Internet DRAFT - draft-nygren-httpbis-httpssvc
draft-nygren-httpbis-httpssvc
HTTP Working Group B. Schwartz
Internet-Draft Google
Intended status: Standards Track M. Bishop
Expires: January 9, 2020 E. Nygren
Akamai Technologies
July 8, 2019
HTTPSSVC service location and parameter specification via the DNS (DNS
HTTPSSVC)
draft-nygren-httpbis-httpssvc-03
Abstract
This document specifies an "HTTPSSVC" DNS resource record type to
facilitate the lookup of information needed to make connections for
HTTPS URIs. The HTTPSSVC DNS RR mechanism allows an HTTPS origin
hostname to be served from multiple network services, each with
associated parameters (such as transport protocol and keying material
for encrypting TLS SNI). It also provides a solution for the
inability of the DNS to allow a CNAME to be placed at the apex of a
domain name. Finally, it provides a way to indicate that the origin
supports HTTPS without having to resort to redirects, allowing
clients to remove HTTP from the bootstrapping process.
By allowing this information to be bootstrapped in the DNS, it allows
for clients to learn of alternative services before their first
contact with the origin. This arrangement offers 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.
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 . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Introductory Example . . . . . . . . . . . . . . . . . . 4
1.2. Goals of the HTTPSSVC RR . . . . . . . . . . . . . . . . 4
1.3. Overview of the HTTPSSVC RR . . . . . . . . . . . . . . . 5
1.4. Additional Alt-Svc parameters . . . . . . . . . . . . . . 6
1.5. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
2. The HTTPSSVC record type . . . . . . . . . . . . . . . . . . 7
2.1. HTTPSSVC RDATA Wire Format . . . . . . . . . . . . . . . 7
2.2. RRNames . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.3. SvcRecordType . . . . . . . . . . . . . . . . . . . . . . 8
2.4. HTTPSSVC records: alias form . . . . . . . . . . . . . . 9
2.5. HTTPSSVC records: alternative service form . . . . . . . 9
3. Differences from Alt-Svc as transmitted over HTTP . . . . . . 10
3.1. Omitting Max Age and Persist . . . . . . . . . . . . . . 10
3.2. Multiple records and preference ordering . . . . . . . . 11
3.3. Constructing Alt-Svc equivalent headers . . . . . . . . . 11
3.4. Granularity and lifetime control . . . . . . . . . . . . 12
4. Client behaviors . . . . . . . . . . . . . . . . . . . . . . 12
4.1. Client resolution . . . . . . . . . . . . . . . . . . . . 12
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4.2. HTTP Strict Transport Security . . . . . . . . . . . . . 13
4.3. Cache interaction . . . . . . . . . . . . . . . . . . . . 14
5. DNS Server Behaviors . . . . . . . . . . . . . . . . . . . . 14
6. Performance optimizations . . . . . . . . . . . . . . . . . . 14
7. Extensions to enhance privacy . . . . . . . . . . . . . . . . 15
7.1. Alt-Svc parameter for ESNI keys . . . . . . . . . . . . . 15
7.2. Interaction with other standards . . . . . . . . . . . . 15
8. Security Considerations . . . . . . . . . . . . . . . . . . . 16
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
10. Acknowledgements and Related Proposals . . . . . . . . . . . 17
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
11.1. Normative References . . . . . . . . . . . . . . . . . . 17
11.2. Informative References . . . . . . . . . . . . . . . . . 19
Appendix A. Additional examples . . . . . . . . . . . . . . . . 20
A.1. Equivalence to Alt-Svc records . . . . . . . . . . . . . 20
Appendix B. Comparison with alternatives . . . . . . . . . . . . 20
B.1. Differences from the SRV RRTYPE . . . . . . . . . . . . . 20
B.2. Differences from the proposed HTTP record . . . . . . . . 21
B.3. Differences from the proposed ANAME record . . . . . . . 21
B.4. Differences from the proposed ESNI record . . . . . . . . 21
B.5. SNI Alt-Svc parameter . . . . . . . . . . . . . . . . . . 22
Appendix C. Design Considerations and Open Issues . . . . . . . 22
C.1. Record Name . . . . . . . . . . . . . . . . . . . . . . . 22
C.2. Applicability to other schemes . . . . . . . . . . . . . 22
C.3. Wire Format . . . . . . . . . . . . . . . . . . . . . . . 22
C.4. Extensibility of SvcRecordType . . . . . . . . . . . . . 22
C.5. Where to include Priority . . . . . . . . . . . . . . . . 22
C.6. Whether to include Weight . . . . . . . . . . . . . . . . 23
Appendix D. Change history . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction
The HTTPSSVC RR is intended to address a number of challenges facing
HTTPS clients and services, while also providing an extensible model
to handle similar use-cases without forcing clients to perform
additional DNS lookups and without forcing them to first make
connections to a default service for the origin.
