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+Network Working Group O. Gudmundsson
+Request for Comments: 3658 December 2003
+Updates: 3090, 3008, 2535, 1035
+Category: Standards Track
+
+
+ Delegation Signer (DS) Resource Record (RR)
+
+Status 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.
+
+Copyright Notice
+
+ Copyright (C) The Internet Society (2003). All Rights Reserved.
+
+Abstract
+
+ The delegation signer (DS) resource record (RR) is inserted at a zone
+ cut (i.e., a delegation point) to indicate that the delegated zone is
+ digitally signed and that the delegated zone recognizes the indicated
+ key as a valid zone key for the delegated zone. The DS RR is a
+ modification to the DNS Security Extensions definition, motivated by
+ operational considerations. The intent is to use this resource
+ record as an explicit statement about the delegation, rather than
+ relying on inference.
+
+ This document defines the DS RR, gives examples of how it is used and
+ describes the implications on resolvers. This change is not
+ backwards compatible with RFC 2535. This document updates RFC 1035,
+ RFC 2535, RFC 3008 and RFC 3090.
+
+
+
+
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+Gudmundsson Standards Track [Page 1]
+\f
+RFC 3658 Delegation Signer (DS) Resource Record (RR) December 2003
+
+
+Table of Contents
+
+ 1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . 3
+ 1.2. Reserved Words. . . . . . . . . . . . . . . . . . . . . 4
+ 2. Specification of the Delegation key Signer. . . . . . . . . . 4
+ 2.1. Delegation Signer Record Model. . . . . . . . . . . . . 4
+ 2.2. Protocol Change . . . . . . . . . . . . . . . . . . . . 5
+ 2.2.1. RFC 2535 2.3.4 and 3.4: Special Considerations
+ at Delegation Points . . . . . . . . . . . . . 6
+ 2.2.1.1. Special processing for DS queries. . . 6
+ 2.2.1.2. Special processing when child and an
+ ancestor share nameserver. . . . . . . 7
+ 2.2.1.3. Modification on use of KEY RR in the
+ construction of Responses. . . . . . . 8
+ 2.2.2. Signer's Name (replaces RFC3008 section 2.7). . 9
+ 2.2.3. Changes to RFC 3090 . . . . . . . . . . . . . . 9
+ 2.2.3.1. RFC 3090: Updates to section 1:
+ Introduction . . . . . . . . . . . . . 9
+ 2.2.3.2. RFC 3090 section 2.1: Globally
+ Secured. . . . . . . . . . . . . . . . 10
+ 2.2.3.3. RFC 3090 section 3: Experimental
+ Status . . . . . . . . . . . . . . . . 10
+ 2.2.4. NULL KEY elimination. . . . . . . . . . . . . . 10
+ 2.3. Comments on Protocol Changes. . . . . . . . . . . . . . 10
+ 2.4. Wire Format of the DS record. . . . . . . . . . . . . . 11
+ 2.4.1. Justifications for Fields . . . . . . . . . . . 12
+ 2.5. Presentation Format of the DS Record. . . . . . . . . . 12
+ 2.6. Transition Issues for Installed Base. . . . . . . . . . 12
+ 2.6.1. Backwards compatibility with RFC 2535 and
+ RFC 1035. . . . . . . . . . . . . . . . . . . . 12
+ 2.7. KEY and corresponding DS record example . . . . . . . . 13
+ 3. Resolver. . . . . . . . . . . . . . . . . . . . . . . . . . . 14
+ 3.1. DS Example" . . . . . . . . . . . . . . . . . . . . . . 14
+ 3.2. Resolver Cost Estimates for DS Records" . . . . . . . . 15
+ 4. Security Considerations . . . . . . . . . . . . . . . . . . . 15
+ 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
+ 6. Intellectual Property Statement . . . . . . . . . . . . . . . 16
+ 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17
+ 8. References. . . . . . . . . . . . . . . . . . . . . . . . . . 17
+ 8.1. Normative References. . . . . . . . . . . . . . . . . . 17
+ 8.2. Informational References. . . . . . . . . . . . . . . . 17
+ 9. Author's Address. . . . . . . . . . . . . . . . . . . . . . . 18
+ 10. Full Copyright Statement. . . . . . . . . . . . . . . . . . . 19
+
+
+
+
+
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+Gudmundsson Standards Track [Page 2]
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+RFC 3658 Delegation Signer (DS) Resource Record (RR) December 2003
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+
+1. Introduction
+
+ Familiarity with the DNS system [RFC1035], DNS security extensions
+ [RFC2535], and DNSSEC terminology [RFC3090] is important.
