DNSOP Working Group B. Schwartz
Internet-Draft Google
Intended status: Standards Track M. Bishop
Expires: December 14, 2020 E. Nygren
Akamai Technologies
June 12, 2020
Service binding and parameter specification via the DNS (DNS SVCB and
HTTPS RRs)
draft-ietf-dnsop-svcb-https-00
Abstract
This document specifies the "SVCB" and "HTTPS" DNS resource record
(RR) types to facilitate the lookup of information needed to make
connections for origin resources, such as for HTTPS URLs. SVCB
records allow an origin to be served from multiple network locations,
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 proposal is inspired by and based on recent DNS
usage proposals such as ALTSVC, ANAME, and ESNIKEYS (as well as long
standing desires to have SRV or a functional equivalent implemented
for HTTP). These proposals each provide an important function but
are potentially incompatible with each other, such as when an origin
is load-balanced across multiple hosting providers (multi-CDN).
Furthermore, these each add potential cases for adding additional
record lookups in addition to AAAA/A lookups. This design attempts
to provide a unified framework that encompasses the key functionality
of these proposals, as well as providing some extensibility for
addressing similar future challenges.
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.
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Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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 . . . . . . . . . . . . . . . . . 6
1.3. Parameter for Encrypted ClientHello . . . . . . . . . . . 7
1.4. Terminology . . . . . . . . . . . . . . . . . . . . . . . 7
2. The SVCB record type . . . . . . . . . . . . . . . . . . . . 7
2.1. Presentation format . . . . . . . . . . . . . . . . . . . 8
2.1.1. Presentation format for SvcFieldValue key=value pairs 8
2.2. SVCB RDATA Wire Format . . . . . . . . . . . . . . . . . 9
2.3. SVCB owner names . . . . . . . . . . . . . . . . . . . . 10
2.4. SvcRecordType . . . . . . . . . . . . . . . . . . . . . . 11
2.5. SVCB records: AliasForm . . . . . . . . . . . . . . . . . 11
2.6. SVCB records: ServiceForm . . . . . . . . . . . . . . . . 12
2.6.1. Special handling of "." for SvcDomainName in
ServiceForm . . . . . . . . . . . . . . . . . . . . . 12
2.6.2. SvcFieldPriority . . . . . . . . . . . . . . . . . . 13
3. Client behavior . . . . . . . . . . . . . . . . . . . . . . . 13
3.1. Handling resolution failures . . . . . . . . . . . . . . 14
3.2. Clients using a Proxy . . . . . . . . . . . . . . . . . . 14
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4. DNS Server Behavior . . . . . . . . . . . . . . . . . . . . . 15
4.1. Authoritative servers . . . . . . . . . . . . . . . . . . 15
4.2. Recursive resolvers . . . . . . . . . . . . . . . . . . . 15
4.3. General requirements . . . . . . . . . . . . . . . . . . 16
5. Performance optimizations . . . . . . . . . . . . . . . . . . 16
5.1. Optimistic pre-connection and connection reuse . . . . . 16
5.2. Generating and using incomplete responses . . . . . . . . 17
5.3. Structuring zones for performance . . . . . . . . . . . . 17
6. Initial SvcParamKeys . . . . . . . . . . . . . . . . . . . . 18
6.1. "alpn" and "no-default-alpn" . . . . . . . . . . . . . . 18
6.2. "port" . . . . . . . . . . . . . . . . . . . . . . . . . 19
6.3. "echconfig" . . . . . . . . . . . . . . . . . . . . . . . 20
6.4. "ipv4hint" and "ipv6hint" . . . . . . . . . . . . . . . . 20
7. Using SVCB with HTTPS and HTTP . . . . . . . . . . . . . . . 21
7.1. Owner names for HTTPS RRs . . . . . . . . . . . . . . . . 21
7.2. Relationship to Alt-Svc . . . . . . . . . . . . . . . . . 22
7.2.1. ALPN usage . . . . . . . . . . . . . . . . . . . . . 22
7.2.2. Untrusted channel . . . . . . . . . . . . . . . . . . 22
7.2.3. TTL and granularity . . . . . . . . . . . . . . . . . 23
7.3. Interaction with Alt-Svc . . . . . . . . . . . . . . . . 23
7.4. Requiring Server Name Indication . . . . . . . . . . . . 23
7.5. HTTP Strict Transport Security . . . . . . . . . . . . . 24
7.6. HTTP-based protocols . . . . . . . . . . . . . . . . . . 24
8. SVCB/HTTPS RR parameter for ECH configuration . . . . . . . . 25
8.1. Client behavior . . . . . . . . . . . . . . . . . . . . . 25
8.2. Deployment considerations . . . . . . . . . . . . . . . . 25
9. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 26
9.1. Protocol enhancements . . . . . . . . . . . . . . . . . . 26
9.2. Apex aliasing . . . . . . . . . . . . . . . . . . . . . . 26
9.3. Parameter binding . . . . . . . . . . . . . . . . . . . . 27
