Internet DRAFT - draft-nygren-dnsop-svcb-httpssvc
draft-nygren-dnsop-svcb-httpssvc
DNSOP Working Group B. Schwartz
Internet-Draft Google
Intended status: Standards Track M. Bishop
Expires: March 25, 2020 E. Nygren
Akamai Technologies
September 22, 2019
Service binding and parameter specification via the DNS (DNS SVCB and
HTTPSSVC)
draft-nygren-dnsop-svcb-httpssvc-00
Abstract
This document specifies the "SVCB" and "HTTPSSVC" DNS resource record
types to facilitate the lookup of information needed to make
connections for origin resources, such as for HTTPS URLs. SVCB
records allow an origin to be served from multiple network locations,
each with associated parameters (such as transport protocol
configuration and keying material for encrypting TLS SNI). They also
enable aliasing of apex domains, which is not possible with CNAME.
The HTTPSSVC DNS RR is a variation of SVCB for HTTPS and HTTP
origins. By providing more information to the client before it
attempts to establish a connection, these records offer potential
benefits to both performance and privacy.
TO BE REMOVED: This proposal is inspired by and based on recent DNS
usage proposals such as ALTSVC, ANAME, and ESNIKEYS (as well as long
standing desires to have SRV or a functional equivalent implemented
for HTTP). These proposals each provide an important function but
are potentially incompatible with each other, such as when an origin
is load-balanced across multiple hosting providers (multi-CDN).
Furthermore, these each add potential cases for adding additional
record lookups in-addition to AAAA/A lookups. This design attempts
to provide a unified framework that encompasses the key functionality
of these proposals, as well as providing some extensibility for
addressing similar future challenges.
TO BE REMOVED: The specific name for this RR type is an open topic
for discussion. "SVCB" and "HTTPSSVC" are meant as placeholders as
they are easy to replace. Other names might include "B", "SRV2",
"SVCHTTPS", "HTTPS", and "ALTSVC".
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Introductory Example . . . . . . . . . . . . . . . . . . 5
1.2. Goals of the SVCB RR . . . . . . . . . . . . . . . . . . 6
1.3. Overview of the SVCB RR . . . . . . . . . . . . . . . . . 7
1.4. Parameter for ESNI . . . . . . . . . . . . . . . . . . . 8
1.5. Terminology . . . . . . . . . . . . . . . . . . . . . . . 8
2. The SVCB record type . . . . . . . . . . . . . . . . . . . . 8
2.1. Parameter specification via ServiceFieldValue . . . . . . 9
2.1.1. Presentation format . . . . . . . . . . . . . . . . . 9
2.2. SVCB RDATA Wire Format . . . . . . . . . . . . . . . . . 10
2.3. SVCB owner names . . . . . . . . . . . . . . . . . . . . 10
2.4. SvcRecordType . . . . . . . . . . . . . . . . . . . . . . 11
2.5. SVCB records: AliasForm . . . . . . . . . . . . . . . . . 11
2.6. SVCB records: ServiceForm . . . . . . . . . . . . . . . . 12
2.6.1. Special handling of "." for SvcDomainName in
ServiceForm . . . . . . . . . . . . . . . . . . . . . 12
2.6.2. SvcFieldPriority . . . . . . . . . . . . . . . . . . 13
3. Client behavior . . . . . . . . . . . . . . . . . . . . . . . 13
3.1. Clients using a Proxy . . . . . . . . . . . . . . . . . . 14
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4. DNS Server Behavior . . . . . . . . . . . . . . . . . . . . . 14
5. Performance optimizations . . . . . . . . . . . . . . . . . . 15
6. Initial SvcParamKeys . . . . . . . . . . . . . . . . . . . . 15
6.1. "alpn" . . . . . . . . . . . . . . . . . . . . . . . . . 16
6.2. "port" . . . . . . . . . . . . . . . . . . . . . . . . . 16
6.3. "esnikeys" . . . . . . . . . . . . . . . . . . . . . . . 16
6.4. "ipv4hint" and "ipv6hint" . . . . . . . . . . . . . . . . 16
7. Using SVCB with HTTPS and HTTP . . . . . . . . . . . . . . . 17
7.1. Owner names for HTTPSSVC records . . . . . . . . . . . . 18
7.2. Mapping between ServiceForm and Alt-Svc . . . . . . . . . 18
7.3. Differences from Alt-Svc as transmitted over HTTP . . . . 19
7.3.1. Max Age and Persist . . . . . . . . . . . . . . . . . 19
7.4. Multiple records and preference ordering . . . . . . . . 20
7.4.1. Constructing Alt-Svc equivalent headers . . . . . . . 20
7.5. Granularity and lifetime control . . . . . . . . . . . . 20
7.6. HTTP Strict Transport Security . . . . . . . . . . . . . 20
7.7. Cache interaction . . . . . . . . . . . . . . . . . . . . 21
8. Extensions to enhance privacy . . . . . . . . . . . . . . . . 22
8.1. Alt-Svc and SVCB/HTTPSSVC parameter for ESNI keys . . . . 22
8.1.1. Handling a mixture of alternatives not supporting
esnikeys . . . . . . . . . . . . . . . . . . . . . . 22
9. Interaction with other standards . . . . . . . . . . . . . . 22
10. Security Considerations . . . . . . . . . . . . . . . . . . . 23
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
11.1. New registry for Service Parameters . . . . . . . . . . 23
11.1.1. Procedure . . . . . . . . . . . . . . . . . . . . . 23
11.1.2. Initial contents . . . . . . . . . . . . . . . . . . 24
11.2. Registry updates . . . . . . . . . . . . . . . . . . . . 25
12. Acknowledgments and Related Proposals . . . . . . . . . . . . 25
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 26
13.1. Normative References . . . . . . . . . . . . . . . . . . 26
13.2. Informative References . . . . . . . . . . . . . . . . . 28
Appendix A. Additional examples . . . . . . . . . . . . . . . . 29
A.1. Equivalence to Alt-Svc records . . . . . . . . . . . . . 29
Appendix B. Comparison with alternatives . . . . . . . . . . . . 29
B.1. Differences from the SRV RR type . . . . . . . . . . . . 29
B.2. Differences from the proposed HTTP record . . . . . . . . 30
B.3. Differences from the proposed ANAME record . . . . . . . 30
B.4. Differences from the proposed ESNI record . . . . . . . . 30
B.5. SNI Alt-Svc parameter . . . . . . . . . . . . . . . . . . 31
Appendix C. Design Considerations and Open Issues . . . . . . . 31
C.1. Record Name . . . . . . . . . . . . . . . . . . . . . . . 31
C.2. Generality . . . . . . . . . . . . . . . . . . . . . . . 31
C.3. Wire Format . . . . . . . . . . . . . . . . . . . . . . . 31
C.4. Where to include Priority . . . . . . . . . . . . . . . . 31
C.5. Whether to include Weight . . . . . . . . . . . . . . . . 32
Appendix D. Change history . . . . . . . . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 33
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1. Introduction
The SVCB and HTTPSSVC RRs provide clients with complete instructions
for access to an origin. This information enables improved
performance and privacy by avoiding transient connections to a sub-
optimal default server, negotiating a preferred protocol, and
providing relevant public keys.
