Extensible Delegation for DNS

Internet-Draft	DELEG	October 2024
April, et al.	Expires 24 April 2025	[Page]

Abstract

A delegation in the Domain Name System (DNS) is a mechanism that enables efficient and distributed management of the DNS namespace. It involves delegating authority over subdomains to specific DNS servers via NS records, allowing for a hierarchical structure and distributing the responsibility for maintaining DNS records.¶

An NS record contains the hostname of the nameserver for the delegated namespace. Any facilities of that nameserver must be discovered through other mechanisms. This document proposes a new extensible DNS record type, DELEG, which contains additional information about the delegated namespace and the capabilities of authoritative nameservers for the delegated namespace.¶

1. Introduction

In the Domain Name System [STD13], subdomains within the domain name hierarchy are indicated by delegations to servers which are authoritative for their portion of the namespace. The DNS records that do this, called NS records, contain hostnames of nameservers, which resolve to addresses. No other information is available to the resolver. It is limited to connect to the authoritative servers over UDP and TCP port 53. This limitation is a barrier for efficient introduction of new DNS technology.¶

The proposed DELEG record type remedies this problem by providing extensible parameters to indicate capabilities that a resolver may use for the delegated authority, for example that it should be contacted using a transport mechanism other than DNS over UDP or TCP on port 53.¶

DELEG records are served with NS and DS records in the Authority section of DNS delegation type responses. Standard behavior of legacy DNS resolvers is to ignore the DELEG type and continue to rely on NS and DS records (see compliance testing described in Appendix A). Resolvers that do understand DELEG and its associated parameters can efficiently switch to the new mechanism.¶

The DELEG record leverages the Service Binding (SVCB) record format defined in [RFC9460], using a subset of the already defined service parameters, however as DELEG creates a zone cut and requires special processing from authoritative name servers as well as resolvers it has to be a new record type similar in handling to the DS record type.¶

DELEG can use AliasMode, inherited from SVCB, to insert a level of indirection to ease the operational maintenance of multiple zones by the same servers. For example, an operator can have numerous customer domains all aliased to nameserver sets whose operational characteristics can be easily updated without intervention from the customers. Most notably, this provides a method for addressing the long-standing problem operators have with maintaining DS records on behalf of their customers. This type of facility will be handled in separate documents.¶

1.1. 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:¶

legacy name servers: An authoritative server that does not support the DELEG record.¶
legacy resolvers: A resolver that does not support the DELEG record.¶

1.2. Motivation and Design Requirements for DELEG

NS based delegation supports DNS over UDP and TCP, but does not have the ability to provide additional connection information like how to contact an authoritative server over an encrypted transport or which port to use when contacting the delegated server. DELEG was designed to support communicating this type of information and more, taking into account various design goals and tradeoffs. The design requirements for DELEG were:¶

Deliberate and Extensible Protocol: A distinct DNS record served from the parent was selected to provide a deliberate and new signal that additional information was available for how a resolver can communicate with an authoritative nameserver. The record existing in the parent eliminates the need for a resolver to send additional queries to a delegated authoritative nameserver in an attempt to learn connection parameters. Introducing a new resource record type rather than using reserved fields from another record type enables future expansion of functionality beyond one or a small number of bits of information for a delegation. This tradeoff does have the limitation that adoption at the root and TLD levels of the hierarchy may be delayed, but this approach can lead to a cleaner and easier to support solution.¶
Extensibility: DELEG as described in this document could be implemented through an existing Resource Record like the TXT record, but doing so would limit the ability to build on the record in the future. To support extending the data that can be communicated, the SVCB record format was used for DELEG to support future growth.¶
Support for Abstract Delegation: the SVCB record format has support for an Alias Form record. In the case of DELEG, the AliasForm record enables a domain owner to indicate that their zone will be hosted elsewhere, like a DNS service provider, in a way that enables the service provider to update their authoritative information without coordination with the domain owner, domain registrar.¶
Authenticated and Parent Centric: While NS records are authoritative at the child, some resolver algorithms are parent centric while others are child centric. With DELEG, this ambiguity is removed and parent centricity and authority is specified. This decision also enables the records to be signed in the parent zone, reducing the potential risk of a denial of service for clients when being delegated. Additionally, a parent centric record is required to support resolution with a cold cache and duplication of data in the child zone can lead to inconsistencies as the records change over time.¶
Able to coexist in the ecosystem: DELEG is defined as being a record in the parent, signifying a zone cut. As a result of that design decision, additional effort and time will be required to deploy DELEG to all levels of the DNS hierarchy, especially when considering the Root zone and TLDs. To support the deployment, DELEG must be able to coexist within the ecosystem with the existing NS based methods of resolution. Testing has been done to show that many deployed resolvers can handle DELEG and NS records side-by-side to enable a rollout.¶
While DELEG can be used in an unsigned zone, it is recommended to use DNSSEC. The DELEG record must be signed or denied in the parent zone when it is signed. Without DNSSEC, certain security properties might not be available and hence certain features only will work when the DELEG record is signed.¶