When clients need to make a connection to fetch resources associated
with an HTTPS URI, they must first resolve A and/or AAAA address
resource records for the origin hostname. This is adequate when
clients default to TCP port 443, do not support Encrypted SNI [ESNI],
and where the origin service operator does not have a desire to put
an CNAME at a zone apex (such as for "https://example.com").
Handling situations beyond this within the DNS requires learning
additional information, and it is highly desirable to minimize the
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number of round-trip and lookups required to learn this additional
information.
1.1. Introductory Example
As an introductory example, a set of example HTTPSSVC and associated
A+AAAA records might be:
www.example.com. 2H IN CNAME svc.example.net.
example.com. 2H IN HTTPSSVC 0 0 svc.example.net.
svc.example.net. 2H IN HTTPSSVC 1 2 svc3.example.net. "h3=\":8003\"; \
esnikeys=\"...\""
svc.example.net. 2H IN HTTPSSVC 1 3 svc2.example.net. "h2=\":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
In the preceding example, both of the "example.com" and
"www.example.com" origin names are aliased to use 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. An ESNI key is specified which allows the SNI values
of "example.com" and "www.example.com" to be encrypted in the
handshake with these service endpoints. When connecting, clients
will continue to treat the authoritative origins as
"https://example.com" and "https://www.example.com", respectively.
1.2. Goals of the HTTPSSVC RR
The goal of the HTTSSVC RR is to allow clients to resolve a single
additional DNS RR in a way that:
o Provides service endpoints authoritative for an origin, along with
parameters associated with each of these endpoints. In
particular:
* to support connecting directly to [HTTP3] (QUIC transport)
service endpoints
* to obtain the [ESNI] keys associated with a service endpoint
o Does not assume that all service endpoints have the same
parameters (such as ESNI keys) or capabilities (such as [HTTP3])
or are even operated by the same entity. This is important as DNS
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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 enviroments, records
(such as A and AAAA) for that name may have been sourced from
different environments.
o Enables the functional equivalent of a CNAME at a zone apex (such
as "example.com") for HTTPS traffic, and generally enables
delegation of operational authority for an HTTPS origin within the
DNS to an alternate name. This addresses 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 a CNAME record as well as NS RRs (as is the case for
zone apex names)
1.3. Overview of the HTTPSSVC RR
This subsection briefly describes the HTTPSSVC RR in a non-normative
manner.
The HTTPSSVC RR has four primary fields:
1. SvcRecordType: A numeric flag indicating how to interpret the
subsequent fields. When "0", it indicates that the RR contains
an alias. When "1", it indicates that the RR contains an
alternative service definition.
2. SvcFieldPriority: The priority of this record (relative to
others, with lower values preferred). Applicable when
SvcRecordType is "1", and otherwise has value "0". (Described in
Section 3.2.)
3. SvcDomainName: The domain name of either the alias target (when
SvcRecordType is "0") or the uri-host domain name of the
alternative service endpoint (when SvcRecordType is "1").
