Network Working Group R. Arends
Internet-Draft Nominum
Expires: August 30, 2002 M. Larson
VeriSign
D. Massey
USC/ISI
S. Rose
NIST
March 1, 2002
DNS Security Introduction and Requirements
draft-ietf-dnsext-dnssec-intro-01
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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This Internet-Draft will expire on August 30, 2002.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
The Domain Name System Security Extensions (DNSSEC) provide data
origin authentication and data integrity. This document introduces
these extensions and describes their capabilities and limitations.
The services that the security extensions provide and do not provide
are discussed. Lastly, the group of documents that describe the DNS
security extensions and their interrelationships is discussed.
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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 [1].
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Threat Model . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1 Packet Interception and/or Modification . . . . . . . . . . 4
2.2 Name Based Attacks (a.k.a. Cache Poisoning) . . . . . . . . 4
2.3 Attacks Not Covered by DNSSEC . . . . . . . . . . . . . . . 5
3. Services Provided by DNS Security . . . . . . . . . . . . . 6
3.1 Data Origin Authentication and Data Integrity . . . . . . . 6
3.1.1 Authenticating Name and Type Non-Existence . . . . . . . . . 7
3.2 Key Distribution . . . . . . . . . . . . . . . . . . . . . . 7
3.3 Transaction Security . . . . . . . . . . . . . . . . . . . . 7
4. Services Not Provided by DNS Security . . . . . . . . . . . 9
5. Resolver Considerations . . . . . . . . . . . . . . . . . . 10
6. Zone Considerations . . . . . . . . . . . . . . . . . . . . 11
7. Server Considerations . . . . . . . . . . . . . . . . . . . 12
8. DNS Security Document Family . . . . . . . . . . . . . . . . 13
8.1 DNS Security Document Roadmap . . . . . . . . . . . . . . . 13
8.2 Categories of DNS Security Documents . . . . . . . . . . . . 13
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . 15
10. Security Considerations . . . . . . . . . . . . . . . . . . 16
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17
References . . . . . . . . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 19
A. Definitions of Important DNSSEC Terms . . . . . . . . . . . 20
Full Copyright Statement . . . . . . . . . . . . . . . . . . 21
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1. Introduction
This document introduces the Domain Name System Security Extensions
(DNSSEC). This document and a collection of related documents update
RFC 2535 [2] and its related documents to further clarify and refine
the security extensions defined in RFC 2535. These security
extensions consist of a set of new resource record types and
modifications to the existing DNS protocol [3]. The new records and
protocol modifications are not fully described in this document, but
in a family of documents outlined in Section 8. The threat model
that the security extensions are designed to protect against is
described in Section 2. The capabilities and limitations of the
security extensions are described in greater detail in Section 3 and
Section 4, respectively. Lastly, the effect that these security
extensions will have on resolvers, zones and servers is discussed in
Section 5, Section 6 and Section 7, respectively.
Appendix A provides definitions for important DNSSEC terms. A term
that appears in Appendix A is followed by an asterisk (*) upon first
use in the body of the document.
The DNS security extensions provide data origin authentication and
data integrity protection as well as a means of public key
distribution. The security extensions do not provide protection
against other types of attack, nor do they provide confidentiality.
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2. Threat Model
The Domain Name System (DNS) protocol has been the target of several
well-publicized attacks since its creation. A more detailed threat
model for DNS is the subject of [4]. A brief description of the
major attacks that the DNS security extensions were designed to
protect against is given below.
2.1 Packet Interception and/or Modification
The attacker has access to the network and observes (and possibly
modifies) DNS message data. This is a form of the "man in the
middle" attack and can be either passive (observation only) or active
(interception and modification). It is also possible that the
attacker responds to a resolver's query with bogus information before
the queried name server's reply reaches the resolver. Message
modification could lead to incorrect information being returned to a
resolver, or a different query being sent to a name server.
DNSSEC uses a digital signature scheme for DNS RRsets (defined in
[5]) or entire DNS messages to provide integrity checks and source
authentication. Any modifications made by an attacker would result
in a signature verification error at the resolver. DNSSEC also
provices a means for authenticating the non-existence of DNS data.