2. DELEG Record Type

The DELEG record wire format is the same as the SVCB record defined in [RFC9460], but with a different value for the resource record type of TBD. For extensions SVCB and DELEG use Service Parameter Keys (SvcParamKeys) and new SvcParamKeys that might be needed also will use the existing IANA Registry.¶

The SVCB record allows for two types of records, the AliasMode and the ServiceMode. The DELEG record uses both. The Target Name either points to a set of name servers answering for the delegated domain if used in ServiceMode or to an SVCB record in AliasMode that then has to be interpreted further to get to the actual name server. The AliasMode DELEG record might point to another AliasMode SVCB record or a CNAME. In order to not allow unbound indirection of DELEG records the maximum number of indirections, CNAME or AliasMode SVCB is 4. The SvcPriority field either can be 0 for AliasMode or 1 for ServiceMode. Different priorities are not used for DELEG delegations as all DELEG records are considered equally important and selection amongst can be decided by the resolver for a good user experience.¶

2.1. Including DELEG RRs in a Zone

A DELEG RRset MAY be present at a delegation point. The DELEG RRset MAY contain multiple records. DELEG RRsets MUST NOT appear at a zone's apex.¶

A DELEG RRset MAY be present with or without NS or DS RRsets at the delegation point.¶

Construction of a DELEG RR requires knowledge which implies communication between the operators of the child and parent zones. This communication is an operational matter not covered by this document.¶

2.2. Difference between the records

This document uses two different resource record types. Both records have the same functionality, with the difference between them being that the DELEG record MUST only be used at a delegation point, while the SVCB is used as a normal resource record and does not indicate that the label is being delegated. For example, take the following DELEG record:¶

Zone com.:
example.com.  86400  IN DELEG 1   config2.example.net.

When a client receives the above record, the resolver should send queries for any name under example.com to the nameserver at config2.example.net unless further delegated. By contrast, when presented with the records below:¶

Zone com.:
example.com.  86400  IN DELEG 0   config3.example.org.

Zone example.org.:
config3.example.org.  86400  IN SVCB 1 . (
                ipv4hint=192.0.2.54,192.0.2.56
                ipv6hint=2001:db8:2423::3,2001:db8:2423::4 )

A resolver trying to resolve a name under example.com would get the first record above from the parent authoritative server, .COM, indicating that the SVCB records found at config3.example.org should be used to locate the authoritative nameservers of example.com, and other parameters.¶

The primary difference between the two records is that the DELEG record means that anything under the record label should be queried at the delegated server while the SVCB record is just for redirection purposes, and any names under the record's label should still be resolved using the same server unless otherwise delegated.¶

It should be noted that both DELEG and SVCB records may exist for the same label, but they will be in different zones. Below is an example of this:¶

Zone com.:
example.com.  86400  IN DELEG 0   c1.example.org.

Zone example.org.:
c1.example.org.  86400  IN DELEG  1   config3.example.net. (
                            ipv6hint=2001:db8:2423::3 )
c1.example.org.  86400  IN NS test.c1.example.org.
test.c1.example.org. 600 IN A 192.0.2.1

Zone c1.example.org:
c1.example.org.  86400  IN SVCB 1   config2.example.net. (
                    ipv6hint=2001:db8:4567::4  )
c1.example.org.  86400  IN NS test.c1.example.org.
test.c1.example.org. 600 IN A 192.0.2.1

In the above case, the DELEG record for c1.example.org would only be used when trying to resolve names at or below c1.example.org. This is why when an AliasMode DELEG or SVCB record is encountered, the resolver MUST query for the SVCB record associated with the given name.¶

2.3. Use of DNSSEC

While DNSSEC is RECOMMENDED, unsigned DELEG records may be retrieved in a secure way from trusted, Privacy-enabling DNS servers using encrypted transports.¶

FOR DISCUSSION: This will lead to cyclical dependencies. A DELEG record can introduce a secure way to communicate with trusted, Privacy-enabling DNS servers. For that, it needs to be DNSSEC signed.¶

2.3.1. Signing DELEG RRs

A DELEG RRset MUST be DNSSEC signed if the zone is signed.¶

If a signed zone contains DELEG records, the referral response containing DELEG records MUST be signed with a DNSKEY that has the DELEG flag set.¶

