Internet-Draft incremental-deleg July 2024
Homburg, et al. Expires 9 January 2025 [Page]
Workgroup:
DNS Delegation
Internet-Draft:
draft-homburg-deleg-incremental-deleg-latest
Published:
Intended Status:
Standards Track
Expires:
Authors:
P. Homburg
NLnet Labs
J. V. Zutphen
University of Amsterdam
W. Toorop
NLnet Labs

Incrementally Deployable Extensible Delegation for DNS

Abstract

This document proposes a mechanism for extensible delegations in the DNS. The mechanism realizes delegations with SVCB resource record sets placed below a _deleg label in the apex of the delegating zone. This authoritative delegation point can be aliased to other names using CNAME and DNAME. The mechanism inherits extensibility from SVCB.

Support in recursive resolvers suffices for the mechanism to be fully functional. The number of subsequent interactions between the recursive resolver and the authoritative name servers is comparable with those for DNS Query Name Minimisation. Additionally, but not required, support in the authoritative name servers enables optimized behavior with reduced (simultaneous) queries. None, mixed or full deployment of the mechanism on authoritative name servers are all fully functional, allowing for the mechanism to be incrementally deployed.

About This Document

This note is to be removed before publishing as an RFC.

Status information for this document may be found at https://datatracker.ietf.org/doc/draft-homburg-deleg-incremental-deleg/.

Discussion of this document takes place on the deleg Working Group mailing list (mailto:dd@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/dd/. Subscribe at https://www.ietf.org/mailman/listinfo/dd/.

Source for this draft and an issue tracker can be found at https://github.com/NLnetLabs/incremental-deleg.

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 9 January 2025.

▲

Table of Contents

1. Introduction

This document describes a delegation mechanism for the Domain Name System (DNS) [STD13] that addresses several matters that, at the time of writing, are suboptimally supported or not at all. These matters are elaborated upon in sections 1.1, 1.2 and 1.3. In addition, the mechanism described in this document aspires to be maximally deployable, which is elaborated upon in Section 1.4.

1.1. Signaling capabilities of the authoritative name servers

The SVCB Resource Record (RR) type [RFC9460] in "dns" service mode [RFC9461] is used in which capability signalling of DNS over Transport Layer Protocol [RFC7858] (DoT), DNS Queries over HTTPS [RFC8484] and DNS over Dedicated QUIC Connections [RFC9250], on default or alternative ports, are already specified. The SVCB RR type is designed to be extensible to support future uses (such as keys for encrypting the TLS ClientHello [I-D.ietf-tls-esni].)

1.2. Outsourcing operation of the delegation

Delegation information is stored at an authoritative location in the zone with this mechanism. Legacy methods to redirect this information to another location, possible under the control of another operator, such as (CNAME Section 3.6.2 of [RFC1034]) and DNAME [RFC6672] remain functional. One could even outsource all delegation operational practice to another party with an DNAME on the _deleg label on the apex of the delegating zone.

Additional to the legacy methods, a delegation may be outsourced to a third party by having an SVCB RRset with a single SVCB RR in AliasMode.

1.3. DNSSEC protection of the delegation

With legacy delegations, the NS RRset at the parent side of a delegation as well as glue records for the names in the NS RRset are not authoritative and not DNSSEC signed. An adversary that is able to spoof a referral response, can alter this information and redirect all traffic for the delegation to a rogue name server undetected. The adversary can then perceive all queries for the redirected zone (Privacy concern) and alter all unsigned parts of responses (such as further referrals, which is a Security concern).

DNSSEC protection of delegation information prevents that, and is the only countermeasure that also works against on-path attackers. At the time of writing, the only way to DNSSEC validate and verify delegations at all levels in the DNS hierarchy is to revalidate delegations [I-D.ietf-dnsop-ns-revalidation], which is done after the fact and has other security concerns (Section 7 of [I-D.ietf-dnsop-ns-revalidation]).

Direct delegation information (provided by SVCB RRs in ServiceMode) includes the hostnames of the authoritative name servers for the delegation as well as IP addresses for those hostnames. Since the information is stored authoritatively in the delegating zone, it will be DNSSEC signed if the zone is signed. When the delegation is outsourced, then it's protected when the zones providing the aliasing resource records and the zones serving the targets of the aliases are all DNSSEC signed.

1.4. Maximize ease of deployment

Delegation information is stored authoritatively within the delegation zone. No semantic changes as to what zones are authoritative for what data are needed. As a consequence, existing DNS software, such as authoritative name servers and DNSSEC signing software, can remain unmodified. Unmodified authoritative name server software will serve the delegation information when queried for. Unmodified signers will sign the delegation information in the delegating zone. Only the recursive resolver needs modification to follow referrals as provided by the delegation information.

