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DNSOP G. Huston
Internet-Draft J. Damas
Intended status: Standards Track APNIC
Expires: August 25, 2018 W. Kumari
Google
February 21, 2018
A Sentinel for Detecting Trusted Keys in DNSSEC
draft-ietf-dnsop-kskroll-sentinel-02
Abstract
The DNS Security Extensions (DNSSEC) were developed to provide origin
authentication and integrity protection for DNS data by using digital
signatures. These digital signatures can be verified by building a
chain of trust starting from a trust anchor and proceeding down to a
particular node in the DNS. This document specifies a mechanism that
will allow an end user to determine the trusted key state for the
root key of the resolvers that handle that user's DNS queries. Note
that this method is only applicable for determing which keys are in
the trust store for the root key.
There is an example / toy implementation of this at http://www.ksk-
test.net .
[ This document is being collaborated on in Github at:
https://github.com/APNIC-Labs/draft-kskroll-sentinel. The most
recent version of the document, open issues, etc should all be
available here. The authors (gratefully) accept pull requests. Text
in square brackets will be removed before publication. ]
[ NOTE: This version uses the labels "kskroll-sentinel-is-ta-<tag-
index>", "kskroll-sentinel-not-ta-<tag-index>"; older versions of
this document used "_is-ta-<tag-index>", "_not-ta-<tag-index>". ]
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 25, 2018.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Use Case . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Sentinel Mechanism . . . . . . . . . . . . . . . . . . . . . 6
4. Sentinel Processing . . . . . . . . . . . . . . . . . . . . . 7
5. Sentinel Test Result Considerations . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
9. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . 11
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
10.1. Normative References . . . . . . . . . . . . . . . . . . 12
10.2. Informative References . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
The DNS Security Extensions (DNSSEC) [RFC4033], [RFC4034] and
[RFC4035] were developed to provide origin authentication and
integrity protection for DNS data by using digital signatures.
DNSSEC uses Key Tags to efficiently match signatures to the keys from
which they are generated. The Key Tag is a 16-bit value computed
from the RDATA portion of a DNSKEY RR using a formula not unlike a
ones-complement checksum. RRSIG RRs contain a Key Tag field whose
value is equal to the Key Tag of the DNSKEY RR that validates the
signature.
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This document specifies how validating resolvers can respond to
certain queries in a manner that allows a querier to deduce whether a
particular key for the root has been loaded into that resolver's
trusted key store. In particular, this response mechanism can be
used to determine whether a certain Root Zone KSK is ready to be used
as a trusted key within the context of a key roll by this resolver.
This new mechanism is OPTIONAL to implement and use, although for
reasons of supporting broad-based measurement techniques, it is
strongly preferred if configurations of DNSSEC-validating resolvers
enabled this mechanism by default, allowing for local configuration
directives to disable this mechanism if desired.
1.1. 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.
Note that example.com, AAAA records and the IPv6 documentation prefix
(2001:db8::/32) are only examples - A records (or CNAMES), other IPs,
other domains work just as well.
2. Use Case
[Ed note: This is currently towards the front of the document; we
will make it an appendix at publication time, but until then it is
worth having up front, as it makes the rest of the document much
easier to understand ]
This section provides a non-normative example of how the sentinel
mechanism could be used, and what each participant does. It is
provided in a conversational tone to be easier to follow.
Alice is in charge of the DNS root KSK (Key Signing Key), and would
like to roll / replace the key with a new one. She publishes the new
KSK, but would like to be able to predict / measure what the impact
will be before removing/revoking the old key. The current KSK has a
key ID of 1111, the new KSK has a key ID of 2222
Bob, Charlie, Dave, Ed are all users. They use the DNS recursive
resolvers supplied by their ISPs. They would like to confirm that
their ISPs have picked up the new KSK and will not break. Bob's ISP
does not perform validation. Charlie's ISP does validate, but the
resolvers have not yet been upgraded to support sentinel. Dave and
Ed's resolvers have been upgraded to support sentinel; Dave's
resolver has the new KSK, Ed's resolver hasn't managed to install the
2222 KSK in its trust store yet.
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Geoff is a researcher, and would like to both provide a means for
Bob, Charlie, Dave and Ed to be able to perform tests, and also would
like to be able to perform Internet wide measurements of what the
impact will be (and report this back to Alice).