+
+ Experience shows that when the same data can reside in two
+ administratively different DNS zones, the data frequently gets out of
+ sync. The presence of an NS RRset in a zone anywhere other than at
+ the apex indicates a zone cut or delegation. The RDATA of the NS
+ RRset specifies the authoritative nameservers for the delegated or
+ "child" zone. Based on actual measurements, 10-30% of all
+ delegations on the Internet have differing NS RRsets at parent and
+ child. There are a number of reasons for this, including a lack of
+ communication between parent and child and bogus name servers being
+ listed to meet registry requirements.
+
+ DNSSEC [RFC2535, RFC3008, RFC3090] specifies that a child zone needs
+ to have its KEY RRset signed by its parent to create a verifiable
+ chain of KEYs. There has been some debate on where the signed KEY
+ RRset should reside, whether at the child [RFC2535] or at the parent.
+ If the KEY RRset resides at the child, maintaining the signed KEY
+ RRset in the child requires frequent two-way communication between
+ the two parties. First, the child transmits the KEY RRset to the
+ parent and then the parent sends the signature(s) to the child.
+ Storing the KEY RRset at the parent was thought to simplify the
+ communication.
+
+ DNSSEC [RFC2535] requires that the parent store a NULL KEY record for
+ an unsecure child zone to indicate that the child is unsecure. A
+ NULL KEY record is a waste: an entire signed RRset is used to
+ communicate effectively one bit of information - that the child is
+ unsecure. Chasing down NULL KEY RRsets complicates the resolution
+ process in many cases, because nameservers for both parent and child
+ need to be queried for the KEY RRset if the child nameserver does not
+ return it. Storing the KEY RRset only in the parent zone simplifies
+ this and would allow the elimination of the NULL KEY RRsets entirely.
+ For large delegation zones, the cost of NULL keys is a significant
+ barrier to deployment.
+
+ Prior to the restrictions imposed by RFC 3445 [RFC3445], another
+ implication of the DNSSEC key model is that the KEY record could be
+ used to store public keys for other protocols in addition to DNSSEC
+ keys. There are a number of potential problems with this, including:
+
+ 1. The KEY RRset can become quite large if many applications and
+ protocols store their keys at the zone apex. Possible protocols
+ are IPSEC, HTTP, SMTP, SSH and others that use public key
+ cryptography.
+
+
+
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+RFC 3658 Delegation Signer (DS) Resource Record (RR) December 2003
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+ 2. The KEY RRset may require frequent updates.
+
+ 3. The probability of compromised or lost keys, which trigger
+ emergency key roll-over procedures, increases.
+
+ 4. The parent may refuse to sign KEY RRsets with non-DNSSEC zone
+ keys.
+
+ 5. The parent may not meet the child's expectations of turnaround
+ time for resigning the KEY RRset.
+
+ Given these reasons, SIG@parent isn't any better than SIG/KEY@Child.
+
+1.2. Reserved Words
+
+ The key words "MAY", "MAY NOT", "MUST", "MUST NOT", "REQUIRED",
+ "RECOMMENDED", "SHOULD", and "SHOULD NOT" in this document are to be
+ interpreted as described in BCP 14, RFC 2119 [RFC2119].
+
+2. Specification of the Delegation key Signer
+
+ This section defines the Delegation Signer (DS) RR type (type code
+ 43) and the changes to DNS to accommodate it.
+
+2.1. Delegation Signer Record Model
+
+ This document presents a replacement for the DNSSEC KEY record chain
+ of trust [RFC2535] that uses a new RR that resides only at the
+ parent. This record identifies the key(s) that the child uses to
+ self-sign its own KEY RRset.
+
+ Even though DS identifies two roles for KEYs, Key Signing Key (KSK)
+ and Zone Signing Key (ZSK), there is no requirement that zone uses
+ two different keys for these roles. It is expected that many small
+ zones will only use one key, while larger zones will be more likely
+ to use multiple keys.
+
+ The chain of trust is now established by verifying the parent KEY
+ RRset, the DS RRset from the parent and the KEY RRset at the child.
+ This is cryptographically equivalent to using just KEY records.
+
+ Communication between the parent and child is greatly reduced, since
+ the child only needs to notify the parent about changes in keys that
+ sign its apex KEY RRset. The parent is ignorant of all other keys in
+ the child's apex KEY RRset. Furthermore, the child maintains full
+ control over the apex KEY RRset and its content. The child can
+ maintain any policies regarding its KEY usage for DNSSEC with minimal
+ impact on the parent. Thus, if the child wants to have frequent key
+
+
+
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+RFC 3658 Delegation Signer (DS) Resource Record (RR) December 2003
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+
+ roll-over for its DNS zone keys, the parent does not need to be aware
+ of it. The child can use one key to sign only its apex KEY RRset and
+ a different key to sign the other RRsets in the zone.
+
+ This model fits well with a slow roll out of DNSSEC and the islands
+ of security model. In this model, someone who trusts "good.example."