4. SvcFieldValue: An Alternative Service field value describing the
alternative service endpoint for the domain name specified in
SvcDomainName (only when SvcRecordType is "1" and otherwise
empty).
Cooperating DNS recursive resolvers will perform subsequent record
resolution (for HTTPSSVC, 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 HTTPSSVC, A, and AAAA record
resolutions. DNS authoritative servers may attach in-bailiwick
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HTTPSSVC, A, AAAA, and CNAME records in the Additional Section to
responses for an HTTPSSVC query.
When SvcRecordType is "1", 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.
Together, these components provide a toolkit that has proven useful
and effective for informing a client of alternative services for an
origin. However, making use of an alternative service requires
contacting the origin server first. This creates an obvious
performance cost: users wait for a full HTTP connection initiation
(multiple roundtrips) before learning of an alternative service that
is preferred by the origin. The first connection also publicly
reveals the user's intended destination to all entities along the
network path.
The SvcFieldValue includes the Alt-Svc Field Value through the DNS.
This is in its standard text format, with the uri-host portion of the
alt-authority component moved into the SvcDomainName field of the
HTTPSSVC RR. A client receiving this information during DNS
resolution can skip the initial connection and proceed directly to an
alternative service.
1.4. Additional Alt-Svc parameters
This document also defines one additional Alt-Svc parameter that can
be used within SvcFieldValue:
o esnikeys (Section 7.1): The ESNIKeys structure from Section 4.1 of
[ESNI] for use in encrypting the actual origin hostname in the TLS
handshake.
1.5. Terminology
For consistency with [AltSvc], we adopt the following definitions:
o An "origin" is an information source as in [RFC6454].
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o The "origin server" is the server that the client would reach when
accessing the origin in the absence of Alt-Svc.
o An "alternative service" is a different server that can serve the
origin.
Abstractly, 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].
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 HTTPSSVC record type
The HTTPSSVC DNS resource record (RR) type (RRTYPE ???) is used to
locate endpoints that can service an "https" origin. The
presentation format of the record is:
RRName TTL Class HTTPSSVC SvcRecordType SvcFieldPriority \
SvcDomainName SvcFieldValue
where SvcRecordType is a numeric value of either "0" or "1",
SvcFieldPriority is a number in the range 0-65535, SvcDomainName is a
domain name, and SvcFieldValue is a string present when SvcRecordType
is "1".
The algorithm for resolving HTTPSSVC records and associated address
records is specified in Section 4.1.
2.1. HTTPSSVC RDATA Wire Format
The RDATA for the HTTPSSVC RR consists of:
o a 1 octet flag field for SvcRecordType, interpreted as an unsigned
numeric value (0 to 255, with only values "0" and "1" defined
here)
o a 2 octet field for SvcFieldPriority as an integer in network byte
order. If SvcRecordType is "0", SvcFieldPriority MUST be 0.
o the uncompressed SvcDomainName, represented as a sequence of
length-prefixed labels as in Section 3.1 of [RFC1035].
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o the SvcFieldValue byte string, consuming the remainder of the
record (so smaller than 65535 octets and constrained by the RRDATA
and DNS message sizes).
When SvcRecordType is "0", the SvcFieldValue SHOULD be empty ("") and
clients MUST ignore the contents of non-empty SvcFieldValue fields.
2.2. RRNames
In the case of the HTTPSSVC RR, an origin is translated into the
RRName in the following manner:
1. If the scheme is "https" and the port is 443, then the RRName is
equal to the origin host name. Otherwise the RRName is
represented by prefixing the port and scheme with "_", then
concatenating them with the host name, resulting in a domain name
like "_8443._https.www.example.com.".
2. When a prior CNAME or HTTPSSVC record has aliased to an HTTPSSVC
record, RRName shall be the name of the alias target.
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.
