DNSEXT Working Group D. Massey
Internet Draft USC/ISI
S. Rose
Expires: December 2001 NIST
Category: Informational June 2001
Updates: RFC 2535
DNS Security Introduction and Requirements
------------------------------
<draft-ietf-dnsext-dnssec-intro-00.txt>
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Abstract
The Domain Name System Security Extensions (DNSSEC) provides data and
origin authentication and integrity. This document introduces the
new DNS extensions and describes the capabilities and limitations of
DNS Security. The services that the security extensions provide and
not provide are discusses. Lastly, the group of documents that
describe the DNS security extensions and their inter-relationships is
discussed.
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1. Introduction
This document introduces the Domain Name System Security Extensions
(DNSSEC). It, and a collection of following documents, are an update
of RFC 2535 and following documents in an attempt to further clarify
and refine the security extensions in RFC 2535. These security
extensions consist of a set of new resource record types and
modifications to the existing DNS protocol [RFC1035]. The new
records and protocol modifications are not fully described in this
document, but in a family of documents that is described in section
8. The threat model DNS security was 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
4 respectively. Lastly, the impacts DNS security will have on
resolvers, zones, and servers are discussed in sections 5 through 7.
The DNS security extensions provide data and source authentication
and integrity protection as well as a means for public key
distribution. The security extensions do not provide protection
against other types of attack or provide confidentiality.
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 [RFC2119]. It is also
assumed the reader has some knowledge of the Domain Name System
[RFC1035].
2. Threat Model
The Domain Name System (DNS) protocol has been the target of several
well-publicized attacks since it's creation. These attacks are
centered on altering DNS traffic in transit, or sending erroneous
referral messages to caching servers (i.e. "cache poisoning"). The
result of these attacks are traffic redirection or complete "zone
hijacking" if the malicious referral code is allowed to propagate.
The goal of the DNS security extensions is to protect the integrity
of DNS data and provide a means of source and data authentication.
No effort has been made to protect against other forms of attack such
as attacks against server implementations and denial of service.
3. Services Provided by DNS Security
The Domain Name System (DNS) protocol security extensions provide
three distinct services: Data origin authentication and integrity,
key distribution, and transaction and request authentication, as
described below.
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3.1 Source and Data Authentication and Integrity
Authentication is provided by cryptographically generated digital
signatures associated with DNS resource record sets (RRsets
[RFC2181]). Commonly, there will be a single private key that
authenticates a zone's data, but there might be multiple keys for
different algorithms, etc. If a security aware resolver reliably
learns a public key of the zone, it can authenticate signed zone
data. The data origin authentication key(s) are associated with the
zone and not with the servers that store copies of the zone data.
A resolver could learn a public key of a zone either by reading it
from the DNS or by having it statically configured. To reliably
learn a public key by reading it from the DNS, the learned key itself
must be signed with a key the resolver trusts. New zone information
is authenticated by forming a "chain of trust" from a newly learned
zone back to a trusted (pre-configured or previously learned) zone
key. Therefore, the resolver must be configured with at least a
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 an RRset with the DNS reply,
providing there is available space in the DNS message packet.
Adding data origin authentication and integrity requires no change to
the "on-the-wire" DNS protocol beyond the addition of the signature
resource type (SIG record) and the key resource type (KEY record)
needed for key distribution. Existing resolver and caching server
implementations can support this service so long as they can support
the new resource record types.
3.2 Key Distribution
A resource record format is defined to associate public keys with DNS
names. This permits the DNS to be used as a public key distribution
mechanism in support of DNS security itself and other protocols.
Security aware resolvers can query for a zone's or other entities'
public key when establishing a chain of trust.
The syntax of a KEY resource record (RR) is described in a separate
document [proto ref]. It includes identifiers for the key algorithm,
and information on the entity the key is associated with.
3.3 Authenticating Name and Type Non-Existence
The above security mechanism only provides a way to sign existing
RRsets in a zone. The problem of sending negative responses with the
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same level of authentication and integrity requires the use of the
Non-existence (NXT) record type. The NXT record allows a negative
reply (either for name or type non-existence) to be authenticated the
same way other DNS replies. NXT records exist to cover the gaps in
the canonical representation of a zone namespace. Each NXT record is
signed and authenticated the same way as any other RRset in the DNS.
3.4 Transaction Security
The data origin authentication service described above protects
retrieved resource records and the non-existence of resource records
but provides no protection for complete DNS messages, including zone
transfers and dynamic updates.
DNS message can be authenticated by including a special signature RR
at the end of the request, either a Transaction SIG (TSIG) or SIG[0]
[Proto doc ref]. A Transaction signature included at the end of a
response can be used to verify the integrity of the DNS message.
Authenticating requests serves no function in older DNS servers and
requests with a non-empty additional information section may produce
error returns or be ignored. Unlike the SIG RR discussed above,
transaction security involves the individual host machines in the DNS
message transaction, so the cryptographic keys used in with
transaction security are associated with individual systems and not
DNS zones.