Such a resolver would explicitly query for the delegations administered as specified in Section 2. The number of round trips from the recursive resolver to the authoritative name server is comparable to what is needed for DNS Query Name Minimisation [RFC9156]. Additional implementation in the authoritative name server optimizes resolution and reduces the number of simultaneous in parallel queries to that what would be needed for legacy delegations. None, mixed or full deployment of the mechanism on authoritative name servers are all fully functional, allowing for the mechanism to be incrementally deployed on the authoritative name servers.

Implementation in the recursive may be less demanding with respect to (among other things) DNSSEC validation because there is no need to make additional exceptions as to what is authoritative at the parent side of a delegation.

1.5. 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.

This document follows terminology as defined in [RFC9499].

Throughout this document we will also use terminology with the meaning as defined below:

Incremental deleg:

The delegation mechanism as specified in this document.

Incremental delegation:

A delegation as specified in this document

Legacy delegations:

The way delegations are done in the DNS traditionally as defined in [STD13].

Delegating zone:

The zone in which the delegation is administered. Sometimes also called the "parent zone" of a delegation.

Subzone:

The zone that is delegated to from the delegating zone.

Delegating name:

The name which realizes the delegation. In legacy delegations, this name is the same as the name of the subzone to which the delegation refers. Delegations described in this document are provided with a different name than the zone that is delegated to.

Delegation point:

The location in the delegating zone where the RRs are provided that make up the delegation. In legacy delegations, this is the parent side of the zone cut and has the same name as the subzone. With incremental deleg, this is the location given by the delegating name.

Triggering query:

The query on which resolution a recursive resolver is working.

Target zone:

The zone for which the authoritative servers, that a resolver contacts while iterating, are authoritative.

2. Delegation administration

An extensible delegation is realized with an SVCB Resource Record set (RRset) [RFC9460] below a specially for the purpose reserved label with the name _deleg at the apex of the delegating zone. The _deleg label scopes the interpretation of the SVCB records and requires registration in the "Underscored and Globally Scoped DNS Node Names" registry (see IANA Considerations (Section 10)). The full scoping of delegations includes the labels that are below the _label and those, together with the name of the delegating domain, make up the name of the subzone to which the delegation refers. For example, if the delegating zone is example., then a delegation to subzone customer.example. is realized by an SVCB RRset at the name customer._deleg.example. in the parent zone. A fully scoped delegating name (such as customer._deleg.example.) is referred to further in this document as the "delegation point".

The use of the SVCB RR type requires a mapping document for each service type. This document uses the SVCB for the "dns" service type and the contents of the SVCB SvcParams MUST be interpreted as specified in Service Binding Mapping for DNS Servers [RFC9461]. At the delegation point (for example customer._deleg.example.), the host names of the authoritative name servers for the subzone, are given in the TargetName RDATA field of SVCB records in ServiceMode. Port Prefix Naming Section 3 of [RFC9461] is not used at the delegation point, but MUST be used when resolving the aliased to name servers with "dns" service type SVCB RRs in AliasMode.

Note that if the delegation is outsourcing to a single operator represented in a single SVCB RRset, it is RECOMMENDED to refer to the name of the operator's SVCB RRset with a CNAME on the delegation point instead of an SVCB RR in AliasMode Section 10.2 of [RFC9460].

2.1. Examples

2.1.1. One name server within the subzone 

$ORIGIN example.
@                  IN  SOA   ns zonemaster ...
customer1._deleg   IN  SVCB  1 ( ns.customer1
                                 ipv4hint=198.51.100.1,203.0.113.1
                                 ipv6hint=2001:db8:1::1,2001:db8:2::1
                               )
Figure 1: One name server within the subzone

2.1.2. Two name servers within the subzone 

$ORIGIN example.
@                  IN  SOA   ns zonemaster ...
customer2._deleg   IN  SVCB  1 ns1.customer2 ( ipv4hint=198.51.100.1
                                               ipv6hint=2001:db8:1::1
                                             )
                   IN  SVCB  1 ns2.customer2 ( ipv4hint=203.0.113.1
                                               ipv6hint=2001:db8:2::1
                                             )
Figure 2: Two name servers within the subzone

2.1.3. Outsourced to an operator 

$ORIGIN example.
@                  IN  SOA   ns zonemaster ...
customer3._deleg   IN  CNAME _dns.ns.operator1
Figure 3: Outsourced with CNAME

Instead of using CNAME, the outsourcing could also been accomplished with an SVCB RRset with a single SVCB RR in AliasMode. The configuration below is operationally equivalent to the CNAME configuration above. It is RECOMMENDED to use a CNAME over an SVCB RRset with a single SVCB RR in AliasMode (Section 10.2 of [RFC9460]). Note that an SVCB RRset refers with TargetName to an DNS service, which will be looked up using Port Prefix Naming Section 3 of [RFC9461], but that CNAME refers to the domain name of the target SVCB RRset (or CNAME) which may have an _dns prefix.