Geoff sets an authoritative DNS server for example.com, and also a
webserver (www.example.com). He adds 3 AAAA records to example.com:
invalid.example.com. IN AAAA 2001:db8::1
kskroll-sentinel-is-ta-2222.example.com. IN AAAA 2001:db8::1
kskroll-sentinel-not-ta-2222.example.com. IN AAAA 2001:db8::1
Geoff then DNSSEC signs the example.com zone, and intentionally makes
the invalid.example.com record invalid/bogus (for example, by editing
the signed zone and entering garbage for the signature). Geoff also
configures his webserver to listen on 2001:db8::1 and serve a
resource (for example, a 1x1 GIF, 1x1.gif) for all of these names.
The webserver also serves a webpage (www.example.com) which contains
links to these 3 resources (http://invalid.example.com/1x1.gif,
http://kskroll-sentinel-is-ta-2222.example.com/1x1.gif,
http://kskroll-sentinel-not-ta-2222.example.com/1x1.gif).
Geoff then asks Bob, Charlie, Dave and Ed to browse to
www.example.com. Using the methods described in this document, the
users can figure out what their fate will be when the 1111 KSK is
removed.
Bob is not using a validating resolver. This means that he will be
able to resolve invalid.example.com (and fetch the 1x1 GIF) - this
tells him that the KSK roll does not affect him, and so he will be
OK.
Charlie's resolvers are validating, but they have not been upgraded
to support the KSK sentinel mechanism. Charlie will not be able to
fetch the http://invalid.example.com/1x1.gif resource (the
invalid.example.com record is bogus, and none of his resolvers will
resolve it). He is able to fetch both of the other resources - from
this he knows (see the logic below) that he is using legacy,
validating resolvers. The KSK sentinel method cannot provided him
with a definitive answer.
Dave's resolvers implement the sentinel method, and have picked up
the new KSK. For the same reason as Charlie, he cannot fetch the
"invalid" resource. His resolver resolves the kskroll-sentinel-is-
ta-2222.example.com name normally (it contacts the example.com
authoritative servers, etc); as it supports the sentinel mechanism,
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just before Dave's recursive server send the reply to Dave's stub, it
performs the KSK Sentinel check (see below). The QNAME starts with
"kskroll-sentinel-is-ta-", and the recursive resolver does indeed
have a key with the Key ID of 2222 in its root trust store. This
means that that this part of the KSK Sentinel check passes (it is
true that 2222 is in the Trust Anchor store), and the recursive
resolver replies normally (with the answer provided by the
authoritative server). Dave's recursive resolver then resolves the
kskroll-sentinel-not-ta-2222.example.com name. Once again, it
performs the normal resolution process, but because it implements KSK
Sentinel (and the QNAME starts with "kskroll-sentinel-not-ta-"), just
before sending the reply, it performs the KSK Sentinel check. As it
has 2222 in it's trust anchor store, the "Is this *not* a trust
anchor" is false, and so the recursive resolver does not reply with
the answer from the authoritative server - instead, it replies with a
SERVFAIL (note that replying with SERVFAIL instead of the original
answer is the only mechanism that KSK Sentinel uses). This means
that Dave cannot fetch "invalid", he can fetch "kskroll-sentinel-is-
ta-2222", but he cannot fetch "kskroll-sentinel-not-ta-2222". From
this, Dave knows that he is behind an upgraded, validating resolver,
which has successfully installed the new, 2222 KSK. Dave has nothing
to worry about - he will be fine with the old (1111) KSK is removed.
Now for Ed. Just like Charlie and Dave, Ed cannot fetch the
"invalid" record. This tells him that his resolvers are validating.
When his (upgraded) resolver performs the KSK Sentinel check for
"kskroll-sentinel-is-ta-2222", it does *not* have the (new, 2222) KSK
in it's trust anchor store. This means check fails, and Ed's
recursive resolver converts the (valid) 2001:db8::1 answer into a
SERVFAIL error response. It performs the same check for kskroll-
sentinel-not-ta-2222.example.com; as it does not have the 2222 KSK,
it is true that this is not a trust anchor for it, and so it replies
normally. This means that Ed cannot fetch the "invalid" resource, he
also cannot fetch the "kskroll-sentinel-is-ta-2222" resource, but he
can fetch the "kskroll-sentinel-not-ta-2222" resource. This tells Ed
that his resolvers have not installed the new KSK, and, when the old
KSK is removed, his DNS will break.
