add B. Schwartz
Internet-Draft Google LLC
Intended status: Standards Track 1 February 2022
Expires: 5 August 2022
Service Binding Mapping for DNS Servers
draft-ietf-add-svcb-dns-02
Abstract
The SVCB DNS record type expresses a bound collection of endpoint
metadata, for use when establishing a connection to a named service.
DNS itself can be such a service, when the server is identified by a
domain name. This document provides the SVCB mapping for named DNS
servers, allowing them to indicate support for new transport
protocols.
Discussion Venues
This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the ADD Working Group
mailing list (add@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/add/.
Source for this draft and an issue tracker can be found at
https://github.com/bemasc/svcb-dns.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 5 August 2022.
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Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Identities and Names . . . . . . . . . . . . . . . . . . . . 3
3.1. Special case: non-default ports . . . . . . . . . . . . . 4
4. Applicable existing SvcParamKeys . . . . . . . . . . . . . . 4
4.1. alpn . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.2. port . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.3. Other applicable SvcParamKeys . . . . . . . . . . . . . . 5
5. New SvcParamKeys . . . . . . . . . . . . . . . . . . . . . . 5
5.1. dohpath . . . . . . . . . . . . . . . . . . . . . . . . . 5
6. Limitations . . . . . . . . . . . . . . . . . . . . . . . . . 6
7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 6
8. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8.1. Adversary on the query path . . . . . . . . . . . . . . . 7
8.1.1. Downgrade attacks . . . . . . . . . . . . . . . . . . 7
8.1.2. Redirection attacks . . . . . . . . . . . . . . . . . 7
8.2. Adversary on the transport path . . . . . . . . . . . . . 8
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
10.1. Normative References . . . . . . . . . . . . . . . . . . 8
10.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Mapping Summary . . . . . . . . . . . . . . . . . . 10
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
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1. Introduction
The SVCB record type [SVCB] provides clients with information about
how to reach alternative endpoints for a service, which may have
improved performance or privacy properties. The service is
identified by a "scheme" indicating the service type, a hostname, and
optionally other information such as a port number. A DNS server is
often identified only by its IP address (e.g. in DHCP), but in some
contexts it can also be identified by a hostname (e.g. "NS" records,
manual resolver configuration) and sometimes also a non-default port
number.
Use of the SVCB record type requires a mapping document for each
service type, indicating how a client for that service can interpret
the contents of the SVCB SvcParams. This document provides the
mapping for the "dns" service type, allowing DNS servers to offer
alternative endpoints and transports, including encrypted transports
like DNS over TLS and DNS over HTTPS.
2. Conventions and Definitions
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.
3. Identities and Names
SVCB record names (i.e. QNAMEs) are formed using Port-Prefix Naming
(Section 2.3 of [SVCB]), with a scheme of "dns". For example, SVCB
records for a DNS service identified as "dns1.example.com" would be
queried at "_dns.dns1.example.com".
In some use cases, the name used for retrieving these DNS records is
different from the server identity used to authenticate the secure
transport. To distinguish them, we use the following terms:
* Binding authority - The service name (Section 1.4 of [SVCB]) and
optional port number used as input to Port-Prefix Naming.
* Authentication name - The name used for secure transport
authentication. It must be a DNS hostname or a literal IP
address. Unless otherwise specified, it is the service name from
the binding authority.
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3.1. Special case: non-default ports
Normally, a DNS service is identified by an IP address or a domain
name. When connecting to the service using unencrypted DNS over UDP
or TCP, clients use the default port number for DNS (53). However,
in rare cases, a DNS service might be identified by both a name and a
port number. For example, the dns: URI scheme [DNSURI] optionally
includes an authority, comprised of a host and a port number (with a
default of 53). DNS URIs normally omit the authority, or specify an
IP address, but a hostname and non-default port number are allowed.
When the binding authority specifies a non-default port number, Port-
Prefix Naming places the port number in an additional a prefix on the
name. For example, if the binding authority is
"dns1.example.com:9953", the client would query for SVCB records at
"_9953._dns.dns1.example.com". If two DNS services operating on
different port numbers provide different behaviors, this arrangement
allows them to preserve the distinction when specifying alternative
endpoints.
4. Applicable existing SvcParamKeys
4.1. alpn
This key indicates the set of supported protocols (Section 6.1 of
[SVCB]). There is no default protocol, so the no-default-alpn key
does not apply, and the alpn key MUST be present.
If the protocol set contains any HTTP versions (e.g. "h2", "h3"),
then the record indicates support for DNS over HTTPS [DOH], and the
"dohpath" key MUST be present (Section 5.1). All keys specified for
use with the HTTPS record are also permissible, and apply to the
resulting HTTP connection.
If the protocol set contains protocols with different default ports,
and no port key is specified, then protocols are contacted separately
on their default ports. Note that in this configuration, ALPN
negotiation does not defend against cross-protocol downgrade attacks.
