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DNS Extensions R. Arends
Internet-Draft
Expires: April 25, 2003 M. Larson
VeriSign
D. Massey
USC/ISI
S. Rose
NIST
October 25, 2002
Protocol Modifications for the DNS Security Extensions
draft-ietf-dnsext-dnssec-protocol-00
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 April 25, 2003.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document is part of a family of documents that describe the
DNS Security Extensions (DNSSEC). The DNS Security Extensions are
a collection of new resource records and protocol modifications
that provide source authentication for the DNS. This document
describes the DNSSEC protocol modifications. The concept of zone
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signing is introduced and the zone format is modified to include
KEY, SIG, NXT, and DS resource records. If a resolver indicates
support for DNSSEC, the response process is modified to include
the appropriate KEY, SIG, NXT, and DS resource records. These
resource records are used by the resolver to authenticate the
response.
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 . . . . . . . . . . . . . . 5
1.3.3 Typos and Minor Corrections . . . . . . . . . . . . . . . . 5
2. Zone Signing and Zone Format . . . . . . . . . . . . . . . . 6
2.1 Inclusion of KEY RRs in a Zone . . . . . . . . . . . . . . . 7
2.2 Inclusion of NXT RRs in a Zone . . . . . . . . . . . . . . . 7
2.3 Inclusion of SIG RRs in a Zone . . . . . . . . . . . . . . . 7
2.4 Changes to the CNAME Resource Record. . . . . . . . . . . . 8
2.5 Inclusion of DS RRs in a Zone . . . . . . . . . . . . . . . 8
2.6 Example of a Secure Zone . . . . . . . . . . . . . . . . . . 9
3. Constructing DNS Responses . . . . . . . . . . . . . . . . . 10
3.1 Indicating Resolver Support for DNSSEC . . . . . . . . . . . 10
3.2 Inclusion of SIG RRs in a Response . . . . . . . . . . . . . 11
3.3 Inclusion of KEY RRs In a Response . . . . . . . . . . . . . 12
3.4 Inclusion of NXT RRs In a Response . . . . . . . . . . . . . 12
3.4.1 Case 1: Query Name Exists, but RR Type Not Present . . . . . 12
3.4.2 Case 2: Query Name Does Not Exist and No Wildcard Matches . 13
3.4.3 Case 3: Query Name Does Not Exist, but Wildcard Matches . . 13
3.5 Inclusion of DS RRs In a Response . . . . . . . . . . . . . 13
3.6 Responding to Queries for DS RRs . . . . . . . . . . . . . . 13
3.7 Special Considerations for Recursive/Caching Servers . . . . 15
3.8 Setting the AD and CD Bits in a Response . . . . . . . . . . 15
3.9 Example DNSSEC Responses . . . . . . . . . . . . . . . . . . 16
4. Authenticating DNS Responses . . . . . . . . . . . . . . . . 17
4.1 Authenticating Referrals . . . . . . . . . . . . . . . . . . 18
4.2 Authenticating an RRSet Using a SIG RR . . . . . . . . . . . 19
4.2.1 Checking the SIG RR Validity . . . . . . . . . . . . . . . . 19
4.2.2 Reconstructing the Signed Data . . . . . . . . . . . . . . . 20
4.2.3 Checking the Signature . . . . . . . . . . . . . . . . . . . 22
4.2.4 Authenticating Wildcard Expanded RRset . . . . . . . . . . . 23
4.3 Authenticated Denial of Existence . . . . . . . . . . . . . 23
4.4 Example . . . . . . . . . . . . . . . . . . . . . . . . . . 24
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4.4.1 Example of Re-Constructing the Original Name . . . . . . . . 24
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . 26
6. Security Considerations . . . . . . . . . . . . . . . . . . 27
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 28
References . . . . . . . . . . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 29
A. Algorithm For Handling Wildcard Expansion . . . . . . . . . 31
Full Copyright Statement . . . . . . . . . . . . . . . . . . 32
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1. Introduction
The DNS Security Extensions (DNSSEC) modify several aspects of the
DNS protocol. The concept of zone signing is introduced and the
zone format is modified to include KEY, SIG, NXT, and DS resource
records as described in Section 2. If the resolver has indicated
support for DNSSEC, the process of constructing a DNS response is
also modified to include the appropriate KEY, SIG, NXT, and DS RR
types. Section 3 defines how a resolver indicates support for
DNSSEC, describes how the DNSSEC RR types are included in a
response, and also describes the specgal processing rules required
to handle queries for the DS RR type. Finally, Section 4 defines
how a resolver uses the DNSSEC RRs to authenticate a response.
