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DNS Extensions R. Arends
Internet-Draft Telematica Instituut
Expires: September 1, 2003 M. Larson
VeriSign
R. Austein
ISC
D. Massey
USC/ISI
S. Rose
NIST
March 3, 2003
Protocol Modifications for the DNS Security Extensions
draft-ietf-dnsext-dnssec-protocol-01
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at http://
www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on September 1, 2003.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document is part of a family of documents which describes the
DNS Security Extensions (DNSSEC). The DNS Security Extensions are a
collection of new resource records and protocol modifications which
add data origin authentication and data integrity to the DNS. This
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document describes the DNSSEC protocol modifications. This document
defines the concept of a signed zone, along with the requirements for
serving and resolving using DNSSEC. These techniques allow a
security-aware resolver to authenticate both DNS resource records and
authoritative DNS error indications.
This document obsoletes RFC 2535 and incorporates changes from all
updates to RFC 2535.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Background and Related Documents . . . . . . . . . . . . . . 4
1.2 Reserved Words . . . . . . . . . . . . . . . . . . . . . . . 4
1.3 Editors' Notes . . . . . . . . . . . . . . . . . . . . . . . 4
1.3.1 Open Technical Issues . . . . . . . . . . . . . . . . . . . 4
1.3.2 Technical Changes or Corrections . . . . . . . . . . . . . . 4
1.3.3 Typos and Minor Corrections . . . . . . . . . . . . . . . . 5
2. Zone Signing . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1 Including KEY RRs in a Zone . . . . . . . . . . . . . . . . 6
2.2 Including SIG RRs in a Zone . . . . . . . . . . . . . . . . 7
2.3 Including NXT RRs in a Zone . . . . . . . . . . . . . . . . 8
2.4 Including DS RRs in a Zone . . . . . . . . . . . . . . . . . 8
2.5 Changes to the CNAME Resource Record. . . . . . . . . . . . 8
2.6 Example of a Secure Zone . . . . . . . . . . . . . . . . . . 8
3. Serving . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1 Including SIG RRs in a Response . . . . . . . . . . . . . . 10
3.2 Including KEY RRs In a Response . . . . . . . . . . . . . . 11
3.3 Including NXT RRs In a Response . . . . . . . . . . . . . . 11
3.3.1 Case 1: Query Name Exists, but RR Type Not Present . . . . . 12
3.3.2 Case 2: Query Name Does Not Exist, and No Wildcard Matches . 12
3.3.3 Case 3: Query Name Does Not Exist, but Wildcard Matches . . 13
3.4 Including DS RRs In a Response . . . . . . . . . . . . . . . 13
3.5 Responding to Queries for DS RRs . . . . . . . . . . . . . . 13
3.6 Responding to Queries for Type AXFR or IXFR . . . . . . . . 15
3.7 Setting the AD and CD Bits in a Response . . . . . . . . . . 15
3.8 Example DNSSEC Responses . . . . . . . . . . . . . . . . . . 16
4. Resolving . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.1 Recursive Name Servers . . . . . . . . . . . . . . . . . . . 23
4.2 Stub resolvers . . . . . . . . . . . . . . . . . . . . . . . 24
5. Authenticating DNS Responses . . . . . . . . . . . . . . . . 25
5.1 Authenticating Referrals . . . . . . . . . . . . . . . . . . 26
5.2 Authenticating an RRSet Using a SIG RR . . . . . . . . . . . 27
5.2.1 Checking the SIG RR Validity . . . . . . . . . . . . . . . . 27
5.2.2 Reconstructing the Signed Data . . . . . . . . . . . . . . . 28
5.2.3 Checking the Signature . . . . . . . . . . . . . . . . . . . 30
5.2.4 Authenticating A Wildcard Expanded RRset Positive Response . 31
5.3 Authenticated Denial of Existence . . . . . . . . . . . . . 31
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5.4 Example . . . . . . . . . . . . . . . . . . . . . . . . . . 32
5.4.1 Example of Re-Constructing the Original Owner Name . . . . . 32
5.4.2 Examples of Authenticating a Response . . . . . . . . . . . 33
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . 34
7. Security Considerations . . . . . . . . . . . . . . . . . . 35
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 36
Normative References . . . . . . . . . . . . . . . . . . . . 37
Informative References . . . . . . . . . . . . . . . . . . . 38
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 38
A. Algorithm For Handling Wildcard Expansion . . . . . . . . . 40
Full Copyright Statement . . . . . . . . . . . . . . . . . . 41
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1. Introduction
The DNS Security Extensions (DNSSEC) modify several aspects of the
DNS protocol. Section 2 defines the concept of a signed zone and
lists the requirements for zone signing. Section 3 describes the
modifications to authoritative name server behavior necessary to
handle signed zones. Section 4 describes the behavior of entities
which include security-aware resolver functions Finally, Section 5
defines how to use DNSSEC RRs to authenticate a response.
1.1 Background and Related Documents
The reader is assumed to be familiar with the basic DNS concepts
described in RFC1034 [1] and RFC1035 [2].
This document is part of a family of documents which define the DNS
security extensions (DNSSEC). The DNS Security Extensions are a
collection of new resource records and protocol modifications which
add data origin authentication and data integrity to the DNS. An
introduction to DNSSEC and definition of common terms can be found in
[9]. A definition of the DNSSEC resource records can be found in
[10]. This document defines the DNSSEC protocol modifications.
1.2 Reserved Words
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119. [4].
1.3 Editors' Notes
1.3.1 Open Technical Issues
Use of NXT RRs throughout this document set will have to be modified
if DNSSEC-Opt-In [11] becomes part of DNSSEC. The use of the NXT
record requires input from the working group. This text describes
the NXT record as it was defined in RFC 2535, and substantial
portions of this document would need to be updated to incorporate
opt-in. The updates will be made if the WG adopts opt-in.
Use of the AD bit requires input from the working group. Since the
AD bit usage is not resolved, this text attempts to capture current
ideas and drafts, but further input from the working group is
required.
1.3.2 Technical Changes or Corrections
Please report technical corrections to dnssec-editors@east.isi.edu.
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To assist the editors, please indicate the text in error and point
out the RFC that defines the correct behavior. For a technical
change where no RFC that defines the correct behavior, or if there's
more than one applicable RFC and the definitions conflict, please
post the issue to namedroppers.
An example correction to dnssec-editors might be: Page X says
"DNSSEC RRs SHOULD be automatically returned in responses." This was
true in RFC 2535, but RFC 3225 (Section 3, 3rd paragraph) says the
DNSSEC RR types MUST NOT be included in responses unless the resolver
indicated support for DNSSEC.
1.3.3 Typos and Minor Corrections
Please report any typos corrections to dnssec-editors@east.isi.edu.
To assist the editors, please provide enough context for us to find
the incorrect text quickly.
An example message to dnssec-editors might be: page X says "the
DNSSEC standard has been in development for over 1 years". It
should read "over 10 years".
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2. Zone Signing
DNSSEC defines the concept of a signed zone. A signed zone includes
KEY, SIG, NXT and (optionally) DS records according to the rules
specified in Section 2.1, Section 2.2, Section 2.3 and Section 2.4,
respectively. Any zone which does not include these records
according to the rules in this section must be considered unsigned.