+ can preconfigure a key from "good.example." as a trusted key, and
+ from then on trusts any data signed by that key or that has a chain
+ of trust to that key. If "example." starts advertising DS records,
+ "good.example." does not have to change operations by suspending
+ self-signing. DS records can be used in configuration files to
+ identify trusted keys instead of KEY records. Another significant
+ advantage is that the amount of information stored in large
+ delegation zones is reduced: rather than the NULL KEY record at every
+ unsecure delegation demanded by RFC 2535, only secure delegations
+ require additional information in the form of a signed DS RRset.
+
+ The main disadvantage of this approach is that verifying a zone's KEY
+ RRset requires two signature verification operations instead of the
+ one in RFC 2535 chain of trust. There is no impact on the number of
+ signatures verified for other types of RRsets.
+
+2.2. Protocol Change
+
+ All DNS servers and resolvers that support DS MUST support the OK bit
+ [RFC3225] and a larger message size [RFC3226]. In order for a
+ delegation to be considered secure the delegation MUST contain a DS
+ RRset. If a query contains the OK bit, a nameserver returning a
+ referral for the delegation MUST include the following RRsets in the
+ authority section in this order:
+
+ If DS RRset is present:
+ parent's copy of child's NS RRset
+ DS and SIG(DS)
+
+ If no DS RRset is present:
+ parent's copy of child's NS RRset
+ parent's zone NXT and SIG(NXT)
+
+ This increases the size of referral messages, possibly causing some
+ or all glue to be omitted. If the DS or NXT RRsets with signatures
+ do not fit in the DNS message, the TC bit MUST be set. Additional
+ section processing is not changed.
+
+ A DS RRset accompanying a NS RRset indicates that the child zone is
+ secure. If a NS RRset exists without a DS RRset, the child zone is
+ unsecure (from the parents point of view). DS RRsets MUST NOT appear
+ at non-delegation points or at a zone's apex.
+
+
+
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+RFC 3658 Delegation Signer (DS) Resource Record (RR) December 2003
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+ Section 2.2.1 defines special considerations related to authoritative
+ nameservers responding to DS queries and replaces RFC 2535 sections
+ 2.3.4 and 3.4. Section 2.2.2 replaces RFC 3008 section 2.7, and
+ section 2.2.3 updates RFC 3090.
+
+2.2.1. RFC 2535 2.3.4 and 3.4: Special Considerations at Delegation
+ Points
+
+ DNS security views each zone as a unit of data completely under the
+ control of the zone owner with each entry (RRset) signed by a special
+ private key held by the zone manager. But the DNS protocol views the
+ leaf nodes in a zone that are also the apex nodes of a child zone
+ (i.e., delegation points) as "really" belonging to the child zone.
+ The corresponding domain names appear in two master files and might
+ have RRsets signed by both the parent and child zones' keys. A
+ retrieval could get a mixture of these RRsets and SIGs, especially
+ since one nameserver could be serving both the zone above and below a
+ delegation point [RFC2181].
+
+ Each DS RRset stored in the parent zone MUST be signed by at least
+ one of the parent zone's private keys. The parent zone MUST NOT
+ contain a KEY RRset at any delegation point. Delegations in the
+ parent MAY contain only the following RR types: NS, DS, NXT and SIG.
+ The NS RRset MUST NOT be signed. The NXT RRset is the exceptional
+ case: it will always appear differently and authoritatively in both
+ the parent and child zones, if both are secure.
+
+ A secure zone MUST contain a self-signed KEY RRset at its apex. Upon
+ verifying the DS RRset from the parent, a resolver MAY trust any KEY
+ identified in the DS RRset as a valid signer of the child's apex KEY
+ RRset. Resolvers configured to trust one of the keys signing the KEY
+ RRset MAY now treat any data signed by the zone keys in the KEY RRset
+ as secure. In all other cases, resolvers MUST consider the zone
+ unsecure.
+
+ An authoritative nameserver queried for type DS MUST return the DS
+ RRset in the answer section.
+
+2.2.1.1. Special processing for DS queries
+
+ When a nameserver is authoritative for the parent zone at a
+ delegation point and receives a query for the DS record at that name,
+ it MUST answer based on data in the parent zone, return DS or
+ negative answer. This is true whether or not it is also
+ authoritative for the child zone.
+
+
+
+
+
+
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+RFC 3658 Delegation Signer (DS) Resource Record (RR) December 2003
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+
+ When the nameserver is authoritative for the child zone at a
+ delegation point but not the parent zone, there is no natural
+ response, since the child zone is not authoritative for the DS record
+ at the zone's apex. As these queries are only expected to originate
+ from recursive nameservers which are not DS-aware, the authoritative
+ nameserver MUST answer with:
+
+ RCODE: NOERROR
+ AA bit: set
+ Answer Section: Empty
+ Authority Section: SOA [+ SIG(SOA) + NXT + SIG(NXT)]
+
+ That is, it answers as if it is authoritative and the DS record does
+ not exist. DS-aware recursive nameservers will query the parent zone
+ at delegation points, so will not be affected by this.
