16.1. Normative References . . . . . . . . . . . . . . . . . . 37
16.2. Informative References . . . . . . . . . . . . . . . . . 40
16.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 41
Appendix A. Decoding text in zone files . . . . . . . . . . . . 41
A.1. Decoding a comma-separated list . . . . . . . . . . . . . 41
Appendix B. HTTP Mapping Summary . . . . . . . . . . . . . . . . 42
Appendix C. Comparison with alternatives . . . . . . . . . . . . 43
C.1. Differences from the SRV RR type . . . . . . . . . . . . 43
C.2. Differences from the proposed HTTP record . . . . . . . . 43
C.3. Differences from the proposed ANAME record . . . . . . . 43
C.4. Comparison with separate RR types for AliasMode and
ServiceMode . . . . . . . . . . . . . . . . . . . . . . . 44
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Appendix D. Change history . . . . . . . . . . . . . . . . . . . 44
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 47
1. Introduction
The SVCB ("Service Binding") and HTTPS RRs provide clients with
complete instructions for access to a service. This information
enables improved performance and privacy by avoiding transient
connections to a sub-optimal default server, negotiating a preferred
protocol, and providing relevant public keys.
For example, when clients need to make a connection to fetch
resources associated with an HTTPS URI, they currently resolve only A
and/or AAAA records for the origin hostname. This is adequate for
services that use basic HTTPS (fixed port, no QUIC, no [ECH]). Going
beyond basic HTTPS confers privacy, performance, and operational
advantages, but it requires the client to learn additional
information, and it is highly desirable to minimize the number of
round-trips and lookups required to learn this additional
information.
The SVCB and HTTPS RRs also help when the operator of a service
wishes to delegate operational control to one or more other domains,
e.g. delegating the origin "https://example.com" to a service
operator endpoint at "svc.example.net". While this case can
sometimes be handled by a CNAME, that does not cover all use-cases.
CNAME is also inadequate when the service operator needs to provide a
bound collection of consistent configuration parameters through the
DNS (such as network location, protocol, and keying information).
This document first describes the SVCB RR as a general-purpose
resource record that can be applied directly and efficiently to a
wide range of services (Section 2). The HTTPS RR is then defined as
a special case of SVCB that improves efficiency and convenience for
use with HTTPS (Section 8) by avoiding the need for an Attrleaf label
[Attrleaf] (Section 8.1). Other protocols with similar needs may
follow the pattern of HTTPS and assign their own SVCB-compatible RR
types.
All behaviors described as applying to the SVCB RR also apply to the
HTTPS RR unless explicitly stated otherwise. Section 8 describes
additional behaviors specific to the HTTPS RR. Apart from Section 8
and introductory examples, much of this document refers only to the
SVCB RR, but those references should be taken to apply to SVCB,
HTTPS, and any future SVCB-compatible RR types.
The SVCB RR has two modes: 1) "AliasMode" simply delegates
operational control for a resource; 2) "ServiceMode" binds together
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configuration information for a service endpoint. ServiceMode
provides additional key=value parameters within each RDATA set.
1.1. Goals of the SVCB RR
The goal of the SVCB RR is to allow clients to resolve a single
additional DNS RR in a way that:
o Provides alternative endpoints that are authoritative for the
service, along with parameters associated with each of these
endpoints.
o Does not assume that all alternative endpoints have the same
parameters or capabilities, or are even operated by the same
entity. This is important as DNS does not provide any way to tie
together multiple RRs for the same name. For example, if
www.example.com is a CNAME alias that switches between one of
three CDNs or hosting environments, successive queries for that
name may return records that correspond to different environments.
o Enables CNAME-like functionality at a zone apex (such as
"example.com") for participating protocols, and generally enables
delegation of operational authority for an origin within the DNS
to an alternate name.
Additional goals specific to HTTPS RRs and the HTTPS use-case
include:
o Connect directly to HTTP3 (QUIC transport) alternative endpoints
[HTTP3]
o Obtain the Encrypted ClientHello [ECH] keys associated with an
alternative endpoint
o Support non-default TCP and UDP ports
o Enable SRV-like benefits (e.g. apex delegation, as mentioned
above) for HTTP(S), where SRV [SRV] has not been widely adopted
o Provide an HSTS-like indication [HSTS] signaling that the HTTPS
scheme should be used instead of HTTP for this request (see
Section 8.5).
1.2. Overview of the SVCB RR
This subsection briefly describes the SVCB RR in a non-normative
manner. (As mentioned above, this all applies equally to the HTTPS
RR which shares the same encoding, format, and high-level semantics.)
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The SVCB RR has two modes: AliasMode, which aliases a name to another
name, and ServiceMode, which provides connection information bound to
a service endpoint domain. Placing both forms in a single RR type
allows clients to fetch the relevant information with a single query.
The SVCB RR has two mandatory fields and one optional. The fields
are:
1. SvcPriority: The priority of this record (relative to others,
with lower values preferred). A value of 0 indicates AliasMode.
(Described in Section 2.4.1.)
2. TargetName: The domain name of either the alias target (for
AliasMode) or the alternative endpoint (for ServiceMode).
3. SvcParams (optional): A list of key=value pairs describing the
alternative endpoint at TargetName (only used in ServiceMode and
otherwise ignored). 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. Clients either use responses
included in the additional section returned by the recursive resolver
or perform necessary SVCB, A, and AAAA record resolutions. DNS
authoritative servers can attach in-bailiwick SVCB, A, AAAA, and
CNAME records in the Additional Section to responses for a SVCB
query.
In ServiceMode, the SvcParams of the SVCB RR provide an extensible
data model for describing alternative endpoints that are
authoritative for the origin, along with parameters associated with
each of these alternative endpoints.
For the HTTPS use-case, the HTTPS RR enables many of the benefits of
Alt-Svc [AltSvc] without waiting for a full HTTP connection
initiation (multiple roundtrips) before learning of the preferred
alternative, and without necessarily revealing the user's intended
destination to all entities along the network path.
1.3. Parameter for Encrypted ClientHello
This document also defines a parameter for Encrypted ClientHello
[ECH] keys. See Section 9.
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1.4. Terminology
Our terminology is based on the common case where the SVCB record is
used to access a resource identified by a URI whose "authority" field
contains a DNS hostname as the "host".
o The "service" is the information source identified by the
"authority" and "scheme" of the URI, capable of providing access
to the resource. For HTTPS URIs, the "service" corresponds to an
HTTPS "origin" [RFC6454].
o The "service name" is the "host" portion of the authority.
o The "authority endpoint" is the authority's hostname and a port
number implied by the scheme or specified in the URI.
o An "alternative endpoint" is a hostname, port number, and other
associated instructions to the client on how to reach an instance
of service.
Additional DNS terminology intends to be consistent with [DNSTerm].
SVCB is a contraction of "service binding". The SVCB RR, HTTPS RR,
and future RR types that share SVCB's format and registry are
collectively known as SVCB-compatible RR types.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. The SVCB record type
The SVCB DNS resource record (RR) type (RR type 64) is used to locate
alternative endpoints for a service.
The algorithm for resolving SVCB records and associated address
records is specified in Section 3.
Other SVCB-compatible resource record types can also be defined as-
needed. In particular, the HTTPS RR (RR type 65) provides special
handling for the case of "https" origins as described in Section 8.