2.3.2. Preventing downgrade attacks

A flag in the DNSKEY record signing the delegation is used as a backwards compatible, secure signal to indicate to a resolver that DELEG records are present or that there is an authenticated denial of a DELEG record. Legacy resolvers will ignore this flag and use the DNSKEY as is.¶

Without this secure signal an on-path adversary can remove DELEG records and its RRsig from a response and effectively downgrade this to a legacy DNSSEC signed response.¶

2.4. Multiple Service Providers

Some zone owners may wish to use multiple providers to serve their zone, in which case multiple DELEG AliasMode records can be used. In the event that multiple DELEG AliasMode records are encountered, the resolver SHOULD treat those as a union the same way this is done with NS records, picking one at random for the first lookup and eventually discovering the others. How exactly DNS questions are directed and split between configuration sets is implementation specific:¶

example.com.    86400    IN  DELEG     0   config1.example.net.
example.com.    86400    IN  DELEG     0   config1.example.org.

[ DRAFT NOTE: SVCB says that there "SHOULD only have a single RR". This ignores that but keeps the randomization part. Section 2.4.2 of SVCB ]¶

2.5. Loop Prevention

The TargetName of an SVCB or DELEG record MAY be the owner of a CNAME record. Resolvers MUST follow CNAMEs as well as further alias SVCB records as normal, but MUST NOT allow more then 4 total lookups per delegation, with the first one being the DELEG referral and then 3 SVCB/CNAME lookups maximal.¶

Special care should be taken by both the zone owner and the delegated zone operator to ensure that a lookup loop is not created by having two AliasMode records rely on each other to serve the zone. Doing so may result in a resolution loop, and likely a denial of service. The mechanism on following CNAME and SVCB alias above should prevent exhaustion of server resources. If a resolution can not be found after 4 lookups the server should reply with a SERVFAIL error code.¶

3. Examples

To introduce DELEG record, this example shows the authority section of a DNS response that delegates a subdomain to another nameserver.¶

example.com.  86400  IN DELEG  1 ns1.example.com. (
                ipv4hint=192.0.2.1 ipv6hint=2001:DB8::1 )
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 the same parameters as regular DNS, but the delegation as well as the glue can be signed.¶

Like with SVCB, DELEG also offer the ability to use the Alias form of 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     ns2.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 . (
                ipv4hint=192.0.2.54,192.0.2.56
                ipv6hint=2001:db8:2423::3,2001:db8:2423::4 )

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.¶

4. Implementation

This document introduces the concept of signaling capabilities to clients on how to connect and validate a subdomain. This section details the implementation specifics of the DELEG record for various DNS components.¶

4.1. Deployment Considerations

The DELEG and SVCB records are intended 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.¶

4.1.1. AliasMode and ServiceMode in the Parent

Both the AliasMode and ServiceMode records can be returned for the DELEG record from the parent. This is different from the SCVB [RFC9460] specification and only applies for the DELEG RRSet in the parent.¶

4.1.2. Rollout

When introduced, the DELEG and SVCB records might not initially be supported by the DNS root or TLD operators. So for initial usage zone owners may place DELEG records into their zones for delegating down the tree into child domains of their zones, as the only way to discover new delegations is with the DELEG record at the parent. When AliasMode is used to the SVCB record the Target SVCB has to exists.¶

4.1.3. 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.¶

4.2. Signaling DELEG support

For a long time there will be both DELEG and NS needed for delegation. As both methods should be configured to get to a proper resolution it is not neccassary to send both in a referral response. We therefore purpose an EDNS flag to be use similar to the DO Bit for DNSSEC to be used to signal that the sender understands DELEG and does not need NS or glue information in the referral.¶

This bit is referred to as the "DELEG" (DE) bit. In the context of the EDNS0 OPT meta-RR, the DO bit is the first bit of the third and fourth bytes of the "extended RCODE and flags" portion of the EDNS0 OPT meta-RR, structured as follows:¶

            +0 (MSB)                +1 (LSB)
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
  0: |   EXTENDED-RCODE      |       VERSION         |
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
  2: |DO|DE|                 Z                       |
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

Setting the DE bit to one in a query indicates to the server that the resolver is able to accept delegations using DELEG only. The DO bit cleared (set to zero) indicates the resolver is unprepared to handle DELEG and hence can only be served NS, DS and glue in a delegation response. The DE bit of the query MUST be copied in the response.¶

4.3. Response Size Considerations

For latency-conscious zones, the overall packet size of the delegation records from a parent zone to child zone should be taken into account when configuring the NS, DELEG and SVCB records. Resolvers that wish to receive DELEG and SVCB records in response SHOULD advertise and support a buffer size that is as large as possible, to allow the authoritative server to respond without truncating whenever possible.¶

