rfc2931.txt
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RFC 2931 DNS SIG(0) September 2000
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
Eastlake Standards Track [Page 6]
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.
Eastlake Standards Track [Page 7]
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
Eastlake Standards Track [Page 8]
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.
Eastlake Standards Track [Page 9]
RFC 2931 DNS SIG(0) September 2000
Full Copyright Statement
Copyright (C) The Internet Society (2000). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Eastlake Standards Track [Page 10]
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