$ORIGIN example.
@                  IN  SOA   ns zonemaster ...
customer3._deleg   IN  SVCB 0 ns.operator1
Figure 4: Outsourced with an AliasMode SVCB RR

The operator SVCB RRset could for example be:

$ORIGIN operator1.example.
@                  IN  SOA   ns zonemaster ...
_dns.ns            IN  SVCB  1 ns ( alpn=h2,dot,h3,doq
                                    ipv4hint=192.0.2.1
                                    ipv6hint=2001:db8:3::1
                                    dohpath=/q{?dns}
                                  )
                   IN  SVCB  2 ns ( ipv4hint=192.0.2.2
                                    ipv6hint=2001:db8:3::2
                                  )
ns                 IN  AAAA  2001:db8:3::1
                   IN  AAAA  2001:db8:3::2
                   IN  A     192.0.2.1
                   IN  A     192.0.2.2
Figure 5: Operator zone

Section 2.4.2 of [RFC9460] states that SVCB RRsets SHOULD only have a single RR in AliasMode, and that if multiple AliasMode RRs are present, clients or recursive resolvers SHOULD pick one at random. Section 2.4.1 of [RFC9460] states that within an SVCB RRset, all RRs SHOULD have the same mode, and that if an RRset contains a record in AliasMode, the recipient MUST ignore any ServiceMode records in the set.

2.1.4. DNSSEC signed name servers within the subzone 

$ORIGIN
@                 IN  SOA    ns zonemaster ...
                  IN  RRSIG  SOA ...
                  IN  DNSKEY 257 3 15 ...
                  IN  RRSIG  DNSKEY ...
                  IN  NS     ns.example.
                  IN  NSEC   customer5._deleg SOA RRSIG NSEC DNSKEY
                  IN  RRSIG  NSEC ...

customer5._deleg  IN  SVCB   1 ns.customer5 alpn=h2,h3 (
                                            ipv4hint=198.51.100.5
                                            ipv6hint=2001:db8:5::1
                                            dohpath=/dns-query{?dns}
                                            )
                  IN  RRSIG  SVCB ...
                  IN  NSEC   customer7._deleg RRSIG NSEC SVCB
                  IN  RRSIG  NSEC ...

customer7._deleg  IN  CNAME  customer5._deleg
                  IN  RRSIG  CNAME ...
                  IN  NSEC   customer5 CNAME RRSIG NSEC
                  IN  RRSIG  NSEC ...

customer5         IN  NS     ns.customer5
ns.customer5      IN  A      198.51.100.5
                  IN  AAAA   2001:db8:5::1
customer5         IN  DS     17405 15 2 ...
                  IN  RRSIG  DS ...
                  IN  NSEC   customer6 NS DS RRSIG NSEC
                  IN  RRSIG  NSEC ...

customer6         IN  NS     ns.customer6
ns.customer6      IN  A      203.0.113.1
                  IN  AAAA   2001:db8:6::1
customer6         IN  DS     ...
                  IN  RRSIG  DS ...
                  IN  NSEC   customer7 NS DS RRSIG NSEC
                  IN  RRSIG  NSEC ...

customer7         IN  NS     ns.customer5
                  IN  DS     ...
                  IN  RRSIG  DS ...
                  IN  NSEC   example. NS DS RRSIG NSEC
                  IN  RRSIG  NSEC ...
Figure 6: DNSSEC signed incremental deleg zone

customer5.example. is delegated to in an extensible way and customer6.example. is delegated only in a legacy way. customer7.example.'s delegation is outsourced to customer5's delegation.

The delegation signals that the authoritative name server supports DoH. customer5.example., customer6.example. and example. are all DNSSEC signed. The DNSSEC authentication chain links from example. to customer5.example. in the for DNSSEC conventional way with the signed customer5.example. DS RRset in the example. zone. Also, customer6.example. is linked to from example. with the signed customer6.example. DS RRset in the example. zone.

Note that both customer5.example. and customer6.example. have legacy delegations in the zone as well. It is important to have those legacy delegations to maintain support for legacy resolvers, that do not support incremental deleg. DNSSEC signers SHOULD construct the NS RRset and glue for the legacy delegation from the SVCB RRset.

3. Minimal implementation

Support in recursive resolvers suffices for the mechanism to be fully functional. Section 3.1 specifies the basic algorithm for resolving incremental delegations. In Section 3.2, an optimization is presented that will reduce the number of (parallel) queries especially for when authoritative name server support is still lacking and there are still many zones that do not contain incremental delegations.

3.1. Recursive Resolver behavior

If the triggering query name is the same as the name of the target zone apex, then no further delegation will occur, and resolution will complete. No special behavior or processing is needed.

Otherwise, the triggering query is below the target zone apex and a delegation may exist in the target zone. In this case two parallel queries MUST be sent. One for the triggering query in the way that is conventional with legacy delegations (which could be just the triggering query or a minimised query [RFC9156]), and one incremental deleg query with query type SVCB.