SVCB RRs are extensible by a list of SvcParams, which are pairs
consisting of a SvcParamKey and a SvcParamValue. Each SvcParamKey
has a presentation name and a registered number. Values are in a
format specific to the SvcParamKey. Their definition should specify
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both their presentation format and wire encoding (e.g., domain names,
binary data, or numeric values). The initial SvcParamKeys and
formats are defined in Section 6.
2.1. Zone file presentation format
The presentation format of the record is:
Name TTL IN SVCB SvcPriority TargetName SvcParams
The SVCB record is defined specifically within the Internet ("IN")
Class ([RFC1035]).
SvcPriority is a number in the range 0-65535, TargetName is a
"<domain-name>" ([RFC1035] Section 5.1), and the SvcParams are a
whitespace-separated list, with each SvcParam consisting of a
SvcParamKey=SvcParamValue pair or a standalone SvcParamKey.
SvcParamKeys are subject to IANA control (Section 14.3).
Each SvcParamKey SHALL appear at most once in the SvcParams. In
presentation format, SvcParamKeys are lower-case alphanumeric
strings. Key names should contain 1-63 characters from the ranges
"a"-"z", "0"-"9", and "-". In ABNF [RFC5234],
alpha-lc = %x61-7A ; a-z
SvcParamKey = 1*63(alpha-lc / DIGIT / "-")
SvcParam = SvcParamKey ["=" SvcParamValue]
SvcParamValue = char-string
value = *OCTET
The SvcParamValue is parsed using the character-string decoding
algorithm (Appendix A), producing a "value". The "value" is then
validated and converted into wire-format in a manner specific to each
key.
When the "=" is omitted, the "value" is interpreted as empty.
Unrecognized keys are represented in presentation format as
"keyNNNNN" where NNNNN is the numeric value of the key type without
leading zeros. A SvcParam in this form SHALL be parsed as specified
above, and the decoded "value" SHALL be used as its wire format
encoding.
For some SvcParamKeys, the "value" corresponds to a list or set of
items. Presentation formats for such keys SHOULD use a comma-
separated list (Appendix A.1).
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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:
o a 2 octet field for SvcPriority as an integer in network byte
order.
o the uncompressed, fully-qualified TargetName, represented as a
sequence of length-prefixed labels as in Section 3.1 of [RFC1035].
o the SvcParams, consuming the remainder of the record (so smaller
than 65535 octets and constrained by the RDATA and DNS message
sizes).
When the list of SvcParams is non-empty (ServiceMode), it contains a
series of SvcParamKey=SvcParamValue pairs, represented as:
o a 2 octet field containing the SvcParamKey as an integer in
network byte order. (See Section 14.3.2 for the defined values.)
o a 2 octet field containing the length of the SvcParamValue as an
integer between 0 and 65535 in network byte order (but constrained
by the RDATA and DNS message sizes).
o an octet string of this length whose contents are in a format
determined by the SvcParamKey.
SvcParamKeys SHALL appear in increasing numeric order.
Clients MUST consider an RR malformed if:
o the end of the RDATA occurs within a SvcParam.
o SvcParamKeys are not in strictly increasing numeric order.
o the SvcParamValue for an SvcParamKey does not have the expected
format.
Note that the second condition implies that there are no duplicate
SvcParamKeys.
If any RRs are malformed, the client MUST reject the entire RRSet and
fall back to non-SVCB connection establishment.
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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.
We term this behavior "Port Prefix Naming", and use it in the
examples throughout this document.
See Section 8.1 for the HTTPS RR behavior.
When a prior CNAME or SVCB record has aliased to a SVCB record, each
RR shall be returned under its own owner name.
Note that none of these forms alter the origin or authority for
validation purposes. For example, TLS clients MUST continue to
validate TLS certificates for the original service name.
As an example, the owner of example.com could publish this record:
_8443._foo.api.example.com. 7200 IN SVCB 0 svc4.example.net.
to indicate that "foo://api.example.com:8443" is aliased to
"svc4.example.net". The owner of example.net, in turn, could publish
this record:
svc4.example.net. 7200 IN SVCB 3 svc4.example.net. (
alpn="bar" port="8004" echconfig="..." )
to indicate that these services are served on port number 8004, which
supports the protocol "bar" and its associated transport in addition
to the default transport protocol for "foo://".
(Parentheses are used to ignore a line break ([RFC1035]
Section 5.1).)
2.4. Interpretation
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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. SVCB RRs with a smaller
SvcPriority value SHOULD be given preference over RRs with a larger
SvcPriority value.
When receiving an RRSet containing multiple SVCB records with the
same SvcPriority value, clients SHOULD apply a random shuffle within
a priority level to the records before using them, to ensure uniform
load-balancing.
2.4.2. AliasMode
In AliasMode, the SVCB record aliases a service to a TargetName.
SVCB RRSets SHOULD only have a single resource record in AliasMode.
If multiple are present, clients or recursive resolvers SHOULD pick
one at random.
The primary purpose of AliasMode is to allow aliasing at the zone
apex, where CNAME is not allowed. In AliasMode, the TargetName will
be the name of a domain that resolves to SVCB (or other SVCB-
compatible record such as HTTPS), AAAA, and/or A records. The
TargetName SHOULD NOT be equal to the owner name, as this would
result in a loop.
In AliasMode, records SHOULD NOT include any SvcParams, and
recipients MUST ignore any SvcParams that are present.
For example, the operator of foo://example.com:8080 could point
requests to a service operating at foosvc.example.net by publishing:
_8080._foo.example.com. 3600 IN SVCB 0 foosvc.example.net.
Using AliasMode maintains a separation of concerns: the owner of
foosvc.example.net can add or remove ServiceMode SVCB records without
requiring a corresponding change to example.com. Note that if
foosvc.example.net promises to always publish a SVCB record, this
AliasMode record can be replaced by a CNAME, which would likely
improve performance.
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AliasMode is especially useful for SVCB-compatible RR types that do
not require an underscore prefix, such as the HTTPS RR type. For
example, the operator of https://example.com could point requests to
a server at svc.example.net by publishing this record at the zone
apex:
example.com. 3600 IN HTTPS 0 svc.example.net.
Note that the SVCB record's owner name MAY be the canonical name of a
CNAME record, and the TargetName MAY be the owner of a CNAME record.
Clients and recursive resolvers MUST follow CNAMEs as normal.
To avoid unbounded alias chains, clients and recursive resolvers MUST
impose a limit on the total number of SVCB aliases they will follow
for each resolution request. This limit MUST NOT be zero, i.e.
implementations MUST be able to follow at least one AliasMode record.
The exact value of this limit is left to implementations.
For compatibility and performance, zone owners SHOULD NOT configure
their zones to require following multiple AliasMode records.
As legacy clients will not know to use this record, service operators
will likely need to retain fallback AAAA and A records alongside this
SVCB record, although in a common case the target of the SVCB record
might offer better performance, and therefore would be preferable for
clients implementing this specification to use.
AliasMode records only apply to queries for the specific RR type.
For example, a SVCB record cannot alias to an HTTPS record, nor vice-
versa.
2.4.3. ServiceMode
In ServiceMode, the TargetName and SvcParams within each resource
record associate an alternative endpoint for the service with its
connection parameters.
Each protocol scheme that uses SVCB MUST define a protocol mapping
that explains how SvcParams are applied for connections of that
scheme. Unless specified otherwise by the protocol mapping, clients
MUST ignore any SvcParam that they do not recognize.