9.4. Non-HTTPS uses . . . . . . . . . . . . . . . . . . . . . 27
10. Interaction with other standards . . . . . . . . . . . . . . 27
11. Security Considerations . . . . . . . . . . . . . . . . . . . 28
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
12.1. New registry for Service Parameters . . . . . . . . . . 28
12.1.1. Procedure . . . . . . . . . . . . . . . . . . . . . 28
12.1.2. Initial contents . . . . . . . . . . . . . . . . . . 29
12.2. Registry updates . . . . . . . . . . . . . . . . . . . . 30
13. Acknowledgments and Related Proposals . . . . . . . . . . . . 31
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 31
14.1. Normative References . . . . . . . . . . . . . . . . . . 31
14.2. Informative References . . . . . . . . . . . . . . . . . 34
14.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Appendix A. Comparison with alternatives . . . . . . . . . . . . 35
A.1. Differences from the SRV RR type . . . . . . . . . . . . 35
A.2. Differences from the proposed HTTP record . . . . . . . . 35
A.3. Differences from the proposed ANAME record . . . . . . . 35
A.4. Comparison with separate RR types for AliasForm and
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ServiceForm . . . . . . . . . . . . . . . . . . . . . . . 36
Appendix B. Change history . . . . . . . . . . . . . . . . . . . 36
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 38
1. Introduction
The SVCB and HTTPS RRs provide clients with complete instructions for
access to an origin. 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 an origin
wishes to delegate operational control to one or more other domains,
e.g. delegating the origin resource "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 7) by avoiding the need for an [Attrleaf]
label (Section 7.1). 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 7 describes
additional behaviors specific to the HTTPS RR. Apart from Section 7
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 forms: 1) the "Alias Form" simply delegates
operational control for a resource; 2) the "Service Form" binds
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together configuration information for a service endpoint. The
Service Form provides additional key=value parameters within each
RDATA set.
TO BE REMOVED: If we use this for providing configuration for DNS
authorities, it is likely we'd specify a distinct "NS2" RR type that
is an instantiation of SVCB for authoritative nameserver delegation
and parameter specification, similar to HTTPS. See
[I-D.draft-tapril-ns2-00] as one example.
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 service endpoints authoritative for the service, along
with parameters associated with each of these endpoints.
o Does not assume that all alternative service 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 service
endpoints
o Obtain the [ECH] keys associated with an alternative service
endpoint
o Support non-default TCP and UDP ports
o Address a set of long-standing issues due to HTTP(S) clients not
implementing support for SRV records, as well as due to a
limitation that a DNS name can not have both CNAME and NS RRs (as
is the case for zone apex names)
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o Provide an HSTS-like indication signaling for the duration of the
DNS RR TTL that the HTTPS scheme should be used instead of HTTP
(see Section 7.5).
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.)
The SVCB RR has two forms: AliasForm, which aliases a name to another
name, and ServiceForm, 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. SvcFieldPriority: The priority of this record (relative to
others, with lower values preferred). A value of 0 indicates
AliasForm. (Described in Section 2.6.2.)
2. SvcDomainName: The domain name of either the alias target (for
AliasForm) or the alternative service endpoint (for ServiceForm).
3. SvcFieldValue (optional): A list of key=value pairs describing
the alternative service endpoint for the domain name specified in
SvcDomainName (only used in ServiceForm and otherwise ignored).
Described in Section 2.1.1.
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 must 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 may attach in-bailiwick SVCB,
A, AAAA, and CNAME records in the Additional Section to responses for
a SVCB query.
When in the ServiceForm, the SvcFieldValue of the SVCB RR provides an
extensible data model for describing network endpoints that are
authoritative for the origin, along with parameters associated with
each of these endpoints.