4.2. port
This key is used to indicate the target port for connection
(Section 6.2 of [SVCB]). If omitted, the client SHALL use the
default port for each transport protocol (853 for DNS over TLS [DOT],
443 for DNS over HTTPS).
This key is automatically mandatory if present. (See Section 7 of
[SVCB] for the definition of "automatically mandatory".)
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4.3. Other applicable SvcParamKeys
These SvcParamKeys from [SVCB] apply to the "dns" scheme without
modification:
* ech
* ipv4hint
* ipv6hint
Future SvcParamKeys may also be applicable.
5. New SvcParamKeys
5.1. dohpath
"dohpath" is a single-valued SvcParamKey whose value (both in
presentation and wire format) MUST be a URI Template [RFC6570]
encoded in UTF-8 [RFC3629]. If the "alpn" SvcParamKey indicates
support for HTTP, "dohpath" MUST be present, and clients MAY
construct a DNS over HTTPS URI Template as follows:
1. Let $HOST be the authentication name encoded as a "host" value
(Section 3.2.2 of [RFC3986]).
2. Let $PORT be the port from the "port" key if present, otherwise
443. (The binding authority's port number MUST NOT be used.)
3. Let $DOHPATH be the "dohpath" value, decoded from UTF-8.
4. The DNS over HTTPS URI Template is "https://$HOST:$PORT$DOHPATH".
The "dohpath" value MUST be chosen such that the resulting URI
Template is valid for use with DNS over HTTPS. For example, DNS over
HTTPS servers are required to support requests using GET and POST
methods. The GET method relies on the "dns" URI Template parameter,
and the POST method does not use it. Therefore, the URI Template is
required to make use of a "dns" variable, and result in a valid URI
whether or not "dns" is defined.
Clients SHOULD NOT query for any "HTTPS" RRs when using the
constructed URI Template. Instead, the SvcParams and address records
associated with this SVCB record SHOULD be used for the HTTPS
connection, with the same semantics as an HTTPS RR. However, for
consistency, service operators SHOULD publish an equivalent HTTPS RR,
especially if clients might learn this URI Template through a
different channel.
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6. Limitations
This document is concerned exclusively with the DNS transport, and
does not affect or inform the construction or interpretation of DNS
messages. For example, nothing in this document indicates whether
the service is intended for use as a recursive or authoritative DNS
server. Clients must know the intended use in their context.
7. Examples
* A resolver at "simple.example" that supports DNS over TLS on port
853 (implicitly, as this is its default port):
_dns.simple.example. 7200 IN SVCB 1 simple.example. alpn=dot
* A resolver at "doh.example" that supports only DNS over HTTPS (DNS
over TLS is not supported):
_dns.doh.example. 7200 IN SVCB 1 doh.example. (
alpn=h2 dohpath=/dns-query{?dns} )
* A resolver at "resolver.example" that supports:
- DNS over TLS on "resolver.example" ports 853 (implicit in
record 1) and 8530 (explicit in record 2), with
"resolver.example" as the Authentication Domain Name,
- DNS over HTTPS at https://resolver.example/dns-query{?dns}
(record 1), and
- an experimental protocol on fooexp.resolver.example:5353
(record 3):
_dns.resolver.example. 7200 IN SVCB 1 resolver.example. (
alpn=dot,h2,h3 dohpath=/dns-query{?dns} )
_dns.resolver.example. 7200 IN SVCB 2 resolver.example. (
alpn=dot port=8530 )
_dns.resolver.example. 7200 IN SVCB 3 fooexp (
port=5353 alpn=foo foo-info=... )
* A nameserver at "ns.example" whose service configuration is
published on a different domain:
_dns.ns.example. 7200 IN SVCB 0 _dns.ns.nic.example.
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8. Security Considerations
8.1. Adversary on the query path
This section considers an adversary who can add or remove responses
to the SVCB query.
During secure transport establishment, clients MUST authenticate the
server to its authentication name, which is not influenced by the
SVCB record contents. Accordingly, this draft does not mandate the
use of DNSSEC. This draft also does not specify how clients
authenticate the name (e.g. selection of roots of trust), which might
vary according to the context.
8.1.1. Downgrade attacks
This attacker cannot impersonate the secure endpoint, but it can
forge a response indicating that the requested SVCB records do not
exist. For a SVCB-reliant client ([SVCB], Section 3) this only
results in a denial of service. However, SVCB-optional clients will
generally fall back to insecure DNS in this case, exposing all DNS
traffic to attacks.
8.1.2. Redirection attacks
SVCB-reliant clients always enforce the authentication domain name,
but they are still subject to attacks using the transport, port
number, and "dohpath" value, which are controlled by this adversary.
By changing these values in the SVCB answers, the adversary can
direct DNS queries for $HOSTNAME to any port on $HOSTNAME, and any
path on "https://$HOSTNAME". If the DNS client uses shared TLS or
HTTP state, the client could be correctly authenticated (e.g. using a
TLS client certificate or HTTP cookie).
