One document matched: draft-ietf-dnsind-sig-zero-00.txt
INTERNET-DRAFT Donald E. Eastlake 3rd
UPDATES RFC 2535 IBM
Expires: April 2000 October 1999
draft-ietf-dnsind-sig-zero-00.txt
DNS Request and Transaction Signatures ( SIG(0)s )
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Status of This Document
This draft, file name draft-ietf-dnsind-sig-zero-00.txt, is intended
to become a Proposed Standard RFC updating Proposed Standard [RFC
2535]. Distribution of this document is unlimited. Comments should
be sent to the DNS Working Group mailing list
<namedroppers@internic.net> or to the author.
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. Internet-Drafts may be updated, replaced, or obsoleted by
other documents at any time. It is not appropriate to use Internet-
Drafts as reference material or to cite them other than as a
``working draft'' or ``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.
Abstract
Extensions to the Domain Name System (DNS) are described in [RFC
2535] that can provide data origin and transaction integrity and
authentication to security aware resolvers and applications through
the use of cryptographic digital signatures.
Implementation experience has indicated the need for minor but non-
interoperable changes in Request and Transaction signature resource
records ( SIG(0)s ). These changes are documented herein.
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Acknowledgments
The significant contributions and suggestions of the following
persons (in alphabetic order) to this draft are gratefully
acknowledged:
Olafur Gudmundsson
Brian Wellington
Table of Contents
[Table of Contents gets moved here from the end]
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1. Introduction
This document makes minor but non-interoperable changes to part of
[RFC 2535], familiarity with which is assumed, and includes
additional explanatory text. These changes concern SIG Resource
Records that are used to sign DNS requests and transactions /
responses. Such a resource record, because it has a type covered
field of zero, is frequently called a SIG(0). The changes are based
on implementation and attempted implementation experience with TSIG
[draft-ietf-dnsind-tsig-*.txt] and the [RFC 2535] specification for
SIG(0).
No changes are made herein related to the KEY or NXT RRs or to the
processing involved with data origin and denial authentication for
DNS data.
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].
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2. SIG(0) Design Rational
The authenticated data origin and data existence denial services of
secure DNS protect only data resource records (RRs) or
authenticatably deny their nonexistence. These services provide no
protection for DNS requests, no protection for message headers on
requests or responses, and no protection of the overall integrity of
a response.
If header bits are falsely set by a bad server, there is little that
can be done. However, it is possible to add transaction
authentication. Such authentication means that a requester can be
sure it is at least getting messages from the server it thinks it
queried and that the response is from the request it sent (i.e., that
these messages have not been diddled in transit). This is
accomplished by optionally adding either a TSIG RR [draft-ietf-
dnsind-tsig-*.txt] or, as described herein, a SIG resource record at
the end of the response which digitally signs both the server's
response and the corresponding resolver query.
Requests can also be authenticated by including a TSIG or, as
described herein, a special SIG RR at the end of the request.
Authenticating requests serves no function in older DNS servers.
Requests with a non-empty additional information section produce
error returns or may even be ignored by a few older DNS servers.
However, this syntax for signing requests is defined for
authenticating dynamic update requests [RFC 2136], TKEY [draft-ietf-
dnsind-tkey-*.txt], or future requests requiring authentication.
The private keys used in transaction security belong to the host
composing the DNS message, not to the zone involved. Request
authentication may also involve the private key of the host or other
entity composing the request or other private keys depending on the
request authority it is sought to establish. The corresponding public
key(s) are normally stored in and retrieved from the DNS for
verification.
Because requests and replies are highly variable, message
authentication SIGs can not be pre-calculated. Thus it will be
necessary to keep the private key on-line, for example in software or
in a directly connected piece of hardware.
2.1 Differences Between TSIG and SIG(0)
There are significant differences between TSIG and SIG(0).
Because TSIG involves secret keys installed at both the requester and
server the presence of such a key implies that the other party
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understands TSIG and likely has the same key installed. Furthermore,
TSIG uses keyed hash authentication codes which are relatively
inexpensive to compute. Thus it is common to sign requests with TSIG
and responses are signed with TSIG if the corresponding request is
signed.