1.1 Background and Related Documents
This document is part of a family of documents that define the DNS
security extensions. The DNS security extensions (DNSSEC) are a
collection of resource records and DNS protocol modifications that
add source authentication the Domain Name System (DNS). An
introduction to DNSSEC and definition of common terms can be found
in [8]. A definition of the DNSSEC resource records can be found
in [9]. This document defines the DNSSEC protocol modificatinos.
The reader to assumed to be familiar with the basic DNS concepts
described in RFC1034 [1] and RFC1035 [2] and should also be
familiar with common DNSSEC terminology as defined in [8].
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
The use of the NXT record requires input from the working group.
Although the opt-in issue is not resolved, progress on this
document was still needed. 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 opt-in is adopted.
The use of the AD bit is described in section 3.8 and requires
input from the working group. Since the AD bit usage is not
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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. 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 there is no RFC that
defines the correct behavior (or RFCs provide conflicting
answers), 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 quickly find the incorrect text.
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 and Zone Format
DNSSEC defines a new process called zone signing and adds the KEY,
SIG, NXT, and DS resource records to the zone format. The zone
signing process is the first step in enabling resource record
authentication for this zone. After a signed zone has been
created and loaded, the KEY, SIG, NXT, and DS resource records can
be included in responses (as decribed in Section 3) and can be
used by resolvers to authenticate responses (as describe in
Section 4). KEY, SIG, NXT, and DS RRs MUST NOT appear in unsigned
zones.
To sign a zone, the zone administrator generates one or more
public/private key pairs. The zone's public key(s) are made
available by storing them in KEY resource records. Other DNS
public keys, such as public keys used by TKEY and SIG(0), can also
be stored in KEY RRs. Once the KEY records have been added to the
zone, the zone is sorted into a canonical form and NXT resource
records are added to enable authenticated denial of existence.
The zone administrator then signs every authoritative RRset in the
zone using the private key(s) and the signatures are stored in SIG
resource records. The resulting signed zone contains all data in
the original (unsigned) zone and also includes the new KEY, NXT,
and SIG RRs.
Section 2.1, Section 2.2, and Section 2.3 present the rules for
the including the KEY, NXT, and SIG resource records in a zone
(respectively).
The zone signing process also requires a change in the definition
of the CNAME resource record and Section 2.4 changes the CNAME RR
to allow SIG and NXT RRs to appear along with the CNAME RR.
To enable authentication chains between DNS zones, a signed zone
includes DS Resource Records for its signed delegations. Section
2.5 presents the rules for including DS resource records.
Note that if a resource record in a signed zone is added,
modified, or deleted, the signatures associaates with this RRset
MUST be updated and the NXT RR associated with the RRset's owner
name MUST also be updated. In addition, the zone MUST be
periodically resigned in order to maintain current SIG expiration
dates and the zone keys SHOULD change periodically. DNSSEC best
practices documents are encouraged to provide recommendations for
signature and key lifetimes.
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2.1 Inclusion of KEY RRs in a Zone
A zone administrator generates a set of public/private key pairs
and uses the private key(s) to sign authoritative RRsets in the
zone. For each private zone key used to create SIG RRs, there
SHOULD be a corresponding public KEY RR stored at the zone apex
and the corresponding KEY RR MUST have the Zone Key Flag (KEY
RDATA Flag bit 7) set to 1. KEY RR's with Zone Flag set MUST only
appear at the zone apex.
A signed zone MUST have at least one zone KEY RR in its apex KEY
set and the apex KEY set MUST be self-signed by at least one
private key whose corresponding public zone KEY RR is stored in
the apex KEY set.
Other DNS public keys, such as those used by TKEY and SIG, can be
stored in the zone using non-Zone KEY RR's (KEY RDATA Flag bit 7
set to 0). Non-zone KEY RR's MUST NOT appear at delegation names,
but MAY appear at any other authoritative name in the zone. A
non-zone KEY RR SHOULD NOT appear at the apex name since this
could lead to large apex KEY sets and requires added processing
time at resolvers.
2.2 Inclusion of NXT RRs in a Zone
Each authoritative name in the zone MUST have an NXT resource
record. The NXT record indicates what are RR types are present at
that name and indicates the next authortitive name in the zone.
The collection of NXT or "next" resource records (RR) provide a
chain of all authoritative names and RRsets in the zone and are
used for authenticated denial of existence. The process for
constructing the NXT RR for a given name is described in [9].