For example, when clients need to make a connection to fetch
resources associated with an HTTPS URI, they currently resolve only A
and/or AAAA records for the origin hostname. This is adequate for
services that use basic HTTPS (fixed port, no QUIC, no [ESNI]).
Going beyond basic HTTPS confers privacy, performance, and
operational advantages, but it requires the client to learn
additional information, and it is highly desirable to minimize the
number of round-trip and lookups required to learn this additional
information.
The SVCB and HTTPSSVC RRs also help when the operator of an origin
wishes to delegate operational control to one or more other domains,
e.g. delegating the origin resource "https://example.com" to a
service operator endpoint at "svc.example.net". While this case can
sometimes be handled by a CNAME, that does not cover all use-cases.
CNAME is also inadequate when the service operator needs to provide a
bound collection of consistent configuration parameters through the
DNS (such as network location, protocol, and keying information).
This document first describes the SVCB RR as a general-purpose
resource record that can be applied directly and efficiently to a
wide range of services. As HTTPS is a primary use-case and has
special requirements, the HTTPSSVC RR is also defined within this
document as a special case of SVCB. Services wishing to avoid the
need for an [Attrleaf] label with SVCB may follow 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
HTTPSSVC RR unless explicitly stated otherwise. Section 7 describes
additional behaviors specific to the HTTPSSVC record. Apart from
Section 7 and introductory examples, much of this document refers
only to the SVCB RR, but those references should be taken to apply to
SVCB, HTTPSSVC, and any future SVCB-compatible RR types.
The SVCB RR has two forms: 1) the "Alias Form" simply delegates
operational control for a resource; 2) the "Service Form" binds
together configuration information for a service endpoint. The
Service Form provides additional key=value parameters within each
RDATA set.
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TO BE REMOVED: If we use this for providing configuration for DNS
authorities, it is likely we'd specify a distinct "NS2" RR type that
is an instantiation of SVCB for authoritative nameserver delegation
and parameter specification, similar to HTTPSSVC.
TO BE REMOVED: Another open question is whether SVCB records should
be self-descriptive and include the service name (eg, "https") in the
RDATA section to avoid ambiguity. Perhaps this could be included as
a svc="baz" parameter for protocols that are not the default for the
RR type? Current inclination is to not do so.
1.1. Introductory Example
As an introductory example for an HTTPS origin resource, a set of
example HTTPSSVC and associated A+AAAA records might be:
www.example.com. 7200 IN CNAME svc.example.net.
; AliasForm
example.com. 7200 IN HTTPSSVC 0 svc.example.net.
; ServiceForm
svc.example.net. 7200 IN HTTPSSVC 2 svc3.example.net. ( alpn=h3
port=8003 esnikeys="..." )
svc.example.net. 7200 IN HTTPSSVC 3 svc2.example.net. ( alpn=h2
port=8002 esnikeys="..." )
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
"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-
default ports), the SVCB RR and an [Attrleaf] label will be used.
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For example, to reach an example resource of
"baz://api.example.com:8765", the following Alias Form SVCB record
would be used to delegate to "svc4-baz.example.net." which in-turn
could return AAAA/A records and/or SVCB records in ServiceForm.
_8765._baz.api.example.com. 7200 IN SVCB 0 svc4-baz.example.net.
1.2. Goals of the SVCB RR
The goal of the SVCB RR is to allow clients to resolve a single
additional DNS RR in a way that:
o Provides service endpoints authoritative for the service, along
with parameters associated with each of these endpoints.
o Does not assume that all alternative service endpoints have the
same parameters or capabilities, or are even operated by the same
entity. This is important as DNS does not provide any way to tie
together multiple RRs for the same name. For example, if
www.example.com is a CNAME alias that switches between one of
three CDNs or hosting environments, successive queries for that
name may return records that correspond to different environments.
o Enables CNAME-like functionality at a zone apex (such as
"example.com") for participating protocols, and generally enables
delegation of operational authority for an origin within the DNS
to an alternate name.
Additional goals specific to HTTPSSVC and the HTTPS use-case include:
o Connect directly to [HTTP3] (QUIC transport) alternative service
endpoints
o Obtain the [ESNI] keys associated with an alternative service
endpoint
o Support non-default TCP and UDP ports
o Address a set of long-standing issues due to HTTP(S) clients not
implementing support for SRV records, as well as due to a
limitation that a DNS name can not have both CNAME and NS RRs (as
is the case for zone apex names)
o Provide an HSTS-like indication signaling for the duration of the
DNS RR TTL that the HTTPS scheme should be used instead of HTTP
(see Section 7.6).
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1.3. Overview of the SVCB RR
This subsection briefly describes the SVCB RR in a non-normative
manner. (As mentioned above, this all applies equally to the
HTTPSSVC RR which shares the same encoding, format, and high-level
semantics.)
The SVCB RR has two forms: AliasForm and ServiceForm. SVCB RR
entries with two non-empty fields are in AliasForm. When more fields
are present, this indicates that the SVCB RR is in ServiceForm. The
fields are:
1. SvcFieldPriority: The priority of this record (relative to
others, with lower values preferred). Applicable for the
ServiceForm, and otherwise has value "0". (Described in
Section 7.4.)
2. SvcDomainName: The domain name of either the alias target (for
AliasForm) or the alternative service endpoint (for ServiceForm).
3. SvcFieldValue: A list of key=value pairs describing the
alternative service endpoint for the domain name specified in
SvcDomainName (only for ServiceForm and otherwise empty).
Described in Section 2.1.
