draft-ietf-dnsop-svcb-httpssvc-01.txt   draft-ietf-dnsop-svcb-httpssvc-02.txt 
DNSOP Working Group B. Schwartz DNSOP Working Group B. Schwartz
Internet-Draft Google Internet-Draft Google
Intended status: Standards Track M. Bishop Intended status: Standards Track M. Bishop
Expires: May 7, 2020 E. Nygren Expires: September 10, 2020 E. Nygren
Akamai Technologies Akamai Technologies
November 4, 2019 March 9, 2020
Service binding and parameter specification via the DNS (DNS SVCB and Service binding and parameter specification via the DNS (DNS SVCB and
HTTPSSVC) HTTPSSVC)
draft-ietf-dnsop-svcb-httpssvc-01 draft-ietf-dnsop-svcb-httpssvc-02
Abstract Abstract
This document specifies the "SVCB" and "HTTPSSVC" DNS resource record This document specifies the "SVCB" and "HTTPSSVC" DNS resource record
types to facilitate the lookup of information needed to make types to facilitate the lookup of information needed to make
connections for origin resources, such as for HTTPS URLs. SVCB connections for origin resources, such as for HTTPS URLs. SVCB
records allow an origin to be served from multiple network locations, records allow an origin to be served from multiple network locations,
each with associated parameters (such as transport protocol each with associated parameters (such as transport protocol
configuration and keying material for encrypting TLS SNI). They also configuration and keying material for encrypting TLS SNI). They also
enable aliasing of apex domains, which is not possible with CNAME. enable aliasing of apex domains, which is not possible with CNAME.
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record lookups in-addition to AAAA/A lookups. This design attempts record lookups in-addition to AAAA/A lookups. This design attempts
to provide a unified framework that encompasses the key functionality to provide a unified framework that encompasses the key functionality
of these proposals, as well as providing some extensibility for of these proposals, as well as providing some extensibility for
addressing similar future challenges. addressing similar future challenges.
TO BE REMOVED: The specific name for this RR type is an open topic TO BE REMOVED: The specific name for this RR type is an open topic
for discussion. "SVCB" and "HTTPSSVC" are meant as placeholders as for discussion. "SVCB" and "HTTPSSVC" are meant as placeholders as
they are easy to replace. Other names might include "B", "SRV2", they are easy to replace. Other names might include "B", "SRV2",
"SVCHTTPS", "HTTPS", and "ALTSVC". "SVCHTTPS", "HTTPS", and "ALTSVC".
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 Status of This Memo
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provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on May 7, 2020. This Internet-Draft will expire on September 10, 2020.
Copyright Notice Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Introductory Example . . . . . . . . . . . . . . . . . . 5 1.1. Example: Protocol enhancements . . . . . . . . . . . . . 5
1.2. Goals of the SVCB RR . . . . . . . . . . . . . . . . . . 6 1.2. Example: Apex aliasing . . . . . . . . . . . . . . . . . 5
1.3. Overview of the SVCB RR . . . . . . . . . . . . . . . . . 7 1.3. Example: Parameter binding . . . . . . . . . . . . . . . 6
1.4. Parameter for ESNI . . . . . . . . . . . . . . . . . . . 8 1.4. Example: Non-HTTPS uses . . . . . . . . . . . . . . . . . 6
1.5. Terminology . . . . . . . . . . . . . . . . . . . . . . . 8 1.5. Goals of the SVCB RR . . . . . . . . . . . . . . . . . . 7
2. The SVCB record type . . . . . . . . . . . . . . . . . . . . 8 1.6. Overview of the SVCB RR . . . . . . . . . . . . . . . . . 8
2.1. Parameter specification via ServiceFieldValue . . . . . . 9 1.7. Parameter for ESNI . . . . . . . . . . . . . . . . . . . 9
2.1.1. Presentation format . . . . . . . . . . . . . . . . . 9 1.8. Terminology . . . . . . . . . . . . . . . . . . . . . . . 9
2.2. SVCB RDATA Wire Format . . . . . . . . . . . . . . . . . 10 2. The SVCB record type . . . . . . . . . . . . . . . . . . . . 9
2.3. SVCB owner names . . . . . . . . . . . . . . . . . . . . 10 2.1. Presentation format . . . . . . . . . . . . . . . . . . . 10
2.4. SvcRecordType . . . . . . . . . . . . . . . . . . . . . . 11 2.1.1. Presentation format for SvcFieldValue key=value pairs 10
2.5. SVCB records: AliasForm . . . . . . . . . . . . . . . . . 11 2.2. SVCB RDATA Wire Format . . . . . . . . . . . . . . . . . 11
2.6. SVCB records: ServiceForm . . . . . . . . . . . . . . . . 12 2.3. SVCB owner names . . . . . . . . . . . . . . . . . . . . 12
2.4. SvcRecordType . . . . . . . . . . . . . . . . . . . . . . 13
2.5. SVCB records: AliasForm . . . . . . . . . . . . . . . . . 13
2.6. SVCB records: ServiceForm . . . . . . . . . . . . . . . . 14
2.6.1. Special handling of "." for SvcDomainName in 2.6.1. Special handling of "." for SvcDomainName in
ServiceForm . . . . . . . . . . . . . . . . . . . . . 12 ServiceForm . . . . . . . . . . . . . . . . . . . . . 14
2.6.2. SvcFieldPriority . . . . . . . . . . . . . . . . . . 13 2.6.2. SvcFieldPriority . . . . . . . . . . . . . . . . . . 14
3. Client behavior . . . . . . . . . . . . . . . . . . . . . . . 13 3. Client behavior . . . . . . . . . . . . . . . . . . . . . . . 14
3.1. Clients using a Proxy . . . . . . . . . . . . . . . . . . 14 3.1. Handling resolution failures . . . . . . . . . . . . . . 15
3.2. Clients using a Proxy . . . . . . . . . . . . . . . . . . 16
4. DNS Server Behavior . . . . . . . . . . . . . . . . . . . . . 14 4. DNS Server Behavior . . . . . . . . . . . . . . . . . . . . . 16
5. Performance optimizations . . . . . . . . . . . . . . . . . . 15 4.1. Authoritative servers . . . . . . . . . . . . . . . . . . 16
5.1. Optimistic pre-connection and connection reuse . . . . . 15 4.2. Recursive resolvers . . . . . . . . . . . . . . . . . . . 16
5.2. Preferring usable records . . . . . . . . . . . . . . . . 16 4.3. General requirements . . . . . . . . . . . . . . . . . . 17
5.3. Structuring zones for performance . . . . . . . . . . . . 16 5. Performance optimizations . . . . . . . . . . . . . . . . . . 18
6. Initial SvcParamKeys . . . . . . . . . . . . . . . . . . . . 16 5.1. Optimistic pre-connection and connection reuse . . . . . 18
6.1. "alpn" . . . . . . . . . . . . . . . . . . . . . . . . . 16 5.2. Generating and using incomplete responses . . . . . . . . 18
6.2. "port" . . . . . . . . . . . . . . . . . . . . . . . . . 17 5.3. Structuring zones for performance . . . . . . . . . . . . 19
6.3. "esniconfig" . . . . . . . . . . . . . . . . . . . . . . 17 6. Initial SvcParamKeys . . . . . . . . . . . . . . . . . . . . 19
6.4. "ipv4hint" and "ipv6hint" . . . . . . . . . . . . . . . . 17 6.1. "alpn" and "no-default-alpn" . . . . . . . . . . . . . . 19
7. Using SVCB with HTTPS and HTTP . . . . . . . . . . . . . . . 18 6.2. "port" . . . . . . . . . . . . . . . . . . . . . . . . . 21
7.1. Owner names for HTTPSSVC records . . . . . . . . . . . . 19 6.3. "esniconfig" . . . . . . . . . . . . . . . . . . . . . . 21
7.2. Populating Alt-Used . . . . . . . . . . . . . . . . . . . 19 6.4. "ipv4hint" and "ipv6hint" . . . . . . . . . . . . . . . . 21
7.3. Differences from Alt-Svc . . . . . . . . . . . . . . . . 19 7. Using SVCB with HTTPS and HTTP . . . . . . . . . . . . . . . 22
7.3.1. Untrusted channel . . . . . . . . . . . . . . . . . . 19 7.1. Owner names for HTTPSSVC records . . . . . . . . . . . . 23
7.3.2. Caching and granularity . . . . . . . . . . . . . . . 20 7.2. Relationship to Alt-Svc . . . . . . . . . . . . . . . . . 23
7.4. HTTP Strict Transport Security . . . . . . . . . . . . . 20 7.2.1. ALPN usage . . . . . . . . . . . . . . . . . . . . . 23
8. Alt-Svc and SVCB/HTTPSSVC parameter for ESNI keys . . . . . . 21 7.2.2. Untrusted channel . . . . . . . . . . . . . . . . . . 24
8.1. Handling a mixture of alternatives not supporting ESNI . 21 7.2.3. TTL and granularity . . . . . . . . . . . . . . . . . 24
9. Interaction with other standards . . . . . . . . . . . . . . 22 7.3. Interaction with Alt-Svc . . . . . . . . . . . . . . . . 24
10. Security Considerations . . . . . . . . . . . . . . . . . . . 22 7.4. HTTP Strict Transport Security . . . . . . . . . . . . . 25
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22 8. SVCB/HTTPSSVC parameter for ESNI keys . . . . . . . . . . . . 25
11.1. New registry for Service Parameters . . . . . . . . . . 22 8.1. Client behavior . . . . . . . . . . . . . . . . . . . . . 26
11.1.1. Procedure . . . . . . . . . . . . . . . . . . . . . 23 8.2. Deployment considerations . . . . . . . . . . . . . . . . 26
11.1.2. Initial contents . . . . . . . . . . . . . . . . . . 23 9. Interaction with other standards . . . . . . . . . . . . . . 26
11.2. Registry updates . . . . . . . . . . . . . . . . . . . . 24 10. Security Considerations . . . . . . . . . . . . . . . . . . . 27
12. Acknowledgments and Related Proposals . . . . . . . . . . . . 25 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 25 11.1. New registry for Service Parameters . . . . . . . . . . 27
13.1. Normative References . . . . . . . . . . . . . . . . . . 25 11.1.1. Procedure . . . . . . . . . . . . . . . . . . . . . 27
13.2. Informative References . . . . . . . . . . . . . . . . . 28 11.1.2. Initial contents . . . . . . . . . . . . . . . . . . 28
Appendix A. Mapping between HTTPSSVC and Alt-Svc . . . . . . . . 29 11.2. Registry updates . . . . . . . . . . . . . . . . . . . . 28
A.1. Multiple records and preference ordering . . . . . . . . 30 12. Acknowledgments and Related Proposals . . . . . . . . . . . . 29
A.2. Additional examples . . . . . . . . . . . . . . . . . . . 30 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 29
Appendix B. Comparison with alternatives . . . . . . . . . . . . 31 13.1. Normative References . . . . . . . . . . . . . . . . . . 29
B.1. Differences from the SRV RR type . . . . . . . . . . . . 31 13.2. Informative References . . . . . . . . . . . . . . . . . 32
B.2. Differences from the proposed HTTP record . . . . . . . . 31 13.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 33
B.3. Differences from the proposed ANAME record . . . . . . . 32 Appendix A. Comparison with alternatives . . . . . . . . . . . . 33
B.4. Differences from the proposed ESNI record . . . . . . . . 32 A.1. Differences from the SRV RR type . . . . . . . . . . . . 33
B.5. SNI Alt-Svc parameter . . . . . . . . . . . . . . . . . . 32 A.2. Differences from the proposed HTTP record . . . . . . . . 33
Appendix C. Design Considerations and Open Issues . . . . . . . 32 A.3. Differences from the proposed ANAME record . . . . . . . 33
C.1. Record Name . . . . . . . . . . . . . . . . . . . . . . . 33 A.4. Differences from the proposed ESNI record . . . . . . . . 34
C.2. Generality . . . . . . . . . . . . . . . . . . . . . . . 33 Appendix B. Design Considerations and Open Issues . . . . . . . 34
C.3. Wire Format . . . . . . . . . . . . . . . . . . . . . . . 33 B.1. Record Name . . . . . . . . . . . . . . . . . . . . . . . 34
C.4. Where to include Priority . . . . . . . . . . . . . . . . 33 B.2. Generality . . . . . . . . . . . . . . . . . . . . . . . 34
C.5. Whether to include Weight . . . . . . . . . . . . . . . . 33 B.3. Wire Format . . . . . . . . . . . . . . . . . . . . . . . 34
Appendix D. Change history . . . . . . . . . . . . . . . . . . . 33 B.4. Whether to include Weight . . . . . . . . . . . . . . . . 35
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34 Appendix C. Change history . . . . . . . . . . . . . . . . . . . 35
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 36
1. Introduction 1. Introduction
The SVCB and HTTPSSVC RRs provide clients with complete instructions The SVCB and HTTPSSVC RRs provide clients with complete instructions
for access to an origin. This information enables improved for access to an origin. This information enables improved
performance and privacy by avoiding transient connections to a sub- performance and privacy by avoiding transient connections to a sub-
optimal default server, negotiating a preferred protocol, and optimal default server, negotiating a preferred protocol, and
providing relevant public keys. providing relevant public keys.
