Network Working Group D. Eastlake 3rd
Request for Comments: 2931 Motorola
Updates: 2535 September 2000
Category: Standards Track
DNS Request and Transaction Signatures ( SIG(0)s )
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
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.
Acknowledgments
The contributions and suggestions of the following persons (in
alphabetic order) to this memo are gratefully acknowledged:
Olafur Gudmundsson
Ed Lewis
Erik Nordmark
Brian Wellington
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RFC 2931 DNS SIG(0) September 2000
Table of Contents
1. Introduction................................................. 2
2. SIG(0) Design Rationale...................................... 3
2.1 Transaction Authentication.................................. 3
2.2 Request Authentication...................................... 3
2.3 Keying...................................................... 3
2.4 Differences Between TSIG and SIG(0)......................... 4
3. The SIG(0) Resource Record................................... 4
3.1 Calculating Request and Transaction SIGs.................... 5
3.2 Processing Responses and SIG(0) RRs......................... 6
3.3 SIG(0) Lifetime and Expiration.............................. 7
4. Security Considerations...................................... 7
5. IANA Considerations.......................................... 7
References...................................................... 7
Author's Address................................................ 8
Appendix: SIG(0) Changes from RFC 2535.......................... 9
Full Copyright Statement........................................ 10
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 (RRs) that are used to digitally 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 [RFC 2845] and the [RFC 2535] specification for
SIG(0).
Sections of [RFC 2535] updated are all of 4.1.8.1 and parts of 4.2
and 4.3. 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 Rationale
SIG(0) provides protection for DNS transactions and requests that is
not provided by the regular SIG, KEY, and NXT RRs specified in [RFC
2535]. The authenticated data origin services of secure DNS either
provide protected data resource records (RRs) or authenticatably deny
their nonexistence. These services provide no protection for glue
records, DNS requests, no protection for message headers on requests
or responses, and no protection of the overall integrity of a
response.
2.1 Transaction Authentication
Transaction authentication means that a requester can be sure it is
at least getting the messages from the server it queried and that the
received messages are in response to the query it sent. This is
accomplished by optionally adding either a TSIG RR [RFC 2845] or, as
described herein, a SIG(0) resource record at the end of the response
which digitally signs the concatenation of the server's response and
the corresponding resolver query.
2.2 Request Authentication
Requests can also be authenticated by including a TSIG or, as
described herein, a special SIG(0) RR at the end of the request.
Authenticating requests serves no function in DNS servers that
predate the specification of dynamic update. Requests with a non-
empty additional information section produce error returns or may
even be ignored by a few such older DNS servers. However, this syntax
for signing requests is defined for authenticating dynamic update
requests [RFC 2136], TKEY requests [RFC 2930], or future requests
requiring authentication.
2.3 Keying
The private keys used in transaction security belong to the host
composing the DNS response message, not to the zone involved.
Request authentication may also involve the private key of the host
or other entity composing the request or of a zone to be affected by
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 as KEY
RRs with a protocol byte of 3 (DNSSEC) or 255 (ANY).
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.
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RFC 2931 DNS SIG(0) September 2000
2.4 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
understands TSIG and very likely has the same key installed.
Furthermore, TSIG uses keyed hash authentication codes which are
relatively inexpensive to compute. Thus it is common to authenticate
requests with TSIG and responses are authenticated with TSIG if the
corresponding request is authenticated.
SIG(0) on the other hand, uses public key authentication, where the
public keys are stored in DNS as KEY RRs and a private key is stored
at the signer. 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 SHOULD only be used on requests when
necessary to authenticate 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. For other replies, whether they are
authenticated by the server or required to be authenticated by the
requester SHOULD be a local configuration option.
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 transaction SIG(0)s, the signer field MUST be a name of the
originating host and there MUST be a KEY RR at that name with the
public key corresponding to the private key used to calculate the
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RFC 2931 DNS SIG(0) September 2000
signature. (The host domain name used may be the inverse IP address
mapping name for an IP address of the host if the relevant KEY is
stored there.)
