7
Network Working Group M. StJohns
8
Request for Comments: 5011 Independent
9
Category: Standards Track September 2007
12
Automated Updates of DNS Security (DNSSEC) Trust Anchors
16
This document specifies an Internet standards track protocol for the
17
Internet community, and requests discussion and suggestions for
18
improvements. Please refer to the current edition of the "Internet
19
Official Protocol Standards" (STD 1) for the standardization state
20
and status of this protocol. Distribution of this memo is unlimited.
24
This document describes a means for automated, authenticated, and
25
authorized updating of DNSSEC "trust anchors". The method provides
26
protection against N-1 key compromises of N keys in the trust point
27
key set. Based on the trust established by the presence of a current
28
anchor, other anchors may be added at the same place in the
29
hierarchy, and, ultimately, supplant the existing anchor(s).
31
This mechanism will require changes to resolver management behavior
32
(but not resolver resolution behavior), and the addition of a single
33
flag bit to the DNSKEY record.
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1. Introduction ....................................................2
66
1.1. Compliance Nomenclature ....................................3
67
2. Theory of Operation .............................................3
68
2.1. Revocation .................................................4
69
2.2. Add Hold-Down ..............................................4
70
2.3. Active Refresh .............................................5
71
2.4. Resolver Parameters ........................................6
72
2.4.1. Add Hold-Down Time ..................................6
73
2.4.2. Remove Hold-Down Time ...............................6
74
2.4.3. Minimum Trust Anchors per Trust Point ...............6
75
3. Changes to DNSKEY RDATA Wire Format .............................6
76
4. State Table .....................................................6
77
4.1. Events .....................................................7
78
4.2. States .....................................................7
79
5. Trust Point Deletion ............................................8
80
6. Scenarios - Informative .........................................9
81
6.1. Adding a Trust Anchor ......................................9
82
6.2. Deleting a Trust Anchor ....................................9
83
6.3. Key Roll-Over .............................................10
84
6.4. Active Key Compromised ....................................10
85
6.5. Stand-by Key Compromised ..................................10
86
6.6. Trust Point Deletion ......................................10
87
7. IANA Considerations ............................................11
88
8. Security Considerations ........................................11
89
8.1. Key Ownership vs. Acceptance Policy .......................11
90
8.2. Multiple Key Compromise ...................................12
91
8.3. Dynamic Updates ...........................................12
92
9. Normative References ...........................................12
93
10. Informative References ........................................12
97
As part of the reality of fielding DNSSEC (Domain Name System
98
Security Extensions) [RFC4033] [RFC4034] [RFC4035], the community has
99
come to the realization that there will not be one signed name space,
100
but rather islands of signed name spaces each originating from
101
specific points (i.e., 'trust points') in the DNS tree. Each of
102
those islands will be identified by the trust point name, and
103
validated by at least one associated public key. For the purpose of
104
this document, we'll call the association of that name and a
105
particular key a 'trust anchor'. A particular trust point can have
106
more than one key designated as a trust anchor.
108
For a DNSSEC-aware resolver to validate information in a DNSSEC
109
protected branch of the hierarchy, it must have knowledge of a trust
110
anchor applicable to that branch. It may also have more than one
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trust anchor for any given trust point. Under current rules, a chain
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of trust for DNSSEC-protected data that chains its way back to ANY
121
known trust anchor is considered 'secure'.
123
Because of the probable balkanization of the DNSSEC tree due to
124
signing voids at key locations, a resolver may need to know literally
125
thousands of trust anchors to perform its duties (e.g., consider an
126
unsigned ".COM"). Requiring the owner of the resolver to manually
127
manage these many relationships is problematic. It's even more
128
problematic when considering the eventual requirement for key
129
replacement/update for a given trust anchor. The mechanism described
130
herein won't help with the initial configuration of the trust anchors
131
in the resolvers, but should make trust point key
132
replacement/rollover more viable.