As an example for schemes and ports other than "https" and port 443:
_8443._wss.api.example.com. 2H IN HTTPSSVC 0 0 svc4.example.net.
svc4.example.net. 2H IN HTTPSSVC 1 3 svc4.example.net. "h2=\":8004\"; \
esnikeys=\"...\""
would indicate that "wss://api.example.com:8443" is aliased to use
HTTP/2 service endpoints offered as "svc4.example.net" on port 8004.
2.3. SvcRecordType
The SvcRecordType field is a numeric value defined to be either "0"
or "1". Within an HTTPSSVC RRSet, all RRs must have the same value
for SvcRecordType. Clients and recursive servers MUST ignore
HTTPSSVC resource records with other SvcRecordType values. If an
RRSet contains a record with type "0", the client MUST ignore any
records in the set with type "1".
When SvcRecordType is "0", the HTTPSSVC is defined to be in "alias
form".
When SvcRecordType is "1", the HTTPSSVC is defined to be in
"alternative service form".
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2.4. HTTPSSVC records: alias form
When SvcRecordType is "0", the HTTPSSVC record is to be treated
similar to a CNAME alias pointing to the domain name specified in
SvcDomainName. HTTPSSVC RRSets MUST only have a single resource
record in this form. If multiple are present, clients or recursive
resolvers SHOULD pick one non-determinstically.
The common use-case for this form of the HTTPSSVC record is as an
alternative to CNAMEs at the zone apex where they are 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 HTTPSSVC 0 0 svc.example.net.
The SvcDomainName MUST point to a domain name that contains another
HTTPSSVC record and/or address (AAAA and/or A) records.
Note that the RRName and the SvcDomainName MAY themselves be CNAMEs.
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 HTTPSSVC and CNAME
records SHOULD be limited to (8?) prior to reaching terminal address
records.
The SvcFieldValue in this form SHOULD be an empty string and clients
MUST ignore its contents.
As legacy clients will not know to use this record, service operators
will likely need to retain fallback AAAA and A records alongside this
HTTPSSVC record, although in a common case the target of the HTTPSSVC
record might have better performance, and therefore would be
preferable for clients implementing this specification to use.
2.5. HTTPSSVC records: alternative service form
When SvcRecordType is "1", the combination of SvcDomainName and
SvcFieldValue within each resource record associates an Alternative
Service Field Value with an origin.
The SvcFieldValue of the HTTPSSVC resource record contains an Alt-Svc
Field Value, exactly as defined in Section 4 of [AltSvc], but with
the uri-host moved to the SvcDomainName field.
For example, if the operator of https://www.example.com intends to
include an HTTP response header like
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Alt-Svc: h3="svc.example.net:8003"; ma=3600, \
h2="svc.example.net:8002"; ma=3600
they could also publish an HTTPSSVC DNS RRSet like
www.example.com. 3600 IN HTTPSSVC 1 2 svc.example.net. "h3=\":8003\""
HTTPSSVC 1 3 svc.example.net. "h2=\":8002\""
This data type can be represented as an Unknown RR as described in
[RFC3597]:
www.example.com. 3600 IN TYPE??? \\# TBD:WRITEME
This construction is intended to be extensible in two ways. First,
any extensions that are made to the Alt-Svc format for transmission
over HTTPS are also applicable here, unless expressly mentioned
otherwise.
Second, by defining a way to map non-HTTPS schemes and non-default
ports (Section 2.2), we provide a way for the HTTPSSVC to be used for
them as needed. However, by using the origin name for the RRName for
scheme https and port 443 we allow HTTPSSVC records to be included at
the end of CNAME chains for existing site implementations without
requiring changes in the zone containing the origin.
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 this field 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.
3.1. Omitting Max Age and Persist
When publishing an HTTPSSVC record in DNS, server operators MUST omit
the "ma" parameter, which encodes the "max age" (i.e. expiration
time) of an Alt-Svc Field Value. Instead, server operators SHOULD
encode the expiration time in the DNS TTL, and MUST NOT set a TTL
longer than the intended "max age".
