Network Working Group W. Hardaker
Internet-Draft USC/ISI
Intended status: Best Current Practice February 21, 2021
Expires: August 25, 2021
Intentionally Temporarily Insecure
draft-hardaker-dnsop-intentionally-temporary-insec-00
Abstract
Performing DNSKEY algorithm transitions with DNSSEC 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.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements notation . . . . . . . . . . . . . . . . . . 3
2. Transitioning temporarily through insecurity . . . . . . . . 3
3. Operational considerations . . . . . . . . . . . . . . . . . 4
4. Security considerations . . . . . . . . . . . . . . . . . . . 4
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
5.1. Normative References . . . . . . . . . . . . . . . . . . 5
5.2. Informative References . . . . . . . . . . . . . . . . . 6
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 6
Appendix B. Github Version of this document . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
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.
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. For example, 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 also requires
that both the old key (K_old) and K_new sign and generate signatures
for the zone, but with only the K_old key is published even though
signatures from K_new are 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.
Although many DNSSEC signing solutions may automate the algorithm
rollover steps (making operator involvement unnecessary), many other
tools do not support automated algorithm updates. In these
environments, the most challenging step is requiring that certain
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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, potentially more operationally
robust but less secure, approach to performing algorithm DNSKEY
rollovers for use in these situations.
1.1. Requirements notation
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.
2. Transitioning temporarily through insecurity
An alternate approach to rolling DNSKEYs, especially when the
toolsets being used do not provide easy algorithm rollover
approaches, 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 during the removal of
the old key and the introduction of a new key using a new algorithm.
Zone TTLs may 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 the zone's DS record (if possible)
and the SOA's negative TTL (MINIMUM) [RFC1035].
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 to expire from caches. This is the most critical timing.
The author of this document failed to wait the required time
once. It was not pretty.
4. Replace the old DNSKEY(s) with the old algorithm with new
DNSKEY(s) with the new algorithm(s) in the zone and publish the
zone.
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5. Wait 2 times the zone SOA's published negative cache time 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, such as
RSAMD5 or RSASHA1 with a more modern one such as RSASHA512 or
ECDSAP256SHA256 can be a daunting task if the zone's current tooling
doesn't provide an easy-to-use solution. This is the case for zone
owners that potentially use command line tools that are 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, understanding of the security considerations of
using this approach is paramount.
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 only 1 TTL may be sufficient assuming
all clocks around the world are operating with perfection.
4. Security considerations
DNSSEC provides an data integrity protection for DNS data. This
document specifically calls out a reason why a zone owner may desire
to deliberately turn off DNSSEC while changing the zone's DNSKEY's
cryptographic algorithms. Thus, this is deliberately turning off
security which 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 child
zones, DANE [RFC6698][RFC7671][RFC7672], PGP keys [RFC7929], and
SSHFP[RFC4255]. Zone owners must carefully consider which records
within their zone 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
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there is one principal answer. Simply put, if the the complexity of
doing it the correct way is difficult with existing tooling then the
chances of performing the more complex procedure and introducing an
error, likely making the entire zone unavailable during that time
period, may be significantly higher than the chances of the zone
being attacked during the transition period of the simpler approach
where zone availability is less likely to be impacted. Simply put,
an invalid zone created by a botched algorithm roll is potentially
worse than an unsigned but still available zone.
5. References
5.1. Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[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/info/rfc2119>.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC4033, March 2005,
<https://www.rfc-editor.org/info/rfc4033>.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
<https://www.rfc-editor.org/info/rfc4035>.
[RFC4509] Hardaker, W., "Use of SHA-256 in DNSSEC Delegation Signer
(DS) Resource Records (RRs)", RFC 4509,
DOI 10.17487/RFC4509, May 2006,
<https://www.rfc-editor.org/info/rfc4509>.
[RFC6781] Kolkman, O., Mekking, W., and R. Gieben, "DNSSEC
Operational Practices, Version 2", RFC 6781,
DOI 10.17487/RFC6781, December 2012,
<https://www.rfc-editor.org/info/rfc6781>.
[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/info/rfc8174>.
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5.2. Informative References
[RFC4255] Schlyter, J. and W. Griffin, "Using DNS to Securely
Publish Secure Shell (SSH) Key Fingerprints", RFC 4255,
DOI 10.17487/RFC4255, January 2006,
<https://www.rfc-editor.org/info/rfc4255>.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
2012, <https://www.rfc-editor.org/info/rfc6698>.
[RFC7671] Dukhovni, V. and W. Hardaker, "The DNS-Based
Authentication of Named Entities (DANE) Protocol: Updates
and Operational Guidance", RFC 7671, DOI 10.17487/RFC7671,
October 2015, <https://www.rfc-editor.org/info/rfc7671>.
[RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via
Opportunistic DNS-Based Authentication of Named Entities
(DANE) Transport Layer Security (TLS)", RFC 7672,
DOI 10.17487/RFC7672, October 2015,
<https://www.rfc-editor.org/info/rfc7672>.
[RFC7929] Wouters, P., "DNS-Based Authentication of Named Entities
(DANE) Bindings for OpenPGP", RFC 7929,
DOI 10.17487/RFC7929, August 2016,
<https://www.rfc-editor.org/info/rfc7929>.
Appendix A. Acknowledgments
The author has discussed the pros and cons of this approach with
multiple people, including Viktor Dukhovni and Warren Kumari.
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
Wes Hardaker
USC/ISI
Email: ietf@hardakers.net
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