+
+ A nameserver authoritative for only the child zone, that is also a
+ caching server MAY (if the RD bit is set in the query) perform
+ recursion to find the DS record at the delegation point, or MAY
+ return the DS record from its cache. In this case, the AA bit MUST
+ NOT be set in the response.
+
+2.2.1.2. Special processing when child and an ancestor share
+ nameserver
+
+ Special rules are needed to permit DS RR aware nameservers to
+ gracefully interact with older caches which otherwise might falsely
+ label a nameserver as lame because of the placement of the DS RR set.
+
+ Such a situation might arise when a nameserver is authoritative for
+ both a zone and it's grandparent, but not the parent. This sounds
+ like an obscure example, but it is very real. The root zone is
+ currently served on 13 machines, and "root-servers.net." is served on
+ 4 of the 13, but "net." is severed on different nameservers.
+
+ When a nameserver receives a query for (<QNAME>, DS, <QCLASS>), the
+ response MUST be determined from reading these rules in order:
+
+ 1) If the nameserver is authoritative for the zone that holds the DS
+ RR set (i.e., the zone that delegates <QNAME>, a.k.a. the "parent"
+ zone), the response contains the DS RR set as an authoritative
+ answer.
+
+ 2) If the nameserver is offering recursive service and the RD bit is
+ set in the query, the nameserver performs the query itself
+ (according to the rules for resolvers described below) and returns
+ its findings.
+
+
+
+
+Gudmundsson Standards Track [Page 7]
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+RFC 3658 Delegation Signer (DS) Resource Record (RR) December 2003
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+ 3) If the nameserver is authoritative for the zone that holds the
+ <QNAME>'s SOA RR set, the response is an authoritative negative
+ answer as described in 2.2.1.1.
+
+ 4) If the nameserver is authoritative for a zone or zones above the
+ QNAME, a referral to the most enclosing (deepest match) zone's
+ servers is made.
+
+ 5) If the nameserver is not authoritative for any part of the QNAME,
+ a response indicating a lame nameserver for QNAME is given.
+
+ Using these rules will require some special processing on the part of
+ a DS RR aware resolver. To illustrate this, an example is used.
+
+ Assuming a nameserver is authoritative for roots.example.net. and for
+ the root zone but not the intervening two zones (or the intervening
+ two label deep zone). Assume that QNAME=roots.example.net.,
+ QTYPE=DS, and QCLASS=IN.
+
+ The resolver will issue this request (assuming no cached data)
+ expecting a referral to a nameserver for .net. Instead, rule number
+ 3 above applies and a negative answer is returned by the nameserver.
+ The reaction by the resolver is not to accept this answer as final,
+ as it can determine from the SOA RR in the negative answer the
+ context within which the nameserver has answered.
+
+ A solution would be to instruct the resolver to hunt for the
+ authoritative zone of the data in a brute force manner.
+
+ This can be accomplished by taking the owner name of the returned SOA
+ RR and striping off enough left-hand labels until a successful NS
+ response is obtained. A successful response here means that the
+ answer has NS records in it. (Entertaining the possibility that a
+ cut point can be two labels down in a zone.)
+
+ Returning to the example, the response will include a negative answer
+ with either the SOA RR for "roots.example.net." or "example.net."
+ depending on whether roots.example.net is a delegated domain. In
+ either case, removing the left most label of the SOA owner name will
+ lead to the location of the desired data.
+
+2.2.1.3. Modification on use of KEY RR in the construction of Responses
+
+ This section updates RFC 2535 section 3.5 by replacing it with the
+ following:
+
+
+
+
+
+
+Gudmundsson Standards Track [Page 8]
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+RFC 3658 Delegation Signer (DS) Resource Record (RR) December 2003
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+
+ A query for KEY RR MUST NOT trigger any additional section
+ processing. Security aware resolvers will include corresponding SIG
+ records in the answer section.
+
+ KEY records SHOULD NOT be added to the additional records section in
+ response to any query.
+
+ RFC 2535 specified that KEY records be added to the additional
+ section when SOA or NS records were included in an answer. This was
+ done to reduce round trips (in the case of SOA) and to force out NULL
+ KEYs (in the NS case). As this document obsoletes NULL keys, there
+ is no need for the inclusion of KEYs with NSs. Furthermore, as SOAs
+ are included in the authority section of negative answers, including
+ the KEYs each time will cause redundant transfers of KEYs.
+
+ RFC 2535 section 3.5 also included a rule for adding the KEY RRset to
+ the response for a query for A and AAAA types. As Restrict KEY
+ [RFC3445] eliminated use of KEY RR by all applications, this rule is
+ no longer needed.