When receiving an HTTPSSVC record, clients SHOULD synthesize a new
"ma" parameter from the DNS TTL if the resulting alt-value is being
passed to a subsystem that might employ caching.
When publishing an HTTPSSVC record, server operators MUST omit the
"persist" parameter, which indicates whether the client should use
this record on other network paths. When receiving an HTTPSSVC
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record, clients MUST discard any records that contain a "persist"
flag. Disabling persistence is important 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.
3.2. Multiple records and preference ordering
Server operators MAY publish multiple SvcRecordType "1" 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 MUST contain exactly one alt-value, as described in Section 3
of [AltSvc].
As RRs within an RRSET are explicitly unordered collections, the
SvcFieldPriority value is introduced to indicate priority. HTTPSSVC
RRs with a smaller SvcFieldPriority value SHOULD be given preference
over RRs with a larger SvcFieldPriority value.
Alt-values received via HTTPS are preferred over any Alt-value
received via DNS.
When receiving an RRSET containing multiple HTTPSSVC records with the
same SvcFieldPriority value, clients SHOULD apply a random shuffle
within a priority level to the records before using them, to ensure
randomized load-balancing.
3.3. Constructing Alt-Svc equivalent headers
For a client to construct the equivalent of an Alt-Svc HTTP response
header:
1. For each RR, the SvcDomainName MUST be inserted as the uri-host.
If SvcDomainName is has the value "." then the RRNAME for the
final HTTPSSVC record MUST be inserted as the uri-host. (In the
case of a CNAME or a HTTPSSVC SvcRecordType "0" record pointing
to an HTTPSSVC record with SvcRecordType "1" and SvcDomainName
"." then it is the RRNAME for the terminal HTTPSSVC record that
must be inserted as the uri-host.)
2. 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.
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3. 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]).
3.4. 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.
4. Client behaviors
4.1. Client resolution
When attempting to resolve a name HOST, clients should follow in-
order:
1. Issue parallel AAAA/A and HTTPSSVC 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 HTTPSSVC record of SvcRecordType "0" is returned for HOST,
clients should loop back to step 1 replacing HOST with
SvcDomainName, subject to loop detection heuristics.
3. If one or more HTTPSSVC record of SvcRecordType "1" is returned
for HOST, clients should synthesize equivalent Alt-Svc Field
Values based on the SvcDomainName and SvcFieldValue. If one of
these alt-values is selected to be used in a connection, the
client will need to resolve AAAA and/or A records for
SvcDomainName.
4. If only AAAA and/or A records are present for HOST (and no
HTTPSSVC), clients should make a connection to one of the IP
addresses contained in these records and proceed normally.
When selecting between AAAA and A records to use, clients may use an
approach such as [HappyEyeballsV2]
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Some possible optimizations are discussed in Section 6 to reduce
latency impact in comparison to ordinary AAAA/A lookups.
4.2. 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
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 RRs, it
SHOULD abandon the connection attempt if the HTTPSSVC RR fails to
validate.
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4.3. 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.
5. DNS Server Behaviors
Recursive DNS servers SHOULD resolve SvcDomainName records and
include them in the Additional Section (along with any relevant CNAME
records). For SvcRecordType=0, recursive DNS servers SHOULD attempt
to resolve and include A, AAAA, and HTTPSSVC records. For
SvcRecordType=1, recursive DNS servers SHOULD attempt to resolve and
include A and AAAA records.
Authoritative DNS servers SHOULD return A, AAAA, and HTTPSSVC records
(as well as any relevant CNAME records) in the Additional Section for
any in-bailiwick SvcDomainNames.
6. Performance optimizations
For optimal performance (i.e. minimum connection setup time), clients
SHOULD issue address (AAAA and/or A) and HTTPSSVC queries
simultaneously, and SHOULD implement a client-side DNS cache. With
these optimizations in place, and conforming DNS servers, using
HTTPSSVC does not add network latency to connection setup.