Some SvcParams impose requirements on other SvcParams in the RR. A
ServiceMode RR is called "self-consistent" if its SvcParams all
comply with each others' requirements. Zone-file implementations
SHOULD enforce self-consistency. Clients MUST reject any RR whose
recognized SvcParams are not self-consistent, and MAY reject the
entire RRSet.
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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.
For example, in the following example "svc2.example.net" is the
effective TargetName:
example.com. 7200 IN HTTPS 0 svc.example.net.
svc.example.net. 7200 IN CNAME svc2.example.net.
svc2.example.net. 7200 IN HTTPS 1 . port=8002 echconfig="..."
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 the priority-ordered
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 7) ServiceMode records are
returned, these represent the alternative endpoints.
5. Otherwise, SVCB resolution has failed, and the list of known
endpoints is empty.
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This procedure does not rely on any recursive or authoritative DNS
server to comply with this specification or have any awareness of
SVCB.
Once SVCB resolution has concluded, the client proceeds with
connection establishment. Clients SHOULD try higher-priority
alternatives first, with fallback to lower-priority alternatives.
Clients issue AAAA and/or A queries for the selected TargetName, and
MAY choose between them using an approach such as Happy Eyeballs
[HappyEyeballsV2].
A client is called "SVCB-optional" if it can connect without the use
of ServiceMode records, and "SVCB-reliant" otherwise. Clients for
pre-existing protocols (e.g. HTTPS) SHALL implement SVCB-optional
behavior (except as noted in Section 3.1 and Section 9.1).
SVCB-optional clients SHOULD issue in parallel any other DNS queries
that might be needed for connection establishment. SVCB-optional
clients SHALL append an alternative endpoint consisting of the final
value of $QNAME, the authority endpoint's port number, and no
SvcParams, to the list of alternative endpoints, which is attempted
before falling back to non-SVCB connection modes. This ensures that
SVCB-optional clients will make use of an AliasMode record whose
TargetName has A and/or AAAA records but no SVCB records.
Some important optimizations are discussed in Section 5 to avoid
additional latency in comparison to ordinary AAAA/A lookups.
3.1. Handling resolution failures
If SVCB resolution fails due to a SERVFAIL error, transport error, or
timeout, and DNS exchanges between the client and the recursive
resolver are cryptographically protected (e.g. using TLS [DoT] or
HTTPS [DoH]), a SVCB-optional client SHOULD abandon the connection
attempt like a SVCB-reliant client would. Otherwise, an active
attacker could mount a downgrade attack by denying the user access to
the SvcParams.
A SERVFAIL error can occur if the domain is DNSSEC-signed, the
recursive resolver is DNSSEC-validating, and the attacker is between
the recursive resolver and the authoritative DNS server. A transport
error or timeout can occur if an active attacker between the client
and the recursive resolver is selectively dropping SVCB queries or
responses, based on their size or other observable patterns.
Similarly, if the client enforces DNSSEC validation on A/AAAA
responses, it SHOULD terminate the connection if a SVCB response
fails to validate.
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If the client is unable to complete SVCB resolution due to its chain
length limit, the client SHOULD fall back to the authority endpoint,
as if the origin's SVCB record did not exist.
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 to
use named destinations, in which case the client does not perform any
A or AAAA queries for destination domains. If the client is using
named destinations with a proxy that does not provide SVCB query
capability (e.g. through an affiliated DNS resolver), the client
would have to perform SVCB resolution separately, likely disclosing
the destinations to additional parties. Clients that support such
proxies SHOULD arrange for a separate SVCB resolution procedure with
appropriate privacy properties, or disable SVCB resolution entirely
if SVCB-optional.
If the client does use SVCB and named destinations, the client SHOULD
follow the standard SVCB resolution process, selecting the smallest-
SvcPriority option that is compatible with the client and the proxy.
When connecting using a SVCB record, clients MUST provide the final
TargetName and port to the proxy, which will perform any required A
and AAAA lookups.
Providing the proxy with the final TargetName has several benefits:
o It allows the client to use the SvcParams, if present, which is
only usable with a specific TargetName. The SvcParams may include
information that enhances performance (e.g. alpn) and privacy
(e.g. echconfig).
o It allows the service to delegate the apex domain.
o It allows the proxy to select between IPv4 and IPv6 addresses for
the server according to its configuration, and receive addresses
based on its network geolocation.
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
in-bailiwick TargetNames. If the zone is signed, the server SHOULD
also include positive or negative DNSSEC responses for these records
in the Additional section.
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4.2. Recursive resolvers
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 response. For
the initial SVCB record query, this is just the normal response
construction process (i.e. unknown RR type resolution under
[RFC3597]). For followup resolutions performed during this
procedure, we define incorporation as adding all useful RRs from the
response to the Additional section without altering the response
code.
Upon receiving a SVCB query, recursive resolvers SHOULD start with
the standard resolution procedure, and then follow this procedure to
construct the full response to the stub resolver:
1. Incorporate the results of SVCB resolution. If the chain length
limit has been reached, terminate successfully (i.e. a NOERROR
response).
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]).
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.
4.3. General requirements
Recursive resolvers SHOULD treat the SvcParams portion of the SVCB RR
as opaque and SHOULD NOT try to alter their behavior based on its
contents.
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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).
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 followup queries. With this behavior, and
conforming DNS servers, using SVCB does not add network latency to
connection setup.
To improve performance when using a non-conforming recursive
resolver, clients SHOULD issue speculative A and/or AAAA queries in
parallel with each SVCB query, based on a predicted value of
TargetName (see Section 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, a TLS
ClientHello can be altered by the "echconfig" value of a SVCB
response (Section 6.3). Clients implementing this optimization
SHOULD wait for 50 milliseconds before starting optimistic pre-
connection, as per the guidance in [HappyEyeballsV2].
A SVCB record is consistent with a connection if the client would
attempt an equivalent connection when making use of that record. If
a SVCB record is consistent with an active or in-progress connection
C, the client MAY prefer that record and use C as its connection.
For example, suppose the client receives this SVCB RRSet for a
protocol that uses TLS over TCP:
_1234._bar.example.com. 300 IN SVCB 1 svc1.example.net. (
echconfig="111..." ipv6hint=2001:db8::1 port=1234 )
SVCB 2 svc2.example.net. (
echconfig="222..." ipv6hint=2001:db8::2 port=1234 )
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If the client has an in-progress TCP connection to
"[2001:db8::2]:1234", it MAY proceed with TLS on that connection
using "echconfig="222..."", even though the other record in the RRSet
has higher priority.
If none of the SVCB records are consistent with any active or in-
progress connection, clients proceed with connection establishment as
described in 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 (Section 4.2 step 1), but might also be appropriate when
the maximum response size is reached, or when responding before fully
chasing dependencies would improve performance. When omitting
certain RRSets, recursive resolvers SHOULD prioritize information for
smaller-SvcPriority records.
As discussed in Section 3, clients MUST be able to fetch additional
information that is required to use a SVCB record, if it is not
included in the initial response. As a performance optimization, if
some of the SVCB records in the response can be used without
requiring additional DNS queries, the client MAY prefer those
records, regardless of their priorities.
6. Initial SvcParamKeys
A few initial SvcParamKeys are defined here. These keys are useful
for HTTPS, and most are applicable to other protocols as well. Each
new protocol mapping document MUST specify which keys are applicable
and safe to use. Protocol mappings MAY alter the interpretation of
SvcParamKeys but MUST NOT alter their presentation or wire formats.