Cooperating DNS recursive resolvers will perform subsequent record
resolution (for SVCB, A, and AAAA records) and return them in the
Additional Section of the response. Clients must either use
responses included in the additional section returned by the
recursive resolver or perform necessary SVCB, A, and AAAA record
resolutions. DNS authoritative servers may attach in-bailiwick SVCB,
A, AAAA, and CNAME records in the Additional Section to responses for
an SVCB query.
When in the ServiceForm, the SvcFieldValue of the SVCB RR provides an
extensible data model for describing network endpoints that are
authoritative for the origin, along with parameters associated with
each of these endpoints.
For the HTTPS use-case with the HTTPSSVC RR, there is also direct
mapping from the SvcDomainName and SvcFieldValue into HTTP
Alternative Services (Alt-Svc) entries [AltSvc]. Encoding this
information here enables many of the benefits of Alt-Svc, without
waiting for a full HTTP connection initiation (multiple roundtrips)
before learning of the preferred alternative, and without necessarily
revealing the user's intended destination to all entities along the
network path.
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1.4. Parameter for ESNI
This document also defines a parameter for Encrypted SNI [ESNI] keys,
both as a general SVCB parameter and also as a corresponding Alt-Svc
parameter. See Section 8.1.
1.5. Terminology
For consistency with [AltSvc], we adopt the following definitions:
o An "origin" is an information source as in [RFC6454]. For
services other than HTTPS, the exact definition will need to be
provided by the document mapping that service onto the SVCB RR.
o The "origin server" is the server that the client would reach when
accessing the origin in the absence of the SVCB record or an HTTPS
Alt-Svc.
o An "alternative service" is a different server that can serve the
origin over a specified protocol.
For example within HTTPS, the origin consists of a scheme (typically
"https"), a host name, and a port (typically "443").
Additional DNS terminology intends to be consistent with [DNSTerm].
SVCB is a contraction of "service binding". HTTPSSVC is a
contraction of "HTTPS service". SVCB, HTTPSSVC, and future RR types
that share SVCB's format and registry are collectively known as SVCB-
compatible RR types.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. The SVCB record type
The SVCB DNS resource record (RR) type (RR type ???) is used to
locate endpoints that can service an origin. There is special
handling for the case of "https" origins. The presentation format of
the record is:
Name TTL IN SVCB SvcFieldPriority SvcDomainName SvcFieldValue
The SVCB record is defined specifically within the Internet ("IN")
Class ([RFC1035]). SvcFieldPriority is a number in the range
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0-65535, SvcDomainName is a domain name, and SvcFieldValue is a set
of key=value pairs present for the ServiceForm. The SvcFieldValue is
empty for the AliasForm.
The algorithm for resolving SVCB records and associated address
records is specified in Section 3.
2.1. Parameter specification via ServiceFieldValue
In ServiceForm, the SvcFieldValue contains key=value pairs. Keys are
IANA-registered SvcParamKeys (Section 11.1) with both a case-
insensitive string representation and a numeric representation in the
range 0-65535. Registered key names should only contain characters
from the ranges "a"-"z", "0"-"9", and "-". In ABNF [RFC5234],
ALPHA_LC = %x61-7A ; a-z
key = ALPHA_LC / DIGIT / "-"
display-key = ALPHA / DIGIT / "-"
Values are in a format specific to the SvcParamKey. Their definition
should specify both their presentation format and wire encoding
(e.g., domain names, binary data, or numeric values).
The 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
list of the key=value pairs. Each pair is presented in the following
form:
basic-visible = %x21 / %x23-5B / %x5D-7E ; VCHAR minus DQUOTE and "\"
escaped-char = "\" (VCHAR / WSP)
contiguous = *(basic-visible / escaped-char)
quoted-string = DQUOTE *(contiguous / WSP) DQUOTE
value = quoted-string / contiguous
pair = display-key "=" value
The value format is intended to match the definition of <character-
string> in [RFC1035] Section 5.1. (Unlike <character-string>, the
length of a value is not limited to 255 characters.)
Unrecognized keys are represented in presentation format as
"keyNNNNN" where NNNNN is the numeric value of the key type without
leading zeros. In presentation format, values of unrecognized keys
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should be represented in wire format, using decimal escape codes
(e.g. \255) when necessary.
2.2. SVCB RDATA Wire Format
The RDATA for the SVCB RR consists of:
o a 2 octet field for SvcFieldPriority as an integer in network byte
order. For AliasForm, SvcFieldPriority MUST be 0.
o the uncompressed SvcDomainName, represented as a sequence of
length-prefixed labels as in Section 3.1 of [RFC1035].
o the SvcFieldValue byte string, consuming the remainder of the
record (so smaller than 65535 octets and constrained by the RDATA
and DNS message sizes).
AliasForm is defined by SvcFieldValue being empty.
When SvcFieldValue is non-empty (ServiceForm), it contains a list of
SvcParamKey=SvcParamValue pairs with length-prefixes for the
SvcParamValues, each of which contains:
o a 2 octet field containing the SvcParamKey as an integer in
network byte order.
o a 2 octet field containing the length of the SvcParamValue as an
integer between 0 and 65535 in network byte order (but constrained
by the RDATA and DNS message sizes).
o an octet string of the length defined by the previous field.
If the parser reaches the end of the RDATA while parsing a
SvcFieldValue, the RR is invalid and MUST be discarded.
TODO: decide if we want special handling for any SvcParamKey ranges?
For example: range for greasing; experimental range; range-of-
mandatory-to-use-the-RR vs range of ignore-just-param-if-unknown.
2.3. SVCB owner names
When querying the SVCB RR, an origin is typically translated into a
QNAME by prefixing the port and scheme with "_", then concatenating
them with the host name, resulting in a domain name like
"_8004._examplescheme.api.example.com.".
Protocol mappings for SVCB MAY remove the port or replace it with
other protocol-specific information, but MUST retain the scheme in
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the QNAME. RR types other than SVCB can define additional behavior
for translating origins to QNAMEs. See Section 7.1 for the HTTPSSVC
behavior.
When a prior CNAME or SVCB record has aliased to an SVCB record, each
RR shall be returned under its own owner name.
Note that none of these forms alter the origin or authority for
validation purposes. For example, clients MUST continue to validate
TLS certificate hostnames based on the origin host.