For the HTTPS use-case, the HTTPS RR enables many of the benefits of
[AltSvc] without waiting for a full HTTP connection initiation
(multiple roundtrips) before learning of the preferred alternative,
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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 8.
1.4. Terminology
For consistency with [AltSvc], we adopt the following definitions:
o An "origin" is an information source as in [RFC6454]. For
services other than HTTPS, the exact definition will need to be
provided by the document mapping that service onto the SVCB RR.
o The "origin server" is the server that the client would reach when
accessing the origin in the absence of the SVCB record or an HTTPS
Alt-Svc.
o An "alternative service" is a different server that can serve the
origin over a specified protocol.
For example within HTTPS, the origin consists of a scheme (typically
"https"), a hostname, and a port (typically "443").
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 ???) is used to
locate endpoints that can service an origin. There is special
handling for the case of "https" origins.
The algorithm for resolving SVCB records and associated address
records is specified in Section 3.
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2.1. Presentation format
The presentation format of the record is:
Name TTL IN SVCB SvcFieldPriority SvcDomainName SvcFieldValue
The SVCB record is defined specifically within the Internet ("IN")
Class ([RFC1035]). SvcFieldPriority is a number in the range
0-65535, SvcDomainName is a domain name (absolute or relative), and
SvcFieldValue is a set of key=value pairs present for the
ServiceForm. Each key SHALL appear at most once in an SvcFieldValue.
The SvcFieldValue is empty for the AliasForm.
2.1.1. Presentation format for SvcFieldValue key=value pairs
In ServiceForm, the SvcFieldValue consists of zero or more elements
separated by whitespace. Each element represents a key=value pair.
Keys are IANA-registered SvcParamKeys (Section 12.1) with both a
case-insensitive string representation and a numeric representation
in the range 0-65535. Registered key names should only contain
characters from the ranges "a"-"z", "0"-"9", and "-". In ABNF
[RFC5234],
ALPHA-LC = %x61-7A ; a-z
key = 1*(ALPHA-LC / DIGIT / "-")
display-key = 1*(ALPHA / DIGIT / "-")
Values are in a format specific to the SvcParamKey. Their definition
should specify both their presentation format and wire encoding
(e.g., domain names, binary data, or numeric values). The initial
keys and formats are defined in Section 6.
The presentation format for SvcFieldValue is a whitespace-separated
list of key=value pairs. When the value is omitted, or both the
value and the "=" are omitted, the presentation value is the empty
string.
; basic-visible is VCHAR minus DQUOTE, ";", "(", ")", and "\".
basic-visible = %x21 / %x23-27 / %2A-3A / %x3C-5B / %x5D-7E
escaped-char = "\" (VCHAR / WSP)
contiguous = 1*(basic-visible / escaped-char)
quoted-string = DQUOTE *(contiguous / WSP) DQUOTE
value = quoted-string / contiguous
pair = display-key "=" value
element = display-key / pair
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The value format is intended to match the definition of <character-
string> in [RFC1035] Section 5.1. (Unlike <character-string>, the
length of a value is not limited to 255 characters.)
Unrecognized keys are represented in presentation format as
"keyNNNNN" where NNNNN is the numeric value of the key type without
leading zeros. In presentation format, values corresponding to
unrecognized keys SHALL be represented in wire format, using decimal
escape codes (e.g. \255) when necessary.
When decoding values of unrecognized keys in the presentation format:
o a character other than "\" represents its ASCII value in wire
format.
o the character "\" followed by three decimal digits, up to 255,
represents an octet in the wire format.
o the character "\" followed by any allowed character, except a
decimal digit, represents the subsequent character's ASCII value.
Elements in presentation format MAY appear in any order, but keys
MUST NOT be repeated.
2.2. SVCB RDATA Wire Format
The RDATA for the SVCB RR consists of:
o a 2 octet field for SvcFieldPriority as an integer in network byte
order.
o the uncompressed, fully-qualified SvcDomainName, represented as a
sequence of length-prefixed labels as in Section 3.1 of [RFC1035].
o the SvcFieldValue byte string, consuming the remainder of the
record (so smaller than 65535 octets and constrained by the RDATA
and DNS message sizes).
AliasForm is defined by SvcFieldPriority being 0.
When SvcFieldValue is non-empty (ServiceForm), 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 12.1.2 for the defined values.)
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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 parser reaches the end of the RDATA while parsing an
SvcFieldValue.