The incremental deleg query name is constructed by concatenating the first label below the part that the triggering query name has in common with the target zone, a _deleg label and the name of the target zone. For example if the triggering query is www.customer.example. and the target zone example., then the incremental deleg query name is customer._deleg.example. For another example, if the triggering query is www.faculty.university.example. and the target zone example. then the incremental deleg name is university._deleg.example.

Normal DNAME, CNAME and SVCB in AliasMode processing should happen as before, though note that when following an SVCB RR in AliasMode, the target name MUST have the _dns label prepended to the TargetName in the SVCB RR. The eventual incremental deleg query response, after following all redirections caused by DNAME, CNAME and AliasMode SVCB RRs, has three possible outcomes:

  1. An SVCB RRset in ServiceMode is returned in the response's answer section containing the delegation for the subzone.

    The SVCB RRs in the RRset MUST be used to follow the referral. The TargetName data field in the SVCB RRs in the RRset MUST be used as the names for the name servers to contact for the subzone, and the ipv4hint and ipv6hint parameters MUST be used as the IP addresses for the TargetName in the same SVCB RR.

    The NS RRset and glue, in the response of the legacy query that was sent in parallel to the incremental deleg query, MUST NOT be used, but the signed DS record (or NSEC(3) records indicating that there was no DS) MUST be used in linking the DNSSEC authentication chain as which would conventionally be done with DNSSEC as well.

  2. The incremental deleg query name does not exist (NXDOMAIN).

    There is no incremental delegation for the subzone, and the referral response for the legacy delegation MUST be processed as would be done with legacy DNS and DNSSEC processing.

  3. The incremental deleg query name does exist, but resulted in a NOERROR no answer response (also known as a NODATA response).

    If the legacy query, did result in a referral for the same number of labels as the subdomain that the incremental deleg query was for, then there was no incremental delegation for the subzone, and the referral response for the legacy delegation MUST be processed as would be done with legacy DNS and DNSSEC processing.

    Otherwise, the subzone may be more than one label below the delegating zone.

    If the response to the legacy query resulted in a referral, then a new incremental deleg query MUST be spawned, matching the zone cut of the legacy referral response. For example if the triggering query is www.university.ac.example. and the target zone example., and the legacy response contained an NS RRset for university.ac.example., then the incremental deleg query name is university.ac._deleg.example. The response to the new incremental deleg query MUST be processed as described above, as if it was the initial incremental deleg query.

    If the legacy query was sent minimised and needs a followup query, then parallel to that followup query a new incremental deleg query will be sent, adding a single label to the previous incremental deleg query name. For example if the triggering query is www.university.ac.example. and the target zone is example. and the minimised legacy query name is ac.example. (which also resulted in a NOERROR no answer response), then the incremental deleg query to be sent along in parallel with the followup legacy query will become university.ac.example. Processing of the responses of those two new queries will be done as described above.

3.2. _deleg label presence

Absence of the _deleg label in a zone is a clear signal that the zone does not contain any incremental deleg delegations. Recursive resolvers SHOULD NOT send any additional incremental deleg queries for zones for which it is known that it does not contain the _deleg label at the apex. The state regarding the presence of the _deleg label within a resolver can be "unknown", "known not to be present", or "known to be present".

The state regarding the presence of the _deleg label can be deduced from the response of the incremental deleg query, if the target zone is signed with DNSSEC. If the target zone is unsigned, the procedure as described in the remainder of this section SHOULD be followed.

When the presence of a _deleg label is "unknown", a _deleg presence test query SHOULD be sent in parallel to the next query for the unsigned target zone (though not when the target name server is known to support incremental deleg, which will be discussed in Section 4.1). The query name for the test query is the _deleg label prepended to the apex of zone for which to test presence, with query type A.

The testing query can have three possible outcomes:

  1. The _deleg label does not exist within the zone, and an NXDOMAIN response is returned.

    The non-existence of the _deleg label MUST be cached for the duration indicated by the "minimum" RDATA field of the SOA resource record in the authority section, adjusted to the boundaries for TTL values that the resolver has (Section 4 of [RFC8767]). For the period the non-existence of the _deleg label is cached, the label is "known not to be present" and the resolver SHOULD NOT send any (additional) incremental deleg queries.

  2. The _deleg label does exist within the zone but contains no data. A NOERROR response is returned with no RRs in the answer section.

    The existence of the _deleg name MUST be cached for the duration indicated by the "minimum" RDATA field of the SOA resource record in the authority section, adjusted to the resolver's TTL boundaries. For the period the existence of the empty non-terminal at the _deleg label is cached, the label is "known to be present" and the resolver MUST send additional incremental deleg queries as described in TODO.

  3. The _deleg label does exist within the zone and contains data. A NOERROR response is return with an A RRset in the answer section.

    The resolver MUST record that the _deleg label is known to be present for a duration indicated by A RRset's TTL value, adjusted to the resolver's TTL boundaries, for example by caching the RRset. For the period any RRset at the _deleg label is cached, the label is "known to be present" and the resolver MUST send additional incremental deleg queries as described in TODO.

