DNSSEC Validators Requirements
draft-mglt-dnsop-dnssec-validator-requirements-02.txt

Abstract

DNSSEC provides data integrity and authentication for DNSSEC validators. However, without valid trust anchor(s) and an acceptable value for the current time, DNSSEC validation cannot be performed. This document lists the requirements to be addressed so resolvers can have DNSSEC validation can be always-on.

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 August 21, 2015.

Copyright Notice

Copyright (c) 2015 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Requirements notation
• 2. Introduction
• 3. Terminology
• 4. Time derivation and absence of Real Time Clock
• 5. Unplugged devices during Trust Anchor KSKs roll over
• 6. Emergency Key rollover
• 6.1. Invalid cached ZSK
• 6.2. Invalid cached RRSIG
• 6.3. Invalid cached KSK
• 6.4. Invalid DS
• 7. Invalid RRSIG
• 8. Private KSK/ZSK
• 9. Requirements
• 10. IANA Considerations
• 11. Security Considerations
• 12. Acknowledgment
• 13. References
• 13.1. Normative References
• 13.2. Informational References
• Appendix A. Document Change Log
• Author's Address

1. Requirements notation

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 [RFC2119].

2. Introduction

DNSSEC [RFC4033], [RFC4034], [RFC4035] adds data authentication and integrity checks to DNS [RFC1034], [RFC1035]. For signature validation, DNSSEC requires a valid trust anchor such as the Key Signing Key (KSK) (the Root Zone KSK for example) and an appropriated time.

Currently few efforts have been made to describe mechanisms that guarantee how a DNSSEC validator can be provisioned with the appropriated KSKs and time so that DNSSEC validation can always be activated. A device that is badly configured or badly provisioned that performs DNSSEC validation may result in disabling the DNS service of the device, and then most of its communications. As a result, non administrated devices that implement DNSSEC validation always need heuristics to disable the DNSSEC validation. This results in an implicit rule that can be stated as: "if DNSSEC validation is performed correctly then do DNSSEC otherwise disable validation and switch to DNS". In a security point of view, this is unacceptable.

This document considers unmanaged devices performing DNSSEC validation and details scenarios where DNSSEC validation cannot be performed properly by the device. In other words, this means that FQDN properly signed are rejected. From these scenarios, this document derives requirements so DNSSEC Validators can have DNSSEC always activated.

3. Terminology

This document uses the following terminology:

- DNSSEC Validator:

the entity that performs DNSSEC resolution and performs signature validation.

4. Time derivation and absence of Real Time Clock

With M2M communication some devices are not expecting to embed Real Time Clock (Rasberry Pi is one example of such devices). When these devices are re-plugged the initial time is set to January 1 1970. Other devices that have clocks that may suffer from time derivation. All these devices cannot rely on their time estimation to perform DNSSEC validation.

Requirement 1: DNSSEC validator MUST be provided means to appropriately update their time.

5. Unplugged devices during Trust Anchor KSKs roll over

In this section we consider a regular Trust Anchor KSK roll over as described in [RFC6781] and [RFC5011]. Unlike regular KSKs, Trust Anchor KSK does not have to update the DS RRset in the parent zone. According to [RFC6781], if TTL_K is the TTL associated to the Trust Anchor KSK and associated RRSIGs, the time of key roll over is around TTL_K with double signed KSKs, and 2 x TTL_K in the case of single singed KSK.

The Root Zone KSK is an example of Trust Anchor KSK and at the time of writing the KSK has a TTL of 172800 seconds which means 2 days. This means that 2 days would be sufficient to perform a Trust Anchor KSK roll over. [RFC5011] recommends to advertise the new / old key for 30 days. This means that a device unplugged for two months may not be aware of a regular Trust Anchor KSK rollover.

A DNSSEC validation may be properly configured by the manufacturer to perform DNSSEC validation. The device may for example be configured by the manufacturer shipped to resellers that store the device for a few months, years before selling the devices to the final end user. Similarly, an operational device may remain unplugged for a while for maintenance reason, or held in reserve when a crash occurs. This fall back is never expected to happen and may happen years after.

Suppose a KSK complete key roll-over occurs (for example at the Root Zone) while the device is offline. Once plugged again, the device will attempt to validate DNSSEC signature with the old Trust Anchor KSK.

The key point in this example is that the device is boot and does not rely on cached information.

Requirement 2: DNSSEC Validator MUST be able to check the validity of their Trust Anchor KSKs.

Requirement 3: DNSSEC Validator MUST be able to retrieve their Trust Anchor KSKs.