As an example:
_8443._foo.api.example.com. 7200 IN SVCB 0 svc4.example.net.
svc4.example.net. 7200 IN SVCB 3 ( svc4.example.net. alpn="bar"
port="8004" esnikeys="..." )
would indicate that "foo://api.example.com:8443" is aliased to use
ALPN protocol "bar" service endpoints offered at "svc4.example.net"
on port 8004.
2.4. SvcRecordType
The SvcRecordType is implicit based on the presence of SvcFieldValue,
and defines the form of the SVCB RR. When SvcFieldValue is empty,
the SVCB SvcRecordType is defined to be in AliasForm. Otherwise, the
SVCB SvcRecordType is defined to be in ServiceForm.
Within an SVCB RRSet, all RRs should have the same SvcRecordType. If
an RRSet contains a record in AliasForm, the client MUST ignore any
records in the set with ServiceForm.
2.5. SVCB records: AliasForm
When SvcRecordType is AliasForm, the SVCB record is to be treated
similar to a CNAME alias pointing to SvcDomainName. SVCB RRSets
SHOULD only have a single resource record in this form. If multiple
are present, clients or recursive resolvers SHOULD pick one at
random.
The AliasForm's primary purpose is to allow aliasing at the zone
apex, where CNAME is not allowed. For example, if an operator of
https://example.com wanted to point HTTPS requests to a service
operating at svc.example.net, they would publish a record such as:
example.com. 3600 IN SVCB 0 svc.example.net.
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The SvcDomainName MUST point to a domain name that contains another
SVCB record, address (AAAA and/or A) records, or both address records
and a ServiceForm SVCB record.
Note that the SVCB record's owner name MAY be the canonical name of a
CNAME record, and the SvcDomainName MAY be the owner of a CNAME
record. Clients and recursive resolvers MUST follow CNAMEs as
normal.
Due to the risk of loops, clients and recursive resolvers MUST
implement loop detection. Chains of consecutive SVCB and CNAME
records SHOULD be limited to (8?) prior to reaching terminal address
records.
As legacy clients will not know to use this record, service operators
will likely need to retain fallback AAAA and A records alongside this
SVCB record, although in a common case the target of the SVCB record
might offer better performance, and therefore would be preferable for
clients implementing this specification to use.
Note that SVCB AliasForm RRs do not alias to RR types other than
address records (AAAA and A), CNAMEs, and ServiceForm SVCB records.
For example, an AliasForm SVCB record does not alias to an HTTPSSVC
record, nor vice-versa.
2.6. SVCB records: ServiceForm
When SvcRecordType is the ServiceForm, the combination of
SvcDomainName and SvcFieldValue parameters within each resource
record associates an alternative service location with its connection
parameters.
Section 7.2 defines a direct mapping between Alt-Svc ([AltSvc])
values and the SVCB ServiceForm. Protocols using SVCB may use this
Alt-Svc mapping or specify their own semantics. Unless specified
otherwise by the protocol mapping, clients MUST ignore SvcFieldValue
parameters that they do not recognize.
2.6.1. Special handling of "." for SvcDomainName in ServiceForm
For ServiceForm SVCB RRs, if SvcDomainName has the value ".", then
the owner name of this record MUST be used as the effective
SvcDomainName. (The SvcDomainName of an SVCB RR in AliasForm MUST
NOT have this value.)
For example, in the following example "svc2.example.net" is the
effective SvcDomainName:
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www.example.com. 7200 IN HTTPSSVC svc.example.net.
svc.example.net. 7200 IN CNAME svc2.example.net.
svc2.example.net. 7200 IN HTTPSSVC 0 . ( alpn=h2
port=8002 esnikeys="..." )
svc2.example.net. 300 IN A 192.0.2.2
svc2.example.net. 300 IN AAAA 2001:db8::2
2.6.2. SvcFieldPriority
As RRs within an RRSet are explicitly unordered collections, the
SvcFieldPriority value serves to indicate priority. SVCB RRs with a
smaller SvcFieldPriority value SHOULD be given preference over RRs
with a larger SvcFieldPriority value.
When receiving an RRSet containing multiple SVCB records with the
same SvcFieldPriority value, clients SHOULD apply a random shuffle
within a priority level to the records before using them, to ensure
uniform load-balancing.
3. Client behavior
An SVCB-aware client resolves an origin by attempting to determine
the preferred SvcFieldValue and IP addresses for its service, using
the following procedure:
1. Issue parallel AAAA/A and SVCB queries for the name HOST. The
answers for these may or may not include CNAME pointers before
reaching one or more of these records.
2. If an SVCB record of AliasForm SvcRecordType is returned for
HOST, clients MUST loop back to step 1 replacing HOST with
SvcDomainName, subject to loop detection heuristics.
3. If one or more SVCB records of ServiceForm SvcRecordType are
returned for HOST, clients should select the highest-priority
option with acceptable parameters, and resolve AAAA and/or A
records for its SvcDomainName if they are not already available.
These are the preferred SvcFieldValue and IP addresses. If
connection fails, the client MAY try to connect using values from
a lower-priority record. If none of the options are acceptable
and working, the client SHOULD connect to the origin server
directly.
4. If an SVCB record for HOST does not exist, the received AAAA and/
or A records are the preferred IP addresses, and there is no
SvcFieldValue.
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This procedure does not rely on any recursive or authoritative server
to comply with this specification or have any awareness of SVCB.
When selecting between AAAA and A records to use, clients may use an
approach such as [HappyEyeballsV2].
Some important optimizations are discussed in Section 5 to avoid
additional latency in comparison to ordinary AAAA/A lookups.
3.1. Clients using a Proxy
Clients using a domain-oriented transport proxy like HTTP CONNECT
([RFC7231] Section 4.3.6) or SOCKS5 ([RFC1928]) SHOULD disable SVCB
support if performing SVCB queries would violate the client's privacy
intent.
If the client can safely perform SVCB queries (e.g. via the proxy or
an affiliated resolver), the client SHOULD follow the standard SVCB
resolution process, selecting the highest priority option that is
compatible with the client and the proxy. The client SHOULD provide
the final SvcDomainName and port (if present) to the proxy as the
destination host and port.
Providing the proxy with the final SvcDomainName has several
benefits:
o It allows the client to use the SvcFieldValue, if present, which
is only usable with a specific SvcDomainName. The SvcFieldValue
may include information that enhances performance (e.g. alpn) and
privacy (e.g. esnikeys).
o It allows the origin to delegate the apex domain.
o It allows the proxy to select between IPv4 and IPv6 addresses for
the server according to its configuration, and receive addresses
based on its network geolocation.
