Movatterモバイル変換

Network Working Group                                         M. StJohnsRequest for Comments: 5011                                   IndependentCategory: Standards Track                                 September 2007Automated Updates of DNS Security (DNSSEC) Trust AnchorsStatus of This Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Abstract   This document describes a means for automated, authenticated, and   authorized updating of DNSSEC "trust anchors".  The method provides   protection against N-1 key compromises of N keys in the trust point   key set.  Based on the trust established by the presence of a current   anchor, other anchors may be added at the same place in the   hierarchy, and, ultimately, supplant the existing anchor(s).   This mechanism will require changes to resolver management behavior   (but not resolver resolution behavior), and the addition of a single   flag bit to the DNSKEY record.StJohns                     Standards Track                     [Page 1]

RFC 5011                  Trust Anchor Update             September 2007Table of Contents1. Introduction ....................................................21.1. Compliance Nomenclature ....................................32. Theory of Operation .............................................32.1. Revocation .................................................42.2. Add Hold-Down ..............................................42.3. Active Refresh .............................................52.4. Resolver Parameters ........................................62.4.1. Add Hold-Down Time ..................................62.4.2. Remove Hold-Down Time ...............................62.4.3. Minimum Trust Anchors per Trust Point ...............63. Changes to DNSKEY RDATA Wire Format .............................64. State Table .....................................................64.1. Events .....................................................74.2. States .....................................................75. Trust Point Deletion ............................................86. Scenarios - Informative .........................................96.1. Adding a Trust Anchor ......................................96.2. Deleting a Trust Anchor ....................................96.3. Key Roll-Over .............................................106.4. Active Key Compromised ....................................106.5. Stand-by Key Compromised ..................................106.6. Trust Point Deletion ......................................107. IANA Considerations ............................................118. Security Considerations ........................................118.1. Key Ownership vs. Acceptance Policy .......................118.2. Multiple Key Compromise ...................................128.3. Dynamic Updates ...........................................129. Normative References ...........................................1210. Informative References ........................................121.  Introduction   As part of the reality of fielding DNSSEC (Domain Name System   Security Extensions) [RFC4033] [RFC4034] [RFC4035], the community has   come to the realization that there will not be one signed name space,   but rather islands of signed name spaces each originating from   specific points (i.e., 'trust points') in the DNS tree.  Each of   those islands will be identified by the trust point name, and   validated by at least one associated public key.  For the purpose of   this document, we'll call the association of that name and a   particular key a 'trust anchor'.  A particular trust point can have   more than one key designated as a trust anchor.   For a DNSSEC-aware resolver to validate information in a DNSSEC   protected branch of the hierarchy, it must have knowledge of a trust   anchor applicable to that branch.  It may also have more than oneStJohns                     Standards Track                     [Page 2]

RFC 5011                  Trust Anchor Update             September 2007   trust anchor for any given trust point.  Under current rules, a chain   of trust for DNSSEC-protected data that chains its way back to ANY   known trust anchor is considered 'secure'.   Because of the probable balkanization of the DNSSEC tree due to   signing voids at key locations, a resolver may need to know literally   thousands of trust anchors to perform its duties (e.g., consider an   unsigned ".COM").  Requiring the owner of the resolver to manually   manage these many relationships is problematic.  It's even more   problematic when considering the eventual requirement for key   replacement/update for a given trust anchor.  The mechanism described   herein won't help with the initial configuration of the trust anchors   in the resolvers, but should make trust point key   replacement/rollover more viable.   As mentioned above, this document describes a mechanism whereby a   resolver can update the trust anchors for a given trust point, mainly   without human intervention at the resolver.  There are some corner   cases discussed (e.g., multiple key compromise) that may require   manual intervention, but they should be few and far between.  This   document DOES NOT discuss the general problem of the initial   configuration of trust anchors for the resolver.1.1.  Compliance Nomenclature   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 inBCP 14, [RFC2119].2.  Theory of Operation   The general concept of this mechanism is that existing trust anchors   can be used to authenticate new trust anchors at the same point in   the DNS hierarchy.  When a zone operator adds a new SEP key (i.e., a   DNSKEY with the Secure Entry Point bit set) (see [RFC4034],Section2.1.1) to a trust point DNSKEY RRSet, and when that RRSet is   validated by an existing trust anchor, then the resolver can add the   new key to its set of valid trust anchors for that trust point.   There are some issues with this approach that need to be mitigated.   For example, a compromise of one of the existing keys could allow an   attacker to add their own 'valid' data.  This implies a need for a   method to revoke an existing key regardless of whether or not that   key is compromised.  As another example, assuming a single key   compromise, we need to prevent an attacker from adding a new key and   revoking all the other old keys.StJohns                     Standards Track                     [Page 3]