We think these requirements are not restrictive to the Root Zone KSK, but to any KSK. In fact it is not always possible to build a trusted delegation between the Root Zone and any sub zone. This may happen for example if one of the upper zones does not handle the secure delegation or improperly implement it. A DS RRset may not be properly filled or its associated signature cannot be validated. As the chain of trust between a zone and the root zone may not be validated, the DNSSEC validation for the zone requires a Trust Anchor. Such DNS(SEC) resolutions may be critical for infrastructure management. A company "Example" may for address all its devices under the domain example.com and may not want disruption to happen if the .com delegation cannot be validated for any reason. Such companies may provision there DNSSEC Validator with the Trust Anchor KSK for the zone example.com in addition to the regular DNSSEC delegation.

Note that providing Trust Anchor KSKs is a crucial operation and can be used a vector of attack. As a result, this operation MUST be performed cautiously.

6. Emergency Key rollover

By emergency key roll over, this paper designates any rollover that are not performed as described in Section 4.1 of [RFC6781] and that result in differences between data stored in the cache of the DNSSEC Validators and the authoritative servers (see section 4.2 in [RFC6781])

Emergency key roll over can be intentionally performed or result from an unexpected behavior in the publishing/validation chain. This is out of scope of this document to understand the reasons/motivations for such key roll over. This document assumes such situation are likely to happen and lists the requirement so DNSSEC Validator can recover from such situations.

6.1. Invalid cached ZSK

An emergency ZSK rollover may result in a new ZSK with associated new RRSIG published in the authoritative zone, while DNSSEC Validator may still cache the old value of the ZSK. For a RRset not cached, the DNSSEC Validator performs a DNSSEC query to the authoritative server that returns the RRset signed with the new ZSK. The DNSSEC Validator validates the signature with the old ZSK which results in an invalid signature check.

Suppose that the old ZSK has been corrupted and that old RRsets have been spoofed. Until the ZSK TTL expires, the DNSSEC Validator considers the spoofed RRsets as valid and the newly signed RRsets as invalid.

Requirement 4: DNSSEC Validator MUST be able to be informed a ZSK MUST be flushed from cache.

Note that if the DNSSEC Validator receives an indication that a ZSK is not valid anymore, it is expected to flush its cache entries of the old ZSK as well as all entries that have been validated by the old ZSK. This does not lead to impersonation of ZSK, at most it generates some additional DNSSEC resolutions and validations.

Note also, that constantly inform the DNSSEC Validator of flushing a specific ZSK may lead to service disruption. In order to prevent such attacks, the DNSSEC is expected to have mechanisms to limit the frequency a zone ZSK can be flushed. Similarly, informing the DNSSEC Validator of flushing randomly chosen ZSK may be associate to resource exhaustion attacks, and also affect the resolution service. As a result, mechanisms are expected to limit the overall number of flushing actions. These case are detailed in Section 11

6.2. Invalid cached RRSIG

This would mean the DNSSEC Validator caches a new ZSK, but still has a RRset with a RRSIG signed with the old ZSK.

This situation should not happen as when a ZSK is renewed all RRsets validated by the old ZSK are flushed from the cache.

6.3. Invalid cached KSK

Consequences of invalid KSK are similar to ZSK. None of the RRSIG can be validated even the one signing the ZSK. (cf. Section 6.1)

Requirement 5: DNSSEC Validator MUST be able to be informed a KSK MUST be flushed from cache.

6.4. Invalid DS

The DS RRset is stored in the parent zone to build a chain of trust with the child zone. This DS RRset can be invalid because its RDATA (KSK) is not anymore used in the child zone or because the DS is badly signed and cannot be validated by the DNSSEC Validator.

In both cases the child zone is considered as insecure and the valid child zone's KSK should become a Trust Anchor KSK.

Requirement 6: DNSSEC Validator MUST be able to be informed a KSK SHOULD be trusted as a Trust Anchor KSK.

7. Invalid RRSIG

A zone may have been badly signed, which means that the KSK or ZSK cannot validate the RRSIG associated to the RRsets. This may not be due to a key roll over, but to an incompatibility between the keys (KSK or ZSK) and the signatures.

Requirement 7: DNSSEC Validator MUST be able to be informed that a KSK or a ZSK MUST NOT be used for RRSIG validation. Unlike "flushing", "MUST NOT be used" means the issue is not a synchronization issue, but that legitimate keys are invalid. Such Keys are known as Negative Trust Anchors [I-D.livingood-negative-trust-anchors].

This means that the zone for a given time will be known as "known insecure". The DNSSEC Validator is not expected to perform signature validation for this zone. It is expected that this information is associated to a Time To Live (TTL).

Note that, this information may be used as an attack vector to impersonate a zone, and must be provided in a trusted way, by a trusted party.

If a zone has been badly signed, the administrator of the authoritative DNS server may resign the zone with the same keys or proceed to an emergency key rollover. If the signature is performed with the same keys, the DNSSEC Validator may notice by itself that RRSIG can be validated. On the other hand if a key rollover is performed, the newly received RRSIG will carry a new key id. Upon receiving a new key id in the RRSIG, the DNSSEC Validator is expected to retrieve the new ZSK/KSK. If the RRSIG can be validated, the DNSSEC Validator is expected to remove the "known insecure" flag.