A nonconforming recursive resolver might return an HTTPSSVC response
with a nonempty SvcDomainName, without the corresponding address
records. If all the HTTPSSVC RRs in the response have nonempty
SvcDomainName values, and the client does not have address records
for any of these values in its DNS cache, the client SHOULD perform
an additional address query for the selected SvcDomainName.
The additional DNS query in this case introduces a delay. To avoid
causing a delay for clients using a nonconforming recursive resolver,
domain owners SHOULD choose the SvcDomainName to be a name 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.
Highly performance-sensitive clients MAY implement the following
special- case shortcut to avoid increased connection time: if (1) one
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of the HTTPSSVC records returned has SvcRecordType=0, (2) its
SvcDomainName is not in the DNS cache, and (3) the address queries
for the origin domain return usable IP addresses, then the client MAY
ignore the HTTPSSVC records and connect directly to the origin
domain. When the SvcDomainNames and any needed HTTPSSVC records are
available, the client SHOULD make subsequent requests over
connections specified by the HTTPSSVC records.
Server operators can therefore expect that publishing HTTPSSVC
records with SvcRecordType=0 should not cause an additional DNS query
for performance-sensitive clients. Server operators who wish to
prevent this optimization should use SvcRecordType=1.
7. Extensions to enhance privacy
7.1. Alt-Svc parameter for ESNI keys
An Alt-Svc "esnikeys" parameter is defined for specifying ESNI keys
corresponding to an alternative service. The value of the parameter
is an ESNIKeys structure [ESNI] encoded in [base64], or the empty
string. ESNI-aware clients SHOULD prefer alt-values with nonempty
esnikeys.
This parameter MAY also be sent in Alt-Svc HTTP response headers and
HTTP/2 ALTSVC frames.
The Alt-Svc specification states that "the client MAY fall back to
using the origin" in case of connection failure [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.
7.2. Interaction with other standards
The purpose of this standard is to reduce connection latency and
improve user privacy. Server operators implementing this standard
SHOULD also implement TLS 1.3 [RFC8446] and OCSP Stapling [RFC6066],
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both of which confer substantial performance and privacy benefits
when used in combination with HTTPSSVC records.
To realize the greatest privacy benefits, this proposal is intended
for use with 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.
This RRType could be extended to support schemes other than "https".
Any such scheme MUST have an entry under the HTTPSSVC RRType 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 be one for which Alt-Svc is defined.
8. Security Considerations
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 HTTPSSVC records.
TBD: expand this section in more detail. In particular: * Just as
with [AltSvc], clients must validate the TLS server certificate
against hostname associated with the origin. Clients MUST NOT use
the SvcDomainName as any part of the server TLS certificate
validation. * ...
9. IANA Considerations
Per [RFC6895], please add the following entry to the data type range
of the Resource Record (RR) TYPEs registry:
+----------+------------------------+-----------------+
| TYPE | Meaning | Reference |
+----------+------------------------+-----------------+
| HTTPSSVC | HTTPS Service Location | (This 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) |
+----------+------------+-------------------+-----------------+
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Per [AltSvc], please add the following entries to the HTTP Alt-Svc
Parameter Registry:
+-------------------+--------------------+-----------------+
| Alt-Svc Parameter | Meaning | Reference |
+-------------------+--------------------+-----------------+
| esnikeys | Encrypted SNI keys | (This document) |
+-------------------+--------------------+-----------------+
10. Acknowledgements 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, and others for their feedback and
suggestions on this draft.
11. References
11.1. 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-03 (work in progress), March 2019.
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[HappyEyeballsV2]
Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
Better Connectivity Using Concurrency", RFC 8305,
DOI 10.17487/RFC8305, December 2017,
<https://www.rfc-editor.org/info/rfc8305>.
[HSTS] Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
Transport Security (HSTS)", RFC 6797,
DOI 10.17487/RFC6797, November 2012,
<https://www.rfc-editor.org/info/rfc6797>.