4. DNS Server Behavior
When replying to an SVCB query, authoritative DNS servers SHOULD
return A, AAAA, and SVCB records (as well as any relevant CNAME
records) in the Additional Section for any in-bailiwick
SvcDomainNames.
Recursive resolvers that are aware of SVCB SHOULD ensure that the
client can execute the procedure in Section 3 without issuing a
second round of queries, by following this procedure while
constructing a response to a stub resolver for an SVCB record query:
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1. When processing an SVCB response from an authoritative server,
add it to the Additional section (unless it is the Answer).
2. Inspect whether each record is in AliasForm or ServiceForm. If
at least one record is in AliasForm, ignore all other SVCB
records in the RRSet.
3. If the record is in AliasForm, resolve A, AAAA, and SVCB records
for the SvcDomainName. If the SVCB record does not exist, add
the A and AAAA records to the Additional section. Otherwise, go
to step 1, subject to loop detection heuristics.
4. If the records are in ServiceForm, resolve A and AAAA records for
each SvcDomainName (or for the owner name if the SvcDomainName is
"."), and include all the results in the Additional section.
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
contents.
5. Performance optimizations
For optimal performance (i.e. minimum connection setup time), clients
SHOULD issue address (AAAA and/or A) and SVCB queries simultaneously,
and SHOULD implement a client-side DNS cache. Responses in the
Additional section of an SVCB response SHOULD be placed in cache
before performing any followup queries. With these optimizations in
place, and conforming DNS servers, using SVCB does not add network
latency to connection setup.
A nonconforming recursive resolver might not return all the
information required to use all the records in an SVCB response. If
some of the SVCB RRs in the response can be used without requiring
additional DNS queries, the client MAY prefer those RRs, regardless
of their priorities.
To avoid a delay for clients using a nonconforming recursive
resolver, domain owners SHOULD use a single SVCB record whose
SvcDomainName is in the origin hostname's CNAME chain if possible.
This will ensure that the required address records are already
present in the client's DNS cache as part of the responses to the
address queries that were issued in parallel.
6. Initial SvcParamKeys
A few initial SvcParamKeys are defined here. These keys are useful
for HTTPS, and most are applicable to other protocols as well.
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6.1. "alpn"
The "alpn" SvcParamKey defines the Application Layer Protocol (ALPN,
as defined in {{!RFC7301}) supported by a TLS-based alternative
service. Its value SHOULD be an entry in the IANA registry "TLS
Application-Layer Protocol Negotiation (ALPN) Protocol IDs".
The presentation format and wire format of SvcParamValue is its
registered "Identification Sequence".
Clients MUST ignore SVCB RRs where the "alpn" SvcParamValue is
unknown or unsupported.
6.2. "port"
The "port" SvcParamKey defines the TCP or UDP port that should be
used to contact this alternative service.
The presentation format of the SvcParamValue is a numeric value
between 0 and 65535 inclusive. The wire format of the SvcParamValue
is the corresponding 2 octet numeric value in network byte order.
6.3. "esnikeys"
The SvcParamKey for ESNI is "esnikeys". Its value is defined in
Section 8.1. It is applicable to most TLS-based protocols.
6.4. "ipv4hint" and "ipv6hint"
The "ipv4hint" and "ipv6hint" keys represent IP address hints for the
service. If A and AAAA records for SvcDomainName are locally
available, the client SHOULD ignore these hints. Otherwise, clients
MUST perform A and/or AAAA queries for SvcDomainName as in Section 3,
and clients SHOULD switch to an IP address in those records as soon
as possible.
The wire format for each parameter is a sequence of IP addresses in
network byte order. Like an A or AAAA RRSet, the list of addresses
represents an unordered collection, and clients SHOULD pick addresses
to use in a random order.
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
use an approach such as [HappyEyeballsV2]. When only "ipv4hint"
parameters are present, IPv6-only clients may synthesize IPv6
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addresses as specified in [RFC7050] or ignore the "ipv4hint" key and
wait for AAAA resolution (Section 3). Recursive resolvers MUST NOT
perform DNS64 ([RFC6147]) on parameters within an SVCB record. For
best performance, server operators SHOULD include "ipv6hint"
parameters whenever they publish "ipv4hint" parameters.
The presentation format for each parameter is a comma-separated list
of IP addresses in standard textual format [RFC5952].
These parameters are intended to minimize additional connection
latency when a recursive resolver is not compliant with the
requirements in Section 4, and SHOULD NOT be included if most clients
are using compliant recursive resolvers.
7. Using SVCB with HTTPS and HTTP
Use of any protocol with SVCB requires a protocol-specific mapping
specification. This section specifies the mapping for HTTPS and
HTTP.
To enable special handling for the HTTPS and HTTP use-cases, the
HTTPSSVC RR type is defined as an SVCB-compatible RR type, specific
to the https and http schemes. This handling includes a mapping from
HTTPSSVC records directly into Alt-Svc entries. Clients MUST NOT
perform SVCB queries or accept SVCB responses for https or http
schemes.
The HTTPSSVC wire format and presentation format are identical to
SVCB, and both share the SvcParamKey registry. SVCB semantics apply
equally to HTTPSSVC unless specified otherwise.
The presence of an HTTPSSVC record for an HTTP or HTTPS service also
provides an indication that all resources are available over HTTPS,
as discussed in Section 7.6. 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 extends the concept introduced in the HTTP
Alternative Services proposed standard [AltSvc]. Alt-Svc defines:
o an extensible data model for describing alternative network
endpoints that are authoritative for an origin
o the "Alt-Svc Field Value", a text format for representing this
information
o standards for sending information in this format from a server to
a client over HTTP/1.1 and HTTP/2.
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Each ServiceForm HTTPSSVC RR provides a set of information that can
be mapped into an Alt-Svc Field Value. A client receiving this
information during DNS resolution can skip the initial connection and
proceed directly to an alternative service.
7.1. Owner names for HTTPSSVC records
The HTTPSSVC RR extends the behavior for determining a QNAME
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,
then the client's original QNAME is equal to the origin host name.
For origins other than https with port 443 and http with port 80, 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.
Note that none of these forms alter the HTTPS origin or authority.
For example, clients MUST continue to validate TLS certificate
hostnames based on the origin host.