RFC 5011                  Trust Anchor Update             September 20072.1.  Revocation   Assume two trust anchor keys A and B.  Assume that B has been   compromised.  Without a specific revocation bit, B could invalidate A   simply by sending out a signed trust point key set that didn't   contain A.  To fix this, we add a mechanism that requires knowledge   of the private key of a DNSKEY to revoke that DNSKEY.   A key is considered revoked when the resolver sees the key in a   self-signed RRSet and the key has the REVOKE bit (seeSection 7   below) set to '1'.  Once the resolver sees the REVOKE bit, it MUST   NOT use this key as a trust anchor or for any other purpose except to   validate the RRSIG it signed over the DNSKEY RRSet specifically for   the purpose of validating the revocation.  Unlike the 'Add' operation   below, revocation is immediate and permanent upon receipt of a valid   revocation at the resolver.   A self-signed RRSet is a DNSKEY RRSet that contains the specific   DNSKEY and for which there is a corresponding validated RRSIG record.   It's not a special DNSKEY RRSet, just a way of describing the   validation requirements for that RRSet.   N.B.: A DNSKEY with the REVOKE bit set has a different fingerprint   than one without the bit set.  This affects the matching of a DNSKEY   to DS records in the parent [RFC3755], or the fingerprint stored at a   resolver used to configure a trust point.   In the given example, the attacker could revoke B because it has   knowledge of B's private key, but could not revoke A.2.2.  Add Hold-Down   Assume two trust point keys A and B.  Assume that B has been   compromised.  An attacker could generate and add a new trust anchor   key C (by adding C to the DNSKEY RRSet and signing it with B), and   then invalidate the compromised key.  This would result in both the   attacker and owner being able to sign data in the zone and have it   accepted as valid by resolvers.   To mitigate but not completely solve this problem, we add a hold-down   time to the addition of the trust anchor.  When the resolver sees a   new SEP key in a validated trust point DNSKEY RRSet, the resolver   starts an acceptance timer, and remembers all the keys that validated   the RRSet.  If the resolver ever sees the DNSKEY RRSet without the   new key but validly signed, it stops the acceptance process for that   key and resets the acceptance timer.  If all of the keys that wereStJohns                     Standards Track                     [Page 4]

RFC 5011                  Trust Anchor Update             September 2007   originally used to validate this key are revoked prior to the timer   expiring, the resolver stops the acceptance process and resets the   timer.   Once the timer expires, the new key will be added as a trust anchor   the next time the validated RRSet with the new key is seen at the   resolver.  The resolver MUST NOT treat the new key as a trust anchor   until the hold-down time expires AND it has retrieved and validated a   DNSKEY RRSet after the hold-down time that contains the new key.   N.B.: Once the resolver has accepted a key as a trust anchor, the key   MUST be considered a valid trust anchor by that resolver until   explicitly revoked as described above.   In the given example, the zone owner can recover from a compromise by   revoking B and adding a new key D and signing the DNSKEY RRSet with   both A and B.   The reason this does not completely solve the problem has to do with   the distributed nature of DNS.  The resolver only knows what it sees.   A determined attacker who holds one compromised key could keep a   single resolver from realizing that the key had been compromised by   intercepting 'real' data from the originating zone and substituting   their own (e.g., using the example, signed only by B).  This is no   worse than the current situation assuming a compromised key.2.3.  Active Refresh   A resolver that has been configured for an automatic update of keys   from a particular trust point MUST query that trust point (e.g., do a   lookup for the DNSKEY RRSet and related RRSIG records) no less often   than the lesser of 15 days, half the original TTL for the DNSKEY   RRSet, or half the RRSIG expiration interval and no more often than   once per hour.  The expiration interval is the amount of time from   when the RRSIG was last retrieved until the expiration time in the   RRSIG.  That is, queryInterval = MAX(1 hr, MIN (15 days, 1/2*OrigTTL,   1/2*RRSigExpirationInterval))   If the query fails, the resolver MUST repeat the query until   satisfied no more often than once an hour and no less often than the   lesser of 1 day, 10% of the original TTL, or 10% of the original   expiration interval.  That is, retryTime = MAX (1 hour, MIN (1 day,   .1 * origTTL, .1 * expireInterval)).StJohns                     Standards Track                     [Page 5]