Geoff would like to do a large scale test and provide the information
back to Alice. He uses some mechanism (such as an advertising
network) to cause a large number of users to attempt to resolve the 3
resources, and then analyzes the results of the tests to determine
what percentage of users will be affected by the KSK rollover event.
The above description is a simplified example - it is not anticipated
that Bob, Charlie, Dave and Ed will actually look for the absence or
presence of web resources; instead, the webpage that they load would
likely contain JavaScript (or similar) which displays the result of
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the tests. An example of this is at http://www.ksk-test.net. This
KSK mechanism does not rely on the web - this method can equally be
used by trying to resolve the names (for example, using 'dig') and
checking which result in a SERVFAIL.
[ Note that the KSK Sentinel mechanism measures a very different
(and, in our opinion, much more useful!) metric than RFC8145 --
RFC8145 relied on resolvers reporting the list of keys that they have
-- this doesn't reflect what the *user* impact of the KSK roll will
be. As we cannot get perfect visibility into all resolvers, we will
have to aim for "do the least harm", not "do no harm" ]
3. Sentinel Mechanism
DNSSEC-Validating resolvers that implement this mechanism MUST be
performing validation of responses in accordance with the DNSSEC
response validation specification [RFC4035].
This sentinel mechanism makes use of 2 special labels, "kskroll-
sentinel-is-ta-<tag-index>." (intended to be used in a query where
the response can answer the question: Is this the key tag a trust
anchor which the validating DNS resolver is currently trusting?) and
"kskroll-sentinel-not-ta-<tag-index>." (intended to be used in a
query where the response can answer the question: Is this the key tag
of a key that is NOT in the resolver's current trust store?). The
use of the positive question and its inverse allows for queries to
detect whether resolvers support this sentinel mechanism. Note that
the test is "Is there an active key with this KeyID in the resolver's
current trust store for the DNS root?", not "Is there any key with
this KeyID in the trust store", nor "Was a key with this KeyID used
to validate this query?". An active key is one which could currently
be used for validation (ie not in AddPend or Revoked state
([RFC5011])).
If the outcome of the DNSSEC validation process on the response
indicates that the response is authentic, and if the left-most label
of the original query name matches the template "kskroll-sentinel-is-
ta-<tag-index>.", then the following rule should be applied to the
response: If the resolver has placed a Root Zone Key Signing Key with
tag index value matching the value specified in the query into the
local resolver's store of trusted keys, then the resolver should
return a response indicating that the response contains authenticated
data according to section 5.8 of [RFC6840]. Otherwise, the resolver
MUST return RCODE 2 (server failure). Note that the <tag-index> is
specified in the DNS label using hexadecimal notation.
If the outcome of the DNSSEC validation process applied to the
response indicates that the response is authentic, and if the left-
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most label of the original query name matches the template "kskroll-
sentinel-not-ta-<tag-index>.", then the following rule should be
applied to the response: If the resolver has not placed a Root Zone
Key Signing Key with tag index value matching the value specified in
the query into the local resolver's store of trusted keys, then the
resolver should return a response indicating that the response
contains authenticated data according to section 5.8 of [RFC6840].
Otherwise, the resolver MUST return RCODE 2 (server failure). Note
that the <tag-index> is specified in the DNS label using hexadecimal
notation.
In all other cases the resolver MUST NOT alter the outcome of the DNS
response validation process.
This mechanism is to be applied only by resolvers that are performing
DNSSEC validation, and applies only to responses to an A or AAAA
query (Query Type value 1 or 28) where the resolver has authenticated
the response according to the DNSSEC validation process and where the
query name contains either of the labels described in this section as
its left-most label. In this case, the resolver is to perform an
additional test following the conventional validation function, as
described in this section. The result of this additional test
determines whether the resolver will alter its response that would
have indicated that the RRset is authentic to a response that
indicates DNSSEC validation failure via the use of RCODE 2.
4. Sentinel Processing
This proposed test that uses the sentinel detection mechanism
described in this document is based on the use of three DNS names
that have three distinct DNS resolution behaviours. The test is
intended to allow a user to determine the state of their DNS
resolution system, and, in particular, whether or not they are using
validating DNS resolvers that have picked up an incoming trust anchor
as a trusted key in a root zone KSK roll scenario.
The name format can be defined in a number of ways, and no name form
is intrinsically better than any other in terms of the test itself.
The critical aspect of the DNS names used in any such test is that
they contain the specified label for either the positive and negative
test as the left-most label in the query name.