Editors' note: Should this be "MUST be considered unsigned"?
DNSSEC requires a change to the definition of the CNAME resource
record. Section 2.5 changes the CNAME RR to allow SIG and NXT RRs to
appear at the same owner name as a CNAME RR.
Section 2.6 shows a sample signed zone.
2.1 Including KEY RRs in a Zone
Editors' note: Unresolved inconsistency between paragraphs in this
section, regarding non-zone KEY RRs at the zone apex. SHOULD NOT,
or MUST NOT?
To sign a zone, the zone's administrator generates one or more
public/private key pairs and uses the private key(s) to sign
authoritative RRsets in the zone. For each private key used to
create SIG RRs, there SHOULD be a corresponding KEY RR stored at the
zone apex. All KEY RRs at the zone apex MUST be zone keys. (A zone
key KEY RR has the Zone Key bit of the Flags RDATA field set to one.
See Section 2.1.1 of [10].) Zone key KEY RRs MUST appear only at the
zone apex.
A signed zone MUST have at least one zone key KEY RR in its apex KEY
RRset. The KEY RRset at the zone apex MUST be self-signed by at
least one private key whose corresponding public key is a zone key
stored in the apex KEY RRset.
Editors' note: The requirement listed in this paragraph may not be
necessary anymore, since the KEY RRset is self-signed anyway
(because the whole zone is signed). This is probably a pre-DS
relic, but we spotted this a few hours before the I-D deadline and
were too chicken to remove it on such short notice....
Other public keys associated with other DNS operations can be stored
in KEY RRs that are not marked as zone keys. Non-zone key KEY RRs
MUST NOT appear at delegation names. Non-zone key KEY RRs also
SHOULD NOT appear at the zone apex, because large KEY RRsets add
processing time at resolvers. Non-zone key KEY RRs MAY appear at any
other name in the zone.
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2.2 Including SIG RRs in a Zone
For each authoritative RRset in the zone (which excludes NS RRsets at
delegation points and glue RRsets), there MUST be at least one SIG
record that meets all of the following requirements:
o The SIG owner name is equal to the RRset owner name;
o The SIG class is equal to the RRset class;
o The SIG Type Covered field is equal to the RRset type;
o The SIG Original TTL field is equal to the TTL of the RRset;
o The SIG RR's TTL is equal to the TTL of the RRset;
o The SIG Labels field is equal to the number of labels in the RRset
owner name, not counting the null root label or any wildcard
label;
o The SIG Signer's Name field is equal to the name of the zone
containing the RRset; and
o The SIG Algorithm, Signer's Name, and Key Tag fields identify a
zone key KEY record at the zone apex.
The process for constructing the SIG RR for a given RRset is
described in [10]. An RRset MAY have multiple SIG RRs associated
with it.
A SIG RR itself MUST NOT be signed, since signing a SIG RRset would
add no value and would create an unterminated dependency loop in the
signing process.
The NS RRset which appears at the zone apex name MUST be signed, but
the NS RRsets which appear at delegation points (that is, the NS
RRsets in the parent zone which delegate the name to the child zone's
name servers) MUST NOT be signed. Glue address RRsets associated
with delegations MUST NOT be signed.
The difference between the set of owner names which require SIG
records and the set of owner names which require NXT records is
subtle and worth highlighting. SIG records are present at the owner
names of all authoritative RRsets. NXT records are present at the
owner names of all names for which the signed zone is authoritative
and also at the owner names of delegations from the signed zone to
its children. Neither NXT nor SIG records are present (in the parent
zone) at the owner names of glue address RRsets. Note, however, that
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this distinction is for the most part only visible during the zone
signing process, because NXT RRsets are authoritative data, and
therefore are signed, thus any owner name which has an NXT RRset will
have SIG RRs as well in the signed zone.
2.3 Including NXT RRs in a Zone
Each owner name in the zone MUST have an NXT resource record, except
for the owner names of any glue address RRsets. The process for
constructing the NXT RR for a given name is described in [10].
2.4 Including DS RRs in a Zone
The DS resource record establishes authentication chains between DNS
zones. A DS RRset SHOULD be present at a delegation point when the
child zone is signed. The DS RRset MAY contain multiple records,
each referencing a key used by the child zone to sign its apex KEY
RRset. All DS RRsets in a zone MUST be signed and DS RRsets MUST NOT
appear at non-delegation points nor at a zone's apex.
A DS RR SHOULD point to a KEY RR which is present in the child's apex
KEY RRset, and the child's apex KEY RRset SHOULD be signed by the
corresponding private key.
Construction of a DS RR requires knowledge of the corresponding KEY
RR in the child zone, which implies communication between the child
and parent zones. This communication is an operational matter not
covered by this document.
2.5 Changes to the CNAME Resource Record.
If a CNAME RRset is present at a name in a signed zone, appropriate
SIG and NXT RRsets are REQUIRED at that name. Other types MUST NOT
be present at that name.
This is a modification to the original CNAME definition given in [1].
The original definition of the CNAME RR did not allow any other types
to co-exist with a CNAME record, but a signed zone requires NXT and
SIG RRsets for every authoritative name. To resolve this conflict,
the definition of the CNAME resource record is hereby modified to
allow for the co-existence of NXT and SIG RRsets.
2.6 Example of a Secure Zone
{{secure zone here}}
Editors' note: Zone file example deferred pending hackery to add
zone files in a format usable by xml2rfc. Goal here is to show a
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(small) complete signed zone.
The apex KEY set includes two KEY RRs, and the KEY RDATA Flags
indicate that each of these KEY RRs is a zone key. The first zone
KEY is used to sign the apex KEY RRset, and a DS record for this key
is provided to the parent zone. The second zone KEY is used to sign
all the other RRsets in the zone. A non-zone KEY RR is also stored
at "host1.example.com"; this KEY might be used by SIG(0) to
authenticate transactions from this host.
The zone includes a wildcard entry "*.a.example.com". Note that the
"*.a.example.com" name is used in constructing NXT chains, and that
the SIG covering the "*.a.example.com" MX RRset has a label count of
3.
The zone also includes two delegations. The delegation to
"insecure.example.com" includes an NS RRset, glue address records,
and an NXT RR, but note that only the NXT RRset is signed. The
"secure.example.com" delegation provides a DS RR, and note that only
NXT and DS RRsets are signed.
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3. Serving
This section describes the behavior of a security-aware authoritative
name server. A security-aware authoritative name server MUST support
the EDNS0 [6] message size extension, MUST support a message size of
at least 1220 octets, and SHOULD support a message size of 4000
octets [8]. Since functions specific to security-aware recursive
name servers included components of both resolving and serving,
issues specific to security-aware recursive name servers are
described in Section 4.
Upon receiving a relevant query which has the EDNS [6] OPT pseudo-RR
DO [7] bit set to one, a security-aware authoritative name server for
a signed zone MUST include additional SIG, NXT, and DS RRs according
to the following rules:
o SIG RRs which can be used to authenticate a response MUST be
included in the response automatically according to the rules in
Section 3.1;
o NXT RRs which can be used to provide authenticated denial of
existence MUST be included in the response automatically according
to the rules in Section 3.3;
o Either DS RRs or an NXT RR proving that no DS RRs exist MUST be
included in referrals automatically according to the rules in
Section 3.4.
