Incrementally Deployable Extensible Delegation for DNS
draft-homburg-deleg-incremental-deleg-00
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| Last updated | 2024-07-08 | ||
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draft-homburg-deleg-incremental-deleg-00
DNS Delegation P. Homburg
Internet-Draft NLnet Labs
Intended status: Standards Track J. V. Zutphen
Expires: 9 January 2025 University of Amsterdam
W. Toorop
NLnet Labs
8 July 2024
Incrementally Deployable Extensible Delegation for DNS
draft-homburg-deleg-incremental-deleg-00
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.
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Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Signaling capabilities of the authoritative name
servers . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Outsourcing operation of the delegation . . . . . . . . . 4
1.3. DNSSEC protection of the delegation . . . . . . . . . . . 4
1.4. Maximize ease of deployment . . . . . . . . . . . . . . . 5
1.5. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2. Delegation administration . . . . . . . . . . . . . . . . . . 6
2.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1.1. One name server within the subzone . . . . . . . . . 7
2.1.2. Two name servers within the subzone . . . . . . . . . 7
2.1.3. Outsourced to an operator . . . . . . . . . . . . . . 7
2.1.4. DNSSEC signed name servers within the subzone . . . . 8
3. Minimal implementation . . . . . . . . . . . . . . . . . . . 10
3.1. Recursive Resolver behavior . . . . . . . . . . . . . . . 10
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3.2. _deleg label presence . . . . . . . . . . . . . . . . . . 12
4. Optimized implementation . . . . . . . . . . . . . . . . . . 13
4.1. Authoritative name server support . . . . . . . . . . . . 13
4.2. Resolver behavior with authoritative name server
support . . . . . . . . . . . . . . . . . . . . . . . . . 17
5. Extra optimized implementation . . . . . . . . . . . . . . . 18
6. Limitations . . . . . . . . . . . . . . . . . . . . . . . . . 19
6.1. Outsourcing to more than one operator . . . . . . . . . . 19
6.2. Priming queries . . . . . . . . . . . . . . . . . . . . . 19
7. Comparison with other delegation mechanisms . . . . . . . . . 19
7.1. Comparison with legacy delegations . . . . . . . . . . . 21
7.2. Comparison with Name DNS Query Name Minimisation . . . . 21
7.3. Comparison with I-D.dnsop-deleg . . . . . . . . . . . . . 21
8. Implementation Status . . . . . . . . . . . . . . . . . . . . 21
9. Security Considerations . . . . . . . . . . . . . . . . . . . 22
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
11.1. Normative References . . . . . . . . . . . . . . . . . . 22
11.2. Informative References . . . . . . . . . . . . . . . . . 23
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
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].)
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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.
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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:
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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".
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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
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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:
$ORIGIN operator1.example.
@ IN SOA ns zonemaster ...
_dns.ns IN SVCB 1 ns ( alpn=h2,dot,h3,doq
ipv4hint=192.0.2.1
ipv6hint=2001:db8:3::1
dohpath=/q{?dns}
)
IN SVCB 2 ns ( ipv4hint=192.0.2.2
ipv6hint=2001:db8:3::2
)
ns IN AAAA 2001:db8:3::1
IN AAAA 2001:db8:3::2
IN A 192.0.2.1
IN A 192.0.2.2
Figure 5: Operator zone
Section 2.4.2 of [RFC9460] states that SVCB RRsets SHOULD only have a
single RR in AliasMode, and that if multiple AliasMode RRs are
present, clients or recursive resolvers SHOULD pick one at random.
Section 2.4.1 of [RFC9460] states that within an SVCB RRset, all RRs
SHOULD have the same mode, and that if an RRset contains a record in
AliasMode, the recipient MUST ignore any ServiceMode records in the
set.
2.1.4. DNSSEC signed name servers within the subzone
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$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
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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
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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
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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.
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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:
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;; ->>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:
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;; ->>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:
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;; ->>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.
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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.
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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.
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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.