The sentinel detection process is envisaged to use a test with three
query names:
a. a query name containing the left-most label "kskroll-sentinel-is-
ta-<tag-index>.". This corresponds to a a validly-signed RRset
in the zone, so that responses associated with queried names in
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this zone can be authenticated by a DNSSEC-validating resolver.
Any validly-signed DNS zone can be used for this test.
b. a query name containing the left-most label "kskroll-sentinel-
not-ta-<tag-index>.". This is also a validly-signed name. Any
validly-signed DNS zone can be used for this test.
c. a third query name that is signed with a DNSSEC signature that
cannot be validated (i.e. the corresponding RRset is not signed
with a valid RRSIG record).
The responses received from queries to resolve each of these names
would allow us to infer a trust key state of the resolution
environment. The techniques describes in this document rely on
(DNSSEC validating) resolvers responding with SERVFAIL (RCODE 2) to
valid answers. Note that a slew of other issues can also cause
SERVFAIL responses, so false positive or negative results may
sometimes occur. To describe this process of classification, we can
classify resolvers into four distinct behavior types, for which we
will use the labels: "Vnew", "Vold", "Vleg", and "nonV". These
labels correspond to resolver behaviour types as follows:
o Vnew: A DNSSEC-Validating resolver that is configured to implement
this mechanism has loaded the nominated key into its local trusted
key store will respond with an A or AAAA RRset response for
"kskroll-sentinel-is-ta" queries, SERVFAIL for "kskroll-sentinel-
not-ta" queries and SERVFAIL for the invalidly signed name
queries.
o Vold: A DNSSEC-Validating resolver that is configured to implement
this mechanism that has not loaded the nominated key into its
local trusted key store will respond with an SERVFAIL for
"kskroll-sentinel-is-ta" queries, an A or AAAA RRset response for
"kskroll-sentinel-not-ta" queries and SERVFAIL for the invalidly
signed name queries.
o Vleg: A DNSSEC-Validating resolver that does not implement this
mechanism will respond with an A or AAAA RRset response for
"kskroll-sentinel-is-ta", an A record response for "kskroll-
sentinel-not-ta" and SERVFAIL for the invalid name.
o nonV: A non-DNSSEC-Validating resolver will respond with an A or
AAAA record response for "kskroll-sentinel-is-ta", an A record
response for "kskroll-sentinel-not-ta" and an A record response
for the invalid name.
Given the clear delineation amongst these three cases, if a client
directs these three queries to a simple resolver, the variation in
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response to the three queries should allow the client to determine
the category of the resolver, and if it supports this mechanism,
whether or not it has loaded a particular key into its local trusted
key stash.
+-------------+----------+-----------+------------+
| Type\Query | is-ta | not-ta | invalid |
+-------------+----------+-----------+------------+
| Vnew | A | SERVFAIL | SERVFAIL |
| Vold | SERVFAIL | A | SERVFAIL |
| Vleg | A | A | SERVFAIL |
| nonV | A | A | A |
+-------------+----------+-----------+------------+
A "Vnew" response pattern says that the nominated key is trusted by
the resolver and has been loaded into its local trusted key stash. A
"Vold" response pattern says that the nominated key is not yet
trusted by the resolver in its own right. A "Vleg" response pattern
is indeterminate, and a "nonV" response pattern indicates that the
resolver does not perform DNSSEC validation.
5. Sentinel Test Result Considerations
The description in the previous section describes a simple situation
where the test queries were being passed to a single recursive
resolver that directly queried authoritative name servers, including
the root servers.
There is also the common case where the end client is configured to
use multiple resolvers. In these cases the SERVFAIL responses from
one resolver will prompt the end client to repeat the query against
one of the other configured resolvers.
If any of the client's resolvers are non-validating resolvers, the
tests will result in the client reporting that it has a non-
validating DNS environment ("nonV"), which is effectively the case.
If all of the client resolvers are DNSSEC-validating resolvers, but
some do not support this trusted key mechanism, then the result will
be indeterminate with respect to trusted key status ("Vleg").
Simlarly, if all the client's resolvers support this mechanism, but
some have loaded the key into the trusted key stash and some have
not, then the result is indeterminate ("Vleg").
There is also the common case of a recursive resolver using a
forwarder.
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If the resolver is non-validating, and it has a single forwarder
clause, then the resolver will presumably mirror the capabilities of
the forwarder target resolver. If this non-validating resolver it
has multiple forwarders, then the above considerations will apply.