134
As mentioned above, this document describes a mechanism whereby a
135
resolver can update the trust anchors for a given trust point, mainly
136
without human intervention at the resolver. There are some corner
137
cases discussed (e.g., multiple key compromise) that may require
138
manual intervention, but they should be few and far between. This
139
document DOES NOT discuss the general problem of the initial
140
configuration of trust anchors for the resolver.
142
1.1. Compliance Nomenclature
144
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
145
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
146
document are to be interpreted as described in BCP 14, [RFC2119].
148
2. Theory of Operation
150
The general concept of this mechanism is that existing trust anchors
151
can be used to authenticate new trust anchors at the same point in
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the DNS hierarchy. When a zone operator adds a new SEP key (i.e., a
153
DNSKEY with the Secure Entry Point bit set) (see [RFC4034], Section
154
2.1.1) to a trust point DNSKEY RRSet, and when that RRSet is
155
validated by an existing trust anchor, then the resolver can add the
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new key to its set of valid trust anchors for that trust point.
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There are some issues with this approach that need to be mitigated.
159
For example, a compromise of one of the existing keys could allow an
160
attacker to add their own 'valid' data. This implies a need for a
161
method to revoke an existing key regardless of whether or not that
162
key is compromised. As another example, assuming a single key
163
compromise, we need to prevent an attacker from adding a new key and
164
revoking all the other old keys.
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Assume two trust anchor keys A and B. Assume that B has been
178
compromised. Without a specific revocation bit, B could invalidate A
179
simply by sending out a signed trust point key set that didn't
180
contain A. To fix this, we add a mechanism that requires knowledge
181
of the private key of a DNSKEY to revoke that DNSKEY.
183
A key is considered revoked when the resolver sees the key in a
184
self-signed RRSet and the key has the REVOKE bit (see Section 7
185
below) set to '1'. Once the resolver sees the REVOKE bit, it MUST
186
NOT use this key as a trust anchor or for any other purpose except to
187
validate the RRSIG it signed over the DNSKEY RRSet specifically for
188
the purpose of validating the revocation. Unlike the 'Add' operation
189
below, revocation is immediate and permanent upon receipt of a valid
190
revocation at the resolver.
192
A self-signed RRSet is a DNSKEY RRSet that contains the specific
193
DNSKEY and for which there is a corresponding validated RRSIG record.
194
It's not a special DNSKEY RRSet, just a way of describing the
195
validation requirements for that RRSet.
197
N.B.: A DNSKEY with the REVOKE bit set has a different fingerprint
198
than one without the bit set. This affects the matching of a DNSKEY
199
to DS records in the parent [RFC3755], or the fingerprint stored at a
200
resolver used to configure a trust point.
202
In the given example, the attacker could revoke B because it has
203
knowledge of B's private key, but could not revoke A.
207
Assume two trust point keys A and B. Assume that B has been
208
compromised. An attacker could generate and add a new trust anchor
209
key C (by adding C to the DNSKEY RRSet and signing it with B), and
210
then invalidate the compromised key. This would result in both the
211
attacker and owner being able to sign data in the zone and have it
212
accepted as valid by resolvers.
214
To mitigate but not completely solve this problem, we add a hold-down
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time to the addition of the trust anchor. When the resolver sees a
216
new SEP key in a validated trust point DNSKEY RRSet, the resolver
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starts an acceptance timer, and remembers all the keys that validated
218
the RRSet. If the resolver ever sees the DNSKEY RRSet without the
219
new key but validly signed, it stops the acceptance process for that
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key and resets the acceptance timer. If all of the keys that were
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originally used to validate this key are revoked prior to the timer
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expiring, the resolver stops the acceptance process and resets the
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Once the timer expires, the new key will be added as a trust anchor
236
the next time the validated RRSet with the new key is seen at the
237
resolver. The resolver MUST NOT treat the new key as a trust anchor
238
until the hold-down time expires AND it has retrieved and validated a
239
DNSKEY RRSet after the hold-down time that contains the new key.