RFC 5011                  Trust Anchor Update             September 20072.4.  Resolver Parameters2.4.1.  Add Hold-Down Time   The add hold-down time is 30 days or the expiration time of the   original TTL of the first trust point DNSKEY RRSet that contained the   new key, whichever is greater.  This ensures that at least two   validated DNSKEY RRSets that contain the new key MUST be seen by the   resolver prior to the key's acceptance.2.4.2.  Remove Hold-Down Time   The remove hold-down time is 30 days.  This parameter is solely a key   management database bookeeping parameter.  Failure to remove   information about the state of defunct keys from the database will   not adversely impact the security of this protocol, but may end up   with a database cluttered with obsolete key information.2.4.3.  Minimum Trust Anchors per Trust Point   A compliant resolver MUST be able to manage at least five SEP keys   per trust point.3.  Changes to DNSKEY RDATA Wire Format   Bit 8 of the DNSKEY Flags field is designated as the 'REVOKE' flag.   If this bit is set to '1', AND the resolver sees an RRSIG(DNSKEY)   signed by the associated key, then the resolver MUST consider this   key permanently invalid for all purposes except for validating the   revocation.4.  State Table   The most important thing to understand is the resolver's view of any   key at a trust point.  The following state table describes this view   at various points in the key's lifetime.  The table is a normative   part of this specification.  The initial state of the key is 'Start'.   The resolver's view of the state of the key changes as various events   occur.   This is the state of a trust-point key as seen from the resolver.   The column on the left indicates the current state.  The header at   the top shows the next state.  The intersection of the two shows the   event that will cause the state to transition from the current state   to the next.StJohns                     Standards Track                     [Page 6]

RFC 5011                  Trust Anchor Update             September 2007                             NEXT STATE           --------------------------------------------------    FROM   |Start  |AddPend |Valid  |Missing|Revoked|Removed|   ----------------------------------------------------------   Start   |       |NewKey  |       |       |       |       |   ----------------------------------------------------------   AddPend |KeyRem |        |AddTime|       |       |       |   ----------------------------------------------------------   Valid   |       |        |       |KeyRem |Revbit |       |   ----------------------------------------------------------   Missing |       |        |KeyPres|       |Revbit |       |   ----------------------------------------------------------   Revoked |       |        |       |       |       |RemTime|   ----------------------------------------------------------   Removed |       |        |       |       |       |       |   ----------------------------------------------------------                           State Table4.1.  Events   NewKey   The resolver sees a valid DNSKEY RRSet with a new SEP key.            That key will become a new trust anchor for the named trust            point after it's been present in the RRSet for at least 'add            time'.   KeyPres  The key has returned to the valid DNSKEY RRSet.   KeyRem   The resolver sees a valid DNSKEY RRSet that does not contain            this key.   AddTime  The key has been in every valid DNSKEY RRSet seen for at            least the 'add time'.   RemTime  A revoked key has been missing from the trust-point DNSKEY            RRSet for sufficient time to be removed from the trust set.   RevBit   The key has appeared in the trust anchor DNSKEY RRSet with            its "REVOKED" bit set, and there is an RRSig over the DNSKEY            RRSet signed by this key.4.2.  States   Start    The key doesn't yet exist as a trust anchor at the resolver.            It may or may not exist at the zone server, but either            hasn't yet been seen at the resolver or was seen but was            absent from the last DNSKEY RRSet (e.g., KeyRem event).StJohns                     Standards Track                     [Page 7]