As¶

4.4. Authoritative Name Servers

4.4.1. Including DELEG RRs in a Response

If a DELEG RRset is present at the delegation point, the name server MUST return both the DELEG RRset and its associated RRSIG RR in the Authority section along with the DS RRset and its associated RRSIG RR and the NS RRset.¶

If no DELEG RRset is present at the delegation point, and the delegation was signed with a DNSKEY that has the DELEG flag set, the name server MUST return the NSEC or NSEC3 RR that proves that the DELEG RRset is not present including its associated RRSIG RR along with the DS RRset and its associated RRSIG RR if present and the NS RRset, if present.¶

Including these DELEG, DS, NSEC or NSEC3, and RRSIG RRs increases the size of referral messages. If space does not permit inclusion of these records, including glue address records, the name server MUST set the TC bit on the response.¶

4.4.2. Responding to Queries for Type DELEG

DELEG records, when present, are included in referrals. When a parent and child are served from the same authoritative server, this referral will not be sent because the authoritative server will respond with information from the child zone. In that case, the resolver may query for type DELEG.¶

The DELEG resource record type is unusual in that it appears only on the parent zone's side of a zone cut. For example, the DELEG RRset for the delegation of "foo.example" is part of the "example" zone rather than in the "foo.example" zone. This requires special processing rules for both name servers and resolvers because the name server for the child zone is authoritative for the name at the zone cut by the normal DNS rules, but the child zone does not contain the DELEG RRset.¶

A DELEG-aware resolver sends queries to the parent zone when looking for a DELEG RR at a delegation point. However, special rules are necessary to avoid confusing legacy resolvers which might become involved in processing such a query (for example, in a network configuration that forces a DELEG-aware resolver to channel its queries through a legacy recursive name server). The rest of this section describes how a DELEG-aware name server processes DELEG queries in order to avoid this problem.¶

The need for special processing by a DELEG-aware name server only arises when all the following conditions are met:¶

The name server has received a query for the DELEG RRset at a zone cut.¶
The name server is authoritative for the child zone.¶
The name server is not authoritative for the parent zone.¶
The name server does not offer recursion.¶

In all other cases, the name server either has some way of obtaining the DELEG RRset or could not have been expected to have the DELEG RRset, so the name server can return either the DELEG RRset or an error response according to the normal processing rules.¶

If all the above conditions are met, however, the name server is authoritative for the domain name being searching for, but cannot supply the requested RRset. In this case, the name server MUST return an authoritative "no data" response showing that the DELEG RRset does not exist in the child zone's apex.¶

4.4.3. Priority of DELEG over NS and Glue Address records

DELEG-aware resolvers SHOULD prioritize the information in DELEG records over NS and glue address records. Glue records MUST NOT be considered for a DELEG delegation. If there is an in domain name server for a non alias mode DELEG record ipv4hint and ipv6hint MUST be used to convey the IP address. An a alias mode DELEG record can not have an in domain name server.¶

Ask www.example.com qtype AAAA to a.root-servers.net the answer is: Answer section: (empty) Authority section: com. 172800 IN NS a.gtld-servers.net. Additional section: a.gtld-servers.net. 172800 IN AAAA 2001:db8:a83e::2:30¶
Ask www.example.com qtype AAAA to a.gtld-servers.net the answer is: Answer section: (empty) Authority section: example.com. 172800 IN NS ns1.example.com. example.com. 172800 IN DELEG 1 config1.example.com. ( ipv6hint=2001:db8:440:1:1f::24 ) Additional section: ns1.example.com. 172800 IN AAAA 2001:db8:322c::35:42¶
Ask www.example.com qtype AAAA to config1.example.com (2001:db8:1:1f::24) the answer is: Answer section: www.example.com. 3600 IN AAAA 2001:db8:a0:322c::2 Authority section: (empty) Additional section: (empty)¶

TODO: more resolution examples (e.g out of bailiwick)¶

[RFC4034]: Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "Resource Records for the DNS Security Extensions", RFC 4034, DOI 10.17487/RFC4034, March 2005, <https://www.rfc-editor.org/rfc/rfc4034>.
[RFC6891]: Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms for DNS (EDNS(0))", STD 75, RFC 6891, DOI 10.17487/RFC6891, April 2013, <https://www.rfc-editor.org/rfc/rfc6891>.
[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>.
[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>.

8.2. Informative References

[BCP219]: Best Current Practice 219, <https://www.rfc-editor.org/info/bcp219>.
At the time of writing, this BCP comprises the following:

Hoffman, P. and K. Fujiwara, "DNS Terminology", BCP 219, RFC 9499, DOI 10.17487/RFC9499, March 2024, <https://www.rfc-editor.org/info/rfc9499>.
[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>.
[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>.
[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>.