241
N.B.: Once the resolver has accepted a key as a trust anchor, the key
242
MUST be considered a valid trust anchor by that resolver until
243
explicitly revoked as described above.
245
In the given example, the zone owner can recover from a compromise by
246
revoking B and adding a new key D and signing the DNSKEY RRSet with
249
The reason this does not completely solve the problem has to do with
250
the distributed nature of DNS. The resolver only knows what it sees.
251
A determined attacker who holds one compromised key could keep a
252
single resolver from realizing that the key had been compromised by
253
intercepting 'real' data from the originating zone and substituting
254
their own (e.g., using the example, signed only by B). This is no
255
worse than the current situation assuming a compromised key.
259
A resolver that has been configured for an automatic update of keys
260
from a particular trust point MUST query that trust point (e.g., do a
261
lookup for the DNSKEY RRSet and related RRSIG records) no less often
262
than the lesser of 15 days, half the original TTL for the DNSKEY
263
RRSet, or half the RRSIG expiration interval and no more often than
264
once per hour. The expiration interval is the amount of time from
265
when the RRSIG was last retrieved until the expiration time in the
266
RRSIG. That is, queryInterval = MAX(1 hr, MIN (15 days, 1/2*OrigTTL,
267
1/2*RRSigExpirationInterval))
269
If the query fails, the resolver MUST repeat the query until
270
satisfied no more often than once an hour and no less often than the
271
lesser of 1 day, 10% of the original TTL, or 10% of the original
272
expiration interval. That is, retryTime = MAX (1 hour, MIN (1 day,
273
.1 * origTTL, .1 * expireInterval)).
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2.4. Resolver Parameters
289
2.4.1. Add Hold-Down Time
291
The add hold-down time is 30 days or the expiration time of the
292
original TTL of the first trust point DNSKEY RRSet that contained the
293
new key, whichever is greater. This ensures that at least two
294
validated DNSKEY RRSets that contain the new key MUST be seen by the
295
resolver prior to the key's acceptance.
297
2.4.2. Remove Hold-Down Time
299
The remove hold-down time is 30 days. This parameter is solely a key
300
management database bookeeping parameter. Failure to remove
301
information about the state of defunct keys from the database will
302
not adversely impact the security of this protocol, but may end up
303
with a database cluttered with obsolete key information.
305
2.4.3. Minimum Trust Anchors per Trust Point
307
A compliant resolver MUST be able to manage at least five SEP keys
310
3. Changes to DNSKEY RDATA Wire Format
312
Bit 8 of the DNSKEY Flags field is designated as the 'REVOKE' flag.
313
If this bit is set to '1', AND the resolver sees an RRSIG(DNSKEY)
314
signed by the associated key, then the resolver MUST consider this
315
key permanently invalid for all purposes except for validating the
320
The most important thing to understand is the resolver's view of any
321
key at a trust point. The following state table describes this view
322
at various points in the key's lifetime. The table is a normative
323
part of this specification. The initial state of the key is 'Start'.
324
The resolver's view of the state of the key changes as various events
327
This is the state of a trust-point key as seen from the resolver.
328
The column on the left indicates the current state. The header at
329
the top shows the next state. The intersection of the two shows the
330
event that will cause the state to transition from the current state
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RFC 5011 Trust Anchor Update September 2007
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--------------------------------------------------
345
FROM |Start |AddPend |Valid |Missing|Revoked|Removed|
346
----------------------------------------------------------
347
Start | |NewKey | | | | |
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----------------------------------------------------------
349
AddPend |KeyRem | |AddTime| | | |
350
----------------------------------------------------------
351
Valid | | | |KeyRem |Revbit | |
352
----------------------------------------------------------
353
Missing | | |KeyPres| |Revbit | |
354
----------------------------------------------------------
355
Revoked | | | | | |RemTime|
356
----------------------------------------------------------
357
Removed | | | | | | |
358
----------------------------------------------------------
364
NewKey The resolver sees a valid DNSKEY RRSet with a new SEP key.