DNSSEC does not change the DNS zone transfer protocol. Zone transfer
requirements are reviewed in Section 3.6.
A security-aware name server which receives a DNS query which does
not include the EDNS OPT pseudo-RR, or which has the DO bit set to
zero, MUST treat the SIG, KEY, and NXT RRs as it would any other
RRset, and MUST NOT perform any of the additional processing
described above. Since the DS RR type has the peculiar property of
only existing in the parent zone at delegation points, DS RRs always
require some special processing, as described in Section 3.5.
3.1 Including SIG RRs in a Response
When a query has the DO bit set to one, the authoritative name server
SHOULD attempt to send SIG RRs which can be used to authenticate the
RRsets in the response. Inclusion of SIG RRs in a response is
subject to the following rules:
o When a signed RRset is placed in the Answer section, its SIG RRs
are also placed in the Answer section. The SIG RRs have a higher
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priority for inclusion than any other RRsets which may need to be
included. If space does not permit the inclusion of these SIG
RRs, the response MUST be considered truncated, and the TC bit
MUST be set.
o When a signed RRset is placed in the Authority section, its SIG
RRs are also placed in the Authority section. The SIG RRs have a
higher priority for inclusion than any other RRsets that may need
to be included. If space does not permit the inclusion of these
SIG RRs, the response MUST be considered truncated, and the TC bit
MUST be set.
o When a signed RRset is placed in the Additional section, its SIG
RRs are also placed in the Additional section. If space does not
permit the inclusion of these SIG RRs, the response MUST NOT be
considered truncated just because these SIG RRs didn't fit.
3.2 Including KEY RRs In a Response
When a query has the DO bit set to one and requests the SOA or NS RRs
at the apex of a signed zone, then a security-aware authoritative
name server for that zone SHOULD return the KEY RRset with the same
name in the Additional section. If Additional section processing
results in more data than will fit in the response message, address
glue RRs have higher priority than KEY RRs. SIG RR(s) associated
with the KEY RRset SHOULD also be included in the Additional section
(see Section 3.1).
Editors' note: Didn't the WG decide that DS RR removes the need
for Additional section processing for KEY RRs? If so, this
subsection should be deleted.
3.3 Including NXT RRs In a Response
Editors' note: This whole section uses the phrase "query name
exists", which is somewhat ambiguous when discussing internal
nodes with no RRs. We are reasonably certain that, as used here,
the phrase only refers to names which are the owner name for least
one RR. Better phrasing needed.
When a query has the DO bit set to one, security-aware authoritative
name servers for a signed zone MUST include NXT RRs in each of the
following cases:
Case 1: The query name exists, but the requested RR type does not
exist.
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Case 2: The query name does not exist, and no wildcard can be
expanded to answer the query.
Case 3: The query name does not exist, but a wildcard can be expanded
to positively answer the query.
Note that, in each case, a set of NXT RRs is included to provide
authenticated denial of existence.
NXT RRs are also included in a referral response when no DS RR is
present; in this case, the NXT RR proves that no DS RR exists for the
delegation. Referrals are discussed in more detail in Section 3.4.
3.3.1 Case 1: Query Name Exists, but RR Type Not Present
If the query name exists but the requested RR type is not present at
the name, then the NXT RR associated with the query name MUST be
included in the Authority section. Any SIG(s) associated with the
NXT RRset are also included in the Authority section (see Section
3.1) If space does not permit the inclusion of the NXT RR (or its
associate SIG RRs), the response MUST be considered truncated and the
TC bit MUST be set.
Note that, since the query name exists, no wildcard expansion applies
to this query, and a single NXT RR suffices to prove the requested
type does not exist.
3.3.2 Case 2: Query Name Does Not Exist, and No Wildcard Matches
If the query name does not exist, and no wildcard expansion matches
the query, then the Authority section of the response MUST include
the following NXT RRs:
o An NXT RR proving that there was no exact match for the name; and
o An NXT RR proving that there was no wildcard which would have
matched the query.
If space does not permit the inclusion of these NXT RRs, the response
MUST be considered truncated, and the TC bit MUST be set. Any SIG(s)
associated with the NXT RRsets MUST also be included in the Authority
section (see Section 3.1).
Editors' note: Should lack of space to include the SIGs cause the
packet to be considered truncated?
Appendix A provides an algorithm which computes the appropriate NXT
RRs to prove that no wildcard matches a given query name.
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3.3.3 Case 3: Query Name Does Not Exist, but Wildcard Matches
If the query name does not exist, but a wildcard expansion can be
used to return a positive match to the query, then the wildcard-
expanded answer and any SIG RRs associated with the wildcard RR MUST
be returned in the Answer section. The Authority section of the
response MUST include the following NXT RRs:
o An NXT RR which proves that there were no exact matches for the
QNAME and QTYPE; and
o An NXT RR which proves that there are no closer wildcard entries
which could have been expanded to match the query.
If space does not permit inclusion of these NXT RRs, the response
MUST be considered truncated, and the TC bit MUST be set. Any SIG
RRs associated with the NXT RRsets MUST also be included in the
response.
Editors' note: Should lack of space to include the SIGs cause the
packet to be considered truncated?
Appendix A provides an algorithm which computes the appropriate NXT
RRs to prove that no closer wildcard matches the query name.
3.4 Including DS RRs In a Response
When a query has the DO bit set to one, and a DS RR exists at the
query name, an authoritative security-aware name server returning a
referral for the delegation MUST include both the NS RRset and also
the DS RRset and its associated SIG RR(s). The NS RRset MUST be
placed before the DS RRset and its associated SIG RRs.
When a query has the DO bit set to one, and no DS RR exists at the
query name, an authoritative security-aware name server returning a
referral for the delegation MUST include both the NS RRset and also
the NXT RR and associated SIG RR(s) which proves that the DS RRset
does not exist. The NS RRset MUST be placed before the NXT RRset and
its associated SIG RR(s).
This increases the size of referral messages, and may cause some or
all glue RRs to be omitted. If space does not permit the inclusion
of the DS or NXT RRset and associated SIG RRs, the response MUST be
considered truncated, and the TC bit MUST be set.
3.5 Responding to Queries for DS RRs
The DS record is the first resource record type which appears only on
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the parent zone's side of a zone cut. In other words, the DS record
for the delegation of "example.com" is only stored in the "com" zone.
This introduces novel name server behavior, since the name server for
the child zone is authoritative for the name by the normal DNS rules
but the child zone does not contain the DS RR. A name server's
response to a DS query depends on whether the name server is
authoritative for the parent zone, the child zone, or both, as
described below.
If a name server is authoritative for the parent zone, and receives a
query for the DS record at the delegated name, then the name server
MUST return the DS RRset from the parent zone. This rule applies
regardless of whether or not the name server is also authoritative
for the child zone.
If the name server is authoritative for the child zone, is not
authoritative for the parent zone, and receives a query for the DS
record at the delegated name, there is no obvious response, because
the child zone is not authoritative for the DS record at the child
zone's apex, and the authoritative DS RR is only stored at the
parent.
If the name server allows recursion, and the RD bit is set in the
query, the name server MAY perform recursion to find the DS record
for the delegated name from the parent zone, and MAY return the DS
record from its cache. In this case, the AA bit MUST NOT be set in
the response.