7.2. Mapping between ServiceForm and 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.
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Conversion from HTTPSSVC to Alt-Svc Field Value SHOULD ignore any
unrecognized SvcParamKeys, and conversion from Alt-Svc Field Value to
HTTPSSVC SHOULD ignore any Alt-Svc parameters that do not have a
corresponding SvcParamKey.
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
On connections to an HTTPSSVC alternative service, clients SHOULD
include the same Alt-Used header that they would include if the
corresponding Alt-Svc Field Value were received over HTTPS.
7.3. Differences from Alt-Svc as transmitted over HTTP
Publishing an alternative services form HTTPSSVC record in DNS is
intended to be equivalent to transmitting the corresponding Alt-Svc
value over HTTPS, and receiving an HTTPSSVC record is intended to be
equivalent to receiving this field value over HTTPS. However, there
are some small differences in the intended client and server
behavior.
7.3.1. Max Age and Persist
There is no SvcParamKey corresponding to the Alt-Svc "ma" (max age)
parameter. Instead, server operators SHOULD encode the expiration
time in the DNS TTL, and MUST NOT set a TTL longer than the intended
"max age".
For security reasons, there is no SvcParamKey corresponding to the
Alt-Svc "persist" parameter.
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7.4. 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.
Clients SHOULD prefer Alt-values received via HTTPS over any Alt-
value received via DNS.
7.4.1. Constructing Alt-Svc equivalent headers
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]).
7.5. Granularity and lifetime control
Sending Alt-Svc over HTTP allows the server to tailor the Alt-Svc
Field Value specifically to the client. When using an HTTPSSVC DNS
record, groups of clients will necessarily receive the same Alt-Svc
Field Value. Therefore, this standard is not suitable for uses that
require single-client granularity in Alt-Svc.
Some DNS caching systems incorrectly extend the lifetime of DNS
records beyond the stated TTL. Server operators MUST NOT rely on
HTTPSSVC records expiring on time, and MAY shorten the TTL to
compensate.
7.6. HTTP Strict Transport Security
By publishing an HTTPSSVC record, the server operator indicates that
all useful HTTP resources on that origin are reachable over HTTPS,
similar to HTTP Strict Transport Security [HSTS]. When an HTTPSSVC
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record is present for an origin, all "http" scheme requests for that
origin SHOULD logically be redirected to "https".
Prior to making an "http" scheme request, the client SHOULD perform a
lookup to determine if an HTTPSSVC record is available for that
origin. To do so, the client SHOULD construct a corresponding
"https" URL as follows:
1. Replace the "http" scheme with "https".
2. If the "http" URL explicitly specifies port 80, specify port 443.
3. Do not alter any other aspect of the URL.
This construction is equivalent to Section 8.3 of [HSTS], point 5.
If an HTTPSSVC record is present for this "https" URL, the client
should treat this as the equivalent of receiving an HTTP "307
Temporary Redirect" redirect to the "https" URL. Because HTTPSSVC is
received over an often insecure channel (DNS), clients MUST NOT place
any more trust in this signal than if they had received a 307
redirect over cleartext HTTP.
If the HTTPSSVC query results in a SERVFAIL error, and the connection
between the client and the recursive resolver is cryptographically
protected (e.g. using TLS [RFC7858] or HTTPS [RFC8484]), the client
SHOULD abandon the connection attempt and display an error message.
A SERVFAIL error can occur if the domain is DNSSEC-signed, the
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
responses, it SHOULD abandon the connection attempt if the HTTPSSVC
response fails to validate.
7.7. Cache interaction
If the client has an Alt-Svc cache, and a usable Alt-Svc value is
present in that cache, then the client SHOULD NOT issue an HTTPSSVC
DNS query. Instead, the client SHOULD proceed with alternative
service connection as usual.
If the client has a cached Alt-Svc entry that is expiring, the client
MAY perform an HTTPSSVC query to refresh the entry.
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8. Extensions to enhance privacy
8.1. Alt-Svc and SVCB/HTTPSSVC parameter for ESNI keys
Both SVCB/HTTPSSVC and Alt-Svc "esnikeys" parameters are defined for
specifying ESNI keys corresponding to an alternative service. The
value of the parameter is an ESNIKeys structure [ESNI] or the empty
string. ESNI-aware clients SHOULD prefer alt-values and SVCB/
HTTPSSVC RRs with non-empty esnikeys.
Both the SVCB SvcParamValue presentation format as well as the Alt-
Svc parameter value is the ESNIKeys structure [ESNI] encoded in
[base64] or the empty string. The SVCB SvcParamValue wire format is
the octet string containing the binary ESNIKeys structure.
This parameter MAY also be sent in Alt-Svc HTTP response headers and
HTTP/2 ALTSVC frames. This parameter MUST NOT appear more than once
in a single alt-value.
8.1.1. Handling a mixture of alternatives not supporting esnikeys
The Alt-Svc specification states that "the client MAY fall back to
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
only to another alt-value that also has the esnikeys parameter. If
the parameter's value is the empty string, the client SHOULD connect
as it would in the absence of any ESNIKeys information.
For example, suppose a server operator has two alternatives.
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 esnikeys value in Alternative B,
and mark Alternative A with esnikeys="" to indicate that fallback
from B to A is allowed.
Other clients and services implementing SVCB or HTTPSSVC with
esnikeys are encouraged to take a similar approach.
9. Interaction with other standards
This standard is intended to reduce connection latency and improve
user privacy. Server operators implementing this standard SHOULD
also implement TLS 1.3 [RFC8446] and OCSP Stapling [RFC6066], both of
which confer substantial performance and privacy benefits when used
in combination with SVCB records.
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To realize the greatest privacy benefits, this proposal is intended
for use over a privacy-preserving DNS transport (like DNS over TLS
[RFC7858] or DNS over HTTPS [RFC8484]). However, performance
improvements, and some modest privacy improvements, are possible
without the use of those standards.
Any specification for use of SVCB with a protocol MUST have an entry
for its scheme under the SVCB RR type in the IANA DNS Underscore
Global Scoped Entry Registry [Attrleaf]. The scheme SHOULD have an
entry in the IANA URI Schemes Registry [RFC7595]. The scheme SHOULD
have a defined specification for use with SVCB, unless it already has
a specification for use with Alt-Svc.