For example, when clients need to make a connection to fetch For example, when clients need to make a connection to fetch
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and/or AAAA records for the origin hostname. This is adequate for and/or AAAA records for the origin hostname. This is adequate for
services that use basic HTTPS (fixed port, no QUIC, no [ESNI]). services that use basic HTTPS (fixed port, no QUIC, no [ESNI]).
Going beyond basic HTTPS confers privacy, performance, and Going beyond basic HTTPS confers privacy, performance, and
operational advantages, but it requires the client to learn operational advantages, but it requires the client to learn
additional information, and it is highly desirable to minimize the additional information, and it is highly desirable to minimize the
number of round-trip and lookups required to learn this additional number of round-trip and lookups required to learn this additional
information. information.
The SVCB and HTTPSSVC RRs also help when the operator of an origin The SVCB and HTTPSSVC RRs also help when the operator of an origin
wishes to delegate operational control to one or more other domains, wishes to delegate operational control to one or more other domains,
e.g. delegating the origin resource "https://example.com" to a e.g. delegating the origin resource "https://example.com" to a
service operator endpoint at "svc.example.net". While this case can service operator endpoint at "svc.example.net". While this case can
sometimes be handled by a CNAME, that does not cover all use-cases. 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 CNAME is also inadequate when the service operator needs to provide a
bound collection of consistent configuration parameters through the bound collection of consistent configuration parameters through the
DNS (such as network location, protocol, and keying information). DNS (such as network location, protocol, and keying information).
This document first describes the SVCB RR as a general-purpose This document first describes the SVCB RR as a general-purpose
resource record that can be applied directly and efficiently to a resource record that can be applied directly and efficiently to a
wide range of services. As HTTPS is a primary use-case and has wide range of services (Section 2). The HTTPSSVC RR is then defined
special requirements, the HTTPSSVC RR is also defined within this as a special case of SVCB that improves efficiency and convenience
document as a special case of SVCB. Services wishing to avoid the for use with HTTPS (Section 7) by avoiding the need for an [Attrleaf]
need for an [Attrleaf] label with SVCB may follow the pattern of label (Section 7.1). Other protocols with similar needs may follow
HTTPSSVC and assign their own SVCB-compatible RR types. the pattern of HTTPSSVC and assign their own SVCB-compatible RR
types.
All behaviors described as applying to the SVCB RR also apply to the All behaviors described as applying to the SVCB RR also apply to the
HTTPSSVC RR unless explicitly stated otherwise. Section 7 describes HTTPSSVC RR unless explicitly stated otherwise. Section 7 describes
additional behaviors specific to the HTTPSSVC record. Apart from additional behaviors specific to the HTTPSSVC record. Apart from
Section 7 and introductory examples, much of this document refers Section 7 and introductory examples, much of this document refers
only to the SVCB RR, but those references should be taken to apply to only to the SVCB RR, but those references should be taken to apply to
SVCB, HTTPSSVC, and any future SVCB-compatible RR types. SVCB, HTTPSSVC, and any future SVCB-compatible RR types.
The SVCB RR has two forms: 1) the "Alias Form" simply delegates The SVCB RR has two forms: 1) the "Alias Form" simply delegates
operational control for a resource; 2) the "Service Form" binds operational control for a resource; 2) the "Service Form" binds
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authorities, it is likely we'd specify a distinct "NS2" RR type that authorities, it is likely we'd specify a distinct "NS2" RR type that
is an instantiation of SVCB for authoritative nameserver delegation is an instantiation of SVCB for authoritative nameserver delegation
and parameter specification, similar to HTTPSSVC. and parameter specification, similar to HTTPSSVC.
TO BE REMOVED: Another open question is whether SVCB records should TO BE REMOVED: Another open question is whether SVCB records should
be self-descriptive and include the service name (eg, "https") in the be self-descriptive and include the service name (eg, "https") in the
RDATA section to avoid ambiguity. Perhaps this could be included as RDATA section to avoid ambiguity. Perhaps this could be included as
a svc="baz" parameter for protocols that are not the default for the a svc="baz" parameter for protocols that are not the default for the
RR type? Current inclination is to not do so. RR type? Current inclination is to not do so.
1.1. Introductory Example 1.1. Example: Protocol enhancements
As an introductory example for an HTTPS origin resource, a set of Consider a simple zone of the form
example HTTPSSVC and associated A+AAAA records might be:
www.example.com. 7200 IN CNAME svc.example.net. simple.example. 300 IN A 192.0.2.1
; AliasForm AAAA 2001:db8::1
example.com. 7200 IN HTTPSSVC 0 svc.example.net.
; ServiceForm
svc.example.net. 7200 IN HTTPSSVC 2 svc3.example.net. ( alpn=h3
port=8003 esniconfig="..." )
svc.example.net. 7200 IN HTTPSSVC 3 svc2.example.net. ( alpn=h2
port=8002 esniconfig="..." )
svc2.example.net. 300 IN A 192.0.2.2
svc2.example.net. 300 IN AAAA 2001:db8::2
svc3.example.net. 300 IN A 192.0.2.3
svc3.example.net. 300 IN AAAA 2001:db8::3
; Compatibility records for non-HTTPSSVC-aware clients
example.com. 300 IN A 192.0.2.1
example.com. 300 IN AAAA 2001:db8::1
svc.example.net. 300 IN A 192.0.2.1
svc.example.net. 300 IN AAAA 2001:db8::1
In the preceding example, both of the "example.com" and The domain owner could add a record like
"www.example.com" origin names are aliased to use alternative service
endpoints offered as "svc.example.net" (with "www.example.com"
continuing to use a CNAME alias). HTTP/2 is available on a cluster
of machines located at svc2.example.net with TCP port 8002 and HTTP/3
is available on a cluster of machines located at svc3.example.net
with UDP port 8003. The client can use the specified ESNI keys to
encrypt the SNI values of "example.com" and "www.example.com" in the
handshake with these alternative service endpoints. When connecting,
clients will continue to treat the authoritative origins as
"https://example.com" and "https://www.example.com", respectively.
For services other than HTTPS (as well as for HTTPS origins with non- simple.example. 7200 IN HTTPSSVC 1 . alpn=h3 ...
default ports), the SVCB RR and an [Attrleaf] label will be used.
For example, to reach an example resource of The presence of this record indicates to clients that simple.example
supports HTTPS, and the key=value pairs indicate that it supports
QUIC in addition to HTTPS over TLS (an implicit default). The record
could also include other information (e.g. non-standard port, ESNI
configuration).
1.2. Example: 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 HTTPSSVC, the owner of aliased.example could alias the apex by
adding one additional record:
. 7200 IN HTTPSSVC 0 pool.svc.example.
With this record in place, HTTPSSVC-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-HTTPSSVC-aware
clients will just ignore the new record.
Similar to CNAME, HTTPSSVC has 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.
1.3. Example: 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 HTTPSSVC 1 h3pool alpn=h2,h3 esniconfig="123..."
HTTPSSVC 2 . alpn=h2 esniconfig="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 HTTPSSVC or not. If the client
does support HTTPSSVC, 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, ESNI
configuration, etc.
1.4. Example: 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 "baz://api.example.com:8765", the following Alias Form SVCB record
would be used to delegate to "svc4-baz.example.net." which in-turn would be used to delegate to "svc4-baz.example.net." which in-turn
could return AAAA/A records and/or SVCB records in ServiceForm. could return AAAA/A records and/or SVCB records in ServiceForm.
_8765._baz.api.example.com. 7200 IN SVCB 0 svc4-baz.example.net. _8765._baz.api.example.com. 7200 IN SVCB 0 svc4-baz.example.net.
1.2. Goals of the SVCB RR HTTPSSVC records use similar [Attrleaf] labels if the origin contains
a non-default port.
1.5. Goals of the SVCB RR
The goal of the SVCB RR is to allow clients to resolve a single The goal of the SVCB RR is to allow clients to resolve a single
additional DNS RR in a way that: additional DNS RR in a way that:
o Provides service endpoints authoritative for the service, along o Provides service endpoints authoritative for the service, along
with parameters associated with each of these endpoints. with parameters associated with each of these endpoints.
o Does not assume that all alternative service endpoints have the o Does not assume that all alternative service endpoints have the
same parameters or capabilities, or are even operated by the same same parameters or capabilities, or are even operated by the same
entity. This is important as DNS does not provide any way to tie entity. This is important as DNS does not provide any way to tie
skipping to change at page 7, line 5 skipping to change at page 8, line 5
o Address a set of long-standing issues due to HTTP(S) clients not 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 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 limitation that a DNS name can not have both CNAME and NS RRs (as
is the case for zone apex names) is the case for zone apex names)
o Provide an HSTS-like indication signaling for the duration of the 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 DNS RR TTL that the HTTPS scheme should be used instead of HTTP
(see Section 7.4). (see Section 7.4).
1.3. Overview of the SVCB RR 1.6. Overview of the SVCB RR
This subsection briefly describes the SVCB RR in a non-normative This subsection briefly describes the SVCB RR in a non-normative
manner. (As mentioned above, this all applies equally to the manner. (As mentioned above, this all applies equally to the
HTTPSSVC RR which shares the same encoding, format, and high-level HTTPSSVC RR which shares the same encoding, format, and high-level
semantics.) semantics.)
The SVCB RR has two forms: AliasForm and ServiceForm. SVCB RR The SVCB RR has two forms: AliasForm, which aliases a name to another
entries with two non-empty fields are in AliasForm. When more fields name, and ServiceForm, which provides connection information bound to
are present, this indicates that the SVCB RR is in ServiceForm. The a service endpoint domain. Placing both forms in a single RR type
fields are: 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 1. SvcFieldPriority: The priority of this record (relative to
others, with lower values preferred). Applicable for the others, with lower values preferred). A value of 0 indicates
ServiceForm, and otherwise has value "0". (Described in AliasForm. (Described in Section 2.6.2.)
Appendix A.1.)