[HTTP3] Bishop, M., "Hypertext Transfer Protocol Version 3
(HTTP/3)", draft-ietf-quic-http-20 (work in progress),
April 2019.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[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>.
[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>.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
DOI 10.17487/RFC6454, December 2011,
<https://www.rfc-editor.org/info/rfc6454>.
[RFC7595] Thaler, D., Ed., Hansen, T., and T. Hardie, "Guidelines
and Registration Procedures for URI Schemes", BCP 35,
RFC 7595, DOI 10.17487/RFC7595, June 2015,
<https://www.rfc-editor.org/info/rfc7595>.
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[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>.
11.2. 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.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
DOI 10.17487/RFC2782, February 2000,
<https://www.rfc-editor.org/info/rfc2782>.
[RFC6895] Eastlake 3rd, D., "Domain Name System (DNS) IANA
Considerations", BCP 42, RFC 6895, DOI 10.17487/RFC6895,
April 2013, <https://www.rfc-editor.org/info/rfc6895>.
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Appendix A. Additional examples
A.1. Equivalence to Alt-Svc records
The following:
www.example.com. 2H IN CNAME svc.example.net.
example.com. 2H IN HTTPSSVC 0 0 svc.example.net.
svc.example.net. 2H IN HTTPSSVC 1 2 svc3.example.net. "h3=\":8003\"; \
esnikeys=\"ABC...\""
svc.example.net. 2H IN HTTPSSVC 1 3 . "h2=\":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 HTTPSSVC record type closely resembles 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 RRTYPE
An SRV record [RFC2782] can perform a similar function to the
HTTPSSVC 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
HTTPSSVC.
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 Alt-Svc and thus HTTPSSVC
can be extended with new parameters. For example, this is what
allows the incorporation of ESNI keys in HTTPSSVC.
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o Using SRV records would not allow a 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 HTTPSSVC 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 HTTPSSVC 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.
Unlike ESNI, this is focused on the specific case of HTTPS, although
this approach could be extended for other protocols. It also allows
specifying ESNI keys for a specific port, not an entire host.
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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 HTTPSSVC 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. The "HTTPSSVC" is proposed as a placeholder. Other
names for this record might include ALTSVC, HTTPS, HTTPSSRV, B, or
something else.
C.2. Applicability to other schemes
The focus of this record is on optimizing the common case of the
"https" scheme. It is worth discussing whether this is a valid
assumption or if a more general solution is applicable. Past efforts
to over-generalize have not met with broad success.
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. Extensibility of SvcRecordType
Only values of "0" and "1" are allowed for SvcRecordType. Should we
give more thought to potential future values? The current version
tries to leave this open by indicating that resource records with
unknown SvcRecordType values should be ignored (and perhaps should be
switched to MUST be ignored)?
C.5. 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 when SvcRecordType=0. A related question is whether to omit it
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from the textual representation when SvcRecordType=0. Regardless,
having a series of sequential numeric values in the textual
representation has risk of user error, especially as MX, SRV, and
others all have their own variations here.
C.6. 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 Alt-Svc attribute.
Appendix D. Change history
o draft-nygren-httpbis-httpssvc-03
* Change redirect type for HSTS-style behavior from 302 to 307 to
reduce ambiguities.
o draft-nygren-httpbis-httpssvc-02
* Remove the redundant length fields from the wire format.
* Define a SvcDomainName of "." for SvcRecordType=1 as being the
HTTPSSVC RRNAME.
* Replace "hq" with "h3".
o draft-nygren-httpbis-httpssvc-01
* Fixes of record name. Replace references to "HTTPSVC" with
"HTTPSSVC".
o draft-nygren-httpbis-httpssvc-00
* Initial version
Authors' Addresses
Ben Schwartz
Google
Email: bemasc@google.com
Mike Bishop
Akamai Technologies
Email: mbishop@evequefou.be
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Erik Nygren
Akamai Technologies
Email: erik+ietf@nygren.org
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