However, if the KSK/ZSK are rolledover and RRSIG cannot be validated, it remains hard for the DNSSEC Validator to determine whether the RRSIG cannot be validated or that RRSIG are invalid. As a result:

Requirement 8: The DNSSEC Validator MUST be able to be informed that a KSK or a ZSK is known "back to secure".

8. Private KSK/ZSK

DNSSEC may also be used in some private environment. Corporate networks and home networks, for example, may want to take advantage of DNSSEC for a local scope network. Typically, a corporate network may use a local scope KSK / ZSK to validate DNS RRsets provided by authoritative DNSSEC server in the corporate network. This use case is also known as the "split-zone" use case. These RRsets within the corporate network may differ from those hosted on the public DNS infrastructure. Note that using different KSK/ZSK for a given zone may expose a zone to signature invalidation. This is especially the case for DNSSEC validators that are expected to flip-flop between local and public scope. How validators have to handle the various provisioned KSK/ZSKs is out of scope of the document.

Homenet work may use DNSSEC with TLDs or associated domain names that are of local scope and not even registered in the public DNS infrastructure.

Requirement 9: DNSSEC Validator MAY be able to be provided KSK for private use.

Requirement 10: DNSSEC Validator MAY be able to be provided ZSK for private use.

9. Requirements

The document lists the following requirements:

- Requirement 1:

DNSSEC validator MUST be provided means to appropriately update their time.

- Requirement 2:

DNSSEC Validator MUST be able to check the validity of their Trust Anchor KSKs.

- Requirement 3:

DNSSEC Validator MUST be able to retrieve their Trust Anchor KSKs.

- Requirement 4:

DNSSEC Validator MUST be able to be informed a ZSK MUST be flushed from cache.

- Requirement 5:

DNSSEC Validator MUST be able to be informed a KSK MUST be flushed from cache.

- Requirement 6:

DNSSEC Validator MUST be able to be informed a KSK SHOULD be trusted as a Trust Anchor KSK.

- Requirement 7:

DNSSEC Validator MUST be able to be informed that a KSK or a ZSK MUST NOT be used for RRSIG validation.

- Requirement 8:

The DNSSEC Validator MUST be able to be informed that a KSK or a ZSK is known "back to secure".

- Requirement 9:

DNSSEC Validator MUST be able to be provided KSK for private use.

- Requirement 10:

DNSSEC Validator MUST be able to be provided ZSK for private use.

10. IANA Considerations

There are no IANA consideration for this document.

11. Security Considerations

The requirements listed in this document aim at providing the DNSSEC Validator appropriated information so DNSSEC validation can be performed. On the other hand, providing inappropriate information can lead to misconfiguring the DNSSEC Validator, and thus disrupting the DNSSEC resolution service. As a result, enabling the setting of configuration parameters by a third party may open a wide surface of attacks.

As an appropriate time value is necessary to perform signature check (cf. Section 4), an attacker may provide rogue time value to prevent the DNSSEC Validator to check signatures.

An attacker may also affect the resolution service by regularly asking the DNSSEC Validator to flush the KSK/ZSK from its cache (cf. Section 6.1 Section 6.3). All associated data will also be flushed. This generates additional DNSSEC resolution and additional validations, as RRSet that were cached require a DNSSEC resolution over the Internet. This affects the resolution service by slowing down responses, and increases the load on the DNSSEC Validator.

An attacker may ask the DNSSEC Validator to consider a rogue KSK/ZSK ( cf. Section 6.4, Section 8), thus hijacking the DNS zone. Similarly, (cf. Section 7) an attacker may inform the DNSSEC Validator not to trust a given KSK in order to prevent DNSSEC validation to be performed.

An attacker (cf. Section 7) can advertise a "known insecure" KSK or ZSK is "back to secure" to prevent signature check to be performed correctly.

As a result, information considered by the DNSSEC Validator should be from a trusted party. This trust party should have been authenticated, and the channel used to exchange the information should also be protected and authenticated.

12. Acknowledgment

The need to address DNSSEC issues on the resolver side started in the Home Networks mailing list and during the IETF87 in Berlin. Among others, people involved in the discussion were Ted Lemon, Ralph Weber, Normen Kowalewski, and Mikael Abrahamsson. People involved in the email discussion initiated by Jim Gettys were, with among others, Paul Wouters, Joe Abley and Michael Richardson.

The current document has been initiated after a discussion with Paul Wouter and Evan Hunt.

13. References

13.1. Normative References

13.2. Informational References

Appendix A. Document Change Log

[RFC Editor: This section is to be removed before publication]

-02: Clarification for private ZSK/KSK.

-01: minor editings.

-00: First version published.

Author's Address