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.
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
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.
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
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.
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.
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.
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:
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.
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