2. SvcDomainName: The domain name of either the alias target (for 2. SvcDomainName: The domain name of either the alias target (for
AliasForm) or the alternative service endpoint (for ServiceForm). AliasForm) or the alternative service endpoint (for ServiceForm).
3. SvcFieldValue: A list of key=value pairs describing the 3. SvcFieldValue (optional): A list of key=value pairs describing
alternative service endpoint for the domain name specified in the alternative service endpoint for the domain name specified in
SvcDomainName (only for ServiceForm and otherwise empty). SvcDomainName (only used in ServiceForm and otherwise ignored).
Described in Section 2.1. Described in Section 2.1.1.
Cooperating DNS recursive resolvers will perform subsequent record Cooperating DNS recursive resolvers will perform subsequent record
resolution (for SVCB, A, and AAAA records) and return them in the resolution (for SVCB, A, and AAAA records) and return them in the
Additional Section of the response. Clients must either use Additional Section of the response. Clients must either use
responses included in the additional section returned by the responses included in the additional section returned by the
recursive resolver or perform necessary SVCB, A, and AAAA record recursive resolver or perform necessary SVCB, A, and AAAA record
resolutions. DNS authoritative servers may attach in-bailiwick SVCB, resolutions. DNS authoritative servers may attach in-bailiwick SVCB,
A, AAAA, and CNAME records in the Additional Section to responses for A, AAAA, and CNAME records in the Additional Section to responses for
an SVCB query. an SVCB query.
When in the ServiceForm, the SvcFieldValue of the SVCB RR provides an When in the ServiceForm, the SvcFieldValue of the SVCB RR provides an
extensible data model for describing network endpoints that are extensible data model for describing network endpoints that are
authoritative for the origin, along with parameters associated with authoritative for the origin, along with parameters associated with
each of these endpoints. each of these endpoints.
For the HTTPS use-case, the HTTPSSVC RR enables many of the benefits For the HTTPS use-case, the HTTPSSVC RR enables many of the benefits
of [AltSvc], without waiting for a full HTTP connection initiation of [AltSvc] without waiting for a full HTTP connection initiation
(multiple roundtrips) before learning of the preferred alternative, (multiple roundtrips) before learning of the preferred alternative,
and without necessarily revealing the user's intended destination to and without necessarily revealing the user's intended destination to
all entities along the network path. all entities along the network path.
1.4. Parameter for ESNI 1.7. Parameter for ESNI
This document also defines a parameter for Encrypted SNI [ESNI] keys, This document also defines a parameter for Encrypted SNI [ESNI] keys.
both as a general SVCB parameter and also as a corresponding Alt-Svc See Section 8.
parameter. See Section 8.
1.5. Terminology 1.8. Terminology
For consistency with [AltSvc], we adopt the following definitions: For consistency with [AltSvc], we adopt the following definitions:
o An "origin" is an information source as in [RFC6454]. For o An "origin" is an information source as in [RFC6454]. For
services other than HTTPS, the exact definition will need to be services other than HTTPS, the exact definition will need to be
provided by the document mapping that service onto the SVCB RR. provided by the document mapping that service onto the SVCB RR.
o The "origin server" is the server that the client would reach when 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 accessing the origin in the absence of the SVCB record or an HTTPS
Alt-Svc. Alt-Svc.
skipping to change at page 8, line 46 skipping to change at page 9, line 45
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
2. The SVCB record type 2. The SVCB record type
The SVCB DNS resource record (RR) type (RR type ???) is used to The SVCB DNS resource record (RR) type (RR type ???) is used to
locate endpoints that can service an origin. There is special locate endpoints that can service an origin. There is special
handling for the case of "https" origins. The presentation format of handling for the case of "https" origins.
the record is:
The algorithm for resolving SVCB records and associated address
records is specified in Section 3.
2.1. Presentation format
The presentation format of the record is:
Name TTL IN SVCB SvcFieldPriority SvcDomainName SvcFieldValue Name TTL IN SVCB SvcFieldPriority SvcDomainName SvcFieldValue
The SVCB record is defined specifically within the Internet ("IN") The SVCB record is defined specifically within the Internet ("IN")
Class ([RFC1035]). SvcFieldPriority is a number in the range Class ([RFC1035]). SvcFieldPriority is a number in the range
0-65535, SvcDomainName is a domain name, and SvcFieldValue is a set 0-65535, SvcDomainName is a domain name, and SvcFieldValue is a set
of key=value pairs present for the ServiceForm. The SvcFieldValue is of key=value pairs present for the ServiceForm. Each key SHALL
empty for the AliasForm. appear at most once in a SvcFieldValue. The SvcFieldValue is empty
for the AliasForm.
The algorithm for resolving SVCB records and associated address 2.1.1. Presentation format for SvcFieldValue key=value pairs
records is specified in Section 3.
2.1. Parameter specification via ServiceFieldValue In ServiceForm, the SvcFieldValue consists of zero or more elements
separated by whitespace. Each element represents a key=value pair.
In ServiceForm, the SvcFieldValue contains key=value pairs. Keys are Keys are IANA-registered SvcParamKeys (Section 11.1) with both a
IANA-registered SvcParamKeys (Section 11.1) with both a case- case-insensitive string representation and a numeric representation
insensitive string representation and a numeric representation in the in the range 0-65535. Registered key names should only contain
range 0-65535. Registered key names should only contain characters characters from the ranges "a"-"z", "0"-"9", and "-". In ABNF
from the ranges "a"-"z", "0"-"9", and "-". In ABNF [RFC5234], [RFC5234],
ALPHA_LC = %x61-7A ; a-z ALPHA_LC = %x61-7A ; a-z
key = ALPHA_LC / DIGIT / "-" key = ALPHA_LC / DIGIT / "-"
display-key = ALPHA / DIGIT / "-" display-key = ALPHA / DIGIT / "-"
Values are in a format specific to the SvcParamKey. Their definition Values are in a format specific to the SvcParamKey. Their definition
should specify both their presentation format and wire encoding should specify both their presentation format and wire encoding
(e.g., domain names, binary data, or numeric values). (e.g., domain names, binary data, or numeric values).
The SVCB format preserves the order of values and can encode multiple
values for the same parameter. However, clients MUST consider only
the first appearance of a parameter unless its specification
explicitly allows multiple values.
2.1.1. Presentation format
The presentation format for SvcFieldValue is a whitespace-separated The presentation format for SvcFieldValue is a whitespace-separated
list of the key=value pairs. Each pair is presented in the following list of elements representing a key-value pair, with an absent value
form: or "=" indicating an empty value. Each element is presented in the
following form:
; basic-visible is VCHAR minus DQUOTE, ";", and "\" ; basic-visible is VCHAR minus DQUOTE, ";", and "\"
basic-visible = %x21 / %x23-3A / %x3C-5B / %x5D-7E basic-visible = %x21 / %x23-3A / %x3C-5B / %x5D-7E
escaped-char = "\" (VCHAR / WSP) escaped-char = "\" (VCHAR / WSP)
contiguous = *(basic-visible / escaped-char) contiguous = *(basic-visible / escaped-char)
quoted-string = DQUOTE *(contiguous / WSP) DQUOTE quoted-string = DQUOTE *(contiguous / WSP) DQUOTE
value = quoted-string / contiguous value = quoted-string / contiguous
pair = display-key "=" value pair = display-key "=" value
element = display-key / pair
The value format is intended to match the definition of <character- The value format is intended to match the definition of <character-
string> in [RFC1035] Section 5.1. (Unlike <character-string>, the string> in [RFC1035] Section 5.1. (Unlike <character-string>, the
length of a value is not limited to 255 characters.) length of a value is not limited to 255 characters.)
Unrecognized keys are represented in presentation format as Unrecognized keys are represented in presentation format as
"keyNNNNN" where NNNNN is the numeric value of the key type without "keyNNNNN" where NNNNN is the numeric value of the key type without
leading zeros. In presentation format, values of unrecognized keys leading zeros. In presentation format, values of unrecognized keys
should be represented in wire format, using decimal escape codes SHALL be represented in wire format, using decimal escape codes (e.g.
(e.g. \255) when necessary. \255) when necessary.
Elements in presentation format MAY appear in any order.
2.2. SVCB RDATA Wire Format 2.2. SVCB RDATA Wire Format
The RDATA for the SVCB RR consists of: The RDATA for the SVCB RR consists of:
o a 2 octet field for SvcFieldPriority as an integer in network byte o a 2 octet field for SvcFieldPriority as an integer in network byte
order. order.
o the uncompressed SvcDomainName, represented as a sequence of o the uncompressed, fully-qualified SvcDomainName, represented as a
length-prefixed labels as in Section 3.1 of [RFC1035]. sequence of length-prefixed labels as in Section 3.1 of [RFC1035].
o the SvcFieldValue byte string, consuming the remainder of the o the SvcFieldValue byte string, consuming the remainder of the
record (so smaller than 65535 octets and constrained by the RDATA record (so smaller than 65535 octets and constrained by the RDATA
and DNS message sizes). and DNS message sizes).
AliasForm is defined by SvcFieldPriority being 0. AliasForm is defined by SvcFieldPriority being 0.
When SvcFieldValue is non-empty (ServiceForm), it contains a list of When SvcFieldValue is non-empty (ServiceForm), it contains a series
SvcParamKey=SvcParamValue pairs with length-prefixes for the of SvcParamKey=SvcParamValue pairs, represented as:
SvcParamValues, each of which contains:
o a 2 octet field containing the SvcParamKey as an integer in o a 2 octet field containing the SvcParamKey as an integer in
network byte order. network byte order.
o a 2 octet field containing the length of the SvcParamValue as an o a 2 octet field containing the length of the SvcParamValue as an
integer between 0 and 65535 in network byte order (but constrained integer between 0 and 65535 in network byte order (but constrained
by the RDATA and DNS message sizes). by the RDATA and DNS message sizes).
o an octet string of the length defined by the previous field. o an octet string of the length defined by the previous field.
If the parser reaches the end of the RDATA while parsing a SvcParamKeys SHALL appear in increasing numeric order.
SvcFieldValue, the RR is invalid and MUST be discarded.
Clients MUST consider an RR malformed if
o the parser reaches the end of the RDATA while parsing a
SvcFieldValue.
o SvcParamKeys are not in strictly increasing numeric order.
o a SvcParamValue for a known 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? TODO: decide if we want special handling for any SvcParamKey ranges?
For example: range for greasing; experimental range; range-of- For example: range for greasing; experimental range; range-of-
mandatory-to-use-the-RR vs range of ignore-just-param-if-unknown. mandatory-to-use-the-RR vs range of ignore-just-param-if-unknown.
2.3. SVCB owner names 2.3. SVCB owner names
When querying the SVCB RR, an origin is typically translated into a When querying the SVCB RR, an origin is typically translated into a
QNAME by prefixing the port and scheme with "_", then concatenating QNAME by prefixing the port and scheme with "_", then concatenating
them with the host name, resulting in a domain name like them with the host name, resulting in a domain name like
skipping to change at page 11, line 21 skipping to change at page 12, line 41
When a prior CNAME or SVCB record has aliased to an SVCB record, each When a prior CNAME or SVCB record has aliased to an SVCB record, each
RR shall be returned under its own owner name. RR shall be returned under its own owner name.
Note that none of these forms alter the origin or authority for Note that none of these forms alter the origin or authority for
validation purposes. For example, clients MUST continue to validate validation purposes. For example, clients MUST continue to validate
TLS certificate hostnames based on the origin host. TLS certificate hostnames based on the origin host.