+
+2.2.2. Signer's Name (replaces RFC 3008 section 2.7)
+
+ The signer's name field of a SIG RR MUST contain the name of the zone
+ to which the data and signature belong. The combination of signer's
+ name, key tag, and algorithm MUST identify a zone key if the SIG is
+ to be considered material. This document defines a standard policy
+ for DNSSEC validation; local policy MAY override the standard policy.
+
+ There are no restrictions on the signer field of a SIG(0) record. The
+ combination of signer's name, key tag, and algorithm MUST identify a
+ key if this SIG(0) is to be processed.
+
+2.2.3. Changes to RFC 3090
+
+ A number of sections in RFC 3090 need to be updated to reflect the DS
+ record.
+
+2.2.3.1. RFC 3090: Updates to section 1: Introduction
+
+ Most of the text is still relevant but the words "NULL key" are to be
+ replaced with "missing DS RRset". In section 1.3, the last three
+ paragraphs discuss the confusion in sections of RFC 2535 that are
+ replaced in section 2.2.1 above. Therefore, these paragraphs are now
+ obsolete.
+
+
+
+
+
+
+
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+RFC 3658 Delegation Signer (DS) Resource Record (RR) December 2003
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+
+2.2.3.2. RFC 3090 section 2.1: Globally Secured
+
+ Rule 2.1.b is replaced by the following rule:
+
+ 2.1.b. The KEY RRset at a zone's apex MUST be self-signed by a
+ private key whose public counterpart MUST appear in a zone signing
+ KEY RR (2.a) owned by the zone's apex and specifying a mandatory-to-
+ implement algorithm. This KEY RR MUST be identified by a DS RR in a
+ signed DS RRset in the parent zone.
+
+ If a zone cannot get its parent to advertise a DS record for it, the
+ child zone cannot be considered globally secured. The only exception
+ to this is the root zone, for which there is no parent zone.
+
+2.2.3.3. RFC 3090 section 3: Experimental Status.
+
+ The only difference between experimental status and globally secured
+ is the missing DS RRset in the parent zone. All locally secured
+ zones are experimental.
+
+2.2.4. NULL KEY elimination
+
+ RFC 3445 section 3 eliminates the top two bits in the flags field of
+ KEY RR. These two bits were used to indicate NULL KEY or NO KEY. RFC
+ 3090 defines that zone as either secure or not and these rules
+ eliminate the need to put NULL keys in the zone apex to indicate that
+ the zone is not secured for a algorithm. Along with this document,
+ these other two eliminate all uses for the NULL KEY. This document
+ obsoletes NULL KEY.
+
+2.3. Comments on Protocol Changes
+
+ Over the years, there have been various discussions surrounding the
+ DNS delegation model, declaring it to be broken because there is no
+ good way to assert if a delegation exists. In the RFC 2535 version
+ of DNSSEC, the presence of the NS bit in the NXT bit map proves there
+ is a delegation at this name. Something more explicit is required
+ and the DS record addresses this need for secure delegations.
+
+ The DS record is a major change to DNS: it is the first resource
+ record that can appear only on the upper side of a delegation.
+ Adding it will cause interoperability problems and requires a flag
+ day for DNSSEC. Many old nameservers and resolvers MUST be upgraded
+ to take advantage of DS. Some old nameservers will be able to be
+ authoritative for zones with DS records but will not add the NXT or
+ DS records to the authority section. The same is true for caching
+ nameservers; in fact, some might even refuse to pass on the DS or NXT
+ records.
+
+
+
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+RFC 3658 Delegation Signer (DS) Resource Record (RR) December 2003
+
+
+2.4. Wire Format of the DS record
+
+ The DS (type=43) record contains these fields: key tag, algorithm,
+ digest type, and the digest of a public key KEY record that is
+ allowed and/or used to sign the child's apex KEY RRset. Other keys
+ MAY sign the child's apex KEY RRset.
+
+ 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | key tag | algorithm | Digest type |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | digest (length depends on type) |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | (SHA-1 digest is 20 bytes) |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
+ | |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
+ | |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+
+ The key tag is calculated as specified in RFC 2535. Algorithm MUST
+ be allowed to sign DNS data. The digest type is an identifier for
+ the digest algorithm used. The digest is calculated over the
+ canonical name of the delegated domain name followed by the whole
+ RDATA of the KEY record (all four fields).
+
+ digest = hash( canonical FQDN on KEY RR | KEY_RR_rdata)
+
+ KEY_RR_rdata = Flags | Protocol | Algorithm | Public Key
+
+ Digest type value 0 is reserved, value 1 is SHA-1, and reserving
+ other types requires IETF standards action. For interoperability
+ reasons, keeping number of digest algorithms low is strongly
+ RECOMMENDED. The only reason to reserve additional digest types is
+ to increase security.
+
+ DS records MUST point to zone KEY records that are allowed to
+ authenticate DNS data. The indicated KEY records protocol field MUST
+ be set to 3; flag field bit 7 MUST be set to 1. The value of other
+ flag bits is not significant for the purposes of this document.