6.1. "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.
As with Alt-Svc [AltSvc], the ALPN protocol identifier is used to
identify the application protocol and associated suite of protocols
supported by the endpoint (the "protocol suite"). Clients filter the
set of ALPN identifiers to match the protocol suites they support,
and this informs the underlying transport protocol used (such as
QUIC-over-UDP or TLS-over-TCP).
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ALPNs are identified by their registered "Identification Sequence"
("alpn-id"), which is a sequence of 1-255 octets.
alpn-id = 1*255OCTET
The presentation "value" SHALL be a comma-separated list
(Appendix A.1) of one or more "alpn-id"s.
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
supported ALPNs, which SHOULD NOT be empty. To determine the set of
protocol suites supported by an endpoint (the "SVCB ALPN set"), the
client adds the default set to the list of "alpn-id"s unless the "no-
default-alpn" SvcParamKey is present. The presence of an ALPN
protocol in the SVCB ALPN set indicates that this service endpoint,
described by TargetName and the other parameters (e.g. "port") offers
service with the protocol suite associated with this ALPN protocol.
ALPN protocol names that do not uniquely identify a protocol suite
(e.g. an Identification Sequence that can be used with both TLS and
DTLS) are not compatible with this SvcParamKey and MUST NOT be
included in the SVCB ALPN set.
To establish a connection to the endpoint, clients MUST
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
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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.
With this procedure in place, an attacker who can modify DNS and
network traffic can prevent a successful transport connection, but
cannot otherwise interfere with ALPN protocol selection. This
procedure also ensures that each ProtocolNameList includes at least
one protocol from the SVCB ALPN set.
Clients SHOULD NOT attempt connection to a service endpoint whose
SVCB ALPN set does not contain any supported protocols. To ensure
consistency of behavior, clients MAY reject the entire SVCB RRSet and
fall back to basic connection establishment if all of the RRs
indicate "no-default-alpn", even if connection could have succeeded
using a non-default alpn.
For compatibility with clients that require default transports, zone
operators SHOULD ensure that at least one RR in each RRSet supports
the default transports.
6.2. "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
SvcParam MUST NOT contain escape sequences.
The wire format of the SvcParamValue is the corresponding 2 octet
numeric value in network byte order.
If a port-restricting firewall is in place between some client and
the service endpoint, changing the port number might cause that
client to lose access to the service, so operators should exercise
caution when using this SvcParamKey to specify a non-default port.
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6.3. "echconfig"
The SvcParamKey to enable Encrypted ClientHello (ECH) is "echconfig".
Its value is defined in Section 9. It is applicable to most TLS-
based protocols.
When publishing a record containing an "echconfig" parameter, the
publisher MUST ensure that all IP addresses of TargetName correspond
to servers that have access to the corresponding private key or are
authoritative for the public name. (See Section 7.2.2 of [ECH] for
more details about the public name.) This yields an anonymity set of
cardinality equal to the number of ECH-enabled server domains
supported by a given client-facing server. Thus, even with an
encrypted ClientHello, an attacker who can enumerate the set of ECH-
enabled domains supported by a client-facing server can guess the
correct SNI with probability at least 1/K, where K is the size of
this ECH-enabled server anonymity set. This probability may be
increased via traffic analysis or other mechanisms.
6.4. "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 as in Section 3, and clients SHOULD use the IP address in
those responses for future connections. Clients MAY opt to terminate
any connections using the addresses in hints and instead switch to
the addresses in response to the TargetName query. Failure to use A
and/or AAAA response addresses could negatively impact load balancing
or other geo-aware features and thereby degrade client performance.
The presentation "value" SHALL be a comma-separated list
(Appendix A.1) of one or more IP addresses of the appropriate family
in standard textual format [RFC5952]. To enable simpler parsing,
this SvcParamValue MUST NOT contain escape sequences.
The wire format for each parameter is a sequence of IP addresses in
network byte order. Like an A or AAAA RRSet, the list of addresses
represents an unordered collection, and clients SHOULD pick addresses
to use in a random order. An empty list of addresses is invalid.
When selecting between IPv4 and IPv6 addresses to use, clients may
use an approach such as Happy Eyeballs [HappyEyeballsV2]. When only
"ipv4hint" is present, IPv6-only clients may synthesize IPv6
addresses as specified in [RFC7050] or ignore the "ipv4hint" key and
wait for AAAA resolution (Section 3). Recursive resolvers MUST NOT
perform DNS64 ([RFC6147]) on parameters within a SVCB record. For
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best performance, server operators SHOULD include an "ipv6hint"
parameter whenever they include an "ipv4hint" parameter.
These parameters are intended to minimize additional connection
latency when a recursive resolver is not compliant with the
requirements in Section 4, and SHOULD NOT be included if most clients
are using compliant recursive resolvers. When TargetName is the
origin hostname or the owner name (which can be written as "."),
server operators SHOULD NOT include these hints, because they are
unlikely to convey any performance benefit.
7. 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" with a client if the
client recognizes all the mandatory keys, and their values indicate
that successful connection establishment is possible. If the SVCB
RRSet contains no compatible RRs, the client will generally act as if
the RRSet is empty.
The presentation "value" SHALL be a comma-separated list
(Appendix A.1) of one or more valid SvcParamKeys, either by their
registered name or in the unknown-key format (Section 2.1). Keys MAY
appear in any order, but MUST NOT appear more than once. For self-
consistency (Section 2.4.3), listed keys MUST also appear in the
SvcParams.
To enable simpler parsing, this SvcParamValue MUST NOT contain escape
sequences.
For example, the following is a valid list of SvcParams:
echconfig=... key65333=ex1 key65444=ex2 mandatory=key65444,echconfig
In wire format, the keys are represented by their numeric values in
network byte order, concatenated in ascending order.
This SvcParamKey is always automatically mandatory, and MUST NOT
appear in its own value-list. Other automatically mandatory keys
SHOULD NOT appear in the list either. (Including them wastes space
and otherwise has no effect.)
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8. Using SVCB with HTTPS and HTTP
Use of any protocol with SVCB requires a protocol-specific mapping
specification. This section specifies the mapping for HTTPS and
HTTP.
To enable special handling for the HTTPS and HTTP use-cases, the
HTTPS RR type is defined as a SVCB-compatible RR type, specific to
the https and http schemes. Clients MUST NOT perform SVCB queries or
accept SVCB responses for "https" or "http" schemes.
The HTTPS RR wire format and presentation format are identical to
SVCB, and both share the SvcParamKey registry. SVCB semantics apply
equally to HTTPS RRs unless specified otherwise. The presentation
format of the record is:
Name TTL IN HTTPS SvcPriority TargetName SvcParams
As with SVCB, the record is defined specifically within the Internet
("IN") Class [RFC1035].
All the SvcParamKeys defined in Section 6 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 7) are
"port" and "no-default-alpn". (As described in Section 7, clients
must either implement these keys or ignore any RR in which they
appear.) Clients that restrict the HTTPS destination port (e.g.
using the "bad ports" list from [FETCH]) SHOULD apply the same
restriction to the "port" SvcParam.
The presence of an HTTPS RR for an origin also indicates that all
HTTP resources are available over HTTPS, as discussed in Section 8.5.