2.3 Inclusion of SIG RRs in a Zone
For each authoritative RRset in the zone, there MUST be at least
one SIG record that meets all of the following requirements:
The SIG owner name is equal to the RRset owner name.
The SIG class is equal to the RRset class.
The SIG Type Covered field is equal to the RRset type.
The SIG Original TTL field is greater than or equal to the TTL
of the RRset.
The SIG Labels field is equal to the number of labels in RRset
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owner name.
The SIG Signer's Name is equal to the name of the zone
containing the RRset.
The SIG Algorithm, Signer's Name, and Key Tag fields identify a
zone KEY record at the zone apex.
The process for constructing the SIG RR for a given RRset is
described in [9]. An RRset MAY have multiple SIG RR associated
with it.
The SIG RR itself MUST NOT be signed since signing a SIG RRset
adds no value and creates a unterminated dependency loop in the
signing process.
The NS RRset that appears at the zone apex name MUST be signed,
but NS RRsets that appear at delegation owner names (child zones)
MUST NOT be signed and any glue address RRsets assoicated with
child delegations MUST NOT be signed.
2.4 Changes to the CNAME Resource Record.
If a CNAME RR is present at a name, RRs other than the SIG and NXT
MUST NOT be present at that name.
The above is modification to the original CNAME definition given
in [1]. The original definition of CNAME did not allow any other
resource records to co-exist with a CNAME record, but the zone
signing process associates NXT and SIG resource records with every
authorititative name. To resolve this conflict, the definition of
the CNAME resource record is modified to allow for the co-
existence of NXT and SIG RRs.
2.5 Inclusion of DS RRs in a Zone
The DS resource record is used to establish authentication chains
between DNS zones. A signed delegation (child zone) SHOULD
provide its parent zone with a DS RR for the delegation. All DS
RRsets stored in a zone MUST be signedx and DS RRsets MUST NOT
appear at non-delegation points or at a zone's apex.
The DS RR provided by the child SHOULD point to a KEY RR that is
present in the child's apex KEY set and the child's apex KEY RRset
SHOULD be signed by the corresponding private key. If the KEY RR
is present in the child's apex KEY set, the KEY RR MUST have the
Zone Key Flag set.
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Note that the process of providing a DS RR can be accomplished by
either directly sending the DS RR to the parent or by sending a
KEY RR to the parent and requesting that the parent construct a DS
RR for the given KEY RR. The parent/child communication needs to
be authenticated in order to prevent an adversary from inserting a
false DS RR. DNSSEC operational and best practices documents are
encouraged to provide guidelines for providing a DS RR.
2.6 Example of a Secure Zone
secure 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 set 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 and this KEY and might be used by
SIG(0) to authenticate transactions from this host.
The zone includes a wildcard entry *.a.example.com. Note the
*.a.example.com name is used in constructing NXT chains and the
SIG covering the *.a.example.com MX RRset has a label count of 3.
The zone also includes two delegations. The delegation to
unsecure.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 has provided a DS RR and note that
only NXT and DS RRsets are signed.
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3. Constructing DNS Responses
Unless a resolver has indicated support for DNSSEC, no changes are
made to the standard (non-secure) DNS response and a server simply
behaves as if no DNSSEC RR types were present. This helps avoid
backwards compatability issues and also avoids increasing the size
of (non-secure) DNS responses. Servers MUST NOT include any
DNSSEC RR types (KEY NXT SIG DS) unless the resolver has indicated
support for DNSSEC using one of the mechanisms described in
Section 3.1.
If a resolver has indicated support for DNSSEC:
SIG RRs that can be used to authenticate a response are
automatically included in the response according to the rules
in Section 3.2.
NXT RRs that can be used to provide authenticated denial of
existence are automatically included in the response according
to the rules in Section 3.4.
DS RRs (or an NXT RR if DS RRs are present) are automatically
included in referals according to the rules in Section 3.5.
Since the DS RR is the only RR type that appears only on the
upper side of a delegation, any query for the DS RR type
requires special processing as described in Section 3.6.
Section 3.7 discusses how these changes impact caching servers and
recursive servers.
3.1 Indicating Resolver Support for DNSSEC
A resolver has indicated it supports DNSSEC if any of the
following hold:
The query explictly requests a KEY, NXT, SIG, or DS RR type.
The query implicitly requests a KEY, NXT, SIG, or DS by
requesting a meta-type that matches the KEY, SIG, NXT, or DS
RRs. In particular, ANY, IXFR, and AFXR queries implictly
match the DNSSEC RR types and DNSSEC RRs MUST be returned in
response to a query for ANY, IFXR, or AXFR.