365
That key will become a new trust anchor for the named trust
366
point after it's been present in the RRSet for at least 'add
369
KeyPres The key has returned to the valid DNSKEY RRSet.
371
KeyRem The resolver sees a valid DNSKEY RRSet that does not contain
374
AddTime The key has been in every valid DNSKEY RRSet seen for at
375
least the 'add time'.
377
RemTime A revoked key has been missing from the trust-point DNSKEY
378
RRSet for sufficient time to be removed from the trust set.
380
RevBit The key has appeared in the trust anchor DNSKEY RRSet with
381
its "REVOKED" bit set, and there is an RRSig over the DNSKEY
382
RRSet signed by this key.
386
Start The key doesn't yet exist as a trust anchor at the resolver.
387
It may or may not exist at the zone server, but either
388
hasn't yet been seen at the resolver or was seen but was
389
absent from the last DNSKEY RRSet (e.g., KeyRem event).
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RFC 5011 Trust Anchor Update September 2007
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AddPend The key has been seen at the resolver, has its 'SEP' bit
400
set, and has been included in a validated DNSKEY RRSet.
401
There is a hold-down time for the key before it can be used
404
Valid The key has been seen at the resolver and has been included
405
in all validated DNSKEY RRSets from the time it was first
406
seen through the hold-down time. It is now valid for
407
verifying RRSets that arrive after the hold-down time.
408
Clarification: The DNSKEY RRSet does not need to be
409
continuously present at the resolver (e.g., its TTL might
410
expire). If the RRSet is seen and is validated (i.e.,
411
verifies against an existing trust anchor), this key MUST be
412
in the RRSet, otherwise a 'KeyRem' event is triggered.
414
Missing This is an abnormal state. The key remains a valid trust-
415
point key, but was not seen at the resolver in the last
416
validated DNSKEY RRSet. This is an abnormal state because
417
the zone operator should be using the REVOKE bit prior to
420
Revoked This is the state a key moves to once the resolver sees an
421
RRSIG(DNSKEY) signed by this key where that DNSKEY RRSet
422
contains this key with its REVOKE bit set to '1'. Once in
423
this state, this key MUST permanently be considered invalid
426
Removed After a fairly long hold-down time, information about this
427
key may be purged from the resolver. A key in the removed
428
state MUST NOT be considered a valid trust anchor. (Note:
429
this state is more or less equivalent to the "Start" state,
430
except that it's bad practice to re-introduce previously
431
used keys -- think of this as the holding state for all the
432
old keys for which the resolver no longer needs to track
435
5. Trust Point Deletion
437
A trust point that has all of its trust anchors revoked is considered
438
deleted and is treated as if the trust point was never configured.
439
If there are no superior configured trust points, data at and below
440
the deleted trust point are considered insecure by the resolver. If
441
there ARE superior configured trust points, data at and below the
442
deleted trust point are evaluated with respect to the superior trust
445
Alternately, a trust point that is subordinate to another configured
446
trust point MAY be deleted by a resolver after 180 days, where such a
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subordinate trust point validly chains to a superior trust point.
456
The decision to delete the subordinate trust anchor is a local
457
configuration decision. Once the subordinate trust point is deleted,
458
validation of the subordinate zone is dependent on validating the
459
chain of trust to the superior trust point.
461
6. Scenarios - Informative
463
The suggested model for operation is to have one active key and one
464
stand-by key at each trust point. The active key will be used to
465
sign the DNSKEY RRSet. The stand-by key will not normally sign this
466
RRSet, but the resolver will accept it as a trust anchor if/when it
467
sees the signature on the trust point DNSKEY RRSet.