+
+ The size of the DS RDATA for type 1 (SHA-1) is 24 bytes, regardless
+ of key size. New digest types probably will have larger digests.
+
+
+
+
+
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+RFC 3658 Delegation Signer (DS) Resource Record (RR) December 2003
+
+
+2.4.1. Justifications for Fields
+
+ The algorithm and key tag fields are present to allow resolvers to
+ quickly identify the candidate KEY records to examine. SHA-1 is a
+ strong cryptographic checksum: it is computationally infeasible for
+ an attacker to generate a KEY record that has the same SHA-1 digest.
+ Combining the name of the key and the key rdata as input to the
+ digest provides stronger assurance of the binding. Having the key
+ tag in the DS record adds greater assurance than the SHA-1 digest
+ alone, as there are now two different mapping functions.
+
+ This format allows concise representation of the keys that the child
+ will use, thus keeping down the size of the answer for the
+ delegation, reducing the probability of DNS message overflow. The
+ SHA-1 hash is strong enough to uniquely identify the key and is
+ similar to the PGP key footprint. The digest type field is present
+ for possible future expansion.
+
+ The DS record is well suited to listing trusted keys for islands of
+ security in configuration files.
+
+2.5. Presentation Format of the DS Record
+
+ The presentation format of the DS record consists of three numbers
+ (key tag, algorithm, and digest type) followed by the digest itself
+ presented in hex:
+
+ example. DS 12345 3 1 123456789abcdef67890123456789abcdef67890
+
+2.6. Transition Issues for Installed Base
+
+ No backwards compatibility with RFC 2535 is provided.
+
+ RFC 2535-compliant resolvers will assume that all DS-secured
+ delegations are locally secure. This is bad, but the DNSEXT Working
+ Group has determined that rather than dealing with both RFC 2535-
+ secured zones and DS-secured zones, a rapid adoption of DS is
+ preferable. Thus, the only option for early adopters is to upgrade
+ to DS as soon as possible.
+
+2.6.1. Backwards compatibility with RFC 2535 and RFC 1035
+
+ This section documents how a resolver determines the type of
+ delegation.
+
+
+
+
+
+
+
+Gudmundsson Standards Track [Page 12]
+\f
+RFC 3658 Delegation Signer (DS) Resource Record (RR) December 2003
+
+
+ RFC 1035 delegation (in parent) has:
+
+ RFC 1035 NS
+
+ RFC 2535 adds the following two cases:
+
+ Secure RFC 2535: NS + NXT + SIG(NXT)
+ NXT bit map contains: NS SIG NXT
+ Unsecure RFC 2535: NS + KEY + SIG(KEY) + NXT + SIG(NXT)
+ NXT bit map contains: NS SIG KEY NXT
+ KEY must be a NULL key.
+
+ DNSSEC with DS has the following two states:
+
+ Secure DS: NS + DS + SIG(DS)
+ NXT bit map contains: NS SIG NXT DS
+ Unsecure DS: NS + NXT + SIG(NXT)
+ NXT bit map contains: NS SIG NXT
+
+ It is difficult for a resolver to determine if a delegation is secure
+ RFC 2535 or unsecure DS. This could be overcome by adding a flag to
+ the NXT bit map, but only upgraded resolvers would understand this
+ flag, anyway. Having both parent and child signatures for a KEY
+ RRset might allow old resolvers to accept a zone as secure, but the
+ cost of doing this for a long time is much higher than just
+ prohibiting RFC 2535-style signatures at child zone apexes and
+ forcing rapid deployment of DS-enabled nameservers and resolvers.
+
+ RFC 2535 and DS can, in theory, be deployed in parallel, but this
+ would require resolvers to deal with RFC 2535 configurations forever.
+ This document obsoletes the NULL KEY in parent zones, which is a
+ difficult enough change that to cause a flag day.
+
+2.7. KEY and corresponding DS record example
+
+ This is an example of a KEY record and the corresponding DS record.
+
+ dskey.example. KEY 256 3 1 (
+ AQPwHb4UL1U9RHaU8qP+Ts5bVOU1s7fYbj2b3CCbzNdj
+ 4+/ECd18yKiyUQqKqQFWW5T3iVc8SJOKnueJHt/Jb/wt
+ ) ; key id = 28668
+ DS 28668 1 1 49FD46E6C4B45C55D4AC69CBD3CD34AC1AFE51DE
+
+
+
+
+
+
+
+
+
+Gudmundsson Standards Track [Page 13]
+\f
+RFC 3658 Delegation Signer (DS) Resource Record (RR) December 2003
+
+
+3. Resolver
+
+3.1. DS Example
+
+ To create a chain of trust, a resolver goes from trusted KEY to DS to
+ KEY.