As an example: As an example:
_8443._foo.api.example.com. 7200 IN SVCB 0 svc4.example.net. _8443._foo.api.example.com. 7200 IN SVCB 0 svc4.example.net.
svc4.example.net. 7200 IN SVCB 3 ( svc4.example.net. alpn="bar" svc4.example.net. 7200 IN SVCB 3 svc4.example.net. (
port="8004" esniconfig="..." ) alpn="bar" port="8004" esniconfig="..." )
would indicate that "foo://api.example.com:8443" is aliased to use would indicate that "foo://api.example.com:8443" is aliased to the
ALPN protocol "bar" service endpoints offered at "svc4.example.net" service endpoints offered at "svc4.example.net" on port number 8004,
on port 8004. which support 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 2.4. SvcRecordType
The SvcRecordType is indicated by the SvcFieldPriority, and defines The SvcRecordType is indicated by the SvcFieldPriority, and defines
the form of the SVCB RR. When SvcFieldPriority is 0, the SVCB 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 AliasForm. Otherwise, the SVCB
SvcRecordType is defined to be in ServiceForm. SvcRecordType is defined to be in ServiceForm.
Within an SVCB RRSet, all RRs should have the same SvcRecordType. If Within an SVCB RRSet, all RRs should have the same SvcRecordType. If
an RRSet contains a record in AliasForm, the client MUST ignore any an RRSet contains a record in AliasForm, the client MUST ignore any
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2.6. SVCB records: ServiceForm 2.6. SVCB records: ServiceForm
When SvcRecordType is the ServiceForm, the combination of When SvcRecordType is the ServiceForm, the combination of
SvcDomainName and SvcFieldValue parameters within each resource SvcDomainName and SvcFieldValue parameters within each resource
record associates an alternative service location with its connection record associates an alternative service location with its connection
parameters. parameters.
Each protocol scheme that uses SVCB MUST define a protocol mapping Each protocol scheme that uses SVCB MUST define a protocol mapping
that explains how SvcFieldValues are applied for connections of that that explains how SvcFieldValues are applied for connections of that
scheme. Appendix A defines a limited mapping between Alt-Svc scheme. Unless specified otherwise by the protocol mapping, clients
([AltSvc]) values and the SVCB ServiceForm. Protocols using SVCB may MUST ignore SvcFieldValue parameters that they do not recognize.
use this Alt-Svc mapping if they also use Alt-Svc. 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 2.6.1. Special handling of "." for SvcDomainName in ServiceForm
For ServiceForm SVCB RRs, if SvcDomainName has the value ".", then For ServiceForm SVCB RRs, if SvcDomainName has the value ".", then
the owner name of this record MUST be used as the effective the owner name of this record MUST be used as the effective
SvcDomainName. (The SvcDomainName of an SVCB RR in AliasForm MUST SvcDomainName. (The SvcDomainName of an SVCB RR in AliasForm MUST
NOT have this value.) NOT have this value.)
For example, in the following example "svc2.example.net" is the For example, in the following example "svc2.example.net" is the
effective SvcDomainName: effective SvcDomainName:
www.example.com. 7200 IN HTTPSSVC svc.example.net. www.example.com. 7200 IN HTTPSSVC svc.example.net.
svc.example.net. 7200 IN CNAME svc2.example.net. svc.example.net. 7200 IN CNAME svc2.example.net.
svc2.example.net. 7200 IN HTTPSSVC 1 . ( alpn=h2 svc2.example.net. 7200 IN HTTPSSVC 1 . port=8002 esniconfig="..."
port=8002 esniconfig="..." )
svc2.example.net. 300 IN A 192.0.2.2 svc2.example.net. 300 IN A 192.0.2.2
svc2.example.net. 300 IN AAAA 2001:db8::2 svc2.example.net. 300 IN AAAA 2001:db8::2
2.6.2. SvcFieldPriority 2.6.2. SvcFieldPriority
As RRs within an RRSet are explicitly unordered collections, the As RRs within an RRSet are explicitly unordered collections, the
SvcFieldPriority value serves to indicate priority. SVCB RRs with a SvcFieldPriority value serves to indicate priority. SVCB RRs with a
smaller SvcFieldPriority value SHOULD be given preference over RRs smaller SvcFieldPriority value SHOULD be given preference over RRs
with a larger SvcFieldPriority value. with a larger SvcFieldPriority value.
skipping to change at page 14, line 14 skipping to change at page 15, line 35
This procedure does not rely on any recursive or authoritative server This procedure does not rely on any recursive or authoritative server
to comply with this specification or have any awareness of SVCB. to comply with this specification or have any awareness of SVCB.
When selecting between AAAA and A records to use, clients may use an When selecting between AAAA and A records to use, clients may use an
approach such as [HappyEyeballsV2]. approach such as [HappyEyeballsV2].
Some important optimizations are discussed in Section 5 to avoid Some important optimizations are discussed in Section 5 to avoid
additional latency in comparison to ordinary AAAA/A lookups. additional latency in comparison to ordinary AAAA/A lookups.
3.1. Clients using a Proxy 3.1. Handling resolution failures
If an 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.
3.2. Clients using a Proxy
Clients using a domain-oriented transport proxy like HTTP CONNECT Clients using a domain-oriented transport proxy like HTTP CONNECT
([RFC7231] Section 4.3.6) or SOCKS5 ([RFC1928]) SHOULD disable SVCB ([RFC7231] Section 4.3.6) or SOCKS5 ([RFC1928]) SHOULD disable SVCB
support if performing SVCB queries would violate the client's privacy support if performing SVCB queries would violate the client's privacy
intent. intent.
If the client can safely perform SVCB queries (e.g. via the proxy or If the client can safely perform SVCB queries (e.g. via the proxy or
an affiliated resolver), the client SHOULD follow the standard SVCB an affiliated resolver), the client SHOULD follow the standard SVCB
resolution process, selecting the highest priority option that is resolution process, selecting the highest priority option that is
compatible with the client and the proxy. The client SHOULD provide compatible with the client and the proxy. The client SHOULD provide
skipping to change at page 14, line 44 skipping to change at page 16, line 39
privacy (e.g. esniconfig). privacy (e.g. esniconfig).
o It allows the origin to delegate the apex domain. o It allows the origin to delegate the apex domain.
o It allows the proxy to select between IPv4 and IPv6 addresses for o It allows the proxy to select between IPv4 and IPv6 addresses for
the server according to its configuration, and receive addresses the server according to its configuration, and receive addresses
based on its network geolocation. based on its network geolocation.
4. DNS Server Behavior 4. DNS Server Behavior
4.1. Authoritative servers
When replying to an SVCB query, authoritative DNS servers SHOULD When replying to an SVCB query, authoritative DNS servers SHOULD
return A, AAAA, and SVCB records (as well as any relevant CNAME return A, AAAA, and SVCB records (as well as any relevant CNAME or
records) in the Additional Section for any in-bailiwick [DNAME] records) in the Additional Section for any in-bailiwick
SvcDomainNames. SvcDomainNames.
4.2. Recursive resolvers
Recursive resolvers that are aware of SVCB SHOULD ensure that the Recursive resolvers that are aware of SVCB SHOULD ensure that the
client can execute the procedure in Section 3 without issuing a client can execute the procedure in Section 3 without issuing a
second round of queries, by following this procedure while second round of queries, by incorporating all the necessary
constructing a response to a stub resolver for an SVCB record query: information into a single response. For the initial SVCB record
query, this is just the normal response construction process (i.e.
1. When processing an SVCB response from an authoritative server, unknown RR type resolution under [RFC3597]). For followup
add it to the Additional section (unless it is the Answer). 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.
2. If all records are in ServiceForm, resolve A and AAAA records for Upon receiving an SVCB query, recursive resolvers SHOULD start with
each SvcDomainName (or for the owner name if the SvcDomainName is the standard resolution procedure, and then follow this procedure to
"."), and include all the results in the Additional section. construct the full response to the stub resolver:
3. Otherwise, select an AliasForm record at random, and resolve A, 1. Incorporate the results of SVCB resolution.
AAAA, and SVCB records for the SvcDomainName. If the SVCB record
does not exist, add the A and AAAA records to the Additional 2. If any of the resolved SVCB records are in AliasForm, choose an
section. Otherwise, go to step 1, subject to loop detection AliasForm record at random, and resolve SVCB, A, and AAAA records
heuristics. for its SvcDomainName.
* If any SVCB records are resolved, go to step 1, subject to
loop detection heuristics.
* 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 SvcParam portion of the SVCB RR as All DNS servers SHOULD treat the SvcParam portion of the SVCB RR as
opaque and SHOULD NOT try to alter their behavior based on its opaque and SHOULD NOT try to alter their behavior based on its
contents. contents.
When responding to a query that includes the DNSSEC OK bit When responding to a query that includes the DNSSEC OK bit
([RFC3225]), DNSSEC-capable recursive and authoritative DNS servers ([RFC3225]), DNSSEC-capable recursive and authoritative DNS servers
MUST accompany each RRSet in the Additional section with the same MUST accompany each RRSet in the Additional section with the same
DNSSEC-related records that it would send when providing that RRSet DNSSEC-related records that they would send when providing that RRSet
as an Answer. as an Answer (e.g. RRSIG, NSEC, NSEC3).
5. Performance optimizations 5. Performance optimizations
For optimal performance (i.e. minimum connection setup time), clients For optimal performance (i.e. minimum connection setup time), clients
SHOULD issue address (AAAA and/or A) and SVCB queries simultaneously, SHOULD issue address (AAAA and/or A) and SVCB queries simultaneously,
and SHOULD implement a client-side DNS cache. Responses in the and SHOULD implement a client-side DNS cache. Responses in the
Additional section of an SVCB response SHOULD be placed in cache Additional section of an SVCB response SHOULD be placed in cache
before performing any followup queries. With these optimizations in before performing any followup queries. With these optimizations in
place, and conforming DNS servers, using SVCB does not add network place, and conforming DNS servers, using SVCB does not add network
latency to connection setup. latency to connection setup.
skipping to change at page 16, line 22 skipping to change at page 18, line 47
If the client has an in-progress TCP connection to If the client has an in-progress TCP connection to
"[2001:db8::2]:1234", it MAY proceed with TLS on that connection "[2001:db8::2]:1234", it MAY proceed with TLS on that connection
using "esniconfig="222..."", even though the other record in the using "esniconfig="222..."", even though the other record in the
RRSet has higher priority. RRSet has higher priority.
If none of the SVCB records are consistent with any active or in- If none of the SVCB records are consistent with any active or in-
progress connection, clients must proceed as described in Step 3 of progress connection, clients must proceed as described in Step 3 of
the procedure in Section 3. the procedure in Section 3.
5.2. Preferring usable records 5.2. Generating and using incomplete responses
A nonconforming recursive resolver might not return all the When following the procedure in Section 4.2, recursive resolvers MAY
information required to use all the records in an SVCB response. If terminate the procedure early and produce a reply that omits some of
the associated RRSets. This might 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 fetch additional
information that is required to use an 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 some of the SVCB records in the response can be used without
requiring additional DNS queries, the client MAY prefer those requiring additional DNS queries, the client MAY prefer those
records, regardless of their priorities. records, regardless of their priorities.
5.3. Structuring zones for performance 5.3. Structuring zones for performance
To avoid a delay for clients using a nonconforming recursive To avoid a delay for clients using a nonconforming recursive
resolver, domain owners SHOULD use a single SVCB record whose resolver, domain owners SHOULD use a single SVCB record whose
SvcDomainName is in the origin hostname's CNAME chain if possible. SvcDomainName is "." if possible. This will ensure that the required
This will ensure that the required address records are already address records are already present in the client's DNS cache as part
present in the client's DNS cache as part of the responses to the of the responses to the address queries that were issued in parallel.
address queries that were issued in parallel.