If the name server does not perform recursion to find the DS RR, the
name server MUST reply with:
RCODE: NOERROR
AA bit: set
Answer Section: Empty
Authority Section: SOA [+ SIG(SOA) + NXT + SIG(NXT)]
In other words, a name server which is authoritative for the child
zone but not for the parent zone answers as if the DS record does not
exist. Note that security-aware resolvers will query the parent zone
at delegation points, and thus will not be affected by this behavior.
For example, suppose that "example.com" is a delegation point, and a
name server receives a query for the "example.com" DS RRset.
o If the name server is authoritative for "com", the name server
MUST reply with the "example.com" DS RRset from the "com" zone.
o If the name server is authoritative for "example.com", is not
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authoritative for "com", and the RD bit is set to one in the
query, the name server MAY perform recursion to find the
"example.com" DS record. If the name server does not use
recursion to obtain the DS RR, the name server MUST reply as
though the DS RR did not exist:
RCODE: NOERROR
AA bit: set
Answer Section: Empty
Authority Section: SOA [+ SIG(SOA) + NXT + SIG(NXT)]
3.6 Responding to Queries for Type AXFR or IXFR
DNSSEC does not change the DNS zone transfer process. A signed zone
will contain SIG, KEY, NXT, and DS resource records, but these
records have no special meaning with respect to a zone transfer
operation, and these RRs are treated as any other resource record
type.
An authoritative name server is not required to verify that a zone is
properly signed before sending or accepting a zone transfer.
However, an authoritative name server MAY choose to reject the entire
zone transfer if the zone fails meets any of the signing requirements
described in Section Section 2. The primary objective of a zone
transfer to ensure that all authoritative name servers have identical
copies of the zone. An authoritative name server which chooses to
perform its own zone validation MUST NOT selectively reject some RRs
and accept others.
Note that the DS RR appears only in the parental side of a
delegation, and is authoritative data in the parent zone. For
example, the DS RR for "example.com" is stored in the "com" zone (the
parent zone) rather than in the "example.com" zone (the child zone).
As with any other authoritative RRset, the "example.com" DS RR MUST
be included the "com" zone transfer.
Note that authoritative NXT RRs appear in both the parent and child
zones at a delegated name, and that the NXT RRs for the delegated
name in the parent and child zones are never identical to each other.
As with any other authoritative RRset, the parental NXT RR at a
delegated name MUST be included zone transfers of the parent zone,
while the NXT at the zone apex of the child zone MUST be included in
zone transfers of the child zone.
3.7 Setting the AD and CD Bits in a Response
Editors' note: This section seems a little lost here. Perhaps we
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should rearrange the section ordering slightly, or provide a
pointer to this subsection at the beginning of Section 3.
DNSSEC allocates two new bits in the DNS message header: The CD
(Checking Disabled) bit and the AD (Authentic Data) bit.
The CD bit is set in query messages by the resolver, and MUST be
copied into the response. If the CD bit is set to one, it indicates
that the resolver is willing to perform authentication, and thus that
the name server need not perform authentication on the RRsets in the
response.
Regardless of the setting of the CD bit, the name server MAY choose
whether or not to perform authentication according to the local name
server policy. The CD bit MAY be used in constructing the local name
server policy. If local name server policy does perform
authentication, any RRsets rejected by the local authentication
policy MUST NOT be returned in a response (regardless of the CD bit).
The AD bit is set by name servers, and indicates the data in the
response has been authenticated by the name server, according to the
local name server policy. The AD bit MUST NOT be set on a response
unless all of the RRsets in the Answer and Authority sections have
met the name server's local authentication policy. A resolver MUST
NOT trust the AD bit unless it communicates with the name server over
a secure transport mechanism and is explicitly configured to trust
the name server's policy.
3.8 Example DNSSEC Responses
The examples in this section use the following example zone to
demonstrate the formation of replies by an authoritative name server.
The zone has two name servers, a single child, and a wildcard MX RR.
The zone is completely signed and has a full NXT chain.
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example.com. SOA (...)
SIG SOA ...
NS a.example.com.
NS b.example.com.
SIG NS ...
MX 10 a.example.com
SIG MX ...
KEY ...
SIG KEY ...
NXT *.example.com.
* MX 10 a.example.com.
SIG MX ...
NXT a.example.com.
a A 10.10.10.1
SIG A ...
NXT b.example.com.
b A 10.10.10.2
SIG A ...
NXT c.example.com.
c CNAME a.example.com.
SIG CNAME
NXT sub.example.com.
sub NS ns.sub.example.com.
SIG NS
DS ...
SIG DS
NXT *.example.com.
ns.sub A 10.10.10.3
sub-nosig NS ns.sub-nosig.example.com.
NXT example.com.
ns.sub-nosig A 10.10.10.4
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A query to the authoritative name server for this zone for
QNAME="c.example.com", QCLASS=IN, QTYPE=A would produce:
Flags: QR=1, AA=1, RCODE=0 (NOERROR)
EDNS: DO=1, size=4000
QUERY:
c.example.com. IN A
ANSWER:
c.example.com. IN A a.example.com
IN SIG CNAME
a.example.com. IN A 10.10.10.1
IN SIG A
AUTHORITY:
example.com. IN NS a.example.com.
IN NS b.example.com.
IN SIG NS ...
ADDITIONAL:
a.example.com. IN A 10.10.10.1
IN SIG A ...
b.example.com. IN A 10.10.10.2
IN SIG A ...
example.com. IN KEY ...
IN SIG KEY ...
A query for QNAME="www.sub.example.com", QCLASS=IN, QTYPE=A would
results in a referral to a signed zone. The resolver can determine
that "sub.example.com" is signed because of the presence of the DS RR
with the hash of the "sub.example.com" zone key.
Flags: QR=1, AA=1, RCODE=0 (NOERROR)
EDNS: DO=1, size=4000
QUERY:
www.sub.example.com. IN A
ANSWER:
;; empty
AUTHORITY:
sub.example.com. IN NS ns.sub.example.com.
IN DS ...
IN SIG DS ...
ADDITIONAL:
ns.sub.example.com. IN A 10.10.10.3
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A query for QNAME="www.sub-nosig.example.com", QCLASS=IN, QTYPE=A
would result in a referral to an unsigned zone. The resolver knows
not to expect DNSSEC RRs from "sub-nosig.example.com", because the DS
bit in the NXT RR bitmap in the referral is not set. Even if DNSSEC
RRs are present in responses from "sub-nosig.example.com" name
servers, the resolver will not be able to construct a authentication
chain, since there is a break between "sub-nosig.example.com" and its
delegating parent zone.
Flags: QR=1, AA=1, RCODE=0 (NOERROR)
EDNS: DO=1, size=4000
QUERY:
www.sub-nosig.example.com. IN A
ANSWER:
;; empty
AUTHORITY:
sub-nosig.example.com. IN NS ns.sub-nosig.example.com.
IN NXT ;; (DS bit not set)
IN SIG NXT ...
ADDITIONAL:
ns.sub-nosig.example.com. IN A 10.10.10.4
A query for QNAME="f.example.com", QCLASS=IN, QTYPE=A returns a name
error, because the name does not exist and is not covered by wildcard
expansion. Therefore, the name server must present proof that the
name does not exist, and that no wildcard expansion is present which
could have been used to answer the query.