469
Since the stand-by key is not in active signing use, the associated
470
private key may (and should) be provided with additional protections
471
not normally available to a key that must be used frequently (e.g.,
472
locked in a safe, split among many parties, etc). Notionally, the
473
stand-by key should be less subject to compromise than an active key,
474
but that will be dependent on operational concerns not addressed
477
6.1. Adding a Trust Anchor
479
Assume an existing trust anchor key 'A'.
481
1. Generate a new key pair.
483
2. Create a DNSKEY record from the key pair and set the SEP and Zone
486
3. Add the DNSKEY to the RRSet.
488
4. Sign the DNSKEY RRSet ONLY with the existing trust anchor key -
491
5. Wait for various resolvers' timers to go off and for them to
492
retrieve the new DNSKEY RRSet and signatures.
494
6. The new trust anchor will be populated at the resolvers on the
495
schedule described by the state table and update algorithm -- see
496
Sections 2 and 4 above.
498
6.2. Deleting a Trust Anchor
500
Assume existing trust anchors 'A' and 'B' and that you want to revoke
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1. Set the revocation bit on key 'A'.
513
2. Sign the DNSKEY RRSet with both 'A' and 'B'. 'A' is now revoked.
514
The operator should include the revoked 'A' in the RRSet for at
515
least the remove hold-down time, but then may remove it from the
520
Assume existing keys A and B. 'A' is actively in use (i.e. has been
521
signing the DNSKEY RRSet). 'B' was the stand-by key. (i.e. has been
522
in the DNSKEY RRSet and is a valid trust anchor, but wasn't being
523
used to sign the RRSet).
525
1. Generate a new key pair 'C'.
526
2. Add 'C' to the DNSKEY RRSet.
527
3. Set the revocation bit on key 'A'.
528
4. Sign the RRSet with 'A' and 'B'.
530
'A' is now revoked, 'B' is now the active key, and 'C' will be the
531
stand-by key once the hold-down expires. The operator should include
532
the revoked 'A' in the RRSet for at least the remove hold-down time,
533
but may then remove it from the DNSKEY RRSet.
535
6.4. Active Key Compromised
537
This is the same as the mechanism for Key Roll-Over (Section 6.3)
538
above, assuming 'A' is the active key.
540
6.5. Stand-by Key Compromised
542
Using the same assumptions and naming conventions as Key Roll-Over
545
1. Generate a new key pair 'C'.
546
2. Add 'C' to the DNSKEY RRSet.
547
3. Set the revocation bit on key 'B'.
548
4. Sign the RRSet with 'A' and 'B'.
550
'B' is now revoked, 'A' remains the active key, and 'C' will be the
551
stand-by key once the hold-down expires. 'B' should continue to be
552
included in the RRSet for the remove hold-down time.
554
6.6. Trust Point Deletion
556
To delete a trust point that is subordinate to another configured
557
trust point (e.g., example.com to .com) requires some juggling of the
558
data. The specific process is:
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1. Generate a new DNSKEY and DS record and provide the DS record to
568
the parent along with DS records for the old keys.
570
2. Once the parent has published the DSs, add the new DNSKEY to the
571
RRSet and revoke ALL of the old keys at the same time, while
572
signing the DNSKEY RRSet with all of the old and new keys.
574
3. After 30 days, stop publishing the old, revoked keys and remove
575
any corresponding DS records in the parent.
577
Revoking the old trust-point keys at the same time as adding new keys
578
that chain to a superior trust prevents the resolver from adding the
579
new keys as trust anchors. Adding DS records for the old keys avoids
580
a race condition where either the subordinate zone becomes unsecure
581
(because the trust point was deleted) or becomes bogus (because it
582
didn't chain to the superior zone).
584
7. IANA Considerations
586
The IANA has assigned a bit in the DNSKEY flags field (see Section 7
587
of [RFC4034]) for the REVOKE bit (8).
589
8. Security Considerations
591
In addition to the following sections, see also Theory of Operation
592
above (Section 2) and especially Section 2.2 for related discussions.