A conscious DNSSEC design decision was keeping DNS information in
plaintext. DNSSEC does use any cryptographic techniques to provide
confidentiality for any DNS information.
2.2 Name Based Attacks (a.k.a. Cache Poisoning)
This type of attack allows the attacker to hijack a DNS zone by
injecting false data into a response in the form of resource records
(RRs) in the authority and additional sections, usually pointing to a
false authoritative server. This attack depends on the targeted
resolver caching these records and using them to answer future
queries from other resolvers. This attack allows the attacker to
subvert a single recursive server and redirect all queries for the
targeted domain name to the attacker's name server, essentially
hijacking the DNS zone from its legitimate servers.
Since this attack modifies the contents of DNS messages, the
signature scheme in DNSSEC protects against it. The keys that
resolvers use to verify DNS data must either be configured as a
trusted key*, or be part of a chain of trust* back to a trusted key.
The DNSSEC trust model ensures that as long as zone administrators
follow reasonable key-signing policies, the key used by an attacker
to sign malicious DNS data would not be trusted.
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Note, however, that referrals are not signed in DNSSEC. It is
expected that the a zone's authoritative servers will provide digital
signatures for RRsets in the answer, authority and additional
sections, as necessary.
2.3 Attacks Not Covered by DNSSEC
Denial of Service (DoS) attacks are not addressed by DNSSEC. DoS
attacks are easy to detect, but difficult to protect against, in a
protocol like DNS. Name server and resolver implementations can
protect against some DoS attacks (by providing system security), but
there are no protocol features in DNSSEC to defend against DoS
attacks.
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3. Services Provided by DNS Security
The Domain Name System (DNS) protocol security extensions provide
three distinct services: Data origin authentication and data
integrity, key distribution, and transaction security, as described
below.
3.1 Data Origin Authentication and Data Integrity
Authentication is provided by cryptographically generated digital
signatures associated with DNS RRsets. These digital signatures are
stored in a new resource record, the SIG record. Typically, there
will be a single private key that signs a zone's data, but multiple
keys are possible; e.g., for different digital signature algorithms.
If a security-aware resolver* reliably learns a zone's public key, it
can authenticate that zone's signed data. An important DNSSEC
concept is that the key pair(s) that sign a zone's data are
associated with the zone itself and not with the zone's authoritative
servers (although hosts can also have key pairs in DNSSEC; see the
reference to SIG(0) in Section 3.3 below). Security-aware servers*
attempt to send the signature(s) needed to authenticate an RRset in
the DNS reply message along with the RRset itself, provided there is
space available in the message.
A resolver could learn a zone's public key by having the key
statically configured or by normal DNS resolution. To allow the
latter, public keys are stored in a new resource record, the KEY
record (see Section 3.2 below). (Note that the private keys used to
sign zone data must be kept secure and best practices call for them
to be stored offline.) To reliably discover a public key by DNS
resolution, the key itself needs to be signed by another key that the
resolver trusts. Zone information is authenticated by forming a
chain of trust from a newly learned public key back to a trusted
public key (which is either statically configured or previously
learned and verified). Therefore, the resolver must be configured
with at least one public key that authenticates one zone as a
starting point. To establish this chain of trust, security-aware
servers attempt to send the signature(s) needed to authenticate a
zone's public key in the DNS reply message along with the public key
itself, provided there is space available in the message.
The chain of trust specified in the original DNS security extentions
proceeded from signed KEY record to signed KEY record, as necessary,
and finally to the queried RRset. A new record, the delegation
signer (DS), has been added for additional flexibility. The DS
record is stored at a delegation point in a parent zone and specifies
the keys used by the child zone to self-sign its KEY records. The
child, in turn, uses one of these keys to sign its zone data. The
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chain of trust is therefore DS->KEY->[DS->KEY->...]->RRset.
Adding data origin authentication and data integrity requires minor
changes to the on-the-wire DNS protocol. Several new resource record
types are required, including the aforementioned SIG, KEY and DS.