This allows HTTPS RRs to apply to pre-existing "http" scheme URLs,
while ensuring that the client uses a secure and authenticated HTTPS
connection.
The HTTPS RR parallels the concepts introduced in the HTTP
Alternative Services proposed standard [AltSvc]. Clients and servers
that implement HTTPS RRs are not required to implement Alt-Svc.
8.1. 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.
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By removing the Attrleaf labels [Attrleaf] used in SVCB, this
construction enables offline DNSSEC signing of wildcard domains,
which are commonly used with HTTPS. Reusing the service name also
allows the targets of existing CNAME chains (e.g. CDN hosts) to
start returning HTTPS RR responses without requiring origin domains
to configure and maintain an additional delegation.
Following of HTTPS AliasMode RRs and CNAME aliases is unchanged from
SVCB.
Clients always convert "http" URLs to "https" before performing an
HTTPS RR query using the process described in Section 8.5, 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.
8.2. Relationship to Alt-Svc
Publishing a ServiceMode HTTPS RR in DNS is intended to be similar to
transmitting an Alt-Svc field value over HTTPS, and receiving an
HTTPS RR is intended to be similar to receiving that field value over
HTTPS. However, there are some differences in the intended client
and server behavior.
8.2.1. ALPN usage
Unlike Alt-Svc Field Values, HTTPS RRs can contain multiple ALPN IDs,
and clients are encouraged to offer additional ALPNs that they
support.
8.2.2. Untrusted channel
SVCB does not require or provide any assurance of authenticity.
(DNSSEC signing and verification, which would provide such assurance,
are OPTIONAL.) The DNS resolution process is treated as an untrusted
channel that learns only the QNAME, and is prevented from mounting
any attack beyond denial of service.
Alt-Svc parameters that cannot be safely received in this model MUST
NOT have a corresponding defined SvcParamKey. For example, there is
no SvcParamKey corresponding to the Alt-Svc "persist" parameter,
because this parameter is not safe to accept over an untrusted
channel.
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8.2.3. 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.
8.2.4. 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.
8.3. Interaction with Alt-Svc
Clients that do not implement support for Encrypted ClientHello MAY
skip the HTTPS RR query if a usable Alt-Svc value is available in the
local cache. If Alt-Svc connection fails, these clients SHOULD fall
back to the HTTPS RR client connection procedure (Section 3).
Clients that implement support for ECH MUST perform the HTTPS RR
query first, and MUST only make use of Alt-Svc when operating in
SVCB-optional mode (see Section 9.1).
This specification does not alter the DNS records used when
connecting to an Alt-Svc hostname (typically A and/or AAAA only).
8.4. Requiring Server Name Indication
Clients MUST NOT use an HTTPS RR response unless the client supports
TLS Server Name Indication (SNI) and indicate the origin name when
negotiating TLS. This supports the conservation of IP addresses.
Note that the TLS SNI (and also the HTTP "Host" or ":authority") will
indicate the origin, not the TargetName.
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8.5. HTTP Strict Transport Security
By publishing a usable HTTPS RR, the server operator indicates that
all useful HTTP resources on that origin are reachable over HTTPS,
similar to HTTP Strict Transport Security [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], point 5.
If an HTTPS RR query for this "https" URL returns any AliasMode HTTPS
RRs, or any compatible ServiceMode HTTPS RRs (see Section 7), the
client SHOULD act as if it has received an HTTP "307 Temporary
Redirect" redirect to this "https" URL. (Receipt of an incompatible
ServiceMode RR does not trigger the redirect behavior.) Because
HTTPS RRs are received over an often insecure channel (DNS), clients
MUST NOT place any more trust in this signal than if they had
received a 307 redirect over cleartext HTTP.
When an HTTPS connection fails due to an error in the underlying
secure transport, such as an error in certificate validation, some
clients currently offer a "user recourse" that allows the user to
bypass the security error and connect anyway. When making an "https"
scheme request to an origin with an HTTPS RR, either directly or via
the above redirect, such a client MAY remove the user recourse
option. Origins that publish HTTPS RRs therefore MUST NOT rely on
user recourse for access. For more information, see Section 8.4 and
Section 12.1 of [HSTS].
8.6. HTTP-based protocols
All protocols employing "http://" or "https://" URLs SHOULD respect
HTTPS RRs. For example, clients that support HTTPS RRs and implement
the altered WebSocket [WebSocket] opening handshake from the W3C
Fetch specification [FETCH] SHOULD use HTTPS RRs for the
"requestURL".
An HTTP-based protocol MAY define its own SVCB mapping. Such
mappings MAY be defined to take precedence over HTTPS RRs.
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9. SVCB/HTTPS RR parameter for ECH configuration
The SVCB "echconfig" parameter is defined for conveying the ECH
configuration of an alternative endpoint. In wire format, the value
of the parameter is an ECHConfigList [ECH], including the redundant
length prefix. In presentation format, the value is a single
ECHConfigList encoded in Base64 [base64]. Base64 is used here to
simplify integration with TLS server software. To enable simpler
parsing, this SvcParam MUST NOT contain escape sequences.
When ECH is in use, the TLS ClientHello is divided into an
unencrypted "outer" and an encrypted "inner" ClientHello. The outer
ClientHello is an implementation detail of ECH, and its contents are
controlled by the ECHConfig in accordance with [ECH]. The inner
ClientHello is used for establishing a connection to the service, so
its contents may be influenced by other SVCB parameters. For
example, the requirements on the ProtocolNameList in Section 6.1
apply only to the inner ClientHello. Similarly, it is the inner
ClientHello whose Server Name Indication identifies the desired
service.
9.1. Client behavior
The SVCB-optional client behavior specified in Section 3 permits
clients to fall back to a direct connection if all SVCB options fail.
This behavior is not suitable for ECH, because fallback would negate
the privacy benefits of ECH. Accordingly, ECH-capable SVCB-optional
clients MUST switch to SVCB-reliant connection establishment if SVCB
resolution succeeded (following Section 3) and all alternative
endpoints have an "echconfig" key.
As a latency optimization, clients MAY prefetch DNS records that will
only be used in SVCB-optional mode.
9.2. Deployment considerations
An HTTPS RRSet containing some RRs with "echconfig" and some without
is vulnerable to a downgrade attack. This configuration is NOT
RECOMMENDED. Zone owners who do use such a mixed configuration
SHOULD mark the RRs with "echconfig" as more preferred (i.e. smaller
SvcPriority) than those without, in order to maximize the likelihood
that ECH will be used in the absence of an active adversary.
10. Zone Structures
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10.1. Structuring zones for flexibility
Each ServiceForm RRSet can only serve a single scheme. The scheme is
indicated by the owner name and the RR type. For the generic SVCB RR
type, this means that each owner name can only be used for a single
scheme. The underscore prefixing requirement (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.
10.2. Structuring zones for performance
To avoid a delay for clients using a nonconforming recursive
resolver, domain owners SHOULD minimize the use of AliasMode records,
and SHOULD choose TargetName according to a predictable convention
that is known to the client, so that clients can issue A and/or AAAA
queries for TargetName in advance (see Section 5). Unless otherwise
specified, the convention is to set TargetName to the service name
for an initial ServiceMode record, or to "." if it is reached via an
alias. For foo://foo.example.com:8080, this might look like:
$ORIGIN example.com. ; Origin
foo 3600 IN CNAME foosvc.example.net.
_8080._foo.foo 3600 IN CNAME foosvc.example.net.