594
Security considerations for trust anchor rollover not specific to
595
this protocol are discussed in [RFC4986].
597
8.1. Key Ownership vs. Acceptance Policy
599
The reader should note that, while the zone owner is responsible for
600
creating and distributing keys, it's wholly the decision of the
601
resolver owner as to whether to accept such keys for the
602
authentication of the zone information. This implies the decision to
603
update trust-anchor keys based on trusting a current trust-anchor key
604
is also the resolver owner's decision.
606
The resolver owner (and resolver implementers) MAY choose to permit
607
or prevent key status updates based on this mechanism for specific
608
trust points. If they choose to prevent the automated updates, they
609
will need to establish a mechanism for manual or other out-of-band
610
updates, which are outside the scope of this document.
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RFC 5011 Trust Anchor Update September 2007
623
8.2. Multiple Key Compromise
625
This scheme permits recovery as long as at least one valid trust-
626
anchor key remains uncompromised, e.g., if there are three keys, you
627
can recover if two of them are compromised. The zone owner should
628
determine their own level of comfort with respect to the number of
629
active, valid trust anchors in a zone and should be prepared to
630
implement recovery procedures once they detect a compromise. A
631
manual or other out-of-band update of all resolvers will be required
632
if all trust-anchor keys at a trust point are compromised.
636
Allowing a resolver to update its trust anchor set based on in-band
637
key information is potentially less secure than a manual process.
638
However, given the nature of the DNS, the number of resolvers that
639
would require update if a trust anchor key were compromised, and the
640
lack of a standard management framework for DNS, this approach is no
641
worse than the existing situation.
643
9. Normative References
645
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
646
Requirement Levels", BCP 14, RFC 2119, March 1997.
648
[RFC3755] Weiler, S., "Legacy Resolver Compatibility for Delegation
649
Signer (DS)", RFC 3755, May 2004.
651
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
652
Rose, "DNS Security Introduction and Requirements", RFC
655
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
656
Rose, "Resource Records for the DNS Security Extensions",
657
RFC 4034, March 2005.
659
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
660
Rose, "Protocol Modifications for the DNS Security
661
Extensions", RFC 4035, March 2005.
663
10. Informative References
665
[RFC4986] Eland, H., Mundy, R., Crocker, S., and S. Krishnaswamy,
666
"Requirements Related to DNS Security (DNSSEC) Trust
667
Anchor Rollover", RFC 4986, August 2007.
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RFC 5011 Trust Anchor Update September 2007
684
EMail: mstjohns@comcast.net
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RFC 5011 Trust Anchor Update September 2007
735
Full Copyright Statement
737
Copyright (C) The IETF Trust (2007).
739
This document is subject to the rights, licenses and restrictions
740
contained in BCP 78, and except as set forth therein, the authors
741
retain all their rights.
743
This document and the information contained herein are provided on an
744
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
745
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
746
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
747
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
748
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
749
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
751
Intellectual Property
753
The IETF takes no position regarding the validity or scope of any
754
Intellectual Property Rights or other rights that might be claimed to
755
pertain to the implementation or use of the technology described in
756
this document or the extent to which any license under such rights
757
might or might not be available; nor does it represent that it has
758
made any independent effort to identify any such rights. Information
759
on the procedures with respect to rights in RFC documents can be
760
found in BCP 78 and BCP 79.
762
Copies of IPR disclosures made to the IETF Secretariat and any
763
assurances of licenses to be made available, or the result of an
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attempt made to obtain a general license or permission for the use of
765
such proprietary rights by implementers or users of this
766
specification can be obtained from the IETF on-line IPR repository at
767
http://www.ietf.org/ipr.
769
The IETF invites any interested party to bring to its attention any
770
copyrights, patents or patent applications, or other proprietary
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rights that may cover technology that may be required to implement
772
this standard. Please address the information to the IETF at
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contrib/zkt/doc/rfc5011.txt