EDNS0 support [6] for larger message sizes [7] is required, as is
support for the OK bit [8].
3.1.1 Authenticating Name and Type Non-Existence
The above security mechanism only provides a way to sign existing
RRsets in a zone. The problem of providing negative responses with
the same level of authentication and integrity requires the use of
another new resource record, the non-existence (NXT) record. The NXT
record allows a negative reply (either for name or type non-
existence) to be authenticated the same way as other DNS replies.
NXT records require a canonical representation and order for domain
names in zones. NXT records exist to cover the gaps, or "empty
space", between domain names in a zone, as well as non-existent
record types for existing names. Each NXT record is signed and
authenticated in the same way as any other RRset.
3.2 Key Distribution
The KEY resource record is defined to associate public keys with DNS
names. This record permits the DNS to be used as a public key
distribution mechanism in support of DNSSEC. Security-aware
resolvers can query for a zone's (or another entity's) public key
when establishing a chain of trust.
The syntax of a KEY resource record (and the other additional
resource records required for DNSSEC) is described in [9]. It
includes identifiers for the algorithm the key is used for and
information on the entity the key is associated with.
The original DNSSEC specification allowed other protocols and
applications to use the KEY record as a general-purpose mechanism to
store public keys. For reasons documented in [10], the KEY record is
now restricted for use with DNSSEC only. Work is in progress on
storing public keys [11] and certificates [12] used by other
protocols and applications in the DNS. A secure DNS tree could then
be used as a lightweight trust mechanism. Some administrators and
users may consider a validated DNSSEC signature to be sufficient to
trust a public key stored in the DNS.
3.3 Transaction Security
The data origin authentication and data integrity service described
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above authenticates retrieved RRsets and the non-existence of RRsets,
but provides no protection for complete DNS messages, e.g., when they
occur in zone transfers and dynamic updates.
A DNS message can be authenticated by including a special signature
RR at the end of the message, either a transaction signature (TSIG)
[13] or SIG record with a type covered field of zero (a "SIG(0)", see
[14]). Such a signature can be used to verify the integrity of the
DNS message. (The signature record in the additional section of a
query may produce an error or simply be ignored by older name servers
that don't support transaction security.) Unlike the mechanism
described in Section 3.1, transaction security is specific to the
individual hosts exchanging DNS messages. The cryptographic keys
used with transaction security are associated with individual hosts
and not DNS zones.
In addition to transaction signatures, there is also support for key
agreement protocols to support transaction security using symmetric
encryption keys. This service uses the transaction key (TKEY) [15]
resource record. The use of the TKEY record in a key agreement
transaction depends on the algorithm used, and each key agreement
protocol is described in a separate document specific to each
algorithm.
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4. Services Not Provided by DNS Security
The DNS design philosophy calls for all DNS data to be public and
uniform answers to all inquirers. Accordingly, confidentiality for
queries or responses is not provided, nor are any sort of access
control lists or other means to differentiate inquirers.
Denial of service is not protected against.
Signed zone data and/or the use of transaction signatures will not
protect against errors in DNS zone information or servers incorrectly
interpreting and/or setting DNS message headers.
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5. Resolver Considerations
A security-aware resolver needs to be able to perform necessary
cryptographic functions to verify digital signatures using at least
the mandatory-to-implement algorithms. Also, security-aware
resolvers must be capable of forming a chain of trust from a newly
learned zone back to a trusted key. This might require additional
queries to intermediate DNS zones for necessary KEY, DS and SIG
records. It is assumed that a security-aware resolver will be
configured with at least one trusted key to aid this process.
The stub resolver found in many hosts may be augmented to provide a
different set of security features instead of the full security
awareness found in a complete resolver. The use of transaction
security could help secure the final message passing between a
recursive DNS server and a stub resolver. This a matter of local
security policy.
A security-aware resolver needs to communicate with only security-
aware servers. If an unsecure server* or an unsigned zone* is part
of the DNS resolution path, the resolver cannot ensure security. If
a security-aware resolver must rely on an unsecure server (or
unsigned zone), the resolver cannot verify DNS responses and should
rely on local policy when trusting responses.
