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title: Extensible Delegation for DNS abbrev: DELEG docname: draft-dnsop-deleg-transport-latest date: {DATE} category: std

ipr: trust200902 workgroup: dnsop area: Internet category: info keyword: Internet-Draft

stand_alone: no pi: [toc, sortrefs, symrefs]

author:

ins: T. April
name: Tim April
organization:
email: [email protected]

contributor:

name: Christian Elmerot
organization: Cloudflare
email: [email protected]

--- abstract

This document extends DELEG record, and SVCB records pointed to by DELEG record, as defined in {{?I-D.draft-dnsop-deleg}}, with ability to specify transport protocols and authentication parameters supported by name servers.

--- middle

Introduction

The new delegation mechanism based on DELEG record type allows to specify attributes a resolver can use when talking to a delegated authority. This document introduces parameters specific to different transport mechanism than the default udp/53 protocol.

Legacy DNS resolvers unaware of DELEG mechanism would continue to use the NS and DS records, while resolvers that understand DELEG and its associated parameters can efficiently switch to new transports.

Terminology

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.

Terminology regarding the Domain Name System comes from {{?BCP219}}, with addition terms defined here:

  • [tbd]
  • [tbd]

Introductory Examples

To introduce DELEG record with DNS-over-TLS {{?RFC7858}}, this example shows a possible response from an authoritative in the authority section of the DNS response when delegating to another nameserver.

example.com.  86400  IN DELEG  1 ns2.example.com. (
                alpn=dot
                tlsa="2 0 1 ABC..."
                ipv4hint=192.0.2.54
                ipv6hint=2001:db8:2423::3 )
example.com.  86400  IN NS     ns1.example.com.
ns1.example.com.    86400   IN  A  192.0.2.1
ns1.example.com     86400   IN  AAAA    2001:DB8::1

In this example, the authoritative nameserver is delegating using DNS-over-TLS. Like in SVCB, DELEG also offer the ability to use the AliasMode delegation. The example below shows an example where example.com is being delegated with a DELEG AliasMode record which can then be further resolved using standard SVCB to locate the actual parameters.

example.com.  86400  IN DELEG 0   config2.example.net.
example.com.  86400  IN NS     ns1.example.net.

The example.net authoritative server may return the following SVCB records in response to a query as directed by the above records.

config2.example.net 3600    IN SVCB ns2.example.net. (
                alpn=dot
                tlsa="2 0 1 ABC..."
                ipv4hint=192.0.2.54
                ipv6hint=2001:db8:2423::3 )

The above records indicate to the client that the actual configuration for the example.com zone can be found at config2.example.net.

Later sections of this document will go into more detail on the resolution process using these records.

Goal of the DELEG record

The primary goal of transport specification in DELEG records is to provide zone owners a way to signal to new clients how to connect to servers serving a child domain that can coexist with NS records in the same zone, and do not break software that does not support new transports.

SvcParams

All SvcParamKeys for the "dns" scheme {{?9461}} apply as specified. These are the "transport parameters", describing how to reach an endpoint.

The "alpn" transport parameter is OPTIONAL to include (unlike in SVCB-DNS, where it is generally required). If the "alpn" SvcParamKey is omitted, the only available transport is presumed to be unencrypted DNS over UDP/TCP port 53. Endpoints can indicate that insecure transport is not available by specifying "mandatory=alpn".

When TargetName is below the zone cut, DELEG ServiceMode records MUST include "ipv4hint" or "ipv6hint" SvcParamKeys. These keys provide the address glue, which enables the initial connection to this endpoint.

The following additional DNS SVCB parameters are defined for the DELEG and SVCB ServiceModes.

"tlsa"

The "tlsa" SvcParamKey is a transport parameter representing a TLSA RRset {{?RFC6698}} to be used when connecting to TargetName using a TLS-based transport. If present, this SvcParam SHOULD match the TLSA records whose base domain ({{?RFC6698, Section 3}}) is TargetName. Due to bootstrapping concerns, this SvcParamKey has been added to the DELEG record as the TLSA records might only be resolveable after the initial connection to the delegated nameserver was established. When this field is not present, certificate validation should be performed by either DANE or by traditional TLS certification validation using trusted root certification authorities.