For all SIG(0) RRs, the owner name, class, TTL, and original TTL, are
meaningless. The TTL fields SHOULD be zero and the CLASS field
SHOULD be ANY. 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 of any
additional information for inclusion in the response. If the SIG(0)
RR cannot be added without causing the message to be truncated, the
server MUST alter the response so that a SIG(0) can be included.
This response consists of only the question and a SIG(0) record, and
has the TC bit set and RCODE 0 (NOERROR). The client should at this
point retry the request using TCP.
3.1 Calculating Request and Transaction SIGs
A DNS request may be optionally signed by including one SIG(0)s at
the end of the query additional information section. Such a SIG is
identified by having a "type covered" field of zero. It signs the
preceding DNS request message including DNS header but not including
the UDP/IP header and before the request RR counts have been adjusted
for the inclusions of the request SIG(0).
It is calculated by using a "data" (see [RFC 2535], Section 4.1.8) of
(1) the SIG's RDATA section entirely omitting (not just zeroing) the
signature subfield itself, (2) the DNS query messages, including DNS
header, but not the UDP/IP header and before the reply RR counts have
been adjusted for the inclusion of the SIG(0). That is
data = RDATA | request - SIG(0)
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 but not its UDP/IP header, and (3)
the entire DNS response message, including DNS header but not the
UDP/IP header and before the response RR counts have been adjusted
for the inclusion of the SIG(0).
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That is
data = RDATA | full query | response - SIG(0)
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) | previous packet
where "|" is concatenations, RDATA is as above, and previous packet
is the previous DNS payload including DNS header and the SIG(0) but
not the TCP/IP header. Support of SIG(0) for TCP is OPTIONAL. As an
alternative, TSIG may be used after, if necessary, setting up a key
with TKEY [RFC 2930].
Except where needed to authenticate an update, TKEY, or similar
privileged request, servers are not required to check a request
SIG(0).
Note: requests and responses can either have a single TSIG or one
SIG(0) but not both a TSIG and a SIG(0).
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 TKEY responses, it MUST be checked and the message
rejected if the checks fail unless otherwise specified for the TKEY
mode in use. For all other responses, it MAY be checked and the
message rejected if the checks fail.
If a response's SIG(0) check succeed, such a transaction
authentication SIG does NOT directly authenticate the validity any
data-RRs in the message. However, it authenticates that they were
sent by the queried server and have not been diddled. (Only a proper
SIG(0) RR signed by the zone or a key tracing its authority to the
zone or to static resolver configuration can directly authenticate
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RFC 2931 DNS SIG(0) September 2000
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
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.
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 parameters are created or parameter values assigned by this
document.
References
[RFC 1982] Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
September 1996.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 2136] Vixie, P., Thomson, S., Rekhter, Y. and J. Bound, "Dynamic
Updates in the Domain Name System (DNS UPDATE)", RFC 2136,
April 1997.
[RFC 2535] Eastlake, D., "Domain Name System Security Extensions",
RFC 2535, March 1999.
[RFC 2845] Vixie, P., Gudmundsson, O., Eastlake, D. and B.
Wellington, "Secret Key Transaction Signatures for DNS
(TSIG)", RFC 2845, May 2000.
[RFC 2930] Eastlake, D., "Secret Key Establishment for DNS (RR)", RFC
2930, September 2000.
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RFC 2931 DNS SIG(0) September 2000
Author's Address
Donald E. Eastlake 3rd
Motorola
140 Forest Avenue
Hudson, MA 01749 USA
Phone: +1-978-562-2827(h)
+1-508-261-5434(w)
Fax: +1 978-567-7941(h)
+1-508-261-4447(w)
EMail: Donald.Eastlake@motorola.com
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RFC 2931 DNS SIG(0) September 2000
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 so as 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).
Add wording to indicate that either a TSIG or one or more SIG(0)s may
be present but not both.
Reword some areas for clarity.
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RFC 2931 DNS SIG(0) September 2000
Full Copyright Statement
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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