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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.
Gudmundsson Standards Track [Page 1]
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
Gudmundsson Standards Track [Page 2]
RFC 3658 Delegation Signer (DS) Resource Record (RR) December 2003
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.
Gudmundsson Standards Track [Page 3]
RFC 3658 Delegation Signer (DS) Resource Record (RR) December 2003
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
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
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
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.
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RFC 3658 Delegation Signer (DS) Resource Record (RR) December 2003
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:
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RFC 3658 Delegation Signer (DS) Resource Record (RR) December 2003
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.
Gudmundsson Standards Track [Page 9]
RFC 3658 Delegation Signer (DS) Resource Record (RR) December 2003
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.
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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
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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.
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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.
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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.
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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.
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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
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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.
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