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.
TODO: decide if we want special handling for any SvcParamKey ranges?
For example: range for greasing; experimental range; range-of-
mandatory-to-use-the-RR vs range of ignore-just-param-if-unknown.
2.3. SVCB owner names
When querying the SVCB RR, an origin is translated into a QNAME by
prepending the hostname with a label indicating the scheme, prefixed
with an underscore, resulting in a domain name like
"_examplescheme.api.example.com.".
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 (or the default if no port number is
specified). We term this behavior "Port Prefix Naming", and use it
in the examples throughout this document.
See Section 7.1 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.
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Note that none of these forms alter the origin or authority for
validation purposes. For example, clients MUST continue to validate
TLS certificate hostnames based on the origin host.
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. SvcRecordType
The SvcRecordType is indicated by the SvcFieldPriority, and defines
the form of the SVCB RR. When SvcFieldPriority is 0, the SVCB
SvcRecordType is defined to be in AliasForm. Otherwise, the SVCB
SvcRecordType is defined to be in ServiceForm.
Within a SVCB RRSet, all RRs should have the same SvcRecordType. If
an RRSet contains a record in AliasForm, the client MUST ignore any
records in the set with ServiceForm.
2.5. SVCB records: AliasForm
When SvcRecordType is AliasForm, the SVCB record is to be treated
similar to a CNAME alias pointing to SvcDomainName. SVCB RRSets
SHOULD only have a single resource record in this form. If multiple
are present, clients or recursive resolvers SHOULD pick one at
random.
The AliasForm's primary purpose is to allow aliasing at the zone
apex, where CNAME is not allowed. For example, if an operator of
https://example.com wanted to point HTTPS requests to a service
operating at svc.example.net, they would publish a record such as:
example.com. 3600 IN SVCB 0 svc.example.net.
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In AliasForm, SvcDomainName MUST be the name of a domain that has
SVCB, AAAA, or A records. It MUST NOT be equal to the owner name, as
this would cause a loop.
Note that the SVCB record's owner name MAY be the canonical name of a
CNAME record, and the SvcDomainName 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 AliasForm 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 AliasForm 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.
Note that SVCB AliasForm RRs do not alias to RR types other than
address records (AAAA and A), CNAMEs, and ServiceForm SVCB records.
For example, an AliasForm SVCB record does not alias to an HTTPS RR,
nor vice-versa.
2.6. SVCB records: ServiceForm
When SvcRecordType is the ServiceForm, the combination of
SvcDomainName and SvcFieldValue parameters within each resource
record associates an alternative service location with its connection
parameters.
Each protocol scheme that uses SVCB MUST define a protocol mapping
that explains how SvcFieldValues are applied for connections of that
scheme. Unless specified otherwise by the protocol mapping, clients
MUST ignore SvcFieldValue parameters that they do not recognize.
2.6.1. Special handling of "." for SvcDomainName in ServiceForm
For ServiceForm SVCB RRs, if SvcDomainName has the value "."
(represented in the wire format as a zero-length label), then the
owner name of this record MUST be used as the effective
SvcDomainName.
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For example, in the following example "svc2.example.net" is the
effective SvcDomainName:
www.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
2.6.2. SvcFieldPriority
As RRs within an RRSet are explicitly unordered collections, the
SvcFieldPriority value serves to indicate priority. SVCB RRs with a
smaller SvcFieldPriority value SHOULD be given preference over RRs
with a larger SvcFieldPriority value.
When receiving an RRSet containing multiple SVCB records with the
same SvcFieldPriority value, clients SHOULD apply a random shuffle
within a priority level to the records before using them, to ensure
uniform load-balancing.
3. Client behavior
An SVCB-aware client resolves an origin HOST by attempting to
determine the preferred SvcFieldValue and IP addresses for its
service, using the following procedure:
1. Issue parallel AAAA/A and SVCB queries for the name HOST. The
answers for these may or may not include CNAME pointers before
reaching one or more of these records.
2. If a SVCB record of AliasForm SvcRecordType is returned for HOST,
clients MUST loop back to step 1 replacing HOST with
SvcDomainName, subject to chain length limits and loop detection
heuristics (see Section 3.1).
3. If one or more SVCB records of ServiceForm SvcRecordType are
returned for HOST, clients should select the highest-priority
option with acceptable parameters, and resolve AAAA and/or A
records for its SvcDomainName if they are not already available.