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+=+====+=========+=========++=====================+================+
| |apex| support | _deleg || <sub>._deleg.<apex> | _deleg.<apex> |
| | | | || SVCB | A |
+=+====+=========+=========++=====================+================+
|1|Yes | * | * || | |
+-+----+---------+---------++---------------------+----------------+
|2| No | * | No || | |
+-+----+---------+---------++---------------------+----------------+
|3| No | Yes | * || | |
+-+----+---------+---------++---------------------+----------------+
|4| No | Unknown | Yes || X | |
+-+----+---------+---------++---------------------+----------------+
|5| No | Unknown | Unknown || X | only for |
| | | | || | unsigned zones |
+-+----+---------+---------++---------------------+----------------+
Table 1: Additional queries in parallel to the legacy query
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
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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).
7.1. Comparison with legacy delegations
7.2. Comparison with Name DNS Query Name Minimisation
7.3. Comparison with [I-D.dnsop-deleg]
+=================================+==============================+
| [I-D.dnsop-deleg] | [this document] |
+=================================+==============================+
| Requires implementation in both | Only resolver implementation |
| authoritative name server as | required. But optimized |
| well as in the resolver | with updated authoritative |
| | software. |
+---------------------------------+------------------------------+
| DNSKEY flag needed to signal | No DNSKEY flag needed. |
| DELEG support with all | Separation of concerns. |
| authoritative name servers that | |
| serve the parent (delegating) | |
| domain. Special requirements | |
| for the child domain. | |
+---------------------------------+------------------------------+
| Authoritative name servers need | Authoritative name servers |
| to be updated all at once | may be updated gradually for |
| | optimization |
+---------------------------------+------------------------------+
| New semantics about what is | Works with current DNS and |
| authoritative (BOGUS with | DNSSEC semantics. Easier to |
| current DNSSEC validators) | implement. |
+---------------------------------+------------------------------+
| No extra queries | An extra query, in parallel |
| | to the legacy query, _per |
| | authoritative_ server when |
| | incremental deleg support is |
| | not yet detected, and _per |
| | unsigned zone_ to determine |
| | presence of the _deleg label |
+---------------------------------+------------------------------+
Table 2: Comparison of [I-D.dnsop-deleg] with [this document]
8. Implementation Status
*Note to the RFC Editor*: please remove this entire section before
publication.
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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:
+=========+============+=================+
| RR Type | _NODE NAME | Reference |
+=========+============+=================+
| SVCB | _deleg | [this document] |
+---------+------------+-----------------+
Table 3: Entry in the Underscored and
Globally Scoped DNS Node Names
registry
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, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC6672] Rose, S. and W. Wijngaards, "DNAME Redirection in the
DNS", RFC 6672, DOI 10.17487/RFC6672, June 2012,
<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, May
2016, <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,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
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[RFC8484] 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>.
[RFC8767] Lawrence, D., Kumari, W., and P. Sood, "Serving Stale Data
to Improve DNS Resiliency", RFC 8767,
DOI 10.17487/RFC8767, March 2020,
<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, November 2021,
<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, May 2022,
<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,
November 2023, <https://www.rfc-editor.org/rfc/rfc9460>.
[RFC9461] Schwartz, B., "Service Binding Mapping for DNS Servers",
RFC 9461, DOI 10.17487/RFC9461, November 2023,
<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, November 1987,
<https://www.rfc-editor.org/info/rfc1034>.
Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <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/>.
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[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, 23 January 2024,
<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, 8
July 2024, <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, 4 March 2024,
<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, 13 July 2020,
<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, March 2019,
<https://www.rfc-editor.org/rfc/rfc8552>.
[RFC9499] Hoffman, P. and K. Fujiwara, "DNS Terminology", BCP 219,
RFC 9499, DOI 10.17487/RFC9499, March 2024,
<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].
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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
Email: philip@nlnetlabs.nl
Jesse van Zutphen
University of Amsterdam
Email: Jesse.vanZutphen@os3.nl
Willem Toorop
NLnet Labs
Email: willem@nlnetlabs.nl
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