Flags: QR=1, AA=1, RCODE=3 (NXDOMAIN)
EDNS: DO=1, size=4000
QUERY:
f.example.com. IN A
ANSWER:
;; empty
AUTHORITY:
example.com. IN SOA ...
IN SIG SOA ...
c.example.com. IN NXT sub.example.com. ...
IN SIG NXT ...
*.example.com. IN NXT a.example.com. ...
IN SIG NXT ...
ADDITIONAL:
example.com. IN KEY ...
IN SIG KEY ...
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A query for QNAME="f.example.com" QCLASS=IN, QTYPE=MX returns an MX
RR synthesized via wildcard expansion. The name server must prove
that no exact match exists.
Flags: QR=1, AA=1, RCODE=0 (NOERROR)
EDNS: DO=1, size=4000
QUERY:
f.example.com. IN MX
ANSWER:
f.example.com. IN MX 10 a.example.com.
IN SIG MX ...
AUTHORITY:
example.com. IN NS a.example.com.
IN NS b.example.com.
IN SIG NS ...
c.example.com. IN NXT sub.example.com.
IN SIG NXT ...
ADDITIONAL:
a.example.com. IN A 10.10.10.1
IN SIG A ...
b.example.com. IN A 10.10.10.2
IN SIG A ...
example.com. IN KEY ...
IN SIG KEY ...
If these responses came from a recursive name server which had all of
the necessary RRsets in its cache instead of from an authoritative
server, the only differences would be the TTLs and the header flags.
The AA bit would not be set, and the AD bit would be set if (and only
if) all the RRsets in a response passed the security policy checks of
the recursive name server.
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4. Resolving
Editors' note: This section is still very rough, and some of the
text here duplicates text from other portions of this document.
This needs to be fixed (one way or another) during final editing.
Suggestions for better text would be welcome.
This section describes the behavior of entities which include
security-aware resolver functions. In many cases such functions will
be part of a security-aware recursive name server, but a stand-alone
security-aware resolver has many of the same requirements. Functions
specific to security-aware recursive name servers are described in a
separate subsection.
A security-aware resolver MUST include an EDNS [6] OPT pseudo-RR with
the DO [7] bit set to one when sending queries.
A security-aware resolver MUST support a message size of at least
1220 octets, SHOULD support a message size of 4000 octets, and MUST
advertise the supported message size using the "sender's UDP payload
size" field in the EDNS OPT pseudo-RR. A security-aware resolver
MUST handle fragmented UDP packets correctly regardless of whether
any such fragmented packets were received via IPv4 or IPv6. Please
see [8] for discussion of these requirements.
A security-aware resolver MUST support the signature verification
mechanisms described in Section 5, and MUST apply them to every
received response except when:
o The security-aware resolver is part of a security-aware recursive
name server, and the response is the result of recursion on behalf
of a query received with the CD bit set;
o The response is the result of a query generated directly via some
form of application interface which instructed the security-aware
resolver not to perform validation for this query; or
o Validation for this query has been disabled by local policy.
A security-aware resolver's support for signature verification MUST
include support for verification of wildcard owner names.
A security-aware resolver MUST attempt to retrieve missing DS, KEY,
or SIG RRs via explicit queries if the resolver needs these RRs in
order to perform signature verification.
A security-aware resolver MUST attempt to retrieve missing a NXT RR
which the resolver needs to authenticate a NODATA response. In
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general it is not possible for a resolver to retrieve missing NXT
RRs, since the resolver will have no way of knowing the owner name of
the missing NXT RR, but in the specific case of a NODATA response,
the resolver does know the name of the missing NXT RR, and must
therefore attempt to retrieve it.
A security-aware resolver MUST be able to determine whether or not it
should expect a particular RRset to be signed. More precisely, a
security-aware resolver must be able to distinguish between three
cases:
1. An RRset for which the resolver is able to build a chain of
signed KEY and DS RRs from a trusted starting point to the RRset.
In this case, the RRset should be signed, and is subject to
signature validation as described above.
2. An RRset for which the resolver knows that it has no chain of
signed KEY and DS RRs from any trusted starting point to the
RRset. This can occur when the target RRset lies in an unsigned
zone or in a descendent of an unsigned zone. In this case, the
RRset may or may not be signed, but the resolver will not be able
to verify the signature.
3. An RRset for which the resolver is not able to determine whether
or not the RRset should be signed, because the resolver is not
able to obtain the necessary DNSSEC RRs. This can occur due when
the security-aware resolver is not able to contact security-aware
name servers for the relevant zones.
A security-aware resolver MUST be capable of being preconfigured with
at least one trusted public key, and SHOULD be capable of being
preconfigured with multiple trusted public keys. Since a security-
aware resolver will not be able to validate signatures without such a
preconfigured trusted key, the resolver SHOULD have some reasonably
robust mechanism for obtaining such keys when it boots.
Editors' note: Should support for multiple public keys be a MUST
rather than a SHOULD?
A security-aware resolver SHOULD cache each response as a single
atomic entry, indexed by the triple <QNAME, QTYPE, QCLASS>, with the
single atomic entry containing the entire answer, including the named
RRset and any associated DNSSEC RRs. The resolver SHOULD discard the
entire atomic entry when any of the RRs contained in it expire.
Editors' note: This is implementation advice which came out of
discussions at various workshops and investigations into possible
implementation issues with the (as yet unsettled) opt-in proposal.
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All of this advice has been discussed in WG meetings, and as far
as the editors know these recommendations are not controversial,
but it is up to the WG to decide whether this sort of
implementation advice belongs in this document.
4.1 Recursive Name Servers
As explained in [9], a security-aware recursive name server is an
entity which acts in both the security-aware name server and
security-aware resolver roles. This section uses the terms "name
server side" and "resolver side" to refer to the code within a
security-aware recursive name server which implements the security-
aware name server role and the code which implements the security-
aware resolver role, respectively.
The resolver side of a security-aware recursive name server MUST set
the DO bit when sending requests, regardless of the state of the DO
bit in the initiating request received by the name server side. If
the initiating request does not have the DO bit set, the name server
side MUST remove any DNSSEC RRs from the response sent to the
initiating resolver, but the resolver side MUST follow the usual
rules for caching which would apply to any security-aware resolver.
A security-aware recursive name server SHOULD NOT attempt to answer a
query by piecing together cached data it received in response to
previous queries that requested different QNAMEs, QTYPEs, or
QCLASSes. A security-aware recursive name server SHOULD NOT use NXT
RRs from one negative response to synthesize a response for a
different query. A security-aware recursive name server SHOULD NOT
use a previous wildcard expansion to generate a response to a
different query.
Editors' note: Should any of the SHOULD NOTs in this paragraph be
MUST NOTs instead?
The name server side of a security-aware recursive name server MUST
pass the sense of the CD bit to the resolver side along with the rest
of an initiating query, so that the resolver side will know whether
whether or not it is required to verify the response data it returns
to the name server side.
Editors' note: What should a security-aware recursive name server
do if it receives a query with CD=1 and DO=0?
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4.2 Stub resolvers
A security-aware stub resolver MUST include an EDNS [6] OPT pseudo-RR
with the DO [7] bit set to one when sending queries.