6. Initial SvcParamKeys 6. Initial SvcParamKeys
A few initial SvcParamKeys are defined here. These keys are useful A few initial SvcParamKeys are defined here. These keys are useful
for HTTPS, and most are applicable to other protocols as well. for HTTPS, and most are applicable to other protocols as well.
6.1. "alpn" 6.1. "alpn" and "no-default-alpn"
The "alpn" SvcParamKey defines the Application Layer Protocol (ALPN, The "alpn" and "no-default-alpn" SvcParamKeys together indicate the
as defined in {{!RFC7301}) supported by a TLS-based alternative set of Application Layer Protocol Negotation (ALPN) protocol
service. Its value SHOULD be an entry in the IANA registry "TLS identifiers [ALPN] and associated transport protocols supported by
Application-Layer Protocol Negotiation (ALPN) Protocol IDs". this service endpoint.
The presentation format and wire format of SvcParamValue is its As with [AltSvc], the ALPN protocol identifier is used to identify
registered "Identification Sequence". 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).
Clients MUST ignore SVCB RRs where the "alpn" SvcParamValue is ALPNs are identified by their registered "Identification Sequence"
unknown or unsupported. (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 / "\\" / %5d-%FF
escaped-id = 1*255(escaped-octet)
alpn-value = escaped-id *("," escaped-id)
In the wire format for "alpn", each ALPN identifier ("alpn-id") is
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
(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" 6.2. "port"
The "port" SvcParamKey defines the TCP or UDP port that should be The "port" SvcParamKey defines the TCP or UDP port that should be
used to contact this alternative service. used to contact this alternative service.
The presentation format of the SvcParamValue is a numeric value The presentation format of the SvcParamValue is a numeric value
between 0 and 65535 inclusive. The wire format of the SvcParamValue between 0 and 65535 inclusive. Any other values (e.g. the empty
is the corresponding 2 octet numeric value in network byte order. value) are syntax errors.
The wire format of the SvcParamValue is the corresponding 2 octet
numeric value in network byte order.
6.3. "esniconfig" 6.3. "esniconfig"
The SvcParamKey for ESNI is "esniconfig". Its value is defined in The SvcParamKey for ESNI is "esniconfig". Its value is defined in
Section 8. It is applicable to most TLS-based protocols. Section 8. It is applicable to most TLS-based protocols.
When publishing a record containing an "esniconfig" parameter, the When publishing a record containing an "esniconfig" parameter, the
publisher MUST ensure that all IP addresses of SvcDomainName publisher MUST ensure that all IP addresses of SvcDomainName
correspond to servers that have access to the corresponding private correspond to servers that have access to the corresponding private
key or are authoritative for the fallback domain. (See [ESNI] for key or are authoritative for the fallback domain. (See [ESNI] for
skipping to change at page 17, line 51 skipping to change at page 22, line 8
Section 3, and clients SHOULD use the IP address in those responses Section 3, and clients SHOULD use the IP address in those responses
for future connections. Clients MAY opt to terminate any connections for future connections. Clients MAY opt to terminate any connections
using the addresses in hints and instead switch to the addresses in using the addresses in hints and instead switch to the addresses in
response to the SvcDomainName. Failure to use A and/or AAAA response response to the SvcDomainName. Failure to use A and/or AAAA response
addresses may negatively impact load balancing or other geo-aware addresses may negatively impact load balancing or other geo-aware
features and thereby degrade client performance. features and thereby degrade client performance.
The wire format for each parameter is a sequence of IP addresses in 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 network byte order. Like an A or AAAA RRSet, the list of addresses
represents an unordered collection, and clients SHOULD pick addresses represents an unordered collection, and clients SHOULD pick addresses
to use in a random order. to use in a random order. An empty list of addresses is invalid.
These parameters MAY be repeated multiple times within a record.
When receiving such a record, clients SHOULD combine the sets of
addresses.
When selecting between IPv4 and IPv6 addresses to use, clients may When selecting between IPv4 and IPv6 addresses to use, clients may
use an approach such as [HappyEyeballsV2]. When only "ipv4hint" use an approach such as [HappyEyeballsV2]. When only "ipv4hint" is
parameters are present, IPv6-only clients may synthesize IPv6 present, IPv6-only clients may synthesize IPv6 addresses as specified
addresses as specified in [RFC7050] or ignore the "ipv4hint" key and in [RFC7050] or ignore the "ipv4hint" key and wait for AAAA
wait for AAAA resolution (Section 3). Recursive resolvers MUST NOT resolution (Section 3). Recursive resolvers MUST NOT perform DNS64
perform DNS64 ([RFC6147]) on parameters within an SVCB record. For ([RFC6147]) on parameters within an SVCB record. For best
best performance, server operators SHOULD include "ipv6hint" performance, server operators SHOULD include an "ipv6hint" parameter
parameters whenever they publish "ipv4hint" parameters. whenever they include an "ipv4hint" parameter.
The presentation format for each parameter is a comma-separated list The presentation format for each parameter is a comma-separated list
of IP addresses in standard textual format [RFC5952]. of IP addresses in standard textual format [RFC5952].
These parameters are intended to minimize additional connection These parameters are intended to minimize additional connection
latency when a recursive resolver is not compliant with the latency when a recursive resolver is not compliant with the
requirements in Section 4, and SHOULD NOT be included if most clients requirements in Section 4, and SHOULD NOT be included if most clients
are using compliant recursive resolvers. 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 7. Using SVCB with HTTPS and HTTP
Use of any protocol with SVCB requires a protocol-specific mapping Use of any protocol with SVCB requires a protocol-specific mapping
specification. This section specifies the mapping for HTTPS and specification. This section specifies the mapping for HTTPS and
HTTP. HTTP.
To enable special handling for the HTTPS and HTTP use-cases, the To enable special handling for the HTTPS and HTTP use-cases, the
HTTPSSVC RR type is defined as an SVCB-compatible RR type, specific HTTPSSVC RR type is defined as an SVCB-compatible RR type, specific
to the https and http schemes. Clients MUST NOT perform SVCB queries to the https and http schemes. Clients MUST NOT perform SVCB queries
or accept SVCB responses for https or http schemes. or accept SVCB responses for "https" or "http" schemes.
The HTTPSSVC wire format and presentation format are identical to The HTTPSSVC wire format and presentation format are identical to
SVCB, and both share the SvcParamKey registry. SVCB semantics apply SVCB, and both share the SvcParamKey registry. SVCB semantics apply
equally to HTTPSSVC unless specified otherwise. equally to HTTPSSVC unless specified otherwise.
The presence of an HTTPSSVC record for an HTTP or HTTPS service also All the SvcParamKeys defined in Section 6 are permitted for use in
provides an indication that all resources are available over HTTPS, HTTPSSVC. The default set of ALPN IDs is the single value
as discussed in Section 7.4. This allows HTTPSSVC RRs to apply to "http/1.1".
pre-existing HTTP scheme URLs, while ensuring that the client uses a
secure and authenticated HTTPS connection. The presence of an HTTPSSVC record for an origin also indicates that
all HTTP resources are available over HTTPS, as discussed in
Section 7.4. This allows HTTPSSVC RRs to apply to pre-existing
"http" scheme URLs, while ensuring that the client uses a secure and
authenticated HTTPS connection.
The HTTPSSVC RR parallels the concepts introduced in the HTTP The HTTPSSVC RR parallels the concepts introduced in the HTTP
Alternative Services proposed standard [AltSvc]. Clients and servers Alternative Services proposed standard [AltSvc]. Clients and servers
that implement HTTPSSVC are NOT REQUIRED to implement Alt-Svc. that implement HTTPSSVC are NOT REQUIRED to implement Alt-Svc.
However, many clients and servers will implement both, and a partial
mapping exists between them (Appendix A).
7.1. Owner names for HTTPSSVC records 7.1. Owner names for HTTPSSVC records
The HTTPSSVC RR extends the behavior for determining a QNAME The HTTPSSVC RR extends the behavior for determining a QNAME
specified above in Section 2.3. In particular, if the scheme is specified above in Section 2.3. In particular, if the scheme is
"https" with port 443, or the scheme is "http" and the port is 80, "https" with port 443 then the client's original QNAME is equal to
then the client's original QNAME is equal to the origin host name. the origin host name.
For origins other than https with port 443 and http with port 80, the By removing the [Attrleaf] labels used in SVCB, this construction
port and scheme continue to be prefixed to the hostname as described enables offline DNSSEC signing of wildcard domains, which are
in Section 2.3. Following of HTTPSSVC AliasForm and CNAME aliases is commonly used with HTTPS. Reusing the origin hostname also allows
also unchanged from SVCB. the targets of existing CNAME chains (e.g. CDN hosts) to start
returning HTTPSSVC responses without requiring origin domains to
configure and maintain an additional delegation.
For HTTPS origins with ports other than 443, the port and scheme
continue to be prefixed to the hostname as described in Section 2.3.
Following of HTTPSSVC AliasForm and CNAME aliases is also unchanged
from SVCB.
Clients always convert "http" URLS to "https" before performing an
HTTPSSVC query using the process described in Section 7.4, so domain
owners MUST NOT publish HTTPSSVC records with a prefix of "_http".
Note that none of these forms alter the HTTPS origin or authority. Note that none of these forms alter the HTTPS origin or authority.
For example, clients MUST continue to validate TLS certificate For example, clients MUST continue to validate TLS certificate
hostnames based on the origin host. hostnames based on the origin host.
7.2. Populating Alt-Used 7.2. Relationship to Alt-Svc
When using an HTTPSSVC RR in ServiceForm, all clients SHOULD include
the "Alt-Used" HTTP header (Section 5 of [RFC7838]). The header's
value (in ABNF) SHOULD be
uri-host ":" port Publishing a ServiceForm HTTPSSVC record in DNS is intended to be
similar to transmitting an Alt-Svc field value over HTTPS, and
receiving an HTTPSSVC record is intended to be similar to receiving
that field value over HTTPS. However, there are some differences in
the intended client and server behavior.
where uri-host is the final value of HOST ({client-behavior}) minus 7.2.1. ALPN usage
the trailing ".", and port is the port number in use.
7.3. Differences from Alt-Svc Unlike Alt-Svc Field Values, HTTPSSVC records can contain multiple
ALPN IDs, and clients are encouraged to offer additional ALPNs that
they support (subject to security constraints).
Publishing a ServiceForm HTTPSSVC record in DNS is intended to be TO BE REMOVED: The ALPN semantics in [AltSvc] are ambiguous, and
similar to transmitting the corresponding Alt-Svc field value over problematic in some interpretations. We should update [AltSvc] to
HTTPS, and receiving an HTTPSSVC record is intended to be similar to give it well-defined semantics that match HTTPSSVC.
receiving that field value over HTTPS. However, there are some
differences in the intended client and server behavior.
7.3.1. Untrusted channel 7.2.2. Untrusted channel
SVCB does not require or provide any assurance of authenticity. SVCB does not require or provide any assurance of authenticity.
(DNSSEC signing and verification, which would provide such assurance, (DNSSEC signing and verification, which would provide such assurance,
are OPTIONAL.) The DNS resolution process is treated as an untrusted are OPTIONAL.) The DNS resolution process is treated as an untrusted
channel that learns only the QNAME, and is prevented from mounting channel that learns only the QNAME, and is prevented from mounting
any attack beyond denial of service. any attack beyond denial of service.
Alt-Svc parameters that cannot be safely received in this model MUST Alt-Svc parameters that cannot be safely received in this model MUST
NOT have a corresponding defined SvcParamKey. For example, there is NOT have a corresponding defined SvcParamKey. For example, there is
no SvcParamKey corresponding to the Alt-Svc "persist" parameter, no SvcParamKey corresponding to the Alt-Svc "persist" parameter,
because this parameter is not safe to accept over an untrusted because this parameter is not safe to accept over an untrusted
channel. channel.