$ORIGIN example.net. ; Service provider zone
foosvc 3600 IN SVCB 1 . key65333=...
foosvc 300 IN AAAA 2001:db8::1
Domain owners SHOULD avoid using a TargetName that is below a DNAME,
as this is likely unnecessary and makes responses slower and larger.
Also, zone structures that require following more than 8 aliases
(counting both AliasMode and CNAME records) are NOT RECOMMENDED.
10.3. Examples
10.3.1. 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
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The domain owner could add this record:
simple.example. 7200 IN HTTPS 1 . alpn=h3
to indicate that simple.example uses HTTPS, and supports QUIC in
addition to HTTPS over TCP (an implicit default). The record could
also include other information (e.g. non-standard port, ECH
configuration).
10.3.2. 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 to HTTPS on aliased.example.) Non-HTTPS-RR-aware
clients will just ignore the new record.
Similar to CNAME, HTTPS RRs have no impact on the origin name. When
connecting, clients will continue to treat the authoritative origins
as "https://www.aliased.example" and "https://aliased.example",
respectively, and will validate TLS server certificates accordingly.
10.3.3. Parameter binding
Suppose that svc.example's default server pool supports HTTP/2, and
it has deployed HTTP/3 on a new server pool with a different
configuration. This can be expressed in the following form:
$ORIGIN svc.example. ; A hosting provider.
pool 7200 IN HTTPS 1 h3pool alpn=h2,h3 echconfig="123..."
HTTPS 2 . alpn=h2 echconfig="abc..."
pool 300 IN A 192.0.2.2
AAAA 2001:db8::2
h3pool 300 IN A 192.0.2.3
AAAA 2001:db8::3
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This configuration is entirely compatible with the "Apex aliasing"
example, whether the client supports HTTPS RRs or not. If the client
does support HTTPS RRs, all connections will be upgraded to HTTPS,
and clients will use HTTP/3 if they can. Parameters are "bound" to
each server pool, so each server pool can have its own protocol, ECH
configuration, etc.
10.3.4. Multi-CDN
The HTTPS RR is intended to support HTTPS services operated by
multiple independent entities, such as different Content Delivery
Networks (CDNs) or different hosting providers. This includes the
case where a service is migrated from one operator to another, as
well as the case where the service is multiplexed between multiple
operators for performance, redundancy, etc.
This example shows such a configuration, with www.customer.example
having different DNS responses to different queries, either over time
or due to logic within the authoritative DNS server:
; This zone contains/returns different CNAME records
; at different points-in-time. The RRset for "www" can
; only ever contain a single CNAME.
; Sometimes the zone has:
$ORIGIN customer.example. ; A Multi-CDN customer domain
www 900 IN CNAME cdn1.svc1.example.
; and other times it contains:
$ORIGIN customer.example.
www 900 IN CNAME customer.svc2.example.
; and yet other times it contains:
$ORIGIN customer.example.
www 900 IN CNAME cdn3.svc3.example.
; With the following remaining constant and always included:
$ORIGIN customer.example. ; A Multi-CDN customer domain
; The apex is also aliased to www to match its configuration
@ 7200 IN HTTPS 0 www
; Non-HTTPS-aware clients use non-CDN IPs
A 203.0.113.82
AAAA 2001:db8:203::2
; Resolutions following the cdn1.svc1.example
; path use these records.
; This CDN uses a different alternative service for HTTP/3.
$ORIGIN svc1.example. ; domain for CDN 1
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cdn1 1800 IN HTTPS 1 h3pool alpn=h3 echconfig="123..."
HTTPS 2 . alpn=h2 echconfig="123..."
A 192.0.2.2
AAAA 2001:db8:192::4
h3pool 300 IN A 192.0.2.3
AAAA 2001:db8:192:7::3
; Resolutions following the customer.svc2.example
; path use these records.
; Note that this CDN only supports HTTP/2.
$ORIGIN svc2.example. ; domain operated by CDN 2
customer 300 IN HTTPS 1 . alpn=h2 echconfig="xyz..."
60 IN A 198.51.100.2
A 198.51.100.3
A 198.51.100.4
AAAA 2001:db8:198::7
AAAA 2001:db8:198::12
; Resolutions following the customer.svc2.example
; path use these records.
; Note that this CDN has no HTTPS records
; and thus no ECH support.
$ORIGIN svc3.example. ; domain operated by CDN 3
cdn3 60 IN A 203.0.113.8
AAAA 2001:db8:113::8
Note that in the above example, the different CDNs have different
echconfig and different capabilities, but clients will use HTTPS RRs
as a bound-together unit.
Domain owners should be cautious when using a multi-CDN
configuration, as it introduces a number of complexities highlighted
by this example:
o If CDN 1 supports ECH, and CDN 2 does not, the client is
vulnerable to ECH downgrade by a network adversary who forces
clients to get CDN 2 records.
o 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.
o The A, AAAA, HTTPS resolutions are independent lookups so clients
may observe and follow different CNAMEs to different CDNs.
Clients may thus find a SvcDomainName pointing to a name other
than the one which returned along with the A and AAAA lookups and
will need to do an additional resolution for them. Including
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ipv6hint and ipv4hint will reduce the performance impact of this
case.
o If not all CDNs publish HTTPS records, clients will sometimes
receive NODATA for HTTPS queries (as with cdn3.svc3.example
above), and thus no echconfig, but could receive A/AAAA records
from a different CDN which does support ECH. Clients will be
unable to use ECH in this case.
10.3.5. Non-HTTPS uses
For services other than HTTPS, 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 OCSP Stapling [RFC6066], both of
which confer substantial performance and privacy benefits when used
in combination with SVCB records.
To realize the greatest privacy benefits, this proposal is intended
for use over a privacy-preserving DNS transport (like DNS over TLS
[DoT] or DNS over HTTPS [DoH]). However, performance improvements,
and some modest privacy improvements, are possible without the use of
those standards.
Any specification for use of SVCB with a protocol MUST have an entry
for its scheme under the SVCB RR type in the IANA DNS Underscore
Global Scoped Entry Registry [Attrleaf]. The scheme SHOULD have an
entry in the IANA URI Schemes Registry [RFC7595]. The scheme SHOULD
have a defined specification for use with SVCB.
12. Security Considerations
SVCB/HTTPS RRs are intended for distribution over untrusted channels,
and clients are REQUIRED to verify that the alternative endpoint is
authoritative for the service (similar to Section 2.1 of [AltSvc]).
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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 8.5), and disable the encryption enabled by the echconfig
SvcParamKey (Section 9). 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 AliasForm "." 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 AliasForm "." records.
A hostile DNS intermediary or origin can return SVCB records
indicating any IP address and port number, including IP addresses
inside the local network and port numbers assigned to internal
services. If the attacker can influence the client's payload (e.g.
TLS session ticket contents), and an internal service has a
sufficiently lax parser, it's possible that the attacker could gain
unintended access. (The same concerns apply to SRV records, HTTP
Alt-Svc, and HTTP redirects.) As a mitigation, SVCB mapping
documents SHOULD indicate any port number restrictions that are
appropriate for the supported transports.
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.