4. Optimized implementation

Support for authoritative name servers enables optimized query behavior by resolvers with reduced (simultaneous) queries. Section 4.1 specifies how incremental deleg supporting authoritative name servers return referral responses for delegations. In Section 4.2 we specify how resolvers can benefit from those authoritative servers.

4.1. Authoritative name server support

Incremental delegations supporting authoritative name servers include the incremental delegation information (or the NSEC(3) records showing the non-existence) in the authority section of referral responses to legacy DNS queries. For example, querying the zone from Figure 6 for www.customer5.example. A, will return the following referral response:

;; ->>HEADER<<- opcode: QUERY, rcode: NOERROR, id: 54349
;; flags: qr ; QUERY: 1, ANSWER: 0, AUTHORITY: 5, ADDITIONAL: 2
;; QUESTION SECTION:
;; www.customer5.example.       IN      A

;; ANSWER SECTION:

;; AUTHORITY SECTION:
customer5.example.      3600    IN      NS      ns.customer5.example.
customer5.example.      3600    IN      DS      ...
customer5.example.      3600    IN      RRSIG   DS ...
customer5._deleg.example.       3600    IN      SVCB    1 (
                ns.customer5.example. alpn=h2,h3
                ipv4hint=198.51.100.5 ipv6hint=2001:db8:5::1
                dohpath=/dns-query{?dns}
                )
customer5._deleg.example.       3600    IN      RRSIG   SVCB ...

;; ADDITIONAL SECTION:
ns.customer5.example.   3600    IN      A       198.51.100.5
ns.customer5.example.   3600    IN      AAAA    2001:db8:5::1

;; Query time: 0 msec
;; EDNS: version 0; flags: do ; udp: 1232
;; SERVER: 192.0.2.53
;; WHEN: Mon Jul  1 20:36:25 2024
;; MSG SIZE  rcvd: 456
Figure 7: An incremental deleg referral response

The referral response in Figure 7 includes the signed SVCB RRset in the authority section.

As another example, querying the zone from Figure 6 for www.customer6.example. A, will return the following referral response:

;; ->>HEADER<<- opcode: QUERY, rcode: NOERROR, id: 36574
;; flags: qr ; QUERY: 1, ANSWER: 0, AUTHORITY: 9, ADDITIONAL: 2
;; QUESTION SECTION:
;; www.customer6.example.       IN      A

;; ANSWER SECTION:

;; AUTHORITY SECTION:
customer6.example.      3600    IN      NS      ns.customer6.example.
customer6.example.      3600    IN      DS      ...
customer6.example.      3600    IN      RRSIG   DS ...
customer5._deleg.example.       1234    IN      NSEC    (
                customer7._deleg.example.  RRSIG NSEC SVCB )
customer5._deleg.example.       1234    IN      RRSIG   NSEC ...
example.        1234    IN      NSEC    (
                customer5._deleg.example.  NS SOA RRSIG NSEC DNSKEY )
example.        1234    IN      RRSIG   NSEC ...

;; ADDITIONAL SECTION:
ns.customer6.example.   3600    IN      A       203.0.113.1
ns.customer6.example.   3600    IN      AAAA    2001:db8:6::1

;; Query time: 0 msec
;; EDNS: version 0; flags: do ; udp: 1232
;; SERVER: 192.0.2.53
;; WHEN: Tue Jul  2 10:23:53 2024
;; MSG SIZE  rcvd: 744
Figure 8: Referral response without incremental deleg

Next to the legacy delegation, the incremental deleg supporting authoritative returns the NSEC(3) RRs needed to show that there was no incremental delegation in the referral response in Figure 8.

Querying the zone from Figure 6 for www.customer7.example. A, will return the following referral response:

;; ->>HEADER<<- opcode: QUERY, rcode: NOERROR, id: 9456
;; flags: qr ; QUERY: 1, ANSWER: 0, AUTHORITY: 7, ADDITIONAL: 2
;; QUESTION SECTION:
;; www.customer7.example.       IN      A

;; ANSWER SECTION:

;; AUTHORITY SECTION:
customer7.example.      3600    IN      NS      ns.customer5.example.
customer7.example.      3600    IN      DS      ...
customer7.example.      3600    IN      RRSIG   DS ...
customer7._deleg.example.       3600    IN      CNAME   (
                customer5._deleg.example. )
customer7._deleg.example.       3600    IN      RRSIG   CNAME ...
customer5._deleg.example.       3600    IN      SVCB    1 (
                ns.customer5.example. alpn=h2,h3
                ipv4hint=198.51.100.5 ipv6hint=2001:db8:5::1 )
customer5._deleg.example.       3600    IN      RRSIG   SVCB ...