These are the preferred SvcFieldValue and IP addresses. If the
connection fails, the client MAY try to connect using values from
a lower-priority record. If none of the options succeed, the
client SHOULD connect to the origin server directly.
4. If a SVCB record for HOST does not exist, the received AAAA and/
or A records are the preferred IP addresses and there is no
SvcFieldValue.
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This procedure does not rely on any recursive or authoritative server
to comply with this specification or have any awareness of SVCB.
When selecting between AAAA and A records to use, clients may use an
approach such as [HappyEyeballsV2].
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 a SVCB query results in a SERVFAIL error, transport error, or
timeout, and DNS exchanges between the client and the recursive
resolver are cryptographically protected (e.g. using TLS [RFC7858] or
HTTPS [RFC8484]), the client MUST NOT fall back to non-SVCB
connection establishment. This ensures that an active attacker
cannot mount a downgrade attack by denying the user access to the
SVCB information.
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 MUST NOT fall back to non-SVCB connection establishment
if the SVCB response fails to validate.
If the client is unable to complete SVCB resolution due to its chain
length limit, the client SHOULD fall back to non-SVCB connection, 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]) SHOULD disable SVCB
support if performing SVCB queries would violate the client's privacy
intent.
If the client can safely perform SVCB queries (e.g. via the proxy or
an affiliated resolver), the client SHOULD follow the standard SVCB
resolution process, selecting the highest priority option that is
compatible with the client and the proxy. The client SHOULD provide
the final SvcDomainName and port to the proxy, which will perform any
required A and AAAA lookups.
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Providing the proxy with the final SvcDomainName has several
benefits:
o It allows the client to use the SvcFieldValue, if present, which
is only usable with a specific SvcDomainName. The SvcFieldValue
may include information that enhances performance (e.g. alpn) and
privacy (e.g. echconfig).
o It allows the origin 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 (as well as any relevant CNAME or
[DNAME] records) in the Additional Section for any in-bailiwick
SvcDomainNames.
4.2. Recursive resolvers
Recursive resolvers that are aware of SVCB SHOULD ensure that the
client can execute the procedure in Section 3 without issuing a
second round of queries, by incorporating all the necessary
information into a single 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 Answer and Additional RRs to the Additional section,
and all Authority RRs to the Authority 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 AliasForm, choose an
AliasForm record at random, and resolve SVCB, A, and AAAA records
for its SvcDomainName.
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* 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 ServiceForm. Resolve A and
AAAA queries for each SvcDomainName (or for the owner name if
SvcDomainName 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]).
4.3. General requirements
All DNS servers SHOULD treat the SvcFieldValue portion of the SVCB RR
as opaque and SHOULD NOT try to alter their behavior based on its
contents.
When responding to a query that includes the DNSSEC OK bit
([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 issue address (AAAA and/or A) and SVCB queries simultaneously,
and 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 these optimizations in
place, and conforming DNS servers, using SVCB does not add network
latency to connection setup.
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].
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An 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 ... )
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 must proceed as described in Step 3 of
the procedure 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
from higher priority ServiceForm records over lower priority
ServiceForm 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.
5.3. Structuring zones for performance
To avoid a delay for clients using a nonconforming recursive
resolver, domain owners SHOULD use a single SVCB record whose
SvcDomainName is "." if possible. This will ensure that the required
address records are already present in the client's DNS cache as part
of the responses to the address queries that were issued in parallel.
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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.
6.1. "alpn" and "no-default-alpn"
The "alpn" and "no-default-alpn" SvcParamKeys together indicate the
set of Application Layer Protocol Negotation (ALPN) protocol
identifiers [ALPN] and associated transport protocols supported by
this service endpoint.
As with [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).
ALPNs are identified by their registered "Identification Sequence"
(alpn-id), which is a sequence of 1-255 octets.
alpn-id = 1*255(OCTET)
The presentation value of "alpn" is a comma-separated list of one or
more "alpn-id"s. Any commas present in the protocol-id are escaped
by a backslash:
escaped-octet = %x00-2b / "\," / %x2d-5b / "\\" / %x5D-FF
escaped-id = 1*(escaped-octet)
alpn-value = escaped-id *("," escaped-id)
The wire format value for "alpn" consists of at least one ALPN
identifier ("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.