If the validating resolver has a forwarding configuration, and uses
the CD flag on all forwarded queries, then this resolver is acting in
a manner that is identical to a standalone resolver. The same
consideration applies if any one one of the forwarder targets is a
non-validating resolver. Similarly, if all the forwarder targets do
not apply this trusted key mechanism, the same considerations apply.
A more complex case is where the following conditions all hold:
o both the validating resolver and the forwarder target resolver
support this trusted key sentinel mechanism, and
o the local resolver's queries do not have the CD bit set, and
o the trusted key state differs between the forwarding resolver and
the forwarder target resolver
then either the outcome is indeterminate validating ("Vleg"), or a
case of mixed signals (SERVFAIL in all three responses), which is
similarly an indeterminate response with respect to the trusted key
state.
6. Security Considerations
This document describes a mechanism to allow users to determine the
trust state of root zone key signing keys in the DNS resolution
system that they use.
The mechanism does not require resolvers to set otherwise
unauthenticated responses to be marked as authenticated, and does not
alter the security properties of DNSSEC with respect to the
interpretation of the authenticity of responses that are so marked.
The mechanism does not require any further significant processing of
DNS responses, and queries of the form described in this document do
not impose any additional load that could be exploited in an attack
over the the normal DNSSEC validation processing load.
7. IANA Considerations
[Note to IANA, to be removed prior to publication: there are no IANA
considerations stated in this version of the document.]
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8. Acknowledgements
This document has borrowed extensively from [RFC8145] for the
introductory text, and the authors would like to acknowledge and
thank the authors of that document both for some text excerpts and
for the more general stimulation of thoughts about monitoring the
progress of a roll of the Key Signing Key of the Root Zone of the
DNS.
The authors would like the especially thank Joe Abley, Mehmet Akcin,
Mark Andrews, Richard Barnes, Ray Bellis, Stephane Bortzmeyer, David
Conrad, Ralph Dolmans, John Dickinson, Steinar Haug, Bob Harold, Wes
Hardaker, Paul Hoffman, Matt Larson, Jinmei Tatuya, Edward Lewis,
George Michaelson, Benno Overeinder, Matthew Pounsett, Andreas
Schulze, Mukund Sivaraman, Petr Spacek. Andrew Sullivan, Paul Vixie,
Duane Wessels and Paul Wouters for their helpful feedback.
[TODO: Add people who have contributed!]
9. Change Log
Note that this document is being worked on in GitHub - see Abstract.
The below is mainly large changes, and is not authoritative.
From -01 to 02:
Removed Address Record definition.
Clarified that many things can cause SERVFAIL.
Made examples FQDN.
Fixed a number of typos.
Had accidentally said that Charlie was using a non-validating
resolver in example.
[ TODO(WK): Doc says keytags are hex, is this really what the WG
wants? ]
And active key is one that can be used *now* (not e.g AddPend)
From -00 to 01:
o Added a conversational description of how the system is intended
to work.
o Clarification that this is for the root.
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o Changed the label template from _is-ta-<tag> to kskroll-sentinel-
is-ta-<tag-index>. This is because BIND (at least) will not allow
records which start with an underscore to have address records
(CNAMEs, yes, A/AAAA no). Some browsers / operating systems also
will not fetch resources from names which start with an
underscore.
10. References
10.1. Normative References
[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>.
[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/info/rfc4034>.
[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>.
[RFC5011] StJohns, M., "Automated Updates of DNS Security (DNSSEC)
Trust Anchors", STD 74, RFC 5011, DOI 10.17487/RFC5011,
September 2007, <https://www.rfc-editor.org/info/rfc5011>.
[RFC6840] Weiler, S., Ed. and D. Blacka, Ed., "Clarifications and
Implementation Notes for DNS Security (DNSSEC)", RFC 6840,
DOI 10.17487/RFC6840, February 2013, <https://www.rfc-
editor.org/info/rfc6840>.
10.2. Informative References
[RFC8145] Wessels, D., Kumari, W., and P. Hoffman, "Signaling Trust
Anchor Knowledge in DNS Security Extensions (DNSSEC)", RFC
8145, DOI 10.17487/RFC8145, April 2017, <https://www.rfc-
editor.org/info/rfc8145>.
Authors' Addresses
Geoff Huston
Email: gih@apnic.net
URI: http://www.apnic.net
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Joao Silva Damas
Email: joao@apnic.net
URI: http://www.apnic.net
Warren Kumari
Email: warren@kumari.net
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