A security-aware stub resolver MUST support a message size of at
least 1220 octets, SHOULD support a message size of 4000 octets, and
MUST advertise the supported message size using the "sender's UDP
payload size" field in the EDNS OPT pseudo-RR. A security-aware stub
resolver MUST handle fragmented UDP packets correctly regardless of
whether any such fragmented packets were received via IPv4 or IPv6.
Please see [8] for discussion of these requirements.
A security-aware stub resolver MUST support the DNSSEC RR types, at
least to the extent of not mishandling responses just because they
contain DNSSEC RRs. A security-aware stub resolver MAY include the
DNSSEC RRs returned by a security-aware recursive name server as part
of the data that it the stub resolver hands back to the application
which invoked it, but is not required to do so.
A security-aware stub resolver SHOULD NOT set the CD bit when sending
queries, since, by definition, a security-aware stub resolver does
not validate signatures and thus depends on the security-aware
recursive name server to perform validation on its behalf.
Editors' note: Should this SHOULD NOT be a MUST NOT?
A security-aware stub resolver MUST NOT place any reliance on
signature validation allegedly performed on its behalf except when
the security-aware stub resolver obtained the data in question from a
trusted security-aware recursive name server via a secure channel.
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5. Authenticating DNS Responses
In order to use DNSSEC RRs for authentication, a security-aware
resolver requires preconfigured knowledge of at least one
authenticated KEY RR. The process for obtaining and authenticating
this initial KEY RR is achieved via some external mechanism. For
example, a resolver could use some off-line authenticated exchange to
obtain a zone's KEY RR or obtain a DS RR that identifies and
authenticates a zone's KEY RR. The remainder of this section assumes
that the resolver has somehow obtained an initial set of
authenticated KEY RRs.
An initial KEY RR can be used to authenticate a zone's apex KEY
RRset. To authenticate an apex KEY RRset using an initial key, the
resolver MUST:
1. Verify that the initial KEY RR appears in the apex KEY RRset, and
verify that the KEY RR has the Zone Key Flag (KEY RDATA bit 7)
set to one.
2. Verify that there is some SIG RR which covers the apex KEY RRset,
and that the combination of the SIG RR and the initial KEY RR
authenticates the KEY RRset. The process for using a SIG RR to
authenticate an RRset is described in Section 5.2.
Once the resolver has authenticated the apex KEY RRset using an
initial KEY RR, delegations from that zone can be authenticated using
DS RRs. This allows a resolver to start from an initial key, and use
DS RRsets to proceed recursively down the DNS tree obtaining other
apex KEY RRsets. If the resolver were preconfigured with a root KEY
RR, and if every delegation had a DS RR associated with it, then the
resolver could obtain any apex KEY RRset. The process of using DS
RRs to authenticate referrals is described in Section 5.1.
Once the resolver has authenticated a zone's apex KEY RRset, Section
5.2 shows how the resolver can use KEY RRs in the apex KEY RRset and
SIG RRs from the zone to authenticate any other RRsets in the zone.
Section 5.3 shows how the resolver can use authenticated NXT RRsets
from the zone to prove that an RRset is not present in the zone.
When a resolver indicates support for DNSSEC, a security-aware name
server should attempt to provide the necessary KEY, SIG, NXT, and DS
RRsets in a response (see Section 3). However, a security-aware
resolver may still receive a response which that lacks the
appropriate DNSSEC RRs, whether due to configuration issues such as a
security-oblivious recursive name server which accidently interfere
with DNSSEC RRs or due to a deliberate attack in which an adversary
forges a response, strips DNSSEC RRs from a response, or modifies a
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query so that DNSSEC RRs appear not to be requested. The absence of
DNSSEC data in a response MUST NOT by itself be taken as an
indication that no authentication information exists.
A resolver SHOULD expect authentication information from signed
zones. A resolver SHOULD believe that a zone is signed if the
resolver has been configured with public key information for the
zone, or if the zone's parent is signed and the delegation from the
parent contains a DS RRset.
5.1 Authenticating Referrals
Once the apex KEY RRset for a signed parent zone has been
authenticated, DS RRsets can be used to authenticate the delegation
to a signed child zone. A DS RR identifies a KEY RR in the child
zone's apex KEY RRset, and contains a cryptographic digest of the
child zone's KEY RR. A strong cryptographic digest algorithm ensures
that an adversary can not easily generate a KEY RR that matches the
digest. Thus, authenticating the digest allows a resolver to
authenticate the matching KEY RR. The resolver can then use this
child KEY RR to authenticate the entire child apex KEY RRset.
Given a DS RR for a delegation, the child zone's apex KEY RRset can
be authenticated if all of the following hold:
o The DS RR has been authenticated using some KEY RR in the parent's
apex KEY RRset (see Section 5.2);
o The Algorithm and Key Tag in the DS RR match the Algorithm field
and the key tag of a KEY RR in the child zone's apex KEY RRset
which, when hashed using the digest algorithm specified in the DS
RR's Digest Type field, results in a digest value which matches
the Digest field of the DS RR; and
o The matching KEY RR in the child zone has the Zone Flag bit set to
one, the corresponding private key has signed the child zone's
apex KEY RRset, and the resulting SIG RR authenticates the child
zone's apex KEY RRset.
If the referral from the parent zone did not contain a DS RRset, the
response should have included a signed NXT RRset proving that no DS
RRset exists for the delegated name (see Section 3.4). A security-
aware resolver MUST send the parent a query for the DS RRset if the
referral includes neither a DS RRset nor a NXT RRset proving the
nonexistence of the DS RRset (see Section 4).
If the resolver authenticates an NXT RRset which proves that no DS
RRset is present for this zone, then there is no authentication path
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leading from the parent to the child. If the resolver has an initial
KEY RR which belongs to the child zone or to any delegation below the
child zone, this initial KEY RR MAY be used to re-establish an
authentication path. If no such initial KEY RR exists, the resolver
can not authenticate RRsets at or below the child zone.
Note that, for a signed delegation, there are two NXT RRs associated
with the delegated name. One NXT RR resides in the parent zone, and
can be used to prove whether a DS RRset exists for the delegated
name. The second NXT RR resides in the child zone, and identifies
which RRsets are present at the apex of the child zone. The parent
NXT RR and child NXT RR can always be distinguished, since the SOA
bit will be set in the child NXT RR and clear in the parent NXT RR.
A security-aware resolver MUST use the parent NXT RR when attempting
to prove that a DS RRset does not exist.
5.2 Authenticating an RRSet Using a SIG RR
A resolver can use a SIG RR and its corresponding KEY RR to attempt
to authenticate RRsets. The resolver first checks the SIG RR to
verify that it covers the RRset, has a valid time interval, and
identifies a valid KEY RR. The resolver then constructs the
canonical form of the signed data by appending the SIG RDATA
(excluding the Signature Field) with the canonical form of the
covered RRset. Finally, resolver uses the public key and signature
to authenticate the signed data. Section 5.2.1, Section 5.2.2, and
Section 5.2.3 describe each step in detail.