+
+ Assume the key for domain "example." is trusted. Zone "example."
+ contains at least the following records:
+ example. SOA <soa stuff>
+ example. NS ns.example.
+ example. KEY <stuff>
+ example. NXT secure.example. NS SOA KEY SIG NXT
+ example. SIG(SOA)
+ example. SIG(NS)
+ example. SIG(NXT)
+ example. SIG(KEY)
+ secure.example. NS ns1.secure.example.
+ secure.example. DS tag=12345 alg=3 digest_type=1 <foofoo>
+ secure.example. NXT unsecure.example. NS SIG NXT DS
+ secure.example. SIG(NXT)
+ secure.example. SIG(DS)
+ unsecure.example NS ns1.unsecure.example.
+ unsecure.example. NXT example. NS SIG NXT
+ unsecure.example. SIG(NXT)
+
+ In zone "secure.example." following records exist:
+ secure.example. SOA <soa stuff>
+ secure.example. NS ns1.secure.example.
+ secure.example. KEY <tag=12345 alg=3>
+ secure.example. KEY <tag=54321 alg=5>
+ secure.example. NXT <nxt stuff>
+ secure.example. SIG(KEY) <key-tag=12345 alg=3>
+ secure.example. SIG(SOA) <key-tag=54321 alg=5>
+ secure.example. SIG(NS) <key-tag=54321 alg=5>
+ secure.example. SIG(NXT) <key-tag=54321 alg=5>
+
+ In this example, the private key for "example." signs the DS record
+ for "secure.example.", making that a secure delegation. The DS
+ record states which key is expected to sign the KEY RRset at
+ "secure.example.". Here "secure.example." signs its KEY RRset with
+ the KEY identified in the DS RRset, thus the KEY RRset is validated
+ and trusted.
+
+ This example has only one DS record for the child, but parents MUST
+ allow multiple DS records to facilitate key roll-over and multiple
+ KEY algorithms.
+
+
+
+
+Gudmundsson Standards Track [Page 14]
+\f
+RFC 3658 Delegation Signer (DS) Resource Record (RR) December 2003
+
+
+ The resolver determines the security status of "unsecure.example." by
+ examining the parent zone's NXT record for this name. The absence of
+ the DS bit indicates an unsecure delegation. Note the NXT record
+ SHOULD only be examined after verifying the corresponding signature.
+
+3.2. Resolver Cost Estimates for DS Records
+
+ From a RFC 2535 recursive resolver point of view, for each delegation
+ followed to chase down an answer, one KEY RRset has to be verified.
+ Additional RRsets might also need to be verified based on local
+ policy (e.g., the contents of the NS RRset). Once the resolver gets
+ to the appropriate delegation, validating the answer might require
+ verifying one or more signatures. A simple A record lookup requires
+ at least N delegations to be verified and one RRset. For a DS-
+ enabled recursive resolver, the cost is 2N+1. For an MX record,
+ where the target of the MX record is in the same zone as the MX
+ record, the costs are N+2 and 2N+2, for RFC 2535 and DS,
+ respectively. In the case of a negative answer, the same ratios hold
+ true.
+
+ The recursive resolver has to do an extra query to get the DS record,
+ which will increase the overall cost of resolving this question, but
+ it will never be worse than chasing down NULL KEY records from the
+ parent in RFC 2535 DNSSEC.
+
+ DS adds processing overhead on resolvers and increases the size of
+ delegation answers, but much less than storing signatures in the
+ parent zone.
+
+4. Security Considerations
+
+ This document proposes a change to the validation chain of KEY
+ records in DNSSEC. The change is not believed to reduce security in
+ the overall system. In RFC 2535 DNSSEC, the child zone has to
+ communicate keys to its parent and prudent parents will require some
+ authentication with that transaction. The modified protocol will
+ require the same authentication, but allows the child to exert more
+ local control over its own KEY RRset.
+
+ There is a remote possibility that an attacker could generate a valid
+ KEY that matches all the DS fields, of a specific DS set, and thus
+ forge data from the child. This possibility is considered
+ impractical, as on average more than
+
+ 2 ^ (160 - <Number of keys in DS set>)
+
+ keys would have to be generated before a match would be found.
+
+
+
+
+Gudmundsson Standards Track [Page 15]
+\f
+RFC 3658 Delegation Signer (DS) Resource Record (RR) December 2003
+
+
+ An attacker that wants to match any DS record will have to generate
+ on average at least 2^80 keys.
+
+ The DS record represents a change to the DNSSEC protocol and there is
+ an installed base of implementations, as well as textbooks on how to
+ set up secure delegations. Implementations that do not understand
+ the DS record will not be able to follow the KEY to DS to KEY chain
+ and will consider all zones secured that way as unsecure.
+
+5. IANA Considerations
+
+ IANA has allocated an RR type code for DS from the standard RR type
+ space (type 43).
