The Delegation_Only DNSKEY flag
draft-pwouters-powerbind-03

Abstract

This document introduces a new DNSKEY flag called DELEGATION_ONLY that indicates that the particular zone will never sign zone data across a label. That is, every label (dot) underneath is considered a zone cut and must have its own (signed) delegation. Additionally, it indicates the zone is expecting its parent to never bypass or override the zone. DNSSEC Validating Resolvers can use this bit to mark any data that violates the DELEGATION_ONLY policy as BOGUS.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on May 7, 2020.

Copyright Notice

Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 2. Terminology
• 3. The Deep Link State problem
• 4. The DELEGATION_ONLY DNSKEY flag
• 4.1. _underscore label exception
• 4.2. Parental Transparency
• 4.3. Marking zone keys DELEGATION_ONLY without parental support
• 4.4. Marking the Root DNSKEY DELEGATION_ONLY
• 4.5. Migrating to and from DELEGATION_ONLY
• 5. Operational Considerations
• 6. Security Considerations
• 7. Privacy Considerations
• 8. Human Rights Considerations
• 9. IANA Considerations
• 10. Acknowledgements
• 11. References
• 11.1. Normative References
• 11.2. Informative References
• Authors' Addresses

1. Introduction

The DNS Security Extensions [DNSSEC] use public key cryptography to create an hierarchical trust base with the DNSSEC root public keys at the top, followed by Top Level domain (TLD) keys one level underneath. While the root and TLD zones are asumed to be almost exclusively delegation-only zones, there is currently no method to audit these zones to ensure they behave as a delegation-only zone. This creates an attractive target for malicious use of these zones - either by their owners or through coercion.

This document defines a mechanism for zone owners, at DNSKEY creation time, to indicate they will only delegate the remainder of the tree to lower-level zones, allowing easier logging and auditing of DNS responses they serve.

This document introduces a new DNSKEY flag allowing zone owners to commit that the zone will never sign any DNS data that traverses a single label and if any such signed data is encountered by validating resolvers, that this data should be interpreted as BOGUS.

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

3. The Deep Link State problem

The hierarchical model of DNS and DNSSEC ([RFC4033], [RFC4034] and [RFC4035]) comes with the property that a zone at one point in the hierarchy can define, and therefor override, everything in the DNS tree from their point and below. For example, the DNSSEC root key could ignore the NS records for ".org" and "example.org" and could place a record "www.example.org" directly into its own zone, with a corresponding RRSIG signed by the root key itself. Even if resolvers would defend against this attack by not allowing RRSIG's to span across a potential zone cut, the zone operator (any level higher in the hierarchy than the target victim) could briefly remove the NS and DS records, and create a "legitimate" DNS entry for "www.example.org", hiding the normal zonecuts. The attacker can then publish DNS addresses records (e.g. A and AAAA records), as well as records used for authentication (e.g. TLSA, SMIME, OPENPGPKEY, SSHP or IPSECKEY records).

Exposing such targetted attacks requires a transparency audit setup ([RFC6962]) that needs to log all signed DNS data to prove that data signed by a parental DNSKEY was out of expected policy. The very distributed nature of DNS makes such transparency logs prohibitively expensive and nearly impossible to operate. Additionally, it would expose all zone data to any public log operators, thereby exposing all DNS data to a public archive. This data could then be used for other malicious purposes.

4. The DELEGATION_ONLY DNSKEY flag

This document introduces a new DNSKEY flag called DELEGATION_ONLY. When this flag is set on a DNSKEY with SEP bit set (KSK), the zone owner commits to not sign any data that crosses a label down in the hierarchy. This commits a parent in the DNS hierarchy to only sign NS and DS records (i.e. all non-glue, delegation records) for its child zones. It will no longer be able to ignore (or briefly delete, see below) a child delegation and publish data crossing zone labels by pretending the next label is not a zone cut.

For such a parent to take over data that belongs to its child zone, it has two choices. It can (temporarilly) remove its own DNSKEY DELEGATION_ONLY flag or it can replace the NS and DS records of its child zone with its own data (destinations and key references) so it can sign DNS data that belongs to its own child zone. However, both of these actions cannot be hidden, thus exposing such malicious behavior when combined with DNSSEC Transparency logs.

A zone that publishes a DNSKEY with the DELEGATION_ONLY flag also signifies that it is not expecting its own parent to skip it, thereby bypassing the DELEGATION_ONLY flag.

4.1. _underscore label exception

Some protocols, such as the DANE protocol [RFC6698] use a number of labels that start with an underscore (_) prefix to publish information about the zone itself. For example, the TLSA record for www.example.com is published at the location _443._tcp.www.example.com. These records are semantically part of the zone itself and are not delegated child zones. Any chain of labels that each start with an underscore (_) is not considered to violate the DELEGATION_ONLY flag limitation of being DELEGATION_ONLY, as this data is logically part of the zone itself and is never meant to be interpreted as a indepedent delegated child zone.

4.2. Parental Transparency

A parent zone, such as the root zone, a TLD or any public suffix list delegation point, that has published a key with the DELEGATION_ONLY flag can no longer make an exception for a single delegated zone without removing the DELEGATION_ONLY flag, switching off its published policy. This action would be highly visible, and for some domains such as the root or TLDs, require human interaction to notify the stake holders to prevent loss of trust.

Removing the DELEGATION_ONLY flag from a DNSKEY requires that the zone first publishes an additional updated DS record to its parent.