5.2.1 Checking the SIG RR Validity
A security-aware resolver can use a SIG RR to authenticate an RRset
if all of the following conditions hold:
o The SIG RR and the RRset MUST have the same owner name and the
same class;
o The SIG RR's Signer's Name field MUST be the name of the zone that
contains the RRset;
o The SIG RR's Type Covered field MUST equal the RRset's type;
o The number of labels in the RRset owner name MUST be greater than
or equal to the value in the SIG RR's Labels field;
o The resolver's notion of the current time MUST be less than or
equal to the time listed in the SIG RR's Expiration field;
o The resolver's notion of the current time MUST be greater than or
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equal to the time listed in the SIG RR's Inception field;
o The SIG RR's Signer's Name, Algorithm, and Key Tag fields MUST
match the owner name, algorithm, and key tag for some KEY RR in
the zone's apex KEY RRset;
o The matching KEY RR MUST be present in the zone's apex KEY RRset,
and MUST have the Zone Flag bit (KEY RDATA Flag bit 7) set to one.
It is possible for more than one KEY RR to match the conditions
above. In this case, the resolver can not predetermine which KEY RR
to use to authenticate the signature, MUST try each matching KEY RR
until the resolver has either validated the signature or has run out
of matching keys to try.
Note that this authentication process is only meaningful if the
resolver authenticates the KEY RR before using it to validate
signatures. The matching KEY RR is considered to be authentic if:
o The apex KEY RRset containing the KEY RR is considered authentic;
or
o The RRset covered by the SIG RR is the apex KEY RRset itself, and
the KEY RR either matches an authenticated DS RR from the parent
zone or matches a DS RR or KEY RR which the resolver has been
preconfigured to believe to be authentic.
5.2.2 Reconstructing the Signed Data
Once the SIG RR has met the validity requirements described in
Section 5.2.1, the resolver needs to reconstruct the original signed
data. The original signed data includes SIG RDATA (excluding the
Signature field) and the canonical form of the RRset. Aside from
being ordered, the canonical form of the RRset might also differ from
the received RRset due to DNS name compression, decremented TTLs, or
wildcard expansion. The resolver should use the following to
reconstruct the original signed data:
signed_data = SIG_RDATA | RR(1) | RR(2)... where
"|" denotes concatenation
SIG_RDATA is the wire format of the SIG RDATA fields with
the Signature field excluded and the Signer's Name in
canonical form.
RR(i) = name | class | type | OrigTTL | RDATA length | RDATA
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name is calculated according to the function below
class is the RRset's class
type is the RRset type and all RRs in the class
OrigTTL is the value from the SIG Original TTL field
All names in the RDATA field are in canonical form
The set of all RR(i) is sorted into canonical order.
To calculate the name:
let sig_labels = the value of the SIG Labels field
let fqdn = RRset's fully qualified domain name in
canonical form
let fqdn_labels = RRset's fully qualified domain name in
canonical form
if sig_labels = fqdn_labels,
name = fqdn
if sig_labels < fqdn_labels,
name = "*." | the leftmost sig_label labels of the
fqdn
if sig_labels > fqdn
the SIG RR did not pass the necessary validation
checks and MUST NOT be used to authenticate this
RRset.
Editors' note: The algorithm above needs to be cross-checked very
carefully against the definitions in [10].
Section 5.4.1 gives an example of original name calculation. The
canonical forms for names and RRsets are defined in [10].
NXT RRsets at a delegation boundary require special processing.
There are two distinct NXT RRsets associated with a signed delegated
name. One NXT RRset resides in the parent zone, and specifies which
RRset are present at the parent zone. The second NXT RRset resides
at the child zone, and identifies which RRsets are present at the
apex in the child zone. The parent NXT RRset and child NXT RRset can
always be distinguished since only the child NXT RRs will specify an
SOA RRset exists at the name. When reconstructing the original NXT
RRset for the delegation from the parent zone, the NXT RRs MUST NOT
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be combined with NXT RRs from the child zone, and when reconstructing
the original NXT RRset for the apex of the child zone, the NXT RRs
MUST NOT be combined with NXT RRs from the parent zone.
Note also that each of the two NXT RRsets at a delegation point has a
corresponding SIG RR with an owner name matching the delegated name,
and each of these SIG RRs is authoritative data associated with the
same zone which contains the corresponding NXT RRset. If necessary,
a resolver can tell these SIG RRs apart by checking the Signer's Name
field.
5.2.3 Checking the Signature
Once the resolver has validated the SIG RR as described in Section
5.2.1 and reconstructed the original signed data as described in
Section 5.2.2, the resolver can attempt to use the cryptographic
signature to authenticate the signed data, and thus (finally!)
authenticate the RRset.
The Algorithm field in the SIG RR identifies the cryptographic
algorithm to generate the signature. The signature itself is
contained in the Signature field of the SIG RDATA, and the public key
to used generate the signature is contained in the Public Key field
of the matching KEY RR(s) (found in Section 5.2.1). [10] provides a
list of algorithm types, and provides pointers to the documents that
define each algorithm's use.
Note that it is possible for more than one KEY RR to match the
conditions in Section 5.2.1. In this case, the resolver can only
determine which KEY RR by trying each matching key until the resolver
either succeeds in validating the signature or runs out of keys to
try.
If the Labels field of the SIG RR is not equal to the number of
labels in the RRset's fully qualified owner name, then the RRset is
either invalid or the result of wildcard expansion. The resolver
MUST verify that wildcard expansion was applied properly before
considering the RRset to be authentic. Section 5.2.4 describes how
to determine whether a wildcard was applied properly.
If other SIG RRs also cover this SIG RR, the local resolver security
policy determines whether the resolver also needs to test these SIG
RRs, and determines how to resolve conflicts if these SIG RRs lead to
differing results.
If the resolver accepts the RRset as authentic, the resolver MUST set
the SIG RR's TTL and the TTL of each RR in the authenticated RRset to
the minimum of:
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o The RRset's TTL as received in the response;
o The SIG RR's TTL as received in the response; and
o The value in the SIG RR's Original TTL field.
5.2.4 Authenticating A Wildcard Expanded RRset Positive Response
If the number of labels in an RRset's fully qualified domain name is
greater than the Labels field in the covering SIG RDATA, then the
RRset and its covering SIG RR were created as a result of wildcard
expansion. Once the resolver has verified the signature as described
in Section 5.2, the resolver must take additional steps to verify the
non-existence of an exact match or closer wildcard match for the
query. Section 5.3 discusses these steps.
Note that the response received by the resolver should include all
NXT RRs needed to authenticate the response (see Section 3.3).
5.3 Authenticated Denial of Existence
A resolver can use authenticated NXT RRs to prove that an RRset is
not present in a signed zone. Security-aware name servers should
automatically include any necessary NXT RRs for signed zones in their
responses to security-aware resolvers.
Security-aware resolvers MUST first authenticate NXT RRsets according
to the standard RRset authentication rules described in Section 5.2,
then apply the NXT RRsets as follows:
o If the requested RR name matches the owner name of an
authenticated NXT RR, then the NXT RR's type bit map field lists
all RR types present at that owner name, and a resolver can prove
that the requested RR type does not exist by checking for the RR
type in the bit map. Since the existence of the authenticated NXT
RR proves that the owner name exists in the zone, wildcard
expansion could not have been used to match the requested RR owner
name and type.
o If the requested RR name would appear after an authenticated NXT
RR owner name and before the name listed in that NXT RR's Next
Domain Name field according to the canonical DNS name order
defined in [10], then no exact match for the requested RR name
exists in the zone. However, it is possible that a wildcard could
be used to match the requested RR owner name and type, so proving
that the requested RRset does not exist also requires proving that
no possible wildcard exists which could have been used to generate
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a positive response.