;; ADDITIONAL SECTION:
ns.customer5.example.   3600    IN      A       198.51.100.5
ns.customer5.example.   3600    IN      AAAA    2001:db8:5::1

;; Query time: 0 msec
;; EDNS: version 0; flags: do ; udp: 1232
;; SERVER: 192.0.2.53
;; WHEN: Tue Jul  2 10:55:07 2024
;; MSG SIZE  rcvd: 593
Figure 9: Aliasing referral response

The incremental delegation of customer7.example. is aliased to the one that is also used by customer5.example. Since both delegations are in the same zone, the authoritative name server for example. returns both the CNAME realising the alias, as well as the SVCB RRset which is the target of the alias in Figure 9. In other cases an returned CNAME or SVCB RR in AliasMode may need further chasing by the resolver.

With unsigned zones, only if an incremental deleg delegation exists, the SVCB RRset (or CNAME) will be present in the authority section of referral responses. If the incremental deleg does not exist, then it is simply absent from the authority section and the referral response is indistinguishable from an non supportive authoritative.

4.2. Resolver behavior with authoritative name server support

Incremental deleg supporting authoritative name servers will include the incremental delegation information (or the NSEC(3) records showing the non-existence) in the authority section of referral responses. If it is known that an authoritative name server supports incremental deleg, then no direct queries for the incremental delegation need to be send in parallel to the legacy delegation query. A resolver SHOULD register that an authoritative name server supports incremental deleg when the authority section, of the returned referral responses from that authoritative name server, contains incremental delegegation information.

When the authority section of a referral response contains an SVCB RRset or a CNAME on the incremental delegation name, or valid NSEC(3) RRs showing the non-existence of such SVCB or CNAME RRset, then the resolver SHOULD register that the contacted authoritative name server supports incremental deleg for the duration indicated by the TTL for that SVCB, CNAME or NSEC(3) RRset, adjusted to the resolver's TTL boundaries, but only if it is longer than any already registered duration. Subsequent queries SHOULD NOT include incremental deleg queries, as described in Section 3.1, to be send in parallel for the duration support for incremental deleg is registered for the authoritative name server.

For example, in Figure 7, the SVCB RRset at the incremental delegation point has TTL 3600. The resolver should register that the contacted authoritative name server supports incremental deleg for (at least) 3600 seconds (one hour). All subsequent queries to that authoritative name server SHOULD NOT include incremental deleg queries to be send in parallel.

If the authority section contains more than one RRset making up the incremental delegation, then the RRset with the longest TTL MUST be taken to determine the duration for which incremental deleg support is registered.

For example, in Figure 9, both a CNAME and an SVCB RRset for the incremental delegation are included in the authority section. The longest TTL must be taken for incremental support registration, though because the TTL of both RRsets is 3600, it does not matter in this case.

With DNSSEC signed zones, support is apparent with all referral responses, with unsigned zones only from referral responses for which a incremental delegation exists.

If the resolver knows that the authoritative name server supports incremental deleg, and a DNSSEC signed zone is being served, then all referrals MUST contain either an incremental delegation, or NSEC(3) records showing that the delegation does not exist. If a referral is returned that does not contain an incremental delegation nor an indication that it does not exist, then the resolver MUST send an additional incremental deleg query to find the incremental delegation (or denial of its existence).

5. Extra optimized implementation

An SVCB RRset on an incremental delegation point, with a SVCB RR in AliasMode, aliasing to the root zone, MUST be interpreted to mean that the legacy delegation information MUST be used to follow the referral. All service parameters for such AliasMode (aliasing to the root) SVCB RRs on the incremental delegation point, MUST be ignored.

For example, such an SVCB RRset registered on the wildcard below the _deleg label on the apex of a zone, can signal that legacy DNS referrals MUST be used for both signed and unsigned zones:

$ORIGIN example.
@                  IN  SOA   ns zonemaster ...
*._deleg    86400  IN  SVCB  0 .
customer1._deleg   IN  SVCB  1 ( ns.customer1
                                 ipv4hint=198.51.100.1,203.0.113.1
                                 ipv6hint=2001:db8:1::1,2001:db8:2::1
                               )
customer3._deleg   IN  CNAME _dns.ns.operator1
Figure 10: Wildcard incremental deleg to control duration of detected support

Resolvers SHOULD register that an authoritative name server supports incremental deleg, if such an SVCB RRset is returned in the authority section of referral responses, for the duration of the TTL if the SVCB RRset, adjusted to the resolver's TTL boundaries, but only if it is longer than any already registered duration. Note that this will also be included in referral responses for unsigned zones, which would otherwise not have signalling of incremental deleg support by the authoritative name server. Also, signed zones need fewer RRs to indicate that no incremental delegation exists. The wildcard expansion already shows the closest encloser (i.e. _deleg.<apex>), so only one additional NSEC(3) is needed to show non-existence of the queried for name below the closest encloser.

This method of signalling that the legacy delegation MUST be used, is RECOMMENDED.