In addition to the ability to use transaction signatures, there is
additional support for the use of key agreement protocols to support
transaction security using symmetric encryption keys. This service
uses the transaction key (TKEY) 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
for each algorithm.
4. What Services Are Not Provided
It is part of the design philosophy of the DNS that the data in it is
public and that the DNS gives the same answers to all inquirers.
Following this philosophy, no attempt has been made to include any
sort of access control lists or other means to differentiate
inquirers.
No effort has been made to provide for any confidentiality for
queries or responses.
Denial of service is not protected against.
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The use of SIGs in a zone and/or the use of transaction signatures
will not protect against errors in DNS zone information or servers
incorrectly interpreting/setting DNS message headers.
5. Resolver Considerations
A security aware resolver will have 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 known secure public key. This might require
additional queries to intermediate DNS zones for necessary keys and
signatures. It is assumed that a security aware resolver will be
pre-configured with at least one public key to aid in this.
The stub resolver found in many host systems 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 to secure the final message passing
between a stub resolver and a primary recursive DNS server. It is a
matter of local security policy.
It is necessary for a security aware resolver only communicate with
security aware servers. If an unsecure server or an unsecure zone is
part of the DNS chain, the resolver cannot insure security. If a
security aware resolver must rely on an unsecure server (or unsigned
zone), it is not possible for the resolver to verify DNS responses,
and should rely on local policy when trusting responses.
6. Zone Considerations
A secure zone will have several differences from an unsecure zone. A
secure zone will have additional security records (SIG, KEY, and NXT
records) added to the zone data. It is possible that the security
records are added before the zone is loaded by a server. In this
case, the server must generate the signatures and NXT records before
loading the zone. Also, zone information is only valid and
considered secure for a definable time period. The SIG records that
accompany zone data have a defined inception and expiration times.
These times establish a validity period for the signatures and the
zone data the signatures covers.
7. Server Considerations
A security aware server must be capable of performing the following
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additional operations, in addition to the normal operations of a DNS
zone server described in [RFC1035]:
A security aware server should make all attempts to include necessary
security related records in DNS responses. This includes all
necessary signatures and public key records, as DNS message space
permits.
A caching security aware server must also take the signatures
validation period into consideration when determining the Time-To-
Live (TTL) of cached data.
A security aware server should have a means of employing transaction
security such as transactions 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 administration access to a security aware server falls under the
category of operational security and are not addressed by the DNS
security extensions.
8. DNS Security Document Family
The DNSSEC set of documents can be partitioned into five main groups
as depicted in Figure 1. All of these documents in turn are under
the larger umbrella group of DNS base protocol documents.
8.1 DNS Security Document Roadmap
+--------------------------------+
| |
| Base DNS Protocol Docs. |
| [RFC1035, RFC2181, etc.] |
| |
+--------------------------------+
|
|
+-----------+ +-------------+
| DNSSEC | | New |
| protocol |<-------->| Security |
| documents | | Uses |
+-----------+ +-------------+
|
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|
+------------------------------+
| |
| |
+------------+ +---------------+
| DS | | |
| Algorithm | | Transaction |
| Impl. | | Impl. |
| | | |
+------------+ +---------------+
Figure 1 DNSSEC Document Roadmap
8.2 Categories of DNS Security Documents
The "DNSSEC protocol" document set refers to the document that makes
up the groundwork for adding security to the DNS protocol [protocol
doc ref].
The "DS 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 one of
these documents deals with one specific digital signature algorithm.
The "Transactions" document set refers to the group of documents that
deal with the message transaction sequence of security-related DNS
operations. Message transaction schemes to support DNSSEC operation
fall under this group, 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 and
individual user public keys (PGP, e-mail, etc.)
9. 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 and source
authentication and integrity through the use of 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.
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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.
10. Acknowledgements
This document was created from the input and ideas of several members
of the DNS Extensions WG. The authors would like to acknowledge the
following people for their contributions and comments on this
document. In alphabetical order:
Donald Eastlake
Miek Gieben
Olafur Gudmundsson
Ted Lindgreen
Ed Lewis
Bill Manning
Brian Wellington
11. References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC1034] Mockapetris, P., "Domain Names - Concepts and Facilities".
RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain Names - Implementation and
Specifications", STD 13, RFC 1035, November 1987.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[RFC3090] Lewis, E. "DNS Security Extensions Clarification on Zone
Status". RFC 3090, March 2001.
[RFC3007] Wellington, B. "Secure Domain Name System (DNS) Dynamic
Update". RFC 3007, November 2000.
12. Authors' Addresses
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Dan Massey
USC/ISI
Email: masseyd@isi.edu
Scott Rose
National Institute for Standards and Technology
Gaithersburg, MD
Email: scott.rose@nist.gov
Expiration and File Name:
This draft, titled <draft-ietf-dnsext-dnssec-intro-00.txt> expires December 2001
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