In the case of the root key, it would require updating out-of-band root key meta information and/or perform an [RFC5011] style rollover for the same key with updated DNSKEY flags. Due to the timings of such a rollover, it would take at least 30 days for the first validating resolvers to even pick this policy change. It would also be a highly visible event.

Replacing the NS and DS records of a child zone can still be done in a targetted attack mode, but these events are something that can be easilly tracked by a transparency infrastructure similar to what is now in use for the WebPKI using [RFC6962](bis). With client implementations of transparency, all DELEGATION_ONLY flag changes would be logged and become visible to the owner of attacked child zones, exposing a parent's malicious actions.

4.3. Marking zone keys DELEGATION_ONLY without parental support

Even before a parent DNSKEY (or the root key) has been marked with DELEGATION_ONLY, zones can already signal their own willingness to commit being DELEGATION_ONLY zones. Any changes of that state in a zone DNSKEY will require those zones to submit a new DS record to their parent. Setting the DELEGATION_ONLY flag would trigger DNSSEC Transparency clients to start monitoring for actions by the zone or its parents that would be bypassing the DELEGATION_ONLY policy of the zone. Validating resolvers would mark any data in violation of the DELEGATION_ONLY policy as BOGUS.

4.4. Marking the Root DNSKEY DELEGATION_ONLY

Once the Root DNSKEY is marked with a DELEGATION_ONLY flag and deployed resolvers are configured with the new DNSKEY, all TLDs will be ensured that the Root DNSKEY can no longer be abused to override child zone data. Until the Root KSK DNSKEY sets this bit, software SHOULD imply this bit is always set, as this is the current expectation of the Root Zone.

4.5. Migrating to and from DELEGATION_ONLY

There might be multiple DNSKEYs with the SEP bit set in a zone. For the purpose of delcaring a zone as DELEGATION_ONLY, only those DNSKEY's that have a corresponding DS record at the parent MUST be considered. If multiple DS records appear at the parent, some of which point to DNSKEY's with and some of which point to DNSKEY's without the DELEGATION_ONLY flag set, the zone MUST be considered DELEGATION_ONLY. This situation will occur when a zone is rolling its DNSKEY key at the same time as it is commiting to a DELEGATION_ONLY zone (or the reverse).

5. Operational Considerations

Setting or unsetting the DELEGATION_ONLY flag must be handled like any other Key Signing Key rollover procedure, with the appropriate wait times to give resolvers the chance to update their caches.

Some TLDs offer a service where small domains can be hosted in-zone at the TLD zone itself. In that case, the TLD MUST NOT set the DELEGATION_ONLY flag. Another solution for such TLDs is to create delegations for these child zones with the same or different DNSKEY as used in the parent zone itself.

If a zone is publishing glue records for a number of zones, and the zone that contains the authoritative records for this glue is deleted, a resigning of the zone will make this orphaned glue authoritative within the zone. However, with the DELEGATION_ONLY bit set, this (signed) DNSSEC data will be considered BOGUS as it violations the commitment to only delegate. This may impact domains that depended on this unsigned glue.

For example, if "example.com" and "example.net" use NS records pointing to "ns.example.net", then if "example.net" is deleted from the ".net" zone, and the previously unsigned glue of "ns.example.net" is now signed by the ".net" zone, the "example.com" zone will lose its NS records and fail to resolve.

The bind DNS software has an option called "delegation_only zones" which is an option that means something completely different. It refers to ignoring wildcard records in specified zones that are deemed delegation-only zones.

6. Security Considerations

Some parental attacks cannot be detected when the validating resolver's cache is empty. Care should be taken by resolvers to not unneccessarilly empty their cache. This is specifically important for roaming clients that re-connect frequently to different wireless or mobile data networks.

The DELEGATION_ONLY DNSKEY flag is only valid for DNSKEY's that have the SEP bit set. It MUST be ignored on DNSKEY's without the SEP bit set.

7. Privacy Considerations

Some of the protection offered by the DELEGATION_ONLY flag is only available when DNS resolvers report changes in the signing depth of high level (root or TLD) DNSKEYs to gain DNSSEC Transparency. This reporting can reveal that a particular node is trying to access a certain DNS name. Defensive measures to prevent exposing users should be taken when implementing DNSSEC Transparency. It is expected that DNSSEC Transparency behaviour will be written up in a separate document.

8. Human Rights Considerations

The DNS protocol's hierarchy limits zones authority to themselves and their child zones only. While this provides a finer grained trust model compared to a simple list of trusted entities, such as used in the WebPKI, it consolidates a lot of power in the top of the DNS hierarchy. With the increased reliance on DNSSEC for securely identifying resources, such as DANE records, it becomes very important to audit those entities high up in the hierarchy to not abuse or be co-erced into abusing control of the independent child zones. The protocol extension specifically aims at increasing parental transparency and blocks some parental attacks from those parents who have publicly claimed to never override their child zone data.

Parents using the DELEGATION_ONLY flag publication to increase their public trust are still able to remove child zones from their zone, for example in cases of legal compliance or to prevent malicious activity happening in its child zone. But these parents can only do so publicly and can no longer surreptitiously take control of their own child zones.

9. IANA Considerations

This document defines a new DNSKEY flag, the DELEGATION_ONLY flag, whose value [TBD] has been allocated by IANA from the DNSKEY FLAGS Registry.

10. Acknowledgements

The author wishes to thank Thomas H. Ptacek for his insistence on this matter.

Thanks to the following IETF participants: Viktor Dukhovni, Shumon Huque, Geoff Huston, Rick Lamb and Sam Weiler.

11. References

11.1. Normative References

11.2. Informative References

Authors' Addresses