6. Limitations

6.1. Outsourcing to more than one operator

Section 2.4.1 of [RFC9460] states that within an SVCB RRset, all RRs SHOULD have the same mode, and that if an RRset contains a record in AliasMode, the recipient MUST ignore any ServiceMode records in the set. Section 2.4.2 of [RFC9460] states that SVCB RRsets SHOULD only have a single RR in AliasMode, and that if multiple AliasMode RRs are present, clients or recursive resolvers SHOULD pick one at random.

Currently this means that query load can be spread out over multiple operators (even though that is NOT RECOMMENDED), but operationally it would make more sense to allow a resolver to select from all the name servers from all the operators. Assumingly SVCB currently supports only a single AliasMode RR in an SVCB RRset because it would otherwise be impossible to interpret the SvcPriority from the SVCB RRsets that is aliased to. A possible solution could be to resolve all AliasMode RRs at the delegation point (though limited to a certain amount, say 8) and then let the resolver pick from all the SVCB RRs, ignoring SvcPriority.

6.2. Priming queries

Some zones, such as the root zone, are targeted directly from hints files. Information about which authoritative name servers and with capabilities, MAY be provided in an SVCB RRset directly at the _deleg label under the apex of the zone. Priming queries from a incremental deleg supporting resolver, MUST send an _deleg.<apex> SVCB query in parallel to the legacy <apex> NS query and process the content as if it was found through an incremental referral response.

7. Comparison with other delegation mechanisms

Table Table 1 provides an overview of when extra queries, in parallel to the legacy query, are sent.

Table 1: Additional queries in parallel to the legacy query
  apex support _deleg   <sub>._deleg.<apex> SVCB _deleg.<apex> A
1 Yes * *      
2 No * No      
3 No Yes *      
4 No Unknown Yes   X  
5 No Unknown Unknown   X only for unsigned zones

The three headers on the left side of the table mean the following:

apex:

Whether the query is for the apex of the target zone. "Yes" means an apex query, "No" means a query below the apex which may be delegated

support:

Whether or not the target authoritative server supports incremental deleg. "Yes" means it supports it and "Unknown" means support is not detected. "*" means it does not matter

_deleg:

Whether or not the _deleg label is present in the target zone (and thus incremental delegations)

On the right side of the table are the extra queries, to be sent in parallel with the legacy query. The _deleg presence test query (most right column) only needs to be sent to unsigned target zones, because its non-existence will be show in the NSEC(3) records showing the non-existence of the incremental delegation (second from right column).

If the query name is the same as the apex of the target zone, no extra queries need to be sent (Row 1). If the _deleg label is known not to exist in the target zone (Row 2) or if the target name server is known to support incremental deleg (Row 3), no extra queries need to be sent. Only if it is unknown that the target name server supports incremental deleg, and the _deleg label is known to exist in the target zone (Row 4) or it its not known whether or not the _deleg label exists (Row 5), and extra incremental deleg query is sent in parallel to the legacy query. If the target zone is unsigned, presence of the _deleg label needs to be tested explicitly (Row 5).

7.3. Comparison with [I-D.dnsop-deleg]

Table 2: Comparison of [I-D.dnsop-deleg] with [this document]
[I-D.dnsop-deleg] [this document]
Requires implementation in both authoritative name server as well as in the resolver Only resolver implementation required. But optimized with updated authoritative software.
DNSKEY flag needed to signal DELEG support with all authoritative name servers that serve the parent (delegating) domain. Special requirements for the child domain. No DNSKEY flag needed. Separation of concerns.
Authoritative name servers need to be updated all at once Authoritative name servers may be updated gradually for optimization
New semantics about what is authoritative (BOGUS with current DNSSEC validators) Works with current DNS and DNSSEC semantics. Easier to implement.
No extra queries An extra query, in parallel to the legacy query, per zone to determine presence of the _deleg label, and per authoritative server when incremental deleg support is not yet detected

8. Implementation Status

Note to the RFC Editor: please remove this entire section before publication.

Jesse van Zutphen has built a proof of concept implementation supporting delegations as specified in this document for the Unbound recursive resolver as part of his master thesis for the Security and Network Engineering master program of the University of Amsterdam. [JZUTPHEN] The source code of his implementation is available on github [DELEG4UNBOUND]

9. Security Considerations

TODO Security

10. IANA Considerations

Per [RFC8552], IANA is requested to add the following entry to the DNS "Underscored and Globally Scoped DNS Node Names" registry:

Table 3: Entry in the Underscored and Globally Scoped DNS Node Names registry
RR Type _NODE NAME Reference
SVCB _deleg [this document]