The resolver has explicity requested DNSSEC by setting the
DNSSEC OK bit in the ENDS0 header.
The "DNSSEC OK" (D0) bit is used for explicit notification of
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DNSSEC support. The DO bit is defined in [9] and setting the DO
bit to one in a query indicates that the resolver is able to
accept DNSSEC security RRs. The DO bit cleared (set to zero)
indicates the resolver is unprepared to handle DNSSEC security RRs
and those RRs MUST NOT be returned in the response (unless DNSSEC
security RRs are explicitly requested in the query or implicitly
requested by the use a meta-RR type such as ANY, IXFR, or AFXR).
The DO bit of the query MUST be copied in the response.
In the event a server returns a NOTIMP, FORMERR or SERVFAIL
response to a query that has the DO bit set, the resolver SHOULD
NOT expect DNSSEC security RRs and SHOULD retry the query without
EDNS0 in accordance with Section 5.3 of RFC2671 [6].
The absence of DNSSEC data in response to a query with the DO bit
set MUST NOT be taken to mean no security information is available
for that zone since the response may have be forged or may be a
non-forged response to an altered (DO bit cleared) query.
3.2 Inclusion of SIG RRs in a Response
If the resolver has indicated support for DNSSEC, servers SHOULD
attempt to send SIG RRs that can be used to authenticate the RR
sets in the response. The inclusion of SIG RRs in a response is
subject to the following rules:
When an signed RRset is placed in the answer section, its SIG
RRs are also placed in the answer 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.
When an 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.
When an 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.
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3.3 Inclusion of KEY RRs In a Response
If the resolver has indicated support for DNSSEC and the query
requests the SOA or NS RRs, then a server SHOULD return the KEY
RRset with the same name in the additional section. If not all
additional information will fit in the response, type A and AAAA
glue RRs have higher priority than KEY RRs. The SIG RR(s)
associated with the KEY RR set SHOULD also be included in the
additional section (see including SIG RRs in Section 3.2).
3.4 Inclusion of NXT RRs In a Response
If the resolver has indicated support for DNSSEC, the server MUST
include NXT RRs in each of the following cases:
Case 1: the query name exists, but the requested RR type does
not exist.
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 answer the query.
NXT RRs are also included in a referal response if no DS RR is
present. In this case, the NXT RR is used to prove no DS RR
exists for the delegation and referals are discussed in detail in
Section 3.5.
Note that in every case the NXT RRs are included to provide
authenticated denial of existence.
3.4.1 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
including SIG RRs in Section 3.2) If space does not permit the
inclusion of the NXT RR (or its associate SIG RRs), the response
MUST be considered truncated.
Note that since the query name exists, an single NXT RR suffices
to prove the requested type does not exist. Since the name exists
in the zone, an NXT RR for that name also exists and lists RR
types present at the name. Since the query name exists, no
wildcard expansion applies to this query.
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3.4.2 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, the authority section of the response MUST include an
NXT RR that proves there was no exact match for the name and MUST
also include NXT RRs that prove no wildcard would have matched the
query. Any SIG(s) associated with the NXT RRsets are also
included in the authority section (see including SIG RRs in
Section 3.2) If space does not permit the inclusion of these NXT
RRs, the response MUST be considered truncated.
Appendix A provides an algorithm for computing the appropriate NXT
RRs that prove no wildcard matches the query name.
3.4.3 Case 3: Query Name Does Not Exist, but Wildcard Matches
If the query name does not exist and a wildcard expansion matches
the query, then the wildcard card expanded answer (and any SIG RRs
associated with the wildcard RR) are returned in the answer
section. The authority section of the response MUST include NXT
RRs that prove there were no exact matches for the name and MUST
also include NXT RRs to prove no closer wildcard entry would have
matched this query.
Appendix A provides an algorithm for computing the appropriate NXT
RRs that prove no closer wildcard matches the query name.
3.5 Inclusion of DS RRs In a Response
If the resolver has indicated support for DNSSEC, a server
returning a referral for the delegation MUST include both the NS
RRset and DS RRset if the DS RRset exists. The NS RRset MUST be
placed before the DS RRset (and its assoicated SIG RRs).
If the resolver has indicated support for DNSSEC, a server
returning a referral for the delegation MUST include both parent
NS RRset and the parent NXT RR if the DS RRset does not exist.
The NS RRset MUST be placed before the NXT RRset (and its
assoicated SIG RRs).
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 RRset (NXT RRset) and its assoicated SIG RRs,
the response MUST be considered truncated.