RFC 5011                  Trust Anchor Update             September 2007   AddPend  The key has been seen at the resolver, has its 'SEP' bit            set, and has been included in a validated DNSKEY RRSet.            There is a hold-down time for the key before it can be used            as a trust anchor.   Valid    The key has been seen at the resolver and has been included            in all validated DNSKEY RRSets from the time it was first            seen through the hold-down time.  It is now valid for            verifying RRSets that arrive after the hold-down time.            Clarification: The DNSKEY RRSet does not need to be            continuously present at the resolver (e.g., its TTL might            expire).  If the RRSet is seen and is validated (i.e.,            verifies against an existing trust anchor), this key MUST be            in the RRSet, otherwise a 'KeyRem' event is triggered.   Missing  This is an abnormal state.  The key remains a valid trust-            point key, but was not seen at the resolver in the last            validated DNSKEY RRSet.  This is an abnormal state because            the zone operator should be using the REVOKE bit prior to            removal.   Revoked  This is the state a key moves to once the resolver sees an            RRSIG(DNSKEY) signed by this key where that DNSKEY RRSet            contains this key with its REVOKE bit set to '1'.  Once in            this state, this key MUST permanently be considered invalid            as a trust anchor.   Removed  After a fairly long hold-down time, information about this            key may be purged from the resolver.  A key in the removed            state MUST NOT be considered a valid trust anchor.  (Note:            this state is more or less equivalent to the "Start" state,            except that it's bad practice to re-introduce previously            used keys -- think of this as the holding state for all the            old keys for which the resolver no longer needs to track            state.)5.  Trust Point Deletion   A trust point that has all of its trust anchors revoked is considered   deleted and is treated as if the trust point was never configured.   If there are no superior configured trust points, data at and below   the deleted trust point are considered insecure by the resolver.  If   there ARE superior configured trust points, data at and below the   deleted trust point are evaluated with respect to the superior trust   point(s).   Alternately, a trust point that is subordinate to another configured   trust point MAY be deleted by a resolver after 180 days, where such aStJohns                     Standards Track                     [Page 8]

RFC 5011                  Trust Anchor Update             September 2007   subordinate trust point validly chains to a superior trust point.   The decision to delete the subordinate trust anchor is a local   configuration decision.  Once the subordinate trust point is deleted,   validation of the subordinate zone is dependent on validating the   chain of trust to the superior trust point.6.  Scenarios - Informative   The suggested model for operation is to have one active key and one   stand-by key at each trust point.  The active key will be used to   sign the DNSKEY RRSet.  The stand-by key will not normally sign this   RRSet, but the resolver will accept it as a trust anchor if/when it   sees the signature on the trust point DNSKEY RRSet.   Since the stand-by key is not in active signing use, the associated   private key may (and should) be provided with additional protections   not normally available to a key that must be used frequently (e.g.,   locked in a safe, split among many parties, etc).  Notionally, the   stand-by key should be less subject to compromise than an active key,   but that will be dependent on operational concerns not addressed   here.6.1.  Adding a Trust Anchor   Assume an existing trust anchor key 'A'.   1.  Generate a new key pair.   2.  Create a DNSKEY record from the key pair and set the SEP and Zone       Key bits.   3.  Add the DNSKEY to the RRSet.   4.  Sign the DNSKEY RRSet ONLY with the existing trust anchor key -       'A'.   5.  Wait for various resolvers' timers to go off and for them to       retrieve the new DNSKEY RRSet and signatures.   6.  The new trust anchor will be populated at the resolvers on the       schedule described by the state table and update algorithm -- see       Sections2 and4 above.6.2.  Deleting a Trust Anchor   Assume existing trust anchors 'A' and 'B' and that you want to revoke   and delete 'A'.StJohns                     Standards Track                     [Page 9]

RFC 5011                  Trust Anchor Update             September 2007   1.  Set the revocation bit on key 'A'.   2.  Sign the DNSKEY RRSet with both 'A' and 'B'.  'A' is now revoked.       The operator should include the revoked 'A' in the RRSet for at       least the remove hold-down time, but then may remove it from the       DNSKEY RRSet.6.3.  Key Roll-Over   Assume existing keys A and B. 'A' is actively in use (i.e. has been   signing the DNSKEY RRSet).  'B' was the stand-by key. (i.e. has been   in the DNSKEY RRSet and is a valid trust anchor, but wasn't being   used to sign the RRSet).      1.  Generate a new key pair 'C'.      2.  Add 'C' to the DNSKEY RRSet.      3.  Set the revocation bit on key 'A'.      4.  Sign the RRSet with 'A' and 'B'.   'A' is now revoked, 'B' is now the active key, and 'C' will be the   stand-by key once the hold-down expires.  The operator should include   the revoked 'A' in the RRSet for at least the remove hold-down time,   but may then remove it from the DNSKEY RRSet.6.4.  Active Key Compromised   This is the same as the mechanism for Key Roll-Over (Section 6.3)   above, assuming 'A' is the active key.6.5.  Stand-by Key Compromised   Using the same assumptions and naming conventions as Key Roll-Over   (Section 6.3) above:      1.  Generate a new key pair 'C'.      2.  Add 'C' to the DNSKEY RRSet.      3.  Set the revocation bit on key 'B'.      4.  Sign the RRSet with 'A' and 'B'.   'B' is now revoked, 'A' remains the active key, and 'C' will be the   stand-by key once the hold-down expires.  'B' should continue to be   included in the RRSet for the remove hold-down time.6.6.  Trust Point Deletion   To delete a trust point that is subordinate to another configured   trust point (e.g., example.com to .com) requires some juggling of the   data.  The specific process is:StJohns                     Standards Track                    [Page 10]