11. References

11.1. Normative References

[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/rfc/rfc2119>.
[RFC6672]
Rose, S. and W. Wijngaards, "DNAME Redirection in the DNS", RFC 6672, DOI 10.17487/RFC6672, , <https://www.rfc-editor.org/rfc/rfc6672>.
[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, , <https://www.rfc-editor.org/rfc/rfc7858>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/rfc/rfc8174>.
[RFC8484]
Hoffman, P. and P. McManus, "DNS Queries over HTTPS (DoH)", RFC 8484, DOI 10.17487/RFC8484, , <https://www.rfc-editor.org/rfc/rfc8484>.
[RFC8767]
Lawrence, D., Kumari, W., and P. Sood, "Serving Stale Data to Improve DNS Resiliency", RFC 8767, DOI 10.17487/RFC8767, , <https://www.rfc-editor.org/rfc/rfc8767>.
[RFC9156]
Bortzmeyer, S., Dolmans, R., and P. Hoffman, "DNS Query Name Minimisation to Improve Privacy", RFC 9156, DOI 10.17487/RFC9156, , <https://www.rfc-editor.org/rfc/rfc9156>.
[RFC9250]
Huitema, C., Dickinson, S., and A. Mankin, "DNS over Dedicated QUIC Connections", RFC 9250, DOI 10.17487/RFC9250, , <https://www.rfc-editor.org/rfc/rfc9250>.
[RFC9460]
Schwartz, B., Bishop, M., and E. Nygren, "Service Binding and Parameter Specification via the DNS (SVCB and HTTPS Resource Records)", RFC 9460, DOI 10.17487/RFC9460, , <https://www.rfc-editor.org/rfc/rfc9460>.
[RFC9461]
Schwartz, B., "Service Binding Mapping for DNS Servers", RFC 9461, DOI 10.17487/RFC9461, , <https://www.rfc-editor.org/rfc/rfc9461>.
[STD13]
Internet Standard 13, <https://www.rfc-editor.org/info/std13>.
At the time of writing, this STD comprises the following:
Mockapetris, P., "Domain names - concepts and facilities", STD 13, RFC 1034, DOI 10.17487/RFC1034, , <https://www.rfc-editor.org/info/rfc1034>.
Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, , <https://www.rfc-editor.org/info/rfc1035>.

11.2. Informative References

[DELEG4UNBOUND]
van Zutphen, J., "A proof of concept implementation of incremental deleg", n.d., <https://github.com/jessevz/unbound/>.
[I-D.dnsop-deleg]
April, T., Špaček, P., Weber, R., and Lawrence, "Extensible Delegation for DNS", Work in Progress, Internet-Draft, draft-dnsop-deleg-00, , <https://datatracker.ietf.org/doc/html/draft-dnsop-deleg-00>.
[I-D.ietf-dnsop-ns-revalidation]
Huque, S., Vixie, P. A., and W. Toorop, "Delegation Revalidation by DNS Resolvers", Work in Progress, Internet-Draft, draft-ietf-dnsop-ns-revalidation-07, , <https://datatracker.ietf.org/doc/html/draft-ietf-dnsop-ns-revalidation-07>.
[I-D.ietf-tls-esni]
Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS Encrypted Client Hello", Work in Progress, Internet-Draft, draft-ietf-tls-esni-18, , <https://datatracker.ietf.org/doc/html/draft-ietf-tls-esni-18>.
[I-D.tapril-ns2]
April, T., "Parameterized Nameserver Delegation with NS2 and NS2T", Work in Progress, Internet-Draft, draft-tapril-ns2-01, , <https://datatracker.ietf.org/doc/html/draft-tapril-ns2-01>.
[JZUTPHEN]
van Zutphen, J., "Extensible delegations in DNS Recursive resolvers", n.d., <https://nlnetlabs.nl/downloads/publications/extensible-deleg-in-resolvers_2024-07-08.pdf>.
[RFC8552]
Crocker, D., "Scoped Interpretation of DNS Resource Records through "Underscored" Naming of Attribute Leaves", BCP 222, RFC 8552, DOI 10.17487/RFC8552, , <https://www.rfc-editor.org/rfc/rfc8552>.
[RFC9499]
Hoffman, P. and K. Fujiwara, "DNS Terminology", BCP 219, RFC 9499, DOI 10.17487/RFC9499, , <https://www.rfc-editor.org/rfc/rfc9499>.

Acknowledgments

The idea to utilize SVCB based RRs to signal capabilities was first proposed by Tim April in [I-D.tapril-ns2].

The idea to utilize SVCB for extensible delegations (and also the idea described in this document) emerged from the DNS Hackathon at the IETF 118. The following participants contributed to this brainstorm session: Vandan Adhvaryu, Roy Arends, David Blacka, Manu Bretelle, Vladimír Čunát, Klaus Darilion, Peter van Dijk, Christian Elmerot, Bob Halley, Shumon Huque, Shane Kerr, David C Lawrence, Edward Lewis, George Michaelson, Erik Nygren, Libor Peltan, Ben Schwartz, Petr Špaček, Jan Včelák and Ralf Weber

Authors' Addresses

Philip Homburg
NLnet Labs
Jesse van Zutphen
University of Amsterdam
Willem Toorop
NLnet Labs