3.6 Responding to Queries for DS RRs
The DS record is the first resource record that appears only on
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the upper side of a delegation. In other words, the DS record for
zone "example.com" is only stored in the "com" zone (the parent/
upper side of the delegation). This introduces novel server
behavior since the child server is authoritative for the zone, but
the zone does not contain the DS RR. A server's response to a DS
query depends on whether the server is authoritative for the
parent and/or child zones as described below.
If a server is authoritative for the parent zone at a delegation
point and receives a query for the DS record at the delegation
name, then the server MUST return the DS RRset from the parent
zone. This is true regardless of whether or not the server is
also authoritative for the child zone.
If the server is authoritative for the child zone at a delegation
point and is not authoritative for the parent zone, there is no
natural response. The child zone is not authoritative for the DS
record at the zone's apex and the DS RR is only stored at the
parent.
If the server allows recursion and the RD bit is set in the
query, the server MAY perform recursion to find the DS record
at the delegation point and MAY return the DS record from its
cache. In this case, the AA bit MUST NOT be set in the
response.
If the server does not perform recursion to find the DS RR,
the server MUST reply with:
RCODE: NOERROR
AA bit: set
Answer Section: Empty
Authority Section: SOA [+ SIG(SOA) + NXT + SIG(NXT)]
In other words, an authortative child server answers as if it is
authoritative for the zone and the DS record does not exist. Note
DS-aware recursive servers will query the parent zone at
delegation points and thus will not be affected by this behavior.
For example, suppose "example.com" is a delegation point and a
query for the "example.com" DS RRset is received by a server.
If the server is authoritative for "com", the server MUST
reply with the "example.com" DS RRset from the "com" zone.
If the server is authoritative for "example.com" and is not
authortative for "com", the server MAY perform recursion to
find the "example.com" DS record (provided the RD bit was set
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in the query). If the server does not use recursion to obtain
the DS RR, the 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)]
3.7 Special Considerations for Recursive/Caching Servers
A DNSSEC aware recursive server MUST set the DO bit on recursive
requests, regardless of the status of the DO bit on the initiating
resolver request. If the initiating resolver request does not
have the DO bit set, the recursive DNSSEC-aware server MUST remove
any DNSSEC security RRs before returning the data to the client,
however cached data MUST NOT be modified.
A caching server SHOULD NOT attempt to answer a query by piecing
together the responses it has received previous from other queries
that requested different names or RR types. A cache typically
does not have access to the complete zone and thus it can be
difficult for a caching server to determine the proper SIG, NXT,
KEY, and DS RRs for a given a query. A caching server SHOULD
cache each response single atomic entry indexed by the question
(including the response and the all the assoicated DNSSEC RR
types). The cache SHOULD discard the entire entry when any RR in
the response expires.
3.8 Setting the AD and CD Bits in a Response
DNSSEC allocates two new bits in the DNS message header section:
The CD (checking disabled) bit and the AD (authentic data) bit.
These bits are defined in [9] and their use is described below.
The CD bit is set by the resolver and MUST be copied in the
response. If the CD bit is set to one, it indicates the resolver
is willing to perform authentication and the server need not
perform authentication on the RRsets in the response.
Regardless of the CD bit, the server MAY choose to perform
authentication (or choose not to perform authentication) according
to the local server policy. The CD bit MAY be used in
constructing the local server policy. If local 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).
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The AD bit is set by the server and indicates the data in the
response has been authenticated by the server, according to the
local 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 servers local authentication policy. A resolver MUST NOT
use the AD bit unless unless it communicates with the server over
a secure transport mechanism and is explicitly configured to trust
the server's policy. DNSSEC best practices documents are
encouraged to provide server policy recommendations.
3.9 Example DNSSEC Responses
example of A and SIG
example of apex KEY
example of signed delegation (DS) and unsigned delegation (NXT)
example of auth denial (includes NXT for wildcards)
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4. Authenticating DNS Responses
In order to use DNSSEC RRs for authentication, a resolver requires
some intial 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. In the
remainder of this section assumes the resolver has used some
unspecified off-line mechanism and 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 the initial KEY RR appears in the apex KEY RRset and
verify the KEY RR has the Zone Key Flag (KEY RDATA bit 7) set
to 1.
2. Verify there is some SIG RR that covers the apex KEY RRset
and the combination of the SIG RR and the initial KEY RR
authenticate the KEY RRset. The process for using a SIG RR to
authenticate an RRset is described in Section 4.2.
