Intentionally Temporarily Degraded or Insecure

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Table of Contents

1. Introduction

Performing DNSKEY [RFC4035] algorithm transitions with DNSSEC [RFC4033] signing is unfortunately challenging to get right in practice without decent tooling support. This document weighs the correct, completely secure way of rolling keys against an alternate, significantly simplified, method that takes a zone through an insecure state.¶

Section 4.1.4 of [RFC6781] describes the necessary steps required when a new signing key is published for a zone that uses a different signing algorithm than the currently published keys. These are the steps that MUST be followed when zone owners wish to have uninterrupted DNSSEC protection for their zones. The steps in this document are designed to ensure that all DNSKEY records and all DS [RFC4509] records (and the rest of a zone records) are properly validatable by validating resolvers throughout the entire process. Whenever possible, this procedure SHOULD be followed.¶

Unfortunately, there are a number of these steps that are challenging to accomplish either because the timing is tricky to get right or because current software doesn't support automating the process easily. Some examples:¶

1. The second step in Section 4.1.4 of [RFC6781] requires that a new key with the new algorithm (which we refer to as K_new) be created, but not yet published. This step requires that both the old key (K_old) and K_new both sign and generate signatures for the zone, but without K_new actually being published in the zone even though its signatures. Put another way, only K_old can exist in the zone even though signatures from both keys must be included. After this odd mix has been published for a sufficient time length, based on the TTL, can K_new be safely introduced and published into the zone as well.¶

2. Sometimes one of the goals is to transfer zone management to new authoritative server software. But, if the newly desired algorithm isn't supported in the existing (to be replaced) DNSSEC signing software, then the transfer to the new software must be accomplished first. However, if there isn't an overlap between the algorithms available in both software sets, it becomes practically impossible to even transfer the zone since neither software set can use both K_old and K_new.¶

Although some DNSSEC signing solutions may automate the algorithm rollover steps (making operator involvement unnecessary), many other tools do not yet support automated algorithm updates. In these environments, the most challenging step is requiring that certain RRSIGs be published without the corresponding DNSKEYs that created them. This will likely require operators to use a text editor on the contents of a signed zone to carefully select zone records to extract before publication. This introduces potentially significant operator error(s).¶

This document proposes an alternate approach that MAY be used to perform algorithm DNSKEY rollovers in these situations, which is potentially more operationally robust but less secure.¶

2. Transitioning temporarily through insecurity

An alternate approach to properly rolling DNSKEYs to a new algorithm, is to intentionally make the zone become insecure while the DNSKEYs and algorithms are swapped. At a high level, this means removing all DS records from the parent zone first, then remove the old key and introduce the new key with its new algorithm during this period. Zone TTLs can be significantly shortened during this period to minimize the period of insecurity.¶

Below are the enumerated steps required by this alternate transition mechanism. Note that there are still two critical waiting time requirements (steps 2 and 6) that must be followed carefully.¶

1. Optional: lower the TTLs of both the zone's DS record, and the TTL of the DNSKEY RRset. Note that in some operational deployments the parent zone may set the TTL of the DS record.¶

2. Remove all DS records from the parent zone.¶

3. Ensure the zone is considered unsigned by all validating resolvers by waiting 2 times the maximum TTL length for the DS record, and/or 2 times the largest TTL found in the zone (whichever is larger). This is the most critical timing as all records associated with K_old must be cleared from validating resolver caches. (The author of this document failed to wait the required time once. It was not pretty.)¶

4. Replace the old DNSKEY(s) using the old algorithm with new DNSKEY(s) using the new algorithm(s) in the zone and publish the zone.¶

5. Wait 2 times the largest TTL found in the zone to ensure the new DNSKEYs will be found by validating resolvers.¶

6. Add the DS record(s) for the new DNSKEYs to the parent zone.¶

7. If the TTLs were modified in the optional step 1, change them back to their preferred values.¶

3. Operational considerations

The process of replacing a DNSKEY with an older algorithm [RFC9904], such as RSAMD5 [RFC4034] or RSASHA1 [RFC9905] with a more modern one [RFC9904] such as RSASHA512 [RFC5702] or ECDSAP256SHA256 [RFC6605] can be a daunting task if the zone's current tooling doesn't provide an easy-to-use solution. For example, this may be the case for zone owners using command line tools integrated into their zone production environment.¶

This document describes an alternative approach to rolling DNSKEY algorithms that may be significantly less prone to operational mistakes. However, it is paramount that operators understand of the security considerations of using this approach.¶

The document recommends waiting 2 times TTL values in certain cases for added assurance that the waiting period is long enough for caches to expire. In reality, waiting slightly longer than 1 TTL may be sufficient but requires accepting added risks with propagation timing and clock synchronization.¶

4. Security considerations

DNSSEC provides data integrity protection for DNS data. This document specifically calls out a reason why a zone owner may desire to deliberately turn off this protection while changing the zone's DNSKEY's cryptographic algorithms. Thus, this technique is potentially harmful if an attacker knows when this will occur and can use that time window to launch DNS modification attacks (for example, cache poisoning attacks) against validating resolvers or other validating DNS infrastructure.¶

Most importantly, this will deliberately break certain types of DNS records that must be validatable for them to be effective. This includes for example, but not limited to, all DS records for the zone's own children, DANE [RFC6698][RFC7671][RFC7672], PGP key fingerprints [RFC7929], and SSHFP[RFC4255] fingerprints. Zone owners must carefully consider which records within their zone and their zone's children depend on DNSSEC being available before using the procedure outlined in this document.¶

Given all of this, it leaves the question of: "why would a zone owner want to deliberately turn off security temporarily then?", to which there is one principal answer: if the the complexity of executing an algorithm role the correct way is difficult (or impossible), then the chances of introducing an error that causes an operational outage may be significantly higher than the chances of the zone being attacked during the insecure transition period. Simply put, an invalid zone created by a botched algorithm roll is potentially worse than an unsigned but still available zone.¶

5. References

Appendix A. Acknowledgments

The author has discussed the pros and cons of this approach with multiple people, including:¶

• Vladimír Čunát¶

• Peter van Dijk¶

• Viktor Dukhovni¶

• Warren Kumari.¶

• Scott Rose¶

• Tuomo Soini¶

• Paul Wouters¶

Appendix B. Github Version of this document

While this document is under development, it can be viewed, tracked, issued, pushed with PRs, ... here:¶

https://github.com/hardaker/draft-hardaker-dnsop-intentionally-temporarily-insecure¶

Author's Address