7.3.2. Caching and granularity 7.2.3. TTL and granularity
There is no SvcParamKey corresponding to the Alt-Svc "ma" (max age) There is no SvcParamKey corresponding to the Alt-Svc "ma" (max age)
parameter. Instead, server operators SHOULD encode the expiration parameter. Instead, server operators encode the expiration time in
time in the DNS TTL. the DNS TTL.
Some DNS caching systems incorrectly extend the lifetime of DNS The appropriate TTL value will typically be similar to the "ma" value
records beyond the stated TTL. Server operators MUST NOT rely on used for Alt-Svc, but may vary depending on the desired efficiency
HTTPSSVC records expiring on time, and MAY shorten the TTL to and agility. Some DNS caches incorrectly extend the lifetime of DNS
compensate. records beyond the stated TTL, so server operators cannot rely on
HTTPSSVC records 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 Sending Alt-Svc over HTTP allows the server to tailor the Alt-Svc
Field Value specifically to the client. When using an HTTPSSVC DNS Field Value specifically to the client. When using an HTTPSSVC DNS
record, groups of clients will necessarily receive the same Alt-Svc record, groups of clients will necessarily receive the same
Field Value. Therefore, HTTPSSVC is not suitable for uses that SvcFieldValue. Therefore, HTTPSSVC is not suitable for uses that
require single-client granularity. require single-client granularity.
If the client has an Alt-Svc cache, and a usable Alt-Svc value is 7.3. Interaction with Alt-Svc
present in that cache, then the client MAY skip the HTTPSSVC query.
If the client has a cached Alt-Svc entry that is expiring, the client Clients that do not implement support for ESNI MAY skip the HTTPSSVC
MAY perform an HTTPSSVC query to refresh the entry. 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
HTTPSSVC client connection procedure (Section 3).
For clients that implement support for ESNI, the interaction between
HTTPSSVC 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. HTTP Strict Transport Security 7.4. HTTP Strict Transport Security
By publishing an HTTPSSVC record, the server operator indicates that By publishing an HTTPSSVC record, the server operator indicates that
all useful HTTP resources on that origin are reachable over HTTPS, all useful HTTP resources on that origin are reachable over HTTPS,
similar to HTTP Strict Transport Security [HSTS]. When an HTTPSSVC similar to HTTP Strict Transport Security [HSTS]. When an HTTPSSVC
record is present for an origin, all "http" scheme requests for that record is present for an origin, all "http" scheme requests for that
origin SHOULD logically be redirected to "https". origin SHOULD logically be redirected to "https".
Prior to making an "http" scheme request, the client SHOULD perform a Prior to making an "http" scheme request, the client SHOULD perform a
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This construction is equivalent to Section 8.3 of [HSTS], point 5. This construction is equivalent to Section 8.3 of [HSTS], point 5.
If an HTTPSSVC record is present for this "https" URL, the client If an HTTPSSVC record is present for this "https" URL, the client
should treat this as the equivalent of receiving an HTTP "307 should treat this as the equivalent of receiving an HTTP "307
Temporary Redirect" redirect to the "https" URL. Because HTTPSSVC is Temporary Redirect" redirect to the "https" URL. Because HTTPSSVC is
received over an often insecure channel (DNS), clients MUST NOT place received over an often insecure channel (DNS), clients MUST NOT place
any more trust in this signal than if they had received a 307 any more trust in this signal than if they had received a 307
redirect over cleartext HTTP. redirect over cleartext HTTP.
If the HTTPSSVC query results in a SERVFAIL error, and the connection When making an "https" scheme request to an origin with an HTTPSSVC
between the client and the recursive resolver is cryptographically record, either directly or via the above redirect, the client SHOULD
protected (e.g. using TLS [RFC7858] or HTTPS [RFC8484]), the client terminate the connection if there are any errors with the underlying
SHOULD abandon the connection attempt and display an error message. secure transport, such as errors in certificate validation. This
A SERVFAIL error can occur if the domain is DNSSEC-signed, the aligns with Section 8.4 and Section 12.1 of [HSTS].
recursive resolver is DNSSEC-validating, and an active attacker
between the recursive resolver and the authoritative DNS server is
attempting to prevent the upgrade to HTTPS.
Similarly, if the client enforces DNSSEC validation on A/AAAA 8. SVCB/HTTPSSVC parameter for ESNI keys
responses, it SHOULD abandon the connection attempt if the HTTPSSVC
response fails to validate.
8. Alt-Svc and SVCB/HTTPSSVC parameter for ESNI keys The SVCB "esniconfig" parameter is defined for conveying the ESNI
configuration of an alternative service. The value of the parameter
is an ESNIConfig structure [ESNI]. In presentation format, the
structure is encoded in [base64]. The SVCB SvcParamValue wire format
is the octet string containing the binary ESNIConfig structure.
Both SVCB/HTTPSSVC and Alt-Svc "esniconfig" parameters are defined 8.1. Client behavior
for conveying the ESNI configuration of an alternative service. The
value of the parameter is an ESNIConfig structure [ESNI] or the empty
string. ESNI-aware clients SHOULD prefer alt-values and SVCB/
HTTPSSVC RRs with non-empty esniconfig.
Both the SVCB SvcParamValue presentation format as well as the Alt- The general client behavior specified in Section 3 permits clients to
Svc parameter value is the ESNIConfig structure [ESNI] encoded in retry connection with a less preferred alternative if the preferred
[base64] or the empty string. The SVCB SvcParamValue wire format is option fails, including falling back to a direct connection if all
the octet string containing the binary ESNIConfig structure. SVCB options fail. This behavior is not suitable for ESNI, because
fallback would negate the privacy benefits of ESNI. Accordingly,
ESNI-capable clients SHALL implement the following behavior for
connection establishment.
This parameter MAY also be sent in Alt-Svc HTTP response headers and 1. Perform connection establishment using HTTPSSVC as described in
HTTP/2 ALTSVC frames. This parameter MUST NOT appear more than once Section 3, but do not fall back to the origin's A/AAAA records.
in a single alt-value. If all the HTTPSSVC RRs have esniconfig, and they all fail,
terminate connection establishment.
8.1. Handling a mixture of alternatives not supporting ESNI 2. If the client implements Alt-Svc, try to connect using any
entries from the Alt-Svc cache.
The Alt-Svc specification states that "the client MAY fall back to 3. Fall back to the origin's A/AAAA records if necessary.
using the origin" in case of connection failure (Section 2.4 of
[AltSvc]). This behavior is not suitable for ESNI, because fallback
would negate the privacy benefits of ESNI.
Accordingly, any connection attempt that uses ESNI MUST fall back As a latency optimization, clients MAY prefetch DNS records for later
only to another alt-value that also has the esniconfig parameter. If steps before they are needed.
the parameter's value is the empty string, the client SHOULD connect
as it would in the absence of any ESNIConfig information.
For example, suppose a server operator has two alternatives. 8.2. Deployment considerations
Alternative A is reliably accessible but does not support ESNI.
Alternative B supports ESNI but is not reliably accessible. The
server operator could include a full esniconfig value in Alternative
B, and mark Alternative A with esniconfig="" to indicate that
fallback from B to A is allowed.
Other clients and services implementing SVCB or HTTPSSVC with An HTTPSSVC RRSet containing some RRs with esniconfig and some
esniconfig are encouraged to take a similar approach. 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 esniconfig as more preferred (i.e. smaller
SvcFieldPriority) than those without, in order to maximize the
likelihood that ESNI will be used in the absence of an active
adversary.
9. Interaction with other standards 9. Interaction with other standards
This standard is intended to reduce connection latency and improve This standard is intended to reduce connection latency and improve
user privacy. Server operators implementing this standard SHOULD user privacy. Server operators implementing this standard SHOULD
also implement TLS 1.3 [RFC8446] and OCSP Stapling [RFC6066], both of also implement TLS 1.3 [RFC8446] and OCSP Stapling [RFC6066], both of
which confer substantial performance and privacy benefits when used which confer substantial performance and privacy benefits when used
in combination with SVCB records. in combination with SVCB records.
To realize the greatest privacy benefits, this proposal is intended To realize the greatest privacy benefits, this proposal is intended
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Any specification for use of SVCB with a protocol MUST have an entry 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 for its scheme under the SVCB RR type in the IANA DNS Underscore
Global Scoped Entry Registry [Attrleaf]. The scheme SHOULD have an Global Scoped Entry Registry [Attrleaf]. The scheme SHOULD have an
entry in the IANA URI Schemes Registry [RFC7595]. The scheme SHOULD entry in the IANA URI Schemes Registry [RFC7595]. The scheme SHOULD
have a defined specification for use with SVCB. have a defined specification for use with SVCB.
10. Security Considerations 10. Security Considerations
SVCB/HTTPSSVC RRs are intended for distribution over untrusted SVCB/HTTPSSVC RRs are intended for distribution over untrusted
channels, and clients are REQUIRED to verify that the alternative channels, and clients are REQUIRED to verify that the alternative
service is authoritative for the origin (Section 2.1 of [AltSvc]). service is authoritative for the origin (similar to Section 2.1 of
Therefore, DNSSEC signing and validation are OPTIONAL for publishing [AltSvc]). Therefore, DNSSEC signing and validation are OPTIONAL for
and using SVCB and HTTPSSVC records. publishing and using SVCB and HTTPSSVC records.
Clients MUST ensure that their DNS cache is partitioned for each Clients MUST ensure that their DNS cache is partitioned for each
local network, or flushed on network changes, to prevent a local local network, or flushed on network changes, to prevent a local
adversary in one network from implanting a forged DNS record that adversary in one network from implanting a forged DNS record that
allows them to track users or hinder their connections after they allows them to track users or hinder their connections after they
leave that network. leave that network.
11. IANA Considerations 11. IANA Considerations
11.1. New registry for Service Parameters 11.1. New registry for Service Parameters
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Values to be added to this name space require Expert Review (see Values to be added to this name space require Expert Review (see
[RFC5226], Section 4.1). Apart from the initial contents, the name [RFC5226], Section 4.1). Apart from the initial contents, the name
MUST NOT start with "key". MUST NOT start with "key".
11.1.2. Initial contents 11.1.2. Initial contents
The "Service Binding (SVCB) Parameter Registry" shall initially be The "Service Binding (SVCB) Parameter Registry" shall initially be
populated with the registrations below: populated with the registrations below:
+-------------+------------+------------------------+---------------+ +-------------+-----------------+----------------------+------------+
| SvcParamKey | NAME | Meaning | Reference | | SvcParamKey | NAME | Meaning | Reference |
+-------------+------------+------------------------+---------------+ +-------------+-----------------+----------------------+------------+
| 0 | key0 | Reserved | (This | | 0 | (no name) | Reserved for | (This |
| | | | document) | | | | internal use | document) |
| | | | | | | | | |
| 1 | alpn | ALPN for alternative | (This | | 1 | alpn | Additional supported | (This |
| | | service | document) | | | | protocols | document) |
| | | | | | | | | |
| 2 | port | Port for alternative | (This | | 2 | no-default-alpn | No support for | (This |
| | | service | document) | | | | default protocol | document) |
| | | | | | | | | |
| 3 | esniconfig | Encrypted SNI | (This | | 3 | port | Port for alternative | (This |
| | | configuration | document) | | | | service | document) |
| | | | | | | | | |
| 4 | ipv4hint | IPv4 address hints | (This | | 4 | ipv4hint | IPv4 address hints | (This |
| | | | document) | | | | | document) |
| | | | | | | | | |
| 5 | key5 | Reserved | (This | | 5 | esniconfig | Encrypted SNI | (This |
| | | | document) | | | | configuration | document) |
| | | | | | | | | |
| 6 | ipv6hint | IPv6 address hints | (This | | 6 | ipv6hint | IPv6 address hints | (This |
| | | | document) | | | | | document) |
| | | | | | | | | |
| 65280-65534 | keyNNNNN | Private Use | (This | | 65280-65534 | keyNNNNN | Private Use | (This |
| | | | document) | | | | | document) |
| | | | | | | | | |
| 65535 | key65535 | Reserved | (This | | 65535 | key65535 | Reserved | (This |
| | | | document) | | | | | document) |
+-------------+------------+------------------------+---------------+ +-------------+-----------------+----------------------+------------+
TODO: do we also want to reserve a range for greasing? TODO: do we also want to reserve a range for greasing?