Once the apex KEY RRset has been authenticated using an initial
KEY RR, delegations from that zone can be authenticated using DS
RRs. This allows a resolver to start from an initial externally
authenticated key, and use DS RRsets to recursively proceed down
the DNS tree to obtain other apex KEY RRsets. If the resolver was
initially configured with a root KEY RR and if every delegation
had a DS RR assoicated with it, the resolver could obtain any apex
KEY RRset. The process of using DS RRs to authentic a referal is
described in Section 4.1.
Once a zones apex KEY RRset has been authenticated, Section 4.2
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 4.3 shows how the resolver can use authenticated
NXT RRsets from the zone to prove an RRset is not present in the
zone.
If the resolver has indicated support for DNSSEC, DNSSEC aware
servers SHOULD attempt to provide the necessary KEY, SIG, NXT, and
DS RRets in a response (see Section 3). However, a response that
lacks the approriate DNSSEC RRs may result from configuration
issues such as a non-DNSSEC aware cache that removes or fails
request DNSSEC RRs or may result from an intentional attack where
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an adversary forges a response, strips DNSSEC RRs from a response
forges, or modifies the query so DNSSEC RRs appear not to be
requested. The absence of DNSSEC data in response MUST NOT be
taken to mean that no authentication information is available.
A resolver SHOULD expect authentication information from signed
zones. A SHOULD believe 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 at the parent contains
a DS RRset. DNSSEC best practices documents are encouraged to
provide guidance on how a resolver responds if DNSSEC RRs are
expected, but can not be obtained. DNSSEC best practices
documents are also are encouraged to provide guidance on how a
resolver responds if the expected DNSSEC RRs are obtained but
appear invalid (e.g. all SIG RRs are expired).
4.1 Authenticating Referrals
Once the apex KEY RRset for a (parent) zone has been
authenticated, DS RRsets can be used to authenticate a referal to
a delegation (child zone). A DS RR identifies a KEY RR in the
child's apex KEY RRset. The DS RR contains a digest of the
child's KEY RR and 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
safely declare the matching child KEY RR to is also authentic.
This child KEY RR is then used to authenticate the entire child
apex KEY RRset.
Given a DS RR for a delegation (child zone), the delegation's
(child zone's) apex KEY RRset is considered to be authentic if all
of the following hold:
The DS RR has been authenticated using some KEY RR in the
parent's apex KEY RRset (see Section 4.2).
The Algorithm, Key Tag, and Digest fields in the DS RR match
the algorithm, key tag, and digest of a KEY RR present in the
child's apex KEY RRset.
The matching KEY RR in the child zone has the Zone Flag bit set
to one, the corresponding private key has signed the child apex
KEY RRset, and the resulting SIG RR authenticates the child's
apex KEY RRset.
If the referal from the parent zone did not contain a DS RRset,
the response SHOULD have included an NXT RRset that proves no DS
RRset exists for the delegation name (see Section 3.5). A
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resolver SHOULD send the parent a query for the DS RRset if
neither a DS RRset or NXT RRset is included in the referal.
If the resolver authenticates an NXT RRset that proves no DS RRset
is present for this zone, then there is no authentication path
leading from the parent to the child. If the resolver has an
initial KEY RR that belongs to the child zone (or 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 for a signed delegation there are two NXT RRs associated with
the delegation name. One NXT RR resides at the parent can be used
to prove whether a DS RRset exists for the delegation name. A
second NXT RR resides at the child zone and identifies which
RRsets are present at the apex in the child zone. The parent NXT
RR and child NXT RR can always be distinguished since the only the
child NXT RR will specify an SOA RR set exists at the name. A
resolver MUST only use the parent NXT RR when proving a DS RRset
does not exist.
4.2 Authenticating an RRSet Using a SIG RR
A SIG RR (and its corresponding KEY RR) is used by a resolver to
authentic an RRset. The SIG RR is first checked to verify that it
covers the RRset, has a valid time interval, and identifies a
valid KEY RR. The signed data is then constructed by appending
SIG RDATA (excluding the Signature Field) with the covered RRset
(in canonical form). Finally, the public key and signature and
used to authenticate the signed data. Section 4.2.1, Section
4.2.2, and Section 4.2.3 describe each step in detail.
4.2.1 Checking the SIG RR Validity
An SIG RR can be used to authenicate an RRset if all of the
following conditions hold:
The SIG RR and the RRset MUST have the same owner name and same
class.
The SIG RR's Signer's Name field MUST be the name of the zone
that contains the RRset.
The SIG RR's Type Covered field MUST equal the RRset's type.
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.
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The resolver's current time MUST be less than or equal to the
time listed in the SIG RR's Expiration field.