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 "ALPN set"), the client
parses the set of ALPN identifiers in the "alpn" parameter, and then
adds the default set unless the "no-default-alpn" SvcParamKey is
present. The presence of a value in the alpn set indicates that this
service endpoint, described by SvcDomainName and the other parameters
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(e.g. "port") offers service with the protocol suite associated with
the ALPN ID.
ALPN IDs that do not uniquely identify a protocol suite (e.g. an ID
that can be used with both TLS and DTLS) are not compatible with this
SvcParamKey and MUST NOT be included in the ALPN set.
Clients SHOULD NOT attempt connection to a service endpoint whose
ALPN set does not contain any compatible protocol suites. 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.
Clients MUST include an "application_layer_protocol_negotiation"
extension in their ClientHello with a ProtocolNameList that includes
at least one ID from the ALPN set. Clients SHOULD also include any
other values that they support and could negotiate on that connection
with equivalent or better security properties. For example, if the
ALPN set only contains "http/1.1", the client could include
"http/1.1" and "h2" in the ProtocolNameList.
Once the client has formulated the ClientHello, protocol negotiation
on that connection proceeds as specified in [ALPN], without regard to
the SVCB ALPN set. To preserve the security guarantees of this
process, clients MUST consolidate all compatible ALPN IDs into a
single ProtocolNameList.
6.2. "port"
The "port" SvcParamKey defines the TCP or UDP port that should be
used to contact this alternative service. If this key is not
present, clients SHALL use the origin server's port number.
The presentation format of the SvcParamValue is a numeric value
between 0 and 65535 inclusive. Any other values (e.g. the empty
value) are syntax errors.
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
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client to lose access to the service, so operators should exercise
caution when using this SvcParamKey to specify a non-default port.
6.3. "echconfig"
The SvcParamKey to enable Encrypted ClientHello (ECH) is "echconfig".
Its value is defined in Section 8. 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 SvcDomainName
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
SvcDomainName are locally available, the client SHOULD ignore these
hints. Otherwise, clients SHOULD perform A and/or AAAA queries for
SvcDomainName 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 SvcDomainName 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 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 [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 best
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performance, server operators SHOULD include an "ipv6hint" parameter
whenever they include an "ipv4hint" parameter.
The presentation format for each parameter is a comma-separated list
of IP addresses in standard textual format [RFC5952].
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 SvcDomainName is ".",
server operators SHOULD NOT include these hints, because they are
unlikely to convey any performance benefit.
7. 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.
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 presence of an HTTPS RR for an origin also indicates that all
HTTP resources are available over HTTPS, as discussed in Section 7.5.
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.
7.1. Owner 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 origin hostname, without any
prefix labels.
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By removing the [Attrleaf] labels used in SVCB, this construction
enables offline DNSSEC signing of wildcard domains, which are
commonly used with HTTPS. Reusing the origin hostname 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 RR AliasForm 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 7.5, 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 host.
7.2. Relationship to Alt-Svc
Publishing a ServiceForm 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.
7.2.1. 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 (subject to security constraints).
TO BE REMOVED: The ALPN semantics in [AltSvc] are ambiguous, and
problematic in some interpretations. We should update [AltSvc] to
give it well-defined semantics that match HTTPS RRs.
7.2.2. 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,
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because this parameter is not safe to accept over an untrusted
channel.
7.2.3. TTL and granularity
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 will typically be similar to the "ma" value
used for Alt-Svc, but may vary depending on the desired efficiency
and agility. Some DNS caches incorrectly extend the lifetime of DNS
records beyond the stated TTL, so server operators cannot rely on
HTTPS RRs expiring on time. Shortening the TTL to compensate for
incorrect caching is NOT RECOMMENDED, as this practice impairs the
performance of correctly functioning caches and does not guarantee
faster expiration from incorrect caches. Instead, server operators
SHOULD maintain compatibility with expired records until they observe
that nearly all connections have migrated to the new configuration.
Sending Alt-Svc over HTTP allows the server to tailor the Alt-Svc
Field Value specifically to the client. When using an HTTPS RR,
groups of clients will necessarily receive the same SvcFieldValue.
Therefore, HTTPS RRs are not suitable for uses that require single-
client granularity.
7.3. 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).
For clients that implement support for ECH, the interaction between
HTTPS RRs and Alt-Svc is described in Section 8.1.
This specification does not alter the DNS queries performed when
connecting to an Alt-Svc hostname (typically A and/or AAAA only).