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14. IANA Considerations
14.1. SVCB RRType
This document defines a new DNS RR type, SVCB, whose value 64 has
been allocated by IANA from the "Resource Record (RR) TYPEs"
subregistry of the "Domain Name System (DNS) Parameters" registry:
Type: SVCB
Value: 64
Meaning: General Purpose Service Endpoints
Reference: This document
14.2. HTTPS RRType
This document defines a new DNS RR type, HTTPS, whose value 65 has
been allocated by IANA from the "Resource Record (RR) TYPEs"
subregistry of the "Domain Name System (DNS) Parameters" registry:
Type: HTTPS
Value: 65
Meaning: HTTPS Specific Service Endpoints
Reference: This document
14.3. New registry for Service Parameters
The "Service Binding (SVCB) Parameter Registry" defines the namespace
for parameters, including string representations and numeric
SvcParamKey values. This registry is shared with other SVCB-
compatible RR types, such as the HTTPS RR.
ACTION: create and include a reference to this registry.
14.3.1. Procedure
A registration MUST include the following fields:
o Number: wire format numeric identifier (range 0-65535)
o Name: unique presentation name
o Meaning: a short description
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o Format Reference: pointer to specification text
The characters in the registered Name MUST be lower-case alphanumeric
or "-" (Section 2.1). The name MUST NOT start with "key" or
"invalid".
Entries in this registry are subject to a First Come First Served
registration policy ([RFC8126], Section 4.6). The Format Reference
MUST specify how to convert the SvcParamValue's presentation format
to wire format and MAY detail its intended meaning and use. An entry
MAY specify a Format Reference of the form "Same as (other key Name)"
if it uses the same presentation and wire formats as an existing key.
This arrangement supports the development of new parameters while
ensuring that zone files can be made interoperable.
14.3.2. Initial contents
The "Service Binding (SVCB) Parameter Registry" shall initially be
populated with the registrations below:
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+-------------+-----------------+------------------+----------------+
| Number | Name | Meaning | Format |
| | | | Reference |
+-------------+-----------------+------------------+----------------+
| 0 | mandatory | Mandatory keys | (This |
| | | in this RR | document) |
| | | | Section 7 |
| | | | |
| 1 | alpn | Additional | (This |
| | | supported | document) |
| | | protocols | Section 6.1 |
| | | | |
| 2 | no-default-alpn | No support for | (This |
| | | default protocol | document) |
| | | | Section 6.1 |
| | | | |
| 3 | port | Port for | (This |
| | | alternative | document) |
| | | endpoint | Section 6.2 |
| | | | |
| 4 | ipv4hint | IPv4 address | (This |
| | | hints | document) |
| | | | Section 6.4 |
| | | | |
| 5 | echconfig | Encrypted | (This |
| | | ClientHello info | document) |
| | | | Section 6.3 |
| | | | |
| 6 | ipv6hint | IPv6 address | (This |
| | | hints | document) |
| | | | Section 6.4 |
| | | | |
| 65280-65534 | N/A | Private Use | (This |
| | | | document) |
| | | | |
| 65535 | N/A | Reserved | (This |
| | | ("Invalid key") | document) |
+-------------+-----------------+------------------+----------------+
14.4. Registry updates
Per [RFC6895], please add the following entries to the data type
range of the Resource Record (RR) TYPEs registry:
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+-------+------------------------------------------+----------------+
| TYPE | Meaning | Reference |
+-------+------------------------------------------+----------------+
| SVCB | Service Location and Parameter Binding | (This |
| | | document) |
| | | |
| HTTPS | HTTPS Service Location and Parameter | (This |
| | Binding | document) |
+-------+------------------------------------------+----------------+
Per [Attrleaf], please add the following entry to the DNS Underscore
Global Scoped Entry Registry:
+---------+------------+-----------------+-----------------+
| RR TYPE | _NODE NAME | Meaning | Reference |
+---------+------------+-----------------+-----------------+
| HTTPS | _https | HTTPS SVCB info | (This document) |
+---------+------------+-----------------+-----------------+
15. Acknowledgments and Related Proposals
There have been a wide range of proposed solutions over the years to
the "CNAME at the Zone Apex" challenge proposed. These include
[I-D.bellis-dnsop-http-record], [I-D.ietf-dnsop-aname], and others.
Thank you to Ian Swett, Ralf Weber, Jon Reed, Martin Thomson, Lucas
Pardue, Ilari Liusvaara, Tim Wicinski, Tommy Pauly, Chris Wood, David
Benjamin, Mark Andrews, Emily Stark, Eric Orth, Kyle Rose, Craig
Taylor, Dan McArdle, Brian Dickson, and others for their feedback and
suggestions on this draft.
16. References
16.1. 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>.
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[base64] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>.
[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>.
[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>.
[ECH] Rescorla, E., Oku, K., Sullivan, N., and C. Wood, "TLS
Encrypted Client Hello", draft-ietf-tls-esni-09 (work in
progress), December 2020.
[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>.
[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>.
[HTTP3] Bishop, M., "Hypertext Transfer Protocol Version 3
(HTTP/3)", draft-ietf-quic-http-33 (work in progress),
December 2020.
[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>.
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[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>.
[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>.
[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>.
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[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>.
16.2. 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>.
[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>.
[FETCH] "Fetch Living Standard", May 2020,
<https://fetch.spec.whatwg.org/>.
[I-D.bellis-dnsop-http-record]
Bellis, R., "A DNS Resource Record for HTTP", draft-
bellis-dnsop-http-record-00 (work in progress), November
2018.
[I-D.ietf-dnsop-aname]
Finch, T., Hunt, E., Dijk, P., Eden, A., and W. Mekking,
"Address-specific DNS aliases (ANAME)", draft-ietf-dnsop-
aname-04 (work in progress), July 2019.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
DOI 10.17487/RFC6454, December 2011,
<https://www.rfc-editor.org/info/rfc6454>.
[RFC6895] Eastlake 3rd, D., "Domain Name System (DNS) IANA
Considerations", BCP 42, RFC 6895, DOI 10.17487/RFC6895,
April 2013, <https://www.rfc-editor.org/info/rfc6895>.
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[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>.
16.3. URIs
[1] https://github.com/MikeBishop/dns-alt-svc
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. The
algorithm for decoding these character-strings is defined in
Section 5.1 of [RFC1035]. Here we summarize the allowed input to
that algorithm, using ABNF:
; non-special is VCHAR minus DQUOTE, ";", "(", ")", and "\".
non-special = %x21 / %x23-27 / %x2A-3A / %x3C-5B / %x5D-7E
; non-digit is VCHAR minus DIGIT
non-digit = %x21-2F / %x3A-7E
; dec-octet is a number 0-255 as a three-digit decimal number.
dec-octet = ( "0" / "1" ) 2DIGIT /
"2" ( ( %x30-34 DIGIT ) / ( "5" %x30-35 ) )
escaped = "\" ( non-digit / dec-octet )
contiguous = 1*( non-special / escaped )
quoted = DQUOTE *( contiguous / ( ["\"] WSP ) ) DQUOTE
char-string = contiguous / quoted
The decoding algorithm allows "char-string" to represent any
"*OCTET". In this document, this algorithm is referred to as
"character-string decoding". The algorithm is the same as used by
"<character-string>" in RFC 1035, although the output length in this
document is not limited to 255 octets.
A.1. Decoding a comma-separated list
In order to represent lists of items in zone files, this
specification uses comma-separated lists. When "," is not escaped
(by a preceding "\"), it separates items in the list. (For
simplicity, empty items are not allowed.)