The resolver's current time MUST be greater than or equal to
the time listed in the SIG RR's Inception field.
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.
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 1.
It is possible that more than one KEY RR matches the conditions
above. In this case, the resolver can not determine which KEY RR
is used to authenticate the signature and the resolver MUST try
each matching KEY RR until the resolver has either validated the
signature or all matching KEY RRs have failed.
Note that the authentication process is only meaningful if the
resolver first authenticates a KEY RR before using it to validate
a signature. The matching KEY RR is considered to be authentic if
The apex KEY RRset containing the KEY RR is considered
authentic
The RRset covered by the SIG RR is the apex KEY RRset itself
and the KEY RR matches an authenticated DS RR from the parent
zone or matches some initial KEY RR/DS RR that is known to be
authentic.
4.2.2 Reconstructing the Signed Data
Once the SIG RR has met the validity requirements described in
Section 4.2.1, the original signed data needs to be reconstructed.
The original signed data includes SIG RDATA (excluding the
Signature field) and the RRset in cannonical order and might
differ from the RRset received in the DNS response due to name
compression, TTL decrementing by a cache, or the RRset may be the
result of wildcard expansion. The following algorithm is used to
reconstuct the original signed data:
signed_data = SIG_RDATA | RR(1) | RR(2)... where
"|" denotes append
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SIG_RDATA is the wire format of the SIG RDATA fields with
the Signature field excluded.
the Signer's Name in cannonical form.
RR(i) = name | class | type | OrigTTL | RDATA length | RDATA
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 cannonical 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.
An example of original name calculation is given in Section 4.4.1.
The canonical form for names and RRsets is defined in [9].
NXT RRsets present at a delegaion name require special processing.
There are two distinct NXT RRsets associated with a signed
delegation name. One NXT RRset resides at the parent and
specifies which RRset are present at the parent. A 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 RR set exists at the name. When
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constructing the original NXT RRset, the NXT RRs MUST NOT be
combined with NXT RRs from the child (and vice versa).
4.2.3 Checking the Signature
Once the SIG RR has met the validity requirements described in
Section 4.2.1 and the original signed data has been reconstructed
as described in Section 4.2.2, the cryptographic signature is used
to authenticate the signed data (and thus authenticate the RRset).
The Algorithm field in the SIG RR identifies the cryptographic
algorithm used to validate the signature. The signature itself is
contained in the Signature field of the SIG RDATA and public key
used to authenticate this signature is contained in the Public Key
field of the matching KEY RR(s) (found in Section 4.2.1). [9]
provides a list of algorithm types and provides pointers to the
documents that define each algorithm's use.
Note it is possible that more than one KEY RR matches the
conditions in Section 4.2.1. In this case, the resolver can not
determine which KEY RR is used to authenticate the signature and
the resolver MUST try each matching KEY RR until the resolver has
either validated the signature or all matching KEY RRs have
failed.
If the SIG RR Labels field is not equal to the number of labels in
the RRsets fully qualified domain name, then the RRset is a result
of wildcard expansion. The resolver MUST verify the wildcard was
applied properly before the RRset is considered authentic. The
RRset and SIG RR MUST be discarded if the resolver proves the
wildcard was applied improperly. Section 4.2.4 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 these SIG RRs need to be tested
and determines how to resolve conflicts if these SIG RRs lead to
differing results.
If the RRset is accepted as authentic, the SIG RR TTL and the TTL
of each RR in the authenticated RRset MUST be set to the minimum
of
the RR TTL received in the response
the value in the SIG RRs Original TTL field
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4.2.4 Authenticating Wildcard Expanded RRset
If a SIG RR's fully qualified domain name does not equal the
Labels field in the SIG RDATA, then SIG RR (and its covered RRset)
were created as a result of wildcard expansion. Once the
signature has been verified as described in Section 4.2,
additional steps are required to verify 1) no intermediate name
cancels the use of the wildcard and 2) no more specific wildcard
name could have been used to create this RRset.
Intermediate label names can be formed from the fully qualified
domain name by removing the rightmost labels and are used to prove
the wildcard was used properly. For example,
"www.a.b.c.example.com." has intermediate names of
"a.b.c.example.com", "b.c.example.com", "c.example.com",
"example.com", and "com". For each intermediate label name whose
label count is greater the SIG RR Labels field, the resolver MUST
obtain and authenticate NXT RRs that prove:
the intermediate label name does not exist (otherwise this
label would cancel the wildcard)
the name "*.intermediate_label_name" does not exist (otherwise
this wildcard would take precedence)
Note the response SHOULD include all NXT RRs needed to the
authenticate the response (see Section Section 3.4).