7.4. 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 SvcDomainName.
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7.5. HTTP Strict Transport Security
By publishing an 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]. When an HTTPS RR
is present for an origin, all "http" scheme requests for that origin
SHOULD logically be redirected to "https".
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 HTTPS RRs
(AliasForm or ServiceForm), the client SHOULD act as if it has
received an HTTP "307 Temporary Redirect" redirect to this "https"
URL. 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 making an "https" scheme request to an origin with an HTTPS RR,
either directly or via the above redirect, the client SHOULD
terminate the connection if there are any errors with the underlying
secure transport, such as errors in certificate validation. This
aligns with Section 8.4 and Section 12.1 of [HSTS].
7.6. HTTP-based protocols
We define an "HTTP-based protocol" as one that involves connecting to
an "http:" or "https:" URL. When implementing an HTTP-based
protocol, clients that use HTTPS RRs for HTTP SHOULD also use it for
this URL. For example, clients that support HTTPS RRs and implement
the altered [WebSocket] opening handshake from [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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8. SVCB/HTTPS RR parameter for ECH configuration
The SVCB "echconfig" parameter is defined for conveying the ECH
configuration of an alternative service. In wire format, the value
of the parameter is an ECHConfigs vector [ECH], including the
redundant length prefix. In presentation format, the value is
encoded in [base64].
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 origin.
8.1. Client behavior
The general client behavior specified in Section 3 permits clients to
retry connection with a less preferred alternative if the preferred
option fails, including falling 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 clients SHALL implement the following behavior for connection
establishment.
1. Perform connection establishment using HTTPS RRs as described in
Section 3, but do not fall back to the origin's A/AAAA records.
If all the HTTPS RRs have an "echconfig" key, and they all fail,
terminate connection establishment.
2. If the client implements Alt-Svc, try to connect using any
entries from the Alt-Svc cache.
3. Fall back to the origin's A/AAAA records if necessary.
As a latency optimization, clients MAY prefetch DNS records for later
steps before they are needed.
8.2. 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
SvcFieldPriority) than those without, in order to maximize the
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likelihood that ECH will be used in the absence of an active
adversary.
9. Examples
9.1. Protocol enhancements
Consider a simple zone of the form
simple.example. 300 IN A 192.0.2.1
AAAA 2001:db8::1
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).
9.2. 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.
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9.3. 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
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.
9.4. Non-HTTPS uses
For services other than HTTPS, the SVCB RR and an [Attrleaf] label
will be used. For example, to reach an example resource of
"baz://api.example.com:8765", the following Alias Form SVCB record
would be used to delegate to "svc4-baz.example.net." which in-turn
could return AAAA/A records and/or SVCB records in ServiceForm.
_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.
10. 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
[RFC7858] or DNS over HTTPS [RFC8484]). However, performance
improvements, and some modest privacy improvements, are possible
without the use of those standards.
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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.
11. Security Considerations
SVCB/HTTPS RRs are intended for distribution over untrusted channels,
and clients are REQUIRED to verify that the alternative service is
authoritative for the origin (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.
12. IANA Considerations
12.1. 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.
12.1.1. Procedure
A registration MUST include the following fields:
o Name: Service parameter key name
o SvcParamKey: Service parameter key numeric identifier (range
0-65535)
o Meaning: a short description
o Pointer to specification text
SvcParamKey values to be added to this namespace have different
policies ([RFC5226], Section 4.1) based on their range:
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+-------------+-------------------------+
| SvcParamKey | IANA Policy |
+-------------+-------------------------+
| 0-255 | Standards Action |
| | |
| 256-32767 | Expert Review |
| | |
| 32768-65280 | First Come First Served |
| | |
| 65280-65534 | Private Use |
| | |
| 65535 | Standards Action |
+-------------+-------------------------+
Apart from the initial contents, the SvcParamKey name MUST NOT start
with "key".