RFC 5011                  Trust Anchor Update             September 2007   1.  Generate a new DNSKEY and DS record and provide the DS record to       the parent along with DS records for the old keys.   2.  Once the parent has published the DSs, add the new DNSKEY to the       RRSet and revoke ALL of the old keys at the same time, while       signing the DNSKEY RRSet with all of the old and new keys.   3.  After 30 days, stop publishing the old, revoked keys and remove       any corresponding DS records in the parent.   Revoking the old trust-point keys at the same time as adding new keys   that chain to a superior trust prevents the resolver from adding the   new keys as trust anchors.  Adding DS records for the old keys avoids   a race condition where either the subordinate zone becomes unsecure   (because the trust point was deleted) or becomes bogus (because it   didn't chain to the superior zone).7.  IANA Considerations   The IANA has assigned a bit in the DNSKEY flags field (seeSection 7   of [RFC4034]) for the REVOKE bit (8).8.  Security Considerations   In addition to the following sections, see also Theory of Operation   above (Section 2) and especiallySection 2.2 for related discussions.   Security considerations for trust anchor rollover not specific to   this protocol are discussed in [RFC4986].8.1.  Key Ownership vs. Acceptance Policy   The reader should note that, while the zone owner is responsible for   creating and distributing keys, it's wholly the decision of the   resolver owner as to whether to accept such keys for the   authentication of the zone information.  This implies the decision to   update trust-anchor keys based on trusting a current trust-anchor key   is also the resolver owner's decision.   The resolver owner (and resolver implementers) MAY choose to permit   or prevent key status updates based on this mechanism for specific   trust points.  If they choose to prevent the automated updates, they   will need to establish a mechanism for manual or other out-of-band   updates, which are outside the scope of this document.StJohns                     Standards Track                    [Page 11]

RFC 5011                  Trust Anchor Update             September 20078.2.  Multiple Key Compromise   This scheme permits recovery as long as at least one valid trust-   anchor key remains uncompromised, e.g., if there are three keys, you   can recover if two of them are compromised.  The zone owner should   determine their own level of comfort with respect to the number of   active, valid trust anchors in a zone and should be prepared to   implement recovery procedures once they detect a compromise.  A   manual or other out-of-band update of all resolvers will be required   if all trust-anchor keys at a trust point are compromised.8.3.  Dynamic Updates   Allowing a resolver to update its trust anchor set based on in-band   key information is potentially less secure than a manual process.   However, given the nature of the DNS, the number of resolvers that   would require update if a trust anchor key were compromised, and the   lack of a standard management framework for DNS, this approach is no   worse than the existing situation.9.  Normative References   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC3755]  Weiler, S., "Legacy Resolver Compatibility for Delegation              Signer (DS)",RFC 3755, May 2004.   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.              Rose, "DNS Security Introduction and Requirements",RFC4033, March 2005.   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.              Rose, "Resource Records for the DNS Security Extensions",RFC 4034, March 2005.   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.              Rose, "Protocol Modifications for the DNS Security              Extensions",RFC 4035, March 2005.10.  Informative References   [RFC4986]  Eland, H., Mundy, R., Crocker, S., and S. Krishnaswamy,              "Requirements Related to DNS Security (DNSSEC) Trust              Anchor Rollover",RFC 4986, August 2007.StJohns                     Standards Track                    [Page 12]

RFC 5011                  Trust Anchor Update             September 2007Author's Address   Michael StJohns   Independent   EMail: mstjohns@comcast.netStJohns                     Standards Track                    [Page 13]

RFC 5011                  Trust Anchor Update             September 2007Full Copyright Statement   Copyright (C) The IETF Trust (2007).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.StJohns                     Standards Track                    [Page 14]