4.3 Authenticated Denial of Existence
A resolver can use authenticated NXT RRs to prove that an RRset is
not present in a signed zone. NXT RRsets SHOULD be automatically
included in the response, provided the zone is signed and the
resolver has indicated support for DNSSEC. NXT RRsets are
authenticated according to standard RRset authentication rules
described in Section 4.2 and are applied as follows:
If the requested RR name matches the owner name of an
authenticated NXT RR, then all RR types present at that owner
name MUST be listed in the NXT RR's Type Bit Map field. A
resolver can prove the requested RR type does not exist by
presenting checking for the RR type in NXT RR's Type Bit Map
field. Also, since owner name exists in the zone, no wildcard
expansion could be used to match the requested RR owner name and
type.
If the requested RR name logically appears after an authenticated
NXT RR owner name and logically appears before the name listed in
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that NXT RR's Next Domain Name field, then the requested RR name
is not present in the zone. However, it is possible a wildcard
could be used to match the requested RR owner name and type
Intermediate label names are used to prove no wildcard matches the
requested name. Intermediate label names are formed from the
requested RR's fully qualified domain name by removing the
rightmost labels from the name. For example,
"www.a.b.c.example.com." has intermediate names of
"a.b.c.example.com", "b.c.example.com", "c.example.com",
"example.com", and "com". To prove no wildcard matches, the
resolver MUST start with the longest intermediate label name prove
that:
No wildcard exists at this intermediate label name. In other
words, there is an authenticated NXT RR such the NXT RR's owner
name logically appears before "*.intermediate_label_name" and
the NXT RR's Next Domain field appears logically after
"*.intermediate_lable_name".
The resolver MUST continue testing intermediate label names until
(in order of decreasing label count) until the intermediate label
name matches an authenticated NXT RR's owner name. Note that this
is guaranteed to occur since at some point the intermediate label
will equal the zone name and NXT RR exists at the zone name.
4.4 Example
4.4.1 Example of Re-Constructing the Original Name
Suppose the RRset owner name received in a response is
"www.a.b.c.example.com.". This fully qualified domain name has 6
labels: "www", "a", "b", "c", "example", and "com". The name
used in reconstructing the original signed data depends on the
value of the SIG Labels.
If the SIG Labels field is 6, then the SIG Labels field equals the
number of labels in the RRsets fully qualified domain name.
Wildcard expansion was not used to construct this RRset and the
name "www.a.b.c.example.com." is used to construct the original
signed data.
If the SIG Labels field is 3, then the SIG Labels field is
strictly less than number of labels in the RRset's fully qualified
domain name. Wildcard expansion was used to construct this RRset
and the original wildcard owner name is constructed by appending
"*." to the last 3 labels in the owner name. The name
"*.c.example.com." is is used to construct the original signed
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data.
authentication process for www.example.com starting from a initial
root key
authentication process for non-existent www.a.b.c.example.com
starting from a initial root key
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5. IANA Considerations
This document introduces no IANA considerations.
[9] contains a complete review of the IANA considerations
introduced by the DNSSEC.
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6. 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. 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 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 the revised version of the
security considerations.
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7. 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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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] Eastlake, D., "DNS Request and Transaction Signatures (
SIG(0)s)", RFC 2931, September 2000.
[8] Arends, R., Larson, M., Massey, D. and S. Rose, "DNSSEC
Intro", October 2002.
[9] Arends, R., Larson, M., Massey, D. and S. Rose, "Resource
Records for the DNS Security Extensions", October 2002.
Authors' Addresses
Roy Arends
Bankastraat 41-E
1094 EB Amsterdam
NL
EMail: roy@logmess.com
Matt Larson
VeriSign, Inc.
21345 Ridgetop Circle
Dulles, VA 20166-6503
USA
EMail: mlarson@verisign.com
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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
RR set that proves no wildcard expansion matches N. The algorithm
was written for clarity not efficiency: (EDITORS NOTE: the
algorithm was really written on a redeye flight during dull movie
so it is unlikely to really achieve clarity :)
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
preceed N in S to NXT_SET.
EndIf
3. Replace the leftmost label of N with *
4. If N exists in S
There is a wildcard match for N.
Return all RRsets associated with N
Else
Add the NXT for name that would immediately
preceed N in S to NXT_SET.
EndIf
5. Remove the leading * from N.
6. If N exists in S
There is an 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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The limited permissions granted above are perpetual and will not
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
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