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item = 1*OCTET
; item-allowed is OCTET minus "," and "\".
item-allowed = %x00-2B / %x2D-5B / %x5D-FF
escaped-item = 1*(item-allowed / "\," / "\\")
comma-separated = [escaped-item *("," escaped-item)]
Decoding of value-lists happens after character-string decoding.
For example, consider these "char-string" SvcParamValues:
"part1,part2,part3\\,part4\\\\"
part1\,\p\a\r\t2\044part3\092,part4\092\\
These inputs are equivalent: character-string decoding either of them
would produce the same "value":
part1,part2,part3\,part4\\
Applying comma-separated list decoding to this "value" would produce
a list of three "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 8). 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-alpn" |
| Keys* | |
| | |
| *SvcParam defaults* | "alpn": ["http/1.1"] |
| | |
| *Special behaviors* | HTTP to HTTPS upgrade |
+-----------------------------+-------------------------------------+
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This table does not indicate any SvcParamKeys that servers are
required to publish, or that clients are required to implement,
because there are none in this mapping.
Appendix C. Comparison with alternatives
The SVCB and HTTPS RR types closely resemble, and are inspired by,
some existing record types and proposals. A complaint with all of
the alternatives is that web clients have seemed unenthusiastic about
implementing them. The hope here is that by providing an extensible
solution that solves multiple problems we will overcome the inertia
and have a path to achieve client implementation.
C.1. Differences from the SRV RR type
An SRV record [SRV] can perform a similar function to the SVCB
record, informing a client to look in a different location for a
service. However, there are several differences:
o SRV records are typically mandatory, whereas clients will always
continue to function correctly without making use of SVCB.
o SRV records cannot instruct the client to switch or upgrade
protocols, whereas SVCB can signal such an upgrade (e.g. to
HTTP/2).
o SRV records are not extensible, whereas SVCB and HTTPS RRs can be
extended with new parameters.
o SVCB records use 16 bit for SvcPriority for consistency with SRV
and other RR types that also use 16 bit priorities.
C.2. Differences from the proposed HTTP record
Unlike [I-D.bellis-dnsop-http-record], this approach is extensible to
cover Alt-Svc and Encrypted ClientHello use-cases. Like that
proposal, this addresses the zone apex CNAME challenge.
Like that proposal, it remains necessary to continue to include
address records at the zone apex for legacy clients.
C.3. Differences from the proposed ANAME record
Unlike [I-D.ietf-dnsop-aname], this approach is extensible to cover
Alt-Svc and ECH use-cases. This approach also does not require any
changes or special handling on either authoritative or primary
servers, beyond optionally returning in-bailiwick additional records.
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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 a serious performance impairment,
because clients cannot rely on their recursive resolver to follow
SVCB aliases (unlike CNAME). Thus, clients would have to issue
queries for both RR types in parallel, potentially at each step of
the alias chain. Recursive resolvers that implement the
specification would, upon receipt of a ServiceMode query, emit both a
ServiceMode and an AliasMode query to the authoritative. Thus,
splitting the RR type would double, or in some cases triple, the load
on clients and servers, and would not reduce implementation
complexity.
Appendix D. Change history
o draft-ietf-dnsop-svcb-https-04
* Simplify the IANA instructions (pure First Come First Served)
* Recommend against publishing chains of >8 aliases
* Clarify requirements for using SVCB with a transport proxy
* Adjust guidance for Port Prefix Naming
* Minor editorial updates
o draft-ietf-dnsop-svcb-https-03
* Simplified escaping of comma-separated values
* Reorganized client requirements
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* Added a warning about port filtering for cross-protocol attacks
* Clarified self-consistency rules for SvcParams
* Added a non-normative mapping summary table for HTTPS
o draft-ietf-dnsop-svcb-https-02
* Added a Privacy Considerations section
* Adjusted resolution fallback description
* Clarified status of SvcParams in AliasMode
* Improved advice on zone structuring and use with Alt-Svc
* Improved examples, including a new Multi-CDN example
* Reorganized text on value-list parsing and SvcPriority
* Improved phrasing and other editorial improvements throughout
o draft-ietf-dnsop-svcb-https-01
* Added a "mandatory" SvcParamKey
* Added the ability to indicate that a service does not exist
* Adjusted resolution and ALPN algorithms
* Major terminology revisions for "origin" and CamelCase names
* Revised ABNF
* Include allocated RR type numbers
* Various corrections, explanations, and recommendations
o draft-ietf-dnsop-svcb-https-00
* Rename HTTPSSVC RR to HTTPS RR
* Rename "an SVCB" to "a SVCB"
* Removed "design considerations and open issues" section and
some other "to be removed" text
o draft-ietf-dnsop-svcb-httpssvc-03
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* Revised chain length limit requirements
* Revised IANA registry rules for SvcParamKeys
* Require HTTPS clients to implement SNI
* Update terminology for Encrypted ClientHello
* Clarifications: non-default ports, transport proxies, HSTS
procedure, WebSocket behavior, wire format, IP hints, inner/
outer ClientHello with ECH
* Various textual and ABNF corrections
o draft-ietf-dnsop-svcb-httpssvc-02
* All changes to Alt-Svc have been removed
* Expanded and reorganized examples
* Priority zero is now the definition of AliasForm
* Repeated SvcParamKeys are no longer allowed
* The "=" sign may be omitted in a key=value pair if the value is
also empty
* In the wire format, SvcParamKeys must be in sorted order
* New text regarding how to handle resolution timeouts
* Expanded description of recursive resolver behavior
* Much more precise description of the intended ALPN behavior
* Match the HSTS specification's language on HTTPS enforcement
* Removed 'esniconfig=""' mechanism and simplified ESNI
connection logic
o draft-ietf-dnsop-svcb-httpssvc-01
* Reduce the emphasis on conversion between HTTPSSVC and Alt-Svc
* Make the "untrusted channel" concept more precise.
* Make SvcFieldPriority = 0 the definition of AliasForm, instead
of a requirement.
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o draft-ietf-dnsop-svcb-httpssvc-00
* Document an optimization for optimistic pre-connection. (Chris
Wood)
* Relax IP hint handling requirements. (Eric Rescorla)
o draft-nygren-dnsop-svcb-httpssvc-00
* Generalize to an SVCB record, with special-case handling for
Alt-Svc and HTTPS separated out to dedicated sections.
* Split out a separate HTTPSSVC record for the HTTPS use-case.
* Remove the explicit SvcRecordType=0/1 and instead make the
AliasForm vs ServiceForm be implicit. This was based on
feedback recommending against subtyping RR type.
* Remove one optimization.
o draft-nygren-httpbis-httpssvc-03
* Change redirect type for HSTS-style behavior from 302 to 307 to
reduce ambiguities.
o draft-nygren-httpbis-httpssvc-02
* Remove the redundant length fields from the wire format.
* Define a SvcDomainName of "." for SvcRecordType=1 as being the
HTTPSSVC RRNAME.
* Replace "hq" with "h3".
o draft-nygren-httpbis-httpssvc-01
* Fixes of record name. Replace references to "HTTPSVC" with
"HTTPSSVC".
o draft-nygren-httpbis-httpssvc-00
* Initial version
Authors' Addresses
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Ben Schwartz
Google
Email: bemasc@google.com
Mike Bishop
Akamai Technologies
Email: mbishop@evequefou.be
Erik Nygren
Akamai Technologies
Email: erik+ietf@nygren.org
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