The SvcParamValue is a non-empty value-list. The presentation and wire format of each value is the same as the presentation and wire format described for the TLSA record as defined in {{?RFC6698}}, sections 2.1 and 2.2 respectively. To avoid wasting resources in the parent zone, parents MAY reject RRSets containing "tlsa" SvcParams that use matching type 0 (exact match).

As a special case, the presentation value "disabled", corresponding to an empty value in wire format, indicates that DANE MUST NOT be used with this record.

To avoid a circular dependency, "tlsa" MUST appear in any DELEG record that is used to serve its own TargetName.

Resolvers that support TLS-based transports MUST adopt one of the following behaviors:

  1. Use DANE for authentication, and treat any endpoints lacking DANE support as incompatible.
  2. Use PKI for authentication, and treat any DANE-only endpoint as incompatible. In this behavior, compatibility with tlsa=disabled endpoints is REQUIRED.
  3. Support both DANE and PKI for authentication, preferring DANE if it is available for each endpoint.

This SvcParamKey MAY be used in any SVCB context where TLSA usage is defined.

;; The server is only authenticatable via DANE.  Any resolver that
;; lacks DANE support will treat this record as incompatible.
alpn=doq tlsa="..." mandatory=alpn

;; The server is only authenticatable via DANE, but also offers Do53
;; services.  All resolvers can access it, but only resolvers with
;; DANE support will use DoQ.
alpn=doq tlsa="..."

;; The server is authenticatable via PKI.  If TLSA records exist at
;; the SVCB-DANE owner names, it is also authenticatable via DANE.
;; This record cannot be used to serve the zone containing its TargetName.
alpn=doq

;; The server is only authenticatable via PKI.  Any resolver that
;; lacks PKI validation support will treat this record as incompatible.
alpn=doq tlsa=disabled mandatory=alpn

{: title="tlsa SvcParam Examples for DELEG"}

Deployment Considerations

The DELEG and SVCB records intends to replace the NS record while also adding additional functionality in order to support additional transports for the DNS. Below are discussions of considerations for deployment.

Availability

If a zone operator removes all NS records before DELEG and SVCB records are implemented by all clients, the availability of their zones will be impacted for the clients that are using non-supporting resolvers. In some cases, this may be a desired quality, but should be noted by zone owners and operators.

Privacy Considerations

All of the information handled or transmitted by this protocol is public information published in the DNS.

Security Considerations

TODO: Fill this section out

Resolution procedure

TODO: Define how resolver selects which server connect to, especially when faced with selection of multiple transports. I's probably resolver's policy decision, but it should be said explicitly.

Connection Failures

When a resolver attempts to access nameserver delegated by a DELEG or SVCB record, if a connection error occurs, such as a certificate mismatch or unreachable server, the resolver SHOULD attempt to connect to the other nameservers delegated to until either exhausting the list or the resolver's policy indicates that they should treat the resolution as failed.

The failure action when failing to resolve a name with DELEG/SVCB due to connection errors is dependent on the resolver operators policies. For resolvers which strongly favor privacy, the operators may wish to return a SERVFAIL when the DELEG/SVCB resolution process completes without successfully contacting a delegated nameserver(s) while opportunistic privacy resolvers may wish to attempt resolution using any NS records that may be present.

IANA Considerations

New SvcParamKey Values

This document defines new SvcParamKey values in the "Service Binding (SVCB) Parameter Registry".

SvcParamKey NAME Meaning Reference
TBD1 tlsa TLSA RRset (This Document)

--- back

Acknowledgments

This draft is heavily based on past work (draft-tapril-ns2) done by Tim April and thus extends the thanks to the people helping on this which are: John Levine, Erik Nygren, Jon Reed, Ben Kaduk, Mashooq Muhaimen, Jason Moreau, Jerrod Wiesman, Billy Tiemann, Gordon Marx and Brian Wellington.

TODO

RFC EDITOR: : PLEASE REMOVE THE THIS SECTION PRIOR TO PUBLICATION.

  • Write a security considerations section
  • Add the dohpath references RFC
  • Define and give examples for DNSSEC svcparams
  • worked out resolution example including AliasMode delegation
  • DoH URI teamplte does not include post

Change Log

RFC EDITOR: : PLEASE REMOVE THE THIS SECTION PRIOR TO PUBLICATION.

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