SIG(0) on the other hand, uses KEY RRs that are stored in the DNS
under the host name. Thus, existence of such a KEY RR does not
necessarily imply implementation of SIG(0). In addition, SIG(0)
involves relatively expensive public key cryptographic operations
that should be minimized and the verification of a SIG(0) involves
obtaining and verifying the corresponding KEY which can be an
expensive and lengthy operation. Indeed, a policy of using SIG(0) on
all requests and verifying it before responding would, for some
configurations, lead to a deadly embrace with the attempt to obtain
and verify the KEY needed to authenticate the request SIG(0)
resulting in additional requests accompanied by a SIG(0) leading to
further requests accompanied by a SIG(0), etc. Furthermore, omitting
SIG(0)s when not required on requests halves the number of public key
operations required by the transaction.
For these reasons, SIG(0)s MUST only be used on requests when
necessary to authentication that the requester has some required
privilege or identity. SIG(0)s on replies are defined in such a way
as to not require a SIG(0) on the corresponding request and still
provide transaction protection. Some replies, such as those
involving TKEY [draft-ietf-dnsind-tkey-*.txt], MUST be signed with
TSIG or SIG(0). For other replies, whether they are signed by the
server or required to be signed by the requester SHOULD be a local
configuration option.
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3. The SIG(0) Resource Record
The structure of and type number of SIG resource records (RRs) is
given in [RFC 2535] Section 4.1. However all of Section 4.1.8.1 and
the parts of Sections 4.2 and 4.3 related to SIG(0) should be
considered replaced by the material below. Any conflict between [RFC
2535] and this document concerning SIG(0) RRs should be resolved in
favor of this document.
For all SIG(0)s, the signer field MUST be a name of the originating
server host. (The inverse IP address mapping name MAY be used if the
relevant KEY is stored there.) However, the owner name, class, TTL,
and original TTL, are meaningless. The class and TTL fields SHOULD
be zero. To conserve space, the owner name SHOULD be root (a single
zero octet). When SIG(0) authentication on a response is desired,
that SIG RR must be considered the highest priority for inclusion in
the response.
3.1 Calculating Request and Transaction SIGs
A DNS request may be optionally signed by including one or more SIGs
at the end of the query additional information section. Such SIGs
are identified by having a "type covered" field of zero. They sign
the preceding DNS request message including DNS header but not
including the UDP/IP header or any request SIG(0)s or TSIGs [draft-
ietf-dnsind-tsig-*.txt] at the end and before the request RR counts
have been adjusted for the inclusions of any request SIG(0)s or
TSIGs.
They are calculated by using a "data" (see [RFC 2535], Section 4.1.8)
of (1) the SIG's RDATA section omitting the signature subfield
itself, (2) the entire DNS query messages, including DNS header, but
not the UDP/IP header or any SIG(0) or TSIG and before the reply RR
counts have been adjusted for the inclusion of any SIG(0) or TSIG.
That is
data = RDATA | request (- SIG(0)s & TSIGs)
where "|" is concatenation and RDATA is the RDATA of the SIG(0) being
calculated less the signature itself.
Similarly, a SIG(0) can be used to secure a response and the request
that produced it. Such transaction signatures are calculated by
using a "data" of (1) the SIG's RDATA section omitting the signature
itself, (2) the entire DNS query message that produced this response,
including the query's DNS header and any SIG(0)s or TSIGs but not its
UDP/IP header, and (3) the entire DNS response message, including DNS
header but not the UDP/IP header or any SIG(0) or TSIGs and before
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the response RR counts have been adjusted for the inclusion of any
SIG(0) or TSIG.
That is
data = RDATA | full query | response (- SIG(0)s & TSIGs)
where "|" is concatenation and RDATA is the RDATA of the SIG(0) being
calculated less the signature itself.
Verification of a response SIG(0) (which is signed by the server host
key, not the zone key) by the requesting resolver shows that the
query and response were not tampered with in transit, that the
response corresponds to the intended query, and that the response
comes from the queried server.