To prove non-existence of an RRset, the resolver must be able to
verify both that the queried RRset does not exist and that no
relevant wildcard RRset exists. Proving this may require more than
one NXT RRset from the zone. If the complete set of necessary NXT
RRsets is not present in a response (perhaps due to truncation), then
a security-aware resolver MUST resend the query in order to attempt
to obtain the full collection of NXT RRs necessary to verify non-
existence of the requested RRset. As with all DNS operations,
however, the resolver MUST bound the work it puts into answering any
particular query.
5.4 Example
5.4.1 Example of Re-Constructing the Original Owner Name
Suppose that a security-aware resolver receives a response containing
an answer RRset with an owner name of is "www.a.b.c.example.com".
This fully qualified domain name has 6 labels: "www", "a", "b", "c",
"example", and "com". What name the resolver should use when
reconstructing the original signed data depends on the value of the
SIG RR's Labels field.
If the value of the SIG RR's Labels field is 6, then the SIG RR's
Labels field matches the number of labels in the owner name, and the
resolver should assume that this RRset is not the result of wildcard
expansion. The resolver should therefore use "www.a.b.c.example.com"
as the owner name when reconstructing the original signed data for
the signature check.
If the value of the SIG RR's Labels field is less than 6, then the
SIG RR's Labels count is less than the number of labels in the
RRset's owner name, and the resolver should assume that this RRset is
the result of wildcard expansion. The resolver should therefore
reconstruct the original owner name by replacing the labels which
appear to be the result of wildcard expansion with a single "*."
label. For example, if the SIG RR's Labels field is 3, the resolver
should reconstruct the original owner name by prepending "*." to the
last 3 labels of the owner name of the answer RRset. Thus, the
resolver should use "*.c.example.com" as the owner name when
reconstructing the original signed data.
If the value of the SIG RR's Labels field is greater than 6, then
this SIG RR cannot possibly be valid for the answer RRset, and there
is no point in attempting to validate the signature.
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5.4.2 Examples of Authenticating a Response
Editors' note: Eventually this will be an example of the
authentication process for "www.example.com", starting from an
initial root key.
Editors' note: Eventually this will be an example of the
authentication process for non-existent "www.a.b.c.example.com",
starting from an initial root key.
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6. IANA Considerations
This document introduces no IANA considerations.
[10] contains a complete review of the IANA considerations introduced
by DNSSEC.
Editors' note: This may not be true anymore, since the AD and CD
bit definitions are now in this document rather than in [10].
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7. Security Considerations
This document describes how the DNS security extensions use public
key cryptography to sign and authenticate DNS resource record sets.
At this time, at least two substantial elements of the DNSSEC
specification have yet to be decided by the working group. The open
opt-in issue would change elements such as what RRsets must be
signed, would impact how wildcards are used, and would replace
authenticated denial of existence with authenticated denial of
security. Handling of the AD bit is also undecided. The AD bit (as
currently defined) is used to indicate the security status of RRsets
in the response. These items clearly raise security considerations
and will be addressed here as these issues are resolved in the
working group.
DNSSEC introduces a number of denial of service issues. These issues
will also be addressed in a future version of these security
considerations.
Please see [9] for general security considerations related to DNSSEC.
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8. Acknowledgements
This document was created from the input and ideas of several members
of the DNS Extensions Working Group and working group mailing list.
The co-authors of this draft would like to express their thanks for
the comments and suggestions received during the revision of these
security extension specifications.
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Normative References
[1] Mockapetris, P., "Domain names - concepts and facilities", STD
13, RFC 1034, November 1987.
[2] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[3] Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
August 1996.
[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[5] Elz, R. and R. Bush, "Clarifications to the DNS Specification",
RFC 2181, July 1997.
[6] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC 2671,
August 1999.
[7] Conrad, D., "Indicating Resolver Support of DNSSEC", RFC 3225,
December 2001.
[8] Gudmundsson, O., "DNSSEC and IPv6 A6 aware server/resolver
message size requirements", RFC 3226, December 2001.
[9] Arends, R., Austein, R., Larson, M., Massey, D. and S. Rose,
"DNS Security Introduction and Requirements", draft-ietf-
dnsext-dnssec-intro-05 (work in progress), February 2003.
[10] Arends, R., Austein, R., Larson, M., Massey, D. and S. Rose,
"Resource Records for DNS Security Extensions", draft-ietf-
dnsext-dnssec-records-04 (work in progress), February 2003.
[11] Kosters, M., Blacka, D. and R. Arends, "DNSSEC Opt-in", draft-
ietf-dnsext-dnssec-opt-in-04 (work in progress), February 2003.
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Informative References
[12] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
[13] Eastlake, D., "Secret Key Establishment for DNS (TKEY RR)", RFC
2930, September 2000.
[14] Eastlake, D., "DNS Request and Transaction Signatures (
SIG(0)s)", RFC 2931, September 2000.
[15] Gudmundsson, O., "Delegation Signer Resource Record", draft-
ietf-dnsext-delegation-signer-12 (work in progress), December
2002.
Authors' Addresses
Roy Arends
Telematica Instituut
Drienerlolaan 5
7522 NB Enschede
NL
EMail: roy.arends@telin.nl
Matt Larson
VeriSign, Inc.
21345 Ridgetop Circle
Dulles, VA 20166-6503
USA
EMail: mlarson@verisign.com
Rob Austein
Internet Software Consortium
40 Gavin Circle
Reading, MA 01867
USA
EMail: sra@isc.org
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Dan Massey
USC Information Sciences Institute
3811 N. Fairfax Drive
Arlington, VA 22203
USA
EMail: masseyd@isi.edu
Scott Rose
National Institute for Standards and Technology
100 Bureau Drive
Gaithersburg, MD 20899-8920
USA
EMail: scott.rose@nist.gov
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Appendix A. Algorithm For Handling Wildcard Expansion
For zone (Z) and a name (N) that may occur in Z, the following
algorithm finds all wildcard RRsets that match N or returns an NXT
RRset that proves no wildcard expansion matches N. The algorithm was
written for clarity, not efficiency:
0. INPUT: a name (N) and a zone (Z).
INIT: NXT_SET = NULL
1. Construct S = sequence of all names in Z, sorted
into canonical order.
2. If N exists in S
There is an exact match for N.
Return all RRsets associated with N
Else
Add the name that would immediately
precede N in S to NXT_SET.
EndIf
3. Replace the leftmost label of N with *
4. If N exists in S
There is a positive wildcard match for N.
Return all RRsets associated with N
Else
Add the NXT for name that would immediately
precede N in S to NXT_SET.
EndIf
5. Remove the leading * from N.
6. If N exists in S
There is a name that terminates the wildcard search.
Add the NXT for N to NXT_SET and return NXT_SET.
Else
Goto Step 3
EndIf
Note: the algorithm is guaranteed to terminate since
eventually there will be a match or N will be
reduced to zone name itself and the zone name
must exist in S.
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Full Copyright Statement
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