10. Security Considerations
SVCB/HTTPSSVC RRs and Alt-Svc Field Values are intended for
distribution over untrusted channels, and clients are REQUIRED to
verify that the alternative service is authoritative for the origin
(Section 2.1 of [AltSvc]). Therefore, DNSSEC signing and validation
are OPTIONAL for publishing and using SVCB and HTTPSSVC records.
Clients MUST ensure that their DNS cache is partitioned for each
local network, or flushed on network changes, to prevent a local
adversary in one network from implanting a forged DNS record that
allows them to track users or hinder their connections after they
leave that network.
11. IANA Considerations
11.1. New registry for Service Parameters
The "Service Binding (SVCB) Parameter Registry" defines the name
space for parameters, including string representations and numeric
SvcParamKey values. This registry is shared with other SVCB-
compatible RR types, such as HTTPSSVC.
ACTION: create and include a reference to this registry.
11.1.1. Procedure
A registration MUST include the following fields:
o Name: Service parameter key name
o SvcParamKey: Service parameter key numeric identifier (range
0-65535)
o Meaning: a short description
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o Pointer to specification text
Values to be added to this name space require Expert Review (see
[RFC5226], Section 4.1). Apart from the initial contents, the name
MUST NOT start with "key".
11.1.2. Initial contents
The "Service Binding (SVCB) Parameter Registry" shall initially be
populated with the registrations below:
+-------------+----------+--------------------------+---------------+
| SvcParamKey | NAME | Meaning | Reference |
+-------------+----------+--------------------------+---------------+
| 0 | key0 | Reserved | (This |
| | | | document) |
| | | | |
| 1 | alpn | ALPN for alternative | (This |
| | | service | document) |
| | | | |
| 2 | port | Port for alternative | (This |
| | | service | document) |
| | | | |
| 3 | esnikeys | ESNI keys literal | (This |
| | | | document) |
| | | | |
| 4 | ipv4hint | IPv4 address hints | (This |
| | | | document) |
| | | | |
| 5 | key5 | Reserved | (This |
| | | | document) |
| | | | |
| 6 | ipv6hint | IPv6 address hints | (This |
| | | | document) |
| | | | |
| 65280-65534 | keyNNNNN | Private Use | (This |
| | | | document) |
| | | | |
| 65535 | key65535 | Reserved | (This |
| | | | document) |
+-------------+----------+--------------------------+---------------+
TODO: do we also want to reserve a range for greasing?
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11.2. Registry updates
Per [RFC6895], please add the following entry to the data type range
of the Resource Record (RR) TYPEs registry:
+----------+----------------------------------------+---------------+
| TYPE | Meaning | Reference |
+----------+----------------------------------------+---------------+
| SVCB | Service Location and Parameter Binding | (This |
| | | document) |
| | | |
| HTTPSSVC | HTTPS Service Location and Parameter | (This |
| | Binding | document) |
+----------+----------------------------------------+---------------+
Per [Attrleaf], please add the following entries to the DNS
Underscore Global Scoped Entry Registry:
+----------+------------+-------------------+-----------------+
| RR TYPE | _NODE NAME | Meaning | Reference |
+----------+------------+-------------------+-----------------+
| 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 |
+-------------------+--------------------+-----------------+
| esnikeys | Encrypted SNI keys | (This document) |
+-------------------+--------------------+-----------------+
12. Acknowledgments and Related Proposals
There have been a wide range of proposed solutions over the years to
the "CNAME at the Zone Apex" challenge proposed. These include
[I-D.draft-bellis-dnsop-http-record-00],
[I-D.draft-ietf-dnsop-aname-03], and others.
Thank you to Ian Swett, Ralf Weber, Jon Reed, Martin Thompson, Lucas
Pardue, Ilari Liusvaara, Tim Wicinski, Tommy Pauly, Chris Wood, and
others for their feedback and suggestions on this draft.
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13. References
13.1. Normative 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>.
[AltSvcSNI]
Bishop, M., "The "SNI" Alt-Svc Parameter", draft-bishop-
httpbis-sni-altsvc-02 (work in progress), May 2018.
[Attrleaf]
Crocker, D., "DNS Scoped Data Through "Underscore" Naming
of Attribute Leaves", draft-ietf-dnsop-attrleaf-16 (work
in progress), November 2018.
[base64] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>.
[ESNI] Rescorla, E., Oku, K., Sullivan, N., and C. Wood,
"Encrypted Server Name Indication for TLS 1.3", draft-
ietf-tls-esni-04 (work in progress), July 2019.
[HappyEyeballsV2]
Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
Better Connectivity Using Concurrency", RFC 8305,
DOI 10.17487/RFC8305, December 2017,
<https://www.rfc-editor.org/info/rfc8305>.
[HSTS] Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
Transport Security (HSTS)", RFC 6797,
DOI 10.17487/RFC6797, November 2012,
<https://www.rfc-editor.org/info/rfc6797>.
[HTTP3] Bishop, M., "Hypertext Transfer Protocol Version 3
(HTTP/3)", draft-ietf-quic-http-20 (work in progress),
April 2019.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[RFC1928] Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D., and
L. Jones, "SOCKS Protocol Version 5", RFC 1928,
DOI 10.17487/RFC1928, March 1996,
<https://www.rfc-editor.org/info/rfc1928>.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[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>.
[RFC3597] Gustafsson, A., "Handling of Unknown DNS Resource Record
(RR) Types", RFC 3597, DOI 10.17487/RFC3597, September
2003, <https://www.rfc-editor.org/info/rfc3597>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<https://www.rfc-editor.org/info/rfc5226>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
Address Text Representation", RFC 5952,
DOI 10.17487/RFC5952, August 2010,
<https://www.rfc-editor.org/info/rfc5952>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
<https://www.rfc-editor.org/info/rfc6066>.
[RFC6147] Bagnulo, M., Sullivan, A., Matthews, P., and I. van
Beijnum, "DNS64: DNS Extensions for Network Address
Translation from IPv6 Clients to IPv4 Servers", RFC 6147,
DOI 10.17487/RFC6147, April 2011,
<https://www.rfc-editor.org/info/rfc6147>.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
DOI 10.17487/RFC6454, December 2011,
<https://www.rfc-editor.org/info/rfc6454>.
[RFC7050] Savolainen, T., Korhonen, J., and D. Wing, "Discovery of
the IPv6 Prefix Used for IPv6 Address Synthesis",
RFC 7050, DOI 10.17487/RFC7050, November 2013,
<https://www.rfc-editor.org/info/rfc7050>.
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[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.