12.1.2. Initial contents
The "Service Binding (SVCB) Parameter Registry" shall initially be
populated with the registrations below:
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+-------------+-----------------+----------------------+------------+
| SvcParamKey | NAME | Meaning | Reference |
+-------------+-----------------+----------------------+------------+
| 0 | (no name) | Reserved for | (This |
| | | internal use | document) |
| | | | |
| 1 | alpn | Additional supported | (This |
| | | protocols | document) |
| | | | |
| 2 | no-default-alpn | No support for | (This |
| | | default protocol | document) |
| | | | |
| 3 | port | Port for alternative | (This |
| | | service | document) |
| | | | |
| 4 | ipv4hint | IPv4 address hints | (This |
| | | | document) |
| | | | |
| 5 | echconfig | Encrypted | (This |
| | | ClientHello info | document) |
| | | | |
| 6 | ipv6hint | IPv6 address hints | (This |
| | | | document) |
| | | | |
| 65280-65534 | keyNNNNN | Private Use | (This |
| | | | document) |
| | | | |
| 65535 | key65535 | Reserved | (This |
| | | | document) |
+-------------+-----------------+----------------------+------------+
TODO: do we also want to reserve a range for greasing?
12.2. Registry updates
Per [RFC6895], please add the following entries to the data type
range of the Resource Record (RR) TYPEs registry:
+-------+------------------------------------------+----------------+
| TYPE | Meaning | Reference |
+-------+------------------------------------------+----------------+
| SVCB | Service Location and Parameter Binding | (This |
| | | document) |
| | | |
| HTTPS | HTTPS Service Location and Parameter | (This |
| | Binding | document) |
+-------+------------------------------------------+----------------+
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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) |
+---------+------------+-----------------+-----------------+
13. 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.draft-bellis-dnsop-http-record-00],
[I-D.draft-ietf-dnsop-aname-03], 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, and others for their feedback and suggestions on this
draft.
14. References
14.1. 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., "DNS Scoped Data Through "Underscore" Naming
of Attribute Leaves", draft-ietf-dnsop-attrleaf-16 (work
in progress), November 2018.
[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>.
[ECH] Rescorla, E., Oku, K., Sullivan, N., and C. Wood, "TLS
Encrypted Client Hello", draft-ietf-tls-esni-07 (work in
progress), June 2020.
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[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-20 (work in progress),
April 2019.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[RFC1928] Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D., and
L. Jones, "SOCKS Protocol Version 5", RFC 1928,
DOI 10.17487/RFC1928, March 1996,
<https://www.rfc-editor.org/info/rfc1928>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[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>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<https://www.rfc-editor.org/info/rfc5226>.
[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>.
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[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>.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
DOI 10.17487/RFC6454, December 2011,
<https://www.rfc-editor.org/info/rfc6454>.
[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>.
[RFC7858] 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>.
[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>.
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[RFC8484] 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>.
[WebSocket]
Fette, I. and A. Melnikov, "The WebSocket Protocol",
RFC 6455, DOI 10.17487/RFC6455, December 2011,
<https://www.rfc-editor.org/info/rfc6455>.
14.2. 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.draft-bellis-dnsop-http-record-00]
Bellis, R., "A DNS Resource Record for HTTP", draft-
bellis-dnsop-http-record-00 (work in progress), November
2018.
[I-D.draft-ietf-dnsop-aname-03]
Finch, T., Hunt, E., Dijk, P., Eden, A., and W. Mekking,
"Address-specific DNS aliases (ANAME)", draft-ietf-dnsop-
aname-03 (work in progress), April 2019.
[I-D.draft-tapril-ns2-00]
April, T., "Parameterized Nameserver Delegation with NS2
and NS2T", draft-tapril-ns2-00 (work in progress), March
2020.
[RFC2782] 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>.
[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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[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>.
14.3. URIs
[1] https://github.com/MikeBishop/dns-alt-svc
Appendix A. 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.
A.1. Differences from the SRV RR type
An SRV record [RFC2782] 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.
A.2. Differences from the proposed HTTP record
Unlike [I-D.draft-bellis-dnsop-http-record-00], 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.
A.3. Differences from the proposed ANAME record
Unlike [I-D.draft-ietf-dnsop-aname-03], 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
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master 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.
A.4. Comparison with separate RR types for AliasForm and ServiceForm
Abstractly, functions of AliasForm and ServiceForm 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 ServiceForm query, emit both a
ServiceForm and an AliasForm 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 B. Change history
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
* Revised chain length limit requirements
* Revised IANA registry rules for SvcParamKeys
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* 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.
o draft-ietf-dnsop-svcb-httpssvc-00
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* 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
Ben Schwartz
Google
Email: bemasc@google.com
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Mike Bishop
Akamai Technologies
Email: mbishop@evequefou.be
Erik Nygren
Akamai Technologies
Email: erik+ietf@nygren.org
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