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6. Zone Considerations
A secure zone* will have several differences from an unsigned zone.
A secure zone will contain additional security-related records (SIG,
KEY, DS and NXT records). SIG and NXT records may be generated by a
signing process prior to serving the zone. If SIG and NXT records
are not present in the zone, an authoritative server needs to
generate them before serving the zone. Zone data is only valid and
considered secure for a definable time period. The SIG records that
accompany zone data have defined inception and expiration times.
These times establish a validity period for the signatures and the
zone data the signatures cover.
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7. Server Considerations
A security-aware server must be capable of performing the following
operations in addition to the normal operations of a DNS zone server
described in [3]:
A security-aware server should make all attempts to include
necessary security related records (SIG, KEY, DS and NXT) in
responses as DNS message space permits.
A caching security-aware server must also take a signature's
validation period into consideration when determining the time to
live (TTL) of cached data: signed data should not be cached beyond
the signature validity period.
A security-aware server should have a means of employing
transaction security, such as transaction signatures (TSIG) or
SIG(0). Transaction security is primarily used when performing
other DNS operations such as zone transfers and dynamic updates
(if they are permitted using the server's local policy).
All other means of restricting query, zone transfer, dynamic
update and administrative access to a security-aware server fall
under the category of operational security and are not addressed
by the DNS security extensions.
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8. DNS Security Document Family
The DNSSEC set of documents can be partitioned into five main groups
as depicted in Figure 1. All these documents are in turn under the
larger umbrella of the DNS base protocol documents described in [16].
8.1 DNS Security Document Roadmap
---------------------------------------------------------------------
+--------------------------------+
| |
| Base DNS Protocol Docs |
| [RFC1035, RFC2181, etc.] |
| |
+--------------------------------+
|
|
+-----------+ +-------------+
| DNSSEC | | New |
| Protocol |<-------->| Security |
| Documents | | Uses |
+-----------+ +-------------+
|
|
+------------------------------+
| |
| |
+------------+ +---------------+
| Dig. Sig. | | |
| Algorithm | | Transaction |
| Impl. | | Impl. |
| | | |
+------------+ +---------------+
Figure 1: DNSSEC Document Roadmap
---------------------------------------------------------------------
8.2 Categories of DNS Security Documents
The "DNSSEC protocol" document set refers to the three documents that
form the core of the DNS security extensions:
1. DNS Security Introduction and Requirements (this document)
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2. Resource Records for DNS Security Extensions [9]
3. Protocol Modifications for the DNS Security Extensions (not yet
published) [14]
The "Dig. Sig. Algorithm Impl." document set refers to the group of
documents that describe how a specific digital signature algorithm is
implemented to fit the DNSSEC resource record format. Each of these
documents deals with a specific digital signature algorithm.
The "Transaction Impl." document set refers to the group of documents
that deal with DNS message transaction security, including secret key
establishment and verification.
The final document set, "New Security Uses", refers to documents that
seek to use proposed DNS Security extensions for other security
related purposes. Documents that fall in this category include the
use of DNS in the storage and distribution of certificates [12] and
individual user public keys (PGP, e-mail, etc.) [11].
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9. IANA Considerations
This document introduces no new IANA considerations.
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10. Security Considerations
This document introduces the DNS security extensions and describes
the document set that contains the new security records and DNS
protocol modifications. The capabilities and limitations of these
extensions are discussed. The extensions provide data origin
authentication and data integrity using digital signatures over
resource records and (optionally) DNS messages. The DNS security
extensions can also be used to support key distribution for other
security protocols.
In order for a secure resolver to validate a DNS response, all the
intermediate zones and servers must be capable of DNS security
processing. A security-aware resolver cannot verify responses sent
by an non-security-aware server.
The DNS security extensions do not protect against denial of service
(DoS) attacks or provide confidentiality.