[RFC7595] Thaler, D., Ed., Hansen, T., and T. Hardie, "Guidelines
and Registration Procedures for URI Schemes", BCP 35,
RFC 7595, DOI 10.17487/RFC7595, June 2015,
<https://www.rfc-editor.org/info/rfc7595>.
[RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "Specification for DNS over Transport
Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
2016, <https://www.rfc-editor.org/info/rfc7858>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS
(DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
<https://www.rfc-editor.org/info/rfc8484>.
13.2. Informative References
[DNSTerm] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
January 2019, <https://www.rfc-editor.org/info/rfc8499>.
[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]
Bellis, R., "A DNS Resource Record for HTTP", draft-
bellis-dnsop-http-record-00 (work in progress), November
2018.
[I-D.draft-ietf-dnsop-aname-03]
Finch, T., Hunt, E., Dijk, P., Eden, A., and W. Mekking,
"Address-specific DNS aliases (ANAME)", draft-ietf-dnsop-
aname-03 (work in progress), April 2019.
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[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
DOI 10.17487/RFC2782, February 2000,
<https://www.rfc-editor.org/info/rfc2782>.
[RFC6895] Eastlake 3rd, D., "Domain Name System (DNS) IANA
Considerations", BCP 42, RFC 6895, DOI 10.17487/RFC6895,
April 2013, <https://www.rfc-editor.org/info/rfc6895>.
Appendix A. Additional examples
A.1. Equivalence to Alt-Svc records
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 esnikeys="ABC..." )
svc.example.net. 7200 IN HTTPSSVC 3 . alpn=h2 port=8002 esnikeys="123..."
is equivalent to the Alt-Svc record:
Alt-Svc: h3="svc3.example.net:8003"; esnikeys="ABC..."; ma=7200, \
h2="svc.example.net:8002"; esnikeys="123..."; ma=7200
for the origins of both "https://www.example.com" and
"https://example.com".
Appendix B. Comparison with alternatives
The SVCB and HTTPSSVC record types closely resemble, and are inspired
by, some existing record types and proposals. A complaint with all
of the alternatives is that web clients have seemed unenthusiastic
about implementing them. The hope here is that by providing an
extensible solution that solves multiple problems we will overcome
the inertia and have a path to achieve client implementation.
B.1. Differences from the SRV RR type
An SRV record [RFC2782] can perform a similar function to the SVCB
record, informing a client to look in a different location for a
service. However, there are several differences:
o SRV records are typically mandatory, whereas clients will always
continue to function correctly without making use of Alt-Svc or
SVCB.
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o SRV records cannot instruct the client to switch or upgrade
protocols, whereas Alt-Svc can signal such an upgrade (e.g. to
HTTP/2).
o SRV records are not extensible, whereas SVCB and HTTPSSVC can be
extended with new parameters.
o Using SRV records would not allow an HTTPS client to skip
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
extensible to cover Alt-Svc and ESNIKeys use-cases. Like that
proposal, this addresses the zone apex CNAME challenge.
Like that proposal it remains necessary to continue to include
address records at the zone apex for legacy clients.
B.3. Differences from the proposed ANAME record
Unlike [I-D.draft-ietf-dnsop-aname-03], this approach is extensible
to cover Alt-Svc and ESNIKeys use-cases. This approach also does not
require any changes or special handling on either authoritative or
master servers, beyond optionally returning in-bailiwick additional
records.
Like that proposal, this addresses the zone apex CNAME challenge for
clients that implement this.
However with this SVCB proposal it remains necessary to continue to
include address records at the zone apex for legacy clients. If
deployment of this standard is successful, the number of legacy
clients will fall over time. As the number of legacy clients
declines, the operational effort required to serve these users
without the benefit of SVCB indirection should fall. Server
operators can easily observe how much traffic reaches this legacy
endpoint, and may remove the apex's address records if the observed
legacy traffic has fallen to negligible levels.
B.4. Differences from the proposed ESNI record
Unlike [ESNI], this approach is extensible and covers the Alt-Svc
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
multi-CDN challenges in a general case.
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Unlike ESNI, SVCB allows specifying different ESNI configurations for
different protocols and ports, rather than applying a single
configuration to all ports on a domain.
B.5. SNI Alt-Svc parameter
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
details are expected to change with subsequent refinement. Some
known issues or topics for discussion are listed below.
C.1. Record Name
Naming is hard. "SVCB" and "HTTPSSVC" are proposed as placeholders
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,
HTTPSSVC, SVCHTTPS, or something else.
C.2. Generality
The SVCB record was designed as a generalization of HTTPSSVC, based
on feedback requesting a solution that applied to protocols pther
than HTTP. Past efforts to over-generalize have not met with broad
success, but we hope that HTTPSSVC and SVCB have struck an acceptable
balance between generality and focus.
C.3. Wire Format
Advice from experts in DNS wire format best practices would be
greatly appreciated to refine the proposed details, overall.
C.4. Where to include Priority
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.
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C.5. Whether to include Weight
Some other similar mechanisms such as SRV have a weight in-addition
to priority. That is excluded here for simplicity. It could always
be added as an optional SVCB parameter.
Appendix D. Change history
o draft-nygren-dnsop-svcb-httpssvc-00
* Generalize to an SVCB record, with special-case handling for
Alt-Svc and HTTPS separated out to dedicated sections.
* Split out a separate HTTPSSVC record for the HTTPS use-case.
* Remove the explicit SvcRecordType=0/1 and instead make the
AliasForm vs ServiceForm be implicit. This was based on
feedback recommending against subtyping RR type.
* Remove one optimization.
o draft-nygren-httpbis-httpssvc-03
* Change redirect type for HSTS-style behavior from 302 to 307 to
reduce ambiguities.
o draft-nygren-httpbis-httpssvc-02
* Remove the redundant length fields from the wire format.
* Define a SvcDomainName of "." for SvcRecordType=1 as being the
HTTPSSVC RRNAME.
* Replace "hq" with "h3".
o draft-nygren-httpbis-httpssvc-01
* Fixes of record name. Replace references to "HTTPSVC" with
"HTTPSSVC".
o draft-nygren-httpbis-httpssvc-00
* Initial version
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Authors' Addresses
Ben Schwartz
Google
Email: bemasc@google.com
Mike Bishop
Akamai Technologies
Email: mbishop@evequefou.be
Erik Nygren
Akamai Technologies
Email: erik+ietf@nygren.org
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