Network Working Group A. Mayrhofer
Internet-Draft nic.at GmbH
Intended status: Standards Track July 23, 2015
Expires: January 24, 2016
The EDNS(0) Padding Option
draft-mayrhofer-edns0-padding-00
Abstract
This document specifies the EDNS0 'Padding' option, allowing DNS
clients and servers to pad request and response packets by a variable
number of bytes. This is to be used together with encrypted DNS
transports in order to impede message-size based correlation attacks
on the confidentiality of messages.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. The 'Padding' Option . . . . . . . . . . . . . . . . . . . . 2
4. Client Considerations . . . . . . . . . . . . . . . . . . . . 3
5. Server Considerations . . . . . . . . . . . . . . . . . . . . 3
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
7. Security Considerations . . . . . . . . . . . . . . . . . . . 4
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 4
9. Normative References . . . . . . . . . . . . . . . . . . . . 4
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 4
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 current 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, and hence jeopardizing some of
the confidentiality gained by encryption. One such property is the
message size.
Size-based correlation of encrypted packets can be avoided by padding
application messages with additional data. This document specifies
the Extensions Mechanisms for DNS (EDNS(0)) "Padding" Option, which
allows to artificially increase the size of a DNS packet by a
variable number of bytes, in order to prevent size-based correlation
once the packet is encrypted.
2. Terminology
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].
3. The 'Padding' Option
The EDNS0 specification [RFC6891] specifies a