In the case of a DNS message via TCP, a SIG(0) on the first data
packet is calculated with "data" as above and for each subsequent
packet, it is calculated as follows:
data = RDATA | DNS payload (- SIG(0)s & TSIGs) | previous packet
where RDATA is as above and previous packet is the previous DNS
payload including DNS header and any SIG(0)s and TSIGs but not the
TCP/IP header. Support of TCP SIG(0) for is OPTIONAL. As an
alternative, TSIG may be used after, if necessary, setting up a key
with TKEY [draft-ietf-dnsind-tkey-*.txt].
Except where needed to authenticate an update, TKEY, or similar
privileged request, servers are not required to check request SIGs.
3.2 Processing Responses and SIG(0) RRs
If a SIG RR is at the end of the additional information section of a
response and has a type covered of zero, it is a transaction
signature covering the response and the query that produced the
response. For all TKEY responses, it MUST be checked and the message
rejected if the checks fail. For all other responses, it MAY be
optionally checked and the message rejected if the checks fail.
If response SIG(0) checks succeed, such a transaction authentication
SIG does NOT directly authenticate any data-RRs in the message but
does authenticate TKEY and other meta-RRs. (Only a proper SIG RR
signed by the zone or a key tracing its authority to the zone or to
static resolver configuration can directly authenticate data-RRs,
depending on resolver policy.) If a resolver or server does not
implement transaction and/or request SIGs, it MUST ignore them
without error where they are optional and treat them as failing where
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they are required.
3.3 SIG(0) Lifetime and Expiration
The inception and expiration times in SIG(0)s are for the purpose of
resisting replay attacks. They should be set to form a time bracket
such that messages outside that bracket can be ignored. In IP
networks, this time bracket should not normally extend further than 5
minutes into the past and 5 minutes into the future.
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4. Security Considerations
No additional considerations beyond those in [RFC 2535]. The
inclusion of the SIG(0) inception and expiration time under the
signature improves resistance to replay attacks.
5. IANA Considerations
No new fields are created or field values assigned by the document.
References
[RFC 1982] - Robert Elz, Randy Bush, "Serial Number Arithmetic",
09/03/1996.
[RFC 2119] - S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", March 1997.
[RFC 2136] - P. Vixie, S. Thomson, Y. Rekhter, J. Bound, "Dynamic
Updates in the Domain Name System (DNS UPDATE)", 04/21/1997.
[RFC 2535] - D. Eastlake, "Domain Name System Security Extensions",
March 1999.
[draft-ietf-dnsind-tsig-*.txt] - P. Vixie, O. Gudmundsson, D.
Eastlake, B. Wellington, "Secret Key Transaction Signatures for DNS
(TSIG)".
[draft-ietf-dnsind-tkey-*.txt] - D. Eastlake, "Secret Key
Establishment for DNS (RR)"
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Author's Address
Donald E. Eastlake 3rd
IBM
65 Shindegan Hill Road
Carmel, NY 10512 USA
Telephone: +1-914-784-7913(w)
+1-914-276-2668(h)
fax: +1-914-784-3833(w)
email: dee3@us.ibm.com
Expiration and File Name
This draft expires April 2000.
Its file name is draft-ietf-dnsind-sig-zero-00.txt.
Appendix: SIG(0) Changes from RFC 2535
Add explanatory text concerning the differences between TSIG and
SIG(0).
Change the data over which SIG(0) is calculated to include the SIG(0)
RDATA other than the signature itself to secure the signature
inception and expiration times and resist replay attacks. Specify
SIG(0) for TCP.
Add discussion of appropriate inception and expiration times for
SIG(0).
Change wording to permit mixing TSIG and SIG(0) RRs.
Reword some areas for clarity.
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Status of This Document....................................1
Abstract...................................................1
Acknowledgments............................................2
Table of Contents..........................................2
1. Introduction............................................3
2. SIG(0) Design Rational..................................4
2.1 Differences Between TSIG and SIG(0)....................4
3. The SIG(0) Resource Record..............................6
3.1 Calculating Request and Transaction SIGs...............6
3.2 Processing Responses and SIG(0) RRs....................7
3.3 SIG(0) Lifetime and Expiration.........................8
4. Security Considerations.................................9
5. IANA Considerations.....................................9
References.................................................9
Author's Address..........................................10
Expiration and File Name..................................10
Appendix: SIG(0) Changes from RFC 2535....................10
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