Network Working Group A. Mayrhofer
Internet-Draft nic.at GmbH
Intended status: Standards Track August 14, 2015
Expires: February 15, 2016
The EDNS(0) Padding Option
draft-mayrhofer-edns0-padding-01
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.
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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-mayrhofer-edns0-padding-01 . . . . . . . . . . . . 4
8.2. draft-mayrhofer-edns0-padding-00 . . . . . . . . . . . . 4
9. Normative References . . . . . . . . . . . . . . . . . . . . 4
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
The Domain Name System (DNS) [RFC1035] was specified to transport DNS
packets 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 packets were encrypted,
meta data of these packets could be used to correlate such packets
with well known unencrypted packets, 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, significantly hampering
size-based correlation of the encrypted packet.
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
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
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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.
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 [[TODO-IANA]].
The OPTION-LENGTH for the 'Padding' option is the size (in octects)
of the PADDING. The minimum number of padding octects is 0.
The PADDING octects MUST be set to 0x00. If a Responder detects non-
0x00 octects in the padding of a query, a FORMERR (RCODE=1) MUST be
returned.
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 respective
UDP payload size (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.
Responders MAY pad DNS responses when the respective DNS query
indicated EDNS(0) support of the Requestor.
Responders MUST NOT pad DNS responses when the respective DNS query
did not indicate EDNS(0).
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5. IANA Considerations
IANA is requested to assign an EDNS Option Code (as described in
Section 9 of [RFC6891]) for the 'Padding' option specified in this
document.
6. Security Considerations
Padding DNS packets obviously increases their size, and will
therefore lead to increased traffic, can lead to increased number of
truncated packets when used over UDP-based transport.
The use of the EDNS(0) Padding provides only a benefit when DNS
packets are not transported in clear text. Implementations therefore
SHOULD avoid using this option if the DNS transport is not encrypted.
The payload of the 'Padding' option could be used as a covert
channel. In order to prevent this, padding octets are required to be
set to 0x00. It shall be noted that variations in the OPTION-SIZE
itself could still be abused for expensive and low-bandwith covert
communication.
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.
8. Changes
8.1. draft-mayrhofer-edns0-padding-01
Changed minimum padding size to 0, rewrote Usage Considerations
section, extended Security considerations section
8.2. draft-mayrhofer-edns0-padding-00
Initial version
9. 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>.
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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