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11. Acknowledgements
This document was created from the input and ideas of several members
of the DNS Extensions Working Group. The authors would like to
acknowledge (in alphabetical order) the following people for their
contributions and comments on this document:
Derek Atkins
Rob Austein
Donald Eastlake
Miek Gieben
Olafur Gudmundsson
Olaf Kolkman
Ed Lewis
Ted Lindgreen
Bill Manning
Brian Wellington
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References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
[3] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[4] Atkins, D. and R. Austein, "Threat Analysis Of The Domain Name
System", draft-ietf-dnsext-dns-threats-00 (work in progress),
November 2001.
[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] Gudmundsson, O., "DNSSEC and IPv6 A6 aware server/resolver
message size requirements", RFC 3226, December 2001.
[8] Conrad, D., "Indicating Resolver Support of DNSSEC", RFC 3225,
December 2001.
[9] Arends, R., Larson, M., Massey, D. and S. Rose, "Resource
Records for DNS Security Extensions", draft-ietf-dnsext-dnssec-
records-00 (work in progress), March 2002.
[10] Massey, D. and S. Rose, "Limiting the Scope of the KEY Resource
Record", draft-ietf-dnsext-restrict-key-for-dnssec-01 (work in
progress), January 2002.
[11] Schlyter, J., "Storing application public keys in the DNS",
draft-schlyter-appkey-02 (work in progress), February 2002.
[12] Eastlake, D. and O. Gudmundsson, "Storing Certificates in the
Domain Name System (DNS)", RFC 2538, March 1999.
[13] Vixie, P., Gudmundsson, O., Eastlake, D. and B. Wellington,
"Secret Key Transaction Authentication for DNS (TSIG)", RFC
2845, May 2000.
[14] Arends, R., Larson, M., Massey, D. and S. Rose, "Protocol
Modifications for the DNS Security Extensions (Not yet
published)", draft-ietf-dnsext-dnssec-protocol-00 (work in
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progress), to be published in 2002.
[15] Eastlake, D., "Secret Key Establishment for DNS (TKEY RR)", RFC
2930, September 2000.
[16] Rose, S., "DNS Security Document Roadmap", draft-ietf-dnsext-
dnssec-roadmap-05 (work in progress), November 2001.
[17] Mockapetris, P., "Domain names - concepts and facilities", STD
13, RFC 1034, November 1987.
Authors' Addresses
Roy Arends
Nominum, Inc.
2385 Bay Street
Redwood City, CA 94063
USA
EMail: roy.arends@nominum.com
Matt Larson
VeriSign, Inc.
21345 Ridgetop Circle
Dulles, VA 20166-6503
USA
EMail: mlarson@verisign.com
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 20899-3460
USA
EMail: scott.rose@nist.gov
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Appendix A. Definitions of Important DNSSEC Terms
trusted key: A public key, for a zone or a host, that a resolver
trusts and that can therefore be used to verify data. A key can
become trusted in two ways. First, it can be statically
configured and declared as trusted in the resolver's
configuration. Second, if a new key is referenced by a DS record
that is signed by an already trusted key, and the signature
verifies, the new key becomes trusted.
chain of trust: In DNSSEC, a key signs a DS record, which points to
another key, which in turn signs another DS record, which points
to yet another key, etc. This alternating succession of KEY and
DS records forms a chain of signed data, with each link in the
chain vouching for the next. A resolver starting at a piece of
data in the chain signed by a trusted key can verify all
subsequent signatures. Thus all subsequent data in the chain is
trusted.
security-aware resolver: A resolver (defined in section 2.4 of [17])
that understands the DNS security extensions. In particular, a
security-aware resolver uses trusted keys to verify signatures
over RRsets and (optionally) DNS messages.
security-aware server: A name server (also defined in section 2.4 of
[17]) that understands the DNS security extensions. In
particular, it supports the KEY, SIG, DS and NXT record types, a
larger DNS message size via EDNS0, and other protocol changes such
as support for the OK bit. Also called a "secure server".
unsecure server: The proper temr for the opposite of a security-aware
server.
unsigned zone: The proper term for the opposite of a secure zone.
secure zone: A zone whose RRsets are signed and which contains
properly constructed KEY, SIG, NXT and (optionally) DS records.
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