11.2. Registry updates 11.2. Registry updates
Per [RFC6895], please add the following entry to the data type range Per [RFC6895], please add the following entry to the data type range
of the Resource Record (RR) TYPEs registry: of the Resource Record (RR) TYPEs registry:
+----------+----------------------------------------+---------------+ +----------+----------------------------------------+---------------+
| TYPE | Meaning | Reference | | TYPE | Meaning | Reference |
+----------+----------------------------------------+---------------+ +----------+----------------------------------------+---------------+
| SVCB | Service Location and Parameter Binding | (This | | SVCB | Service Location and Parameter Binding | (This |
| | | document) | | | | document) |
| | | | | | | |
| HTTPSSVC | HTTPS Service Location and Parameter | (This | | HTTPSSVC | HTTPS Service Location and Parameter | (This |
| | Binding | document) | | | Binding | document) |
+----------+----------------------------------------+---------------+ +----------+----------------------------------------+---------------+
Per [Attrleaf], please add the following entries to the DNS
Underscore Global Scoped Entry Registry:
+----------+------------+-------------------+-----------------+ Per [Attrleaf], please add the following entry to the DNS Underscore
| RR TYPE | _NODE NAME | Meaning | Reference | Global Scoped Entry Registry:
+----------+------------+-------------------+-----------------+
| HTTPSSVC | _https | Alt-Svc for HTTPS | (This document) |
| | | | |
| HTTPSSVC | _http | Alt-Svc for HTTPS | (This document) |
+----------+------------+-------------------+-----------------+
Per [AltSvc], please add the following entry to the HTTP Alt-Svc
Parameter Registry:
+-------------------+-----------------------------+-----------------+ +----------+------------+-----------------+-----------------+
| Alt-Svc Parameter | Meaning | Reference | | RR TYPE | _NODE NAME | Meaning | Reference |
+-------------------+-----------------------------+-----------------+ +----------+------------+-----------------+-----------------+
| esniconfig | Encrypted SNI configuration | (This document) | | HTTPSSVC | _https | HTTPS SVCB info | (This document) |
+-------------------+-----------------------------+-----------------+ +----------+------------+-----------------+-----------------+
12. Acknowledgments and Related Proposals 12. Acknowledgments and Related Proposals
There have been a wide range of proposed solutions over the years to There have been a wide range of proposed solutions over the years to
the "CNAME at the Zone Apex" challenge proposed. These include the "CNAME at the Zone Apex" challenge proposed. These include
[I-D.draft-bellis-dnsop-http-record-00], [I-D.draft-bellis-dnsop-http-record-00],
[I-D.draft-ietf-dnsop-aname-03], and others. [I-D.draft-ietf-dnsop-aname-03], and others.
Thank you to Ian Swett, Ralf Weber, Jon Reed, Martin Thompson, Lucas Thank you to Ian Swett, Ralf Weber, Jon Reed, Martin Thomson, Lucas
Pardue, Ilari Liusvaara, Tim Wicinski, Tommy Pauly, Chris Wood, and Pardue, Ilari Liusvaara, Tim Wicinski, Tommy Pauly, Chris Wood, David
others for their feedback and suggestions on this draft. Benjamin, and others for their feedback and suggestions on this
draft.
13. References 13. References
13.1. Normative References 13.1. Normative References
[AltSvc] Nottingham, M., McManus, P., and J. Reschke, "HTTP [ALPN] Friedl, S., Popov, A., Langley, A., and E. Stephan,
Alternative Services", RFC 7838, DOI 10.17487/RFC7838, "Transport Layer Security (TLS) Application-Layer Protocol
April 2016, <https://www.rfc-editor.org/info/rfc7838>. Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
July 2014, <https://www.rfc-editor.org/info/rfc7301>.
[AltSvcSNI]
Bishop, M., "The "SNI" Alt-Svc Parameter", draft-bishop-
httpbis-sni-altsvc-02 (work in progress), May 2018.
[Attrleaf] [Attrleaf]
Crocker, D., "DNS Scoped Data Through "Underscore" Naming Crocker, D., "DNS Scoped Data Through "Underscore" Naming
of Attribute Leaves", draft-ietf-dnsop-attrleaf-16 (work of Attribute Leaves", draft-ietf-dnsop-attrleaf-16 (work
in progress), November 2018. in progress), November 2018.
[base64] Josefsson, S., "The Base16, Base32, and Base64 Data [base64] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006, Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>. <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>.
[ESNI] Rescorla, E., Oku, K., Sullivan, N., and C. Wood, [ESNI] Rescorla, E., Oku, K., Sullivan, N., and C. Wood,
"Encrypted Server Name Indication for TLS 1.3", draft- "Encrypted Server Name Indication for TLS 1.3", draft-
ietf-tls-esni-04 (work in progress), July 2019. ietf-tls-esni-05 (work in progress), November 2019.
[HappyEyeballsV2] [HappyEyeballsV2]
Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2: Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
Better Connectivity Using Concurrency", RFC 8305, Better Connectivity Using Concurrency", RFC 8305,
DOI 10.17487/RFC8305, December 2017, DOI 10.17487/RFC8305, December 2017,
<https://www.rfc-editor.org/info/rfc8305>. <https://www.rfc-editor.org/info/rfc8305>.
[HSTS] Hodges, J., Jackson, C., and A. Barth, "HTTP Strict [HSTS] Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
Transport Security (HSTS)", RFC 6797, Transport Security (HSTS)", RFC 6797,
DOI 10.17487/RFC6797, November 2012, DOI 10.17487/RFC6797, November 2012,
skipping to change at page 26, line 42 skipping to change at page 30, line 42
[RFC1928] Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D., and [RFC1928] Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D., and
L. Jones, "SOCKS Protocol Version 5", RFC 1928, L. Jones, "SOCKS Protocol Version 5", RFC 1928,
DOI 10.17487/RFC1928, March 1996, DOI 10.17487/RFC1928, March 1996,
<https://www.rfc-editor.org/info/rfc1928>. <https://www.rfc-editor.org/info/rfc1928>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,
<https://www.rfc-editor.org/info/rfc2181>.
[RFC3225] Conrad, D., "Indicating Resolver Support of DNSSEC", [RFC3225] Conrad, D., "Indicating Resolver Support of DNSSEC",
RFC 3225, DOI 10.17487/RFC3225, December 2001, RFC 3225, DOI 10.17487/RFC3225, December 2001,
<https://www.rfc-editor.org/info/rfc3225>. <https://www.rfc-editor.org/info/rfc3225>.
[RFC3597] Gustafsson, A., "Handling of Unknown DNS Resource Record [RFC3597] Gustafsson, A., "Handling of Unknown DNS Resource Record
(RR) Types", RFC 3597, DOI 10.17487/RFC3597, September (RR) Types", RFC 3597, DOI 10.17487/RFC3597, September
2003, <https://www.rfc-editor.org/info/rfc3597>. 2003, <https://www.rfc-editor.org/info/rfc3597>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", RFC 5226, IANA Considerations Section in RFCs", RFC 5226,
skipping to change at page 27, line 50 skipping to change at page 32, line 5
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231, Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014, DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>. <https://www.rfc-editor.org/info/rfc7231>.
[RFC7595] Thaler, D., Ed., Hansen, T., and T. Hardie, "Guidelines [RFC7595] Thaler, D., Ed., Hansen, T., and T. Hardie, "Guidelines
and Registration Procedures for URI Schemes", BCP 35, and Registration Procedures for URI Schemes", BCP 35,
RFC 7595, DOI 10.17487/RFC7595, June 2015, RFC 7595, DOI 10.17487/RFC7595, June 2015,
<https://www.rfc-editor.org/info/rfc7595>. <https://www.rfc-editor.org/info/rfc7595>.
[RFC7838] 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>.
[RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D., [RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "Specification for DNS over Transport and P. Hoffman, "Specification for DNS over Transport
Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
2016, <https://www.rfc-editor.org/info/rfc7858>. 2016, <https://www.rfc-editor.org/info/rfc7858>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>. <https://www.rfc-editor.org/info/rfc8446>.
[RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS [RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS
(DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018, (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
<https://www.rfc-editor.org/info/rfc8484>. <https://www.rfc-editor.org/info/rfc8484>.
13.2. Informative References 13.2. 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 [DNSTerm] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499, Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
January 2019, <https://www.rfc-editor.org/info/rfc8499>. January 2019, <https://www.rfc-editor.org/info/rfc8499>.
[HTTP] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
[I-D.draft-bellis-dnsop-http-record-00] [I-D.draft-bellis-dnsop-http-record-00]
Bellis, R., "A DNS Resource Record for HTTP", draft- Bellis, R., "A DNS Resource Record for HTTP", draft-
bellis-dnsop-http-record-00 (work in progress), November bellis-dnsop-http-record-00 (work in progress), November
2018. 2018.
[I-D.draft-ietf-dnsop-aname-03] [I-D.draft-ietf-dnsop-aname-03]
Finch, T., Hunt, E., Dijk, P., Eden, A., and W. Mekking, Finch, T., Hunt, E., Dijk, P., Eden, A., and W. Mekking,
"Address-specific DNS aliases (ANAME)", draft-ietf-dnsop- "Address-specific DNS aliases (ANAME)", draft-ietf-dnsop-
aname-03 (work in progress), April 2019. aname-03 (work in progress), April 2019.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782, specifying the location of services (DNS SRV)", RFC 2782,
DOI 10.17487/RFC2782, February 2000, DOI 10.17487/RFC2782, February 2000,
<https://www.rfc-editor.org/info/rfc2782>. <https://www.rfc-editor.org/info/rfc2782>.
[RFC6895] Eastlake 3rd, D., "Domain Name System (DNS) IANA [RFC6895] Eastlake 3rd, D., "Domain Name System (DNS) IANA
Considerations", BCP 42, RFC 6895, DOI 10.17487/RFC6895, Considerations", BCP 42, RFC 6895, DOI 10.17487/RFC6895,
April 2013, <https://www.rfc-editor.org/info/rfc6895>. April 2013, <https://www.rfc-editor.org/info/rfc6895>.
Appendix A. Mapping between HTTPSSVC and Alt-Svc 13.3. URIs
Conversion between HTTPSSVC's ServiceForm and Alt-Svc is possible.
Note that conversion in either direction can be lossy, because some
parameters are only defined for HTTPSSVC or Alt-Svc.
To construct an Alt-Svc Field Value (as defined in Section 4 of
[AltSvc]) from an HTTPSSVC record:
o The SvcDomainName is mapped into the uri-host portion of alt-
authority with the trailing "." removed. (If SvcDomainName is
".", the special handling described in Section 2.6.1 MUST be
applied first.)
o The SvcParamValue of the "port" service parameter, or 443 if no
such parameter is present, is written to the port portion of the
alt-authority.
o The SvcParamValue of the "alpn" service parameter is mapped to the
protocol-id. This MUST follow the normalization and encoding
requirements for protocol-id specified in [AltSvc] Section 3.