This behavior creates a number of possible attacks for certain server
configurations. For example, if "https://$HOSTNAME/upload" accepts
any POST request as a public file upload, the adversary could forge a
SVCB record containing dohpath=/upload. This would cause the client
to upload and publish every query, resulting in unexpected storage
costs for the server and privacy loss for the client. Similarly, if
two DoH endpoints are available on the same origin, and the service
has designated one of them for use with this specification, this
adversary can cause clients to use the other endpoint instead.
To mitigate redirection attacks, a client of this SVCB mapping MUST
NOT provide client authentication for DNS queries, except to servers
that it specifically knows are not vulnerable to such attacks. If an
endpoint sends an invalid response to a DNS query, the client SHOULD
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NOT send more queries to that endpoint. DNS services that are
identified by a hostname (Section 3) MUST ensure that all
unauthenticated DNS requests to that name receive any promised
privacy and security guarantees, regardless of transport, port
number, or HTTP path.
8.2. Adversary on the transport path
This section considers an adversary who can modify network traffic
between the client and the alternative service (identified by the
TargetName).
For a SVCB-reliant client, this adversary can only cause a denial of
service. However, because DNS is unencrypted by default, this
adversary can execute a downgrade attack against SVCB-optional
clients. Accordingly, when use of this specification is optional,
clients SHOULD switch to SVCB-reliant behavior if SVCB resolution
succeeds. Specifications making using of this mapping MAY adjust
this fallback behavior to suit their requirements.
9. IANA Considerations
Per [SVCB] IANA is directed to add the following entry to the SVCB
Service Parameters registry.
+========+=========+==============================+=================+
| Number | Name | Meaning | Reference |
+========+=========+==============================+=================+
| 7 | dohpath | DNS over HTTPS path template | (This |
| | | | document) |
+--------+---------+------------------------------+-----------------+
Table 1
Per [Attrleaf], IANA is directed to add the following entry to the
DNS Underscore Global Scoped Entry Registry:
+=========+============+===============+=================+
| RR TYPE | _NODE NAME | Meaning | Reference |
+=========+============+===============+=================+
| SVCB | _dns | DNS SVCB info | (This document) |
+---------+------------+---------------+-----------------+
Table 2
10. References
10.1. Normative References
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[DOH] Hoffman, P. and P. McManus, "DNS Queries over HTTPS
(DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
<https://www.rfc-editor.org/rfc/rfc8484>.
[DOT] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "Specification for DNS over Transport
Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
2016, <https://www.rfc-editor.org/rfc/rfc7858>.
[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/rfc/rfc2119>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/rfc/rfc3629>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/rfc/rfc3986>.
[RFC6570] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
and D. Orchard, "URI Template", RFC 6570,
DOI 10.17487/RFC6570, March 2012,
<https://www.rfc-editor.org/rfc/rfc6570>.
[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/rfc/rfc8174>.
[SVCB] Schwartz, B., Bishop, M., and E. Nygren, "Service binding
and parameter specification via the DNS (DNS SVCB and
HTTPS RRs)", Work in Progress, Internet-Draft, draft-ietf-
dnsop-svcb-https-08, 12 October 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-dnsop-
svcb-https-08>.
10.2. Informative References
[Attrleaf] Crocker, D., "Scoped Interpretation of DNS Resource
Records through "Underscored" Naming of Attribute Leaves",
BCP 222, RFC 8552, DOI 10.17487/RFC8552, March 2019,
<https://www.rfc-editor.org/rfc/rfc8552>.
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[DNSURI] Josefsson, S., "Domain Name System Uniform Resource
Identifiers", RFC 4501, DOI 10.17487/RFC4501, May 2006,
<https://www.rfc-editor.org/rfc/rfc4501>.
Appendix A. Mapping Summary
This table serves as a non-normative summary of the DNS mapping for
SVCB.
+=================+====================================+
+=================+====================================+
| *Mapped scheme* | "dns" |
+-----------------+------------------------------------+
| *RR type* | SVCB (64) |
+-----------------+------------------------------------+
| *Name prefix* | _dns for port 53, else _$PORT._dns |
+-----------------+------------------------------------+
| *Required keys* | alpn |
+-----------------+------------------------------------+
| *Automatically | port |
| Mandatory Keys* | |
+-----------------+------------------------------------+
| *Special | Supports all HTTPS RR SvcParamKeys |
| behaviors* | |
+-----------------+------------------------------------+
| | Overrides the HTTPS RR for DoH |
+-----------------+------------------------------------+
| | Default port is per-transport |
+-----------------+------------------------------------+
| | No encrypted -> cleartext fallback |
+-----------------+------------------------------------+
Table 3
Acknowledgments
Thanks to the many reviewers and contributors, including Daniel
Migault, Paul Hoffman, Matt Norhoff, Peter van Dijk, Eric Rescorla,
and Andreas Schulze.
Author's Address
Benjamin Schwartz
Google LLC
Email: bemasc@google.com
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