+
+ IANA has established a new registry for the DS RR type for digest
+ algorithms. Defined types are:
+
+ 0 is Reserved,
+ 1 is SHA-1.
+
+ Adding new reservations requires IETF standards action.
+
+6. Intellectual Property Statement
+
+ The IETF takes no position regarding the validity or scope of any
+ intellectual property 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; neither does it represent that it
+ has made any effort to identify any such rights. Information on the
+ IETF's procedures with respect to rights in standards-track and
+ standards-related documentation can be found in BCP-11. Copies of
+ claims of rights made available for publication 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 implementors or users of this specification can
+ be obtained from the IETF Secretariat.
+
+ The IETF invites any interested party to bring to its attention any
+ copyrights, patents or patent applications, or other proprietary
+ rights which may cover technology that may be required to practice
+ this standard. Please address the information to the IETF Executive
+ Director.
+
+
+
+
+
+
+
+
+Gudmundsson Standards Track [Page 16]
+\f
+RFC 3658 Delegation Signer (DS) Resource Record (RR) December 2003
+
+
+7. Acknowledgments
+
+ Over the last few years a number of people have contributed ideas
+ that are captured in this document. The core idea of using one key
+ to sign only the KEY RRset comes from discussions with Bill Manning
+ and Perry Metzger on how to put in a single root key in all
+ resolvers. Alexis Yushin, Brian Wellington, Sam Weiler, Paul Vixie,
+ Jakob Schlyter, Scott Rose, Edward Lewis, Lars-Johan Liman, Matt
+ Larson, Mark Kosters, Dan Massey, Olaf Kolman, Phillip Hallam-Baker,
+ Miek Gieben, Havard Eidnes, Donald Eastlake 3rd., Randy Bush, David
+ Blacka, Steve Bellovin, Rob Austein, Derek Atkins, Roy Arends, Mark
+ Andrews, Harald Alvestrand, and others have provided useful comments.
+
+8. References
+
+8.1. Normative References
+
+ [RFC1035] Mockapetris, P., "Domain Names - Implementation and
+ Specification", STD 13, RFC 1035, November 1987.
+
+ [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
+ Requirement Levels", BCP 14, RFC 2119, March 1997.
+
+ [RFC2535] Eastlake, D., "Domain Name System Security Extensions",
+ RFC 2535, March 1999.
+
+ [RFC3008] Wellington, B., "Domain Name System Security (DNSSEC)
+ Signing Authority", RFC 3008, November 2000.
+
+ [RFC3090] Lewis, E., "DNS Security Extension Clarification on Zone
+ Status", RFC 3090, March 2001.
+
+ [RFC3225] Conrad, D., "Indicating Resolver Support of DNSSEC", RFC
+ 3225, December 2001.
+
+ [RFC3445] Massey, D. and S. Rose, "Limiting the scope of the KEY
+ Resource Record (RR)", RFC 3445, December 2002.
+
+8.2. Informational References
+
+ [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
+ Specification", RFC 2181, July 1997.
+
+ [RFC3226] Gudmundsson, O., "DNSSEC and IPv6 A6 aware server/resolver
+ message size requirements", RFC 3226, December 2001.
+
+
+
+
+
+
+Gudmundsson Standards Track [Page 17]
+\f
+RFC 3658 Delegation Signer (DS) Resource Record (RR) December 2003
+
+
+9. Author's Address
+
+ Olafur Gudmundsson
+ 3821 Village Park Drive
+ Chevy Chase, MD, 20815
+
+ EMail: ds-rfc@ogud.com
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Gudmundsson Standards Track [Page 18]
+\f
+RFC 3658 Delegation Signer (DS) Resource Record (RR) December 2003
+
+
+10. Full Copyright Statement
+
+ Copyright (C) The Internet Society (2003). All Rights Reserved.
+
+ This document and translations of it may be copied and furnished to
+ others, and derivative works that comment on or otherwise explain it
+ or assist in its implementation may be prepared, copied, published
+ and distributed, in whole or in part, without restriction of any
+ kind, provided that the above copyright notice and this paragraph are
+ included on all such copies and derivative works. However, this
+ document itself may not be modified in any way, such as by removing
+ the copyright notice or references to the Internet Society or other
+ Internet organizations, except as needed for the purpose of
+ developing Internet standards in which case the procedures for
+ copyrights defined in the Internet Standards process must be
+ followed, or as required to translate it into languages other than
+ English.
+
+ The limited permissions granted above are perpetual and will not be
+ revoked by the Internet Society or its successors or assignees.
+
+ This document and the information contained herein is provided on an
+ "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
+ TASK FORCE DISCLAIMS 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.
+
+Acknowledgement
+
+ Funding for the RFC Editor function is currently provided by the
+ Internet Society.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Gudmundsson Standards Track [Page 19]
+\f
projects / thirdparty / bind9.git / commitdiff