This parameter is MANDATORY.
o The DNS TTL is mapped to the "ma" (max age) Alt-Svc parameter.
o For SVCB parameters with defined mappings to HTTPS Alt-Svc, each
should be included as an Alt-Svc parameter, typically as the
SvcParamKey name "=" a defined encoding of the SvcParamValue.
Converting an Alt-Svc Field Value into an HTTPSSVC record follows the
reverse of this procedure.
Conversion between HTTPSSVC and Alt-Svc Field Value MUST ignore any
SvcParamKeys and Alt-Svc parameters that are unrecognized or do not
have a defined mapping.
For example, if the operator of https://www.example.com intends to
include an HTTP response header like
Alt-Svc: h3="svc.example.net:8003"; ma=3600; foo=123, \
h2="svc.example.net:8002"; ma=3600; foo=123
they could also publish an HTTPSSVC DNS RRSet like
www.example.com. 3600 IN HTTPSSVC 2 svc.example.net. (
alpn=h3 port=8003 foo=123 )
HTTPSSVC 3 svc.example.net. (
alpn=h2 port=8002 foo=123 )
Where "foo" is a hypothetical future HTTPSSVC and Alt-Svc parameter.
This data type can also be represented as an Unknown RR as described
in [RFC3597]:
www.example.com. 3600 IN TYPE??? \\# TBD:WRITEME
A.1. Multiple records and preference ordering
Server operators MAY publish multiple ServiceForm HTTPSSVC records as
an RRSet. When converting a collection of alt-values into an
HTTPSSVC RRSet, the server operator MUST set the overall TTL to a
value no larger than the minimum of the "max age" values (following
Section 5.2 of [RFC2181]).
Each RR corresponds to exactly one alt-value, as described in
Section 3 of [AltSvc].
As discussed in Section 2.6.2, HTTPSSVC RRs with a smaller
SvcFieldPriority value SHOULD be sorted ahead of and given preference
over RRs with a larger SvcFieldPriority value.
When constructing equivalent Alt-Svc headers from an RRSet:
1. The RRs SHOULD be ordered by increasing SvcFieldPriority, with
shuffling for equal SvcFieldPriority values. Clients MAY choose
to further prioritize alt-values where address records are
immediately available for the alt-value's SvcDomainName.
2. The client SHOULD concatenate the thus-transformed-and-ordered
SvcFieldValues in the RRSet, separated by commas. (This is
semantically equivalent to receiving multiple Alt-Svc HTTP
response headers, according to Section 3.2.2 of [HTTP]).
A.2. Additional examples
The following:
www.example.com. 7200 IN CNAME svc.example.net.
example.com. 7200 IN HTTPSSVC 0 svc.example.net.
svc.example.net. 7200 IN HTTPSSVC 2 svc3.example.net. (
alpn=h3 port=8003 esniconfig="ABC..." )
svc.example.net. 7200 IN HTTPSSVC 3 . (
alpn=h2 port=8002 esniconfig="123..." )
is equivalent to the Alt-Svc record:
Alt-Svc: h3="svc3.example.net:8003"; esniconfig="ABC..."; ma=7200, \
h2="svc.example.net:8002"; esniconfig="123..."; ma=7200
for the origins of both "https://www.example.com" and [1] https://github.com/MikeBishop/dns-alt-svc
"https://example.com".
Appendix B. Comparison with alternatives Appendix A. Comparison with alternatives
The SVCB and HTTPSSVC record types closely resemble, and are inspired The SVCB and HTTPSSVC record types closely resemble, and are inspired
by, some existing record types and proposals. A complaint with all by, some existing record types and proposals. A complaint with all
of the alternatives is that web clients have seemed unenthusiastic of the alternatives is that web clients have seemed unenthusiastic
about implementing them. The hope here is that by providing an about implementing them. The hope here is that by providing an
extensible solution that solves multiple problems we will overcome extensible solution that solves multiple problems we will overcome
the inertia and have a path to achieve client implementation. the inertia and have a path to achieve client implementation.
B.1. Differences from the SRV RR type A.1. Differences from the SRV RR type
An SRV record [RFC2782] can perform a similar function to the SVCB An SRV record [RFC2782] can perform a similar function to the SVCB
record, informing a client to look in a different location for a record, informing a client to look in a different location for a
service. However, there are several differences: service. However, there are several differences:
o SRV records are typically mandatory, whereas clients will always o SRV records are typically mandatory, whereas clients will always
continue to function correctly without making use of Alt-Svc or continue to function correctly without making use of SVCB.
SVCB.
o SRV records cannot instruct the client to switch or upgrade o SRV records cannot instruct the client to switch or upgrade
protocols, whereas Alt-Svc can signal such an upgrade (e.g. to protocols, whereas SVCB can signal such an upgrade (e.g. to
HTTP/2). HTTP/2).
o SRV records are not extensible, whereas SVCB and HTTPSSVC can be o SRV records are not extensible, whereas SVCB and HTTPSSVC can be
extended with new parameters. extended with new parameters.
o Using SRV records would not allow an HTTPS client to skip A.2. Differences from the proposed HTTP record
processing of the Alt-Svc information in a subsequent connection,
so it does not confer a performance advantage.
B.2. Differences from the proposed HTTP record
Unlike [I-D.draft-bellis-dnsop-http-record-00], this approach is Unlike [I-D.draft-bellis-dnsop-http-record-00], this approach is
extensible to cover Alt-Svc and ESNI use-cases. Like that proposal, extensible to cover Alt-Svc and ESNI use-cases. Like that proposal,
this addresses the zone apex CNAME challenge. this addresses the zone apex CNAME challenge.
Like that proposal it remains necessary to continue to include Like that proposal it remains necessary to continue to include
address records at the zone apex for legacy clients. address records at the zone apex for legacy clients.
B.3. Differences from the proposed ANAME record A.3. Differences from the proposed ANAME record
Unlike [I-D.draft-ietf-dnsop-aname-03], this approach is extensible Unlike [I-D.draft-ietf-dnsop-aname-03], this approach is extensible
to cover Alt-Svc and ESNI use-cases. This approach also does not to cover Alt-Svc and ESNI use-cases. This approach also does not
require any changes or special handling on either authoritative or require any changes or special handling on either authoritative or
master servers, beyond optionally returning in-bailiwick additional master servers, beyond optionally returning in-bailiwick additional
records. records.
Like that proposal, this addresses the zone apex CNAME challenge for Like that proposal, this addresses the zone apex CNAME challenge for
clients that implement this. clients that implement this.
However with this SVCB proposal it remains necessary to continue to However with this SVCB proposal it remains necessary to continue to
include address records at the zone apex for legacy clients. If include address records at the zone apex for legacy clients. If
deployment of this standard is successful, the number of legacy deployment of this standard is successful, the number of legacy
clients will fall over time. As the number of legacy clients clients will fall over time. As the number of legacy clients
declines, the operational effort required to serve these users declines, the operational effort required to serve these users
without the benefit of SVCB indirection should fall. Server without the benefit of SVCB indirection should fall. Server
operators can easily observe how much traffic reaches this legacy operators can easily observe how much traffic reaches this legacy
endpoint, and may remove the apex's address records if the observed endpoint, and may remove the apex's address records if the observed
legacy traffic has fallen to negligible levels. legacy traffic has fallen to negligible levels.
B.4. Differences from the proposed ESNI record A.4. Differences from the proposed ESNI record
Unlike [ESNI], this approach is extensible and covers the Alt-Svc Unlike [ESNI], this approach is extensible and covers the Alt-Svc
case as well as addresses the zone apex CNAME challenge. case as well as addresses the zone apex CNAME challenge.
By using the Alt-Svc model we also provide a way to solve the ESNI By mirroring the Alt-Svc model we also provide a way to solve the
multi-CDN challenges in a general case. ESNI multi-CDN challenges in a general case.
Unlike ESNI, SVCB allows specifying different ESNI configurations for Unlike ESNI, SVCB allows specifying different ESNI configurations for
different protocols and ports, rather than applying a single different protocols and ports, rather than applying a single
configuration to all ports on a domain. configuration to all ports on a domain.
B.5. SNI Alt-Svc parameter Appendix B. Design Considerations and Open Issues
Defining an Alt-Svc sni= parameter (such as from [AltSvcSNI]) would
have provided some benefits to clients and servers not implementing
ESNI, such as for specifying that "_wildcard.example.com" could be
sent as an SNI value rather than the full name. There is nothing
precluding SVCB from being used with an sni= parameter if one were to
be defined, but it is not included here to reduce scope, complexity,
and additional potential security and tracking risks.
Appendix C. Design Considerations and Open Issues
This draft is intended to be a work-in-progress for discussion. Many This draft is intended to be a work-in-progress for discussion. Many
details are expected to change with subsequent refinement. Some details are expected to change with subsequent refinement. Some
known issues or topics for discussion are listed below. known issues or topics for discussion are listed below.
C.1. Record Name B.1. Record Name
Naming is hard. "SVCB" and "HTTPSSVC" are proposed as placeholders Naming is hard. "SVCB" and "HTTPSSVC" are proposed as placeholders
that are easy to search for and replace when a final name is chosen. that are easy to search for and replace when a final name is chosen.
Other names for this record might include B, ALTSVC, HTTPS, HTTPSSRV, Other names for this record might include B, ALTSVC, HTTPS, HTTPSSRV,
HTTPSSVC, SVCHTTPS, or something else. HTTPSSVC, SVCHTTPS, or something else.
C.2. Generality B.2. Generality
The SVCB record was designed as a generalization of HTTPSSVC, based The SVCB record was designed as a generalization of HTTPSSVC, based
on feedback requesting a solution that applied to protocols pther on feedback requesting a solution that applied to protocols pther
than HTTP. Past efforts to over-generalize have not met with broad than HTTP. Past efforts to over-generalize have not met with broad
success, but we hope that HTTPSSVC and SVCB have struck an acceptable success, but we hope that HTTPSSVC and SVCB have struck an acceptable
balance between generality and focus. balance between generality and focus.
C.3. Wire Format B.3. Wire Format
Advice from experts in DNS wire format best practices would be Advice from experts in DNS wire format best practices would be
greatly appreciated to refine the proposed details, overall. greatly appreciated to refine the proposed details, overall.
C.4. Where to include Priority B.4. Whether to include Weight
The SvcFieldPriority could alternately be included as a pri= Alt-Svc
attribute. It wouldn't be applicable for Alt-Svc returned via HTTP,
but it is also not necessarily needed by DNS servers. It is also not
used for AliasForm RRs.
C.5. Whether to include Weight
Some other similar mechanisms such as SRV have a weight in-addition Some other similar mechanisms such as SRV have a weight in-addition
to priority. That is excluded here for simplicity. It could always to priority. That is excluded here for simplicity. It could always
be added as an optional SVCB parameter. be added as an optional SVCB parameter.
Appendix D. Change history Appendix C. Change history
o draft-ietf-dnsop-svcb-httpssvc-01 o draft-ietf-dnsop-svcb-httpssvc-01
* Reduce the emphasis on conversion between HTTPSSVC and Alt-Svc * Reduce the emphasis on conversion between HTTPSSVC and Alt-Svc
* Make the "untrusted channel" concept more precise. * Make the "untrusted channel" concept more precise.
* Make SvcFieldPriority = 0 the definition of AliasForm, instead * Make SvcFieldPriority = 0 the definition of AliasForm, instead
of a requirement. of a requirement.
 End of changes. 119 change blocks. 
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