Internet DRAFT - draft-ietf-dprive-edns0-padding
draft-ietf-dprive-edns0-padding
Network Working Group A. Mayrhofer
Internet-Draft nic.at GmbH
Intended status: Standards Track March 6, 2016
Expires: September 7, 2016
The EDNS(0) Padding Option
draft-ietf-dprive-edns0-padding-03
Abstract
This document specifies the EDNS(0) 'Padding' option, which allows
DNS clients and servers to pad request and response messages by a
variable number of octets.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. The 'Padding' Option . . . . . . . . . . . . . . . . . . . . 3
4. Usage Considerations . . . . . . . . . . . . . . . . . . . . 3
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
6. Security Considerations . . . . . . . . . . . . . . . . . . . 4
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 4
8. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
8.1. draft-ietf-dprive-edns0-padding-03 . . . . . . . . . . . 5
8.2. draft-ietf-dprive-edns0-padding-02 . . . . . . . . . . . 5
8.3. draft-ietf-dprive-edns0-padding-01 . . . . . . . . . . . 5
8.4. draft-ieft-dprive-edns0-padding-00 . . . . . . . . . . . 5
8.5. draft-mayrhofer-edns0-padding-01 . . . . . . . . . . . . 5
8.6. draft-mayrhofer-edns0-padding-00 . . . . . . . . . . . . 5
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
9.1. Normative References . . . . . . . . . . . . . . . . . . 5
9.2. Informative References . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
The Domain Name System (DNS) [RFC1035] was specified to transport DNS
messages in clear text form. Since this can expose significant
amounts of information about the internet activities of an end user,
the IETF has undertaken work to provide confidentiality to DNS
transactions (see the DPRIVE WG). Encrypting the DNS transport is
considered as one of the options to improve the situation.
However, even if both DNS query and response messages were encrypted,
meta data could still be used to correlate such messages with well
known unencrypted messages, hence jeopardizing some of the
confidentiality gained by encryption. One such property is the
message size.
This document specifies the Extensions Mechanisms for DNS (EDNS(0))
"Padding" Option, which allows to artificially increase the size of a
DNS message by a variable number of bytes, hampering size-based
correlation of the encrypted message.
2. Terminology
The terms "Requestor", "Responder" are to be interpreted as specified
in [RFC6891].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
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"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
3. The 'Padding' Option
The EDNS(0) [RFC6891] specifies a mechanism to include new options in
DNS packets, contained in the RDATA of the OPT meta-RR. This
document specifies the 'Padding' option in order to allow clients and
servers pad DNS packets by a variable number of bytes. The 'Padding'
option MUST occur at most once per OPT meta-RR (and hence, at most
once per message).
The figure below specifies the structure of the option in the RDATA
of the OPT RR:
0 8 16
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| OPTION-CODE |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| OPTION-LENGTH |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| (PADDING) ... (PADDING) ... /
+- - - - - - - - - - - - - - - -
Figure 1
The OPTION-CODE for the 'Padding' option is 12.
The OPTION-LENGTH for the 'Padding' option is the size (in octets) of
the PADDING. The minimum number of padding octets is 0.
The PADDING octets SHOULD be set to 0x00. Other values MAY be used;
for example, in cases where there is a concern that the padded
message could be subject to compression before encryption. PADDING
octets of any value MUST be accepted in messages received.
4. Usage Considerations
This document does not specify the actual amount of padding to be
used, since this depends on the situation in which the option is
used. However, padded DNS messages MUST NOT exceed the number of
octets specified in the Requestor's Payload Size field encoded in the
RR Class Field (see Section 6.2.3 and 6.2.4 of [RFC6891]).
Responders MUST pad DNS responses when the respective DNS query
included the 'Padding' option, unless doing so would violate the
maximum UDP payload size.
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Responders MAY pad DNS responses when the respective DNS query
indicated EDNS(0) support of the Requestor and the 'Padding' option
was not included.
Responders MUST NOT pad DNS responses when the respective DNS query
did not indicate EDNS(0) support.
5. IANA Considerations
IANA has assigned EDNS Option Code 12 for Padding.
IANA is requested to update the respective registration record by
changing the Reference field to [[THISRFC]] and the Status field to
'Standard'.
6. Security Considerations
Padding DNS packets obviously increases their size, and will
therefore lead to increased traffic.
The use of the EDNS(0) Padding only provides a benefit when DNS
packets are not transported in clear text. Further, it is possible
EDNS(0) Padding may make DNS amplification attacks easier.
Implementations therefore MUST NOT use this option if the DNS
transport is not encrypted.
Padding length might be affected by lower-level compression.
Therefore (as described in Section 3.3 of [RFC7525]), implementations
and deployments SHOULD disable TLS-level compression.
The payload of the 'Padding' option could (like many other fields in
the DNS protocol) be used as a covert channel.
7. Acknowledgements
This document was inspired by a discussion with Daniel Kahn Gillmor
during IETF93, as an alternative to the proposed padding on the TLS
layer. Allison Mankin, Andreas Gustafsson, Christian Huitema, Jinmei
Tatuya and Shane Kerr suggested text for this document.
8. Changes
Note to RFC Editors: Please remove this whole section before
publication
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8.1. draft-ietf-dprive-edns0-padding-03
Fixed typo in Acknowledgements, added Shane. Do not use over
unencrypted transport is now a MUST. Logic around when responders
may send the option clarified. Reduced "hampering" claim in
introduction.
8.2. draft-ietf-dprive-edns0-padding-02
Clarified that changes section is to be removed before publication.
Clarified that both Requestors and Responders are to ignore padding
contents. changed text about non-zero padding contents based on WGLC
comments. removed security considerations about truncation based on
WGLC comment. added more acknowledgements. replaced "packets" with
"messages" where appropriate.
8.3. draft-ietf-dprive-edns0-padding-01
Fixed 'octects' typo. Changed 'covert channel' text to align with
allowing non-0x00 padding. changed IANA considerations - assigned
option code is 12. Changed field definitions to allow for non-0x00
padding, removed FORMERR requirement. referenced rfc7525 in security
considerations. added acknowledgements.
8.4. draft-ieft-dprive-edns0-padding-00
Adopted by WG. Changed text about message size limit based on
feedback.
8.5. draft-mayrhofer-edns0-padding-01
Changed minimum padding size to 0, rewrote Usage Considerations
section, extended Security considerations section
8.6. draft-mayrhofer-edns0-padding-00
Initial version
9. References
9.1. Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <http://www.rfc-editor.org/info/rfc1035>.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891,
DOI 10.17487/RFC6891, April 2013,
<http://www.rfc-editor.org/info/rfc6891>.
9.2. Informative References
[RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
2015, <http://www.rfc-editor.org/info/rfc7525>.
Author's Address
Alexander Mayrhofer
nic.at GmbH
Karlsplatz 1/2/9
Vienna 1010
Austria
Email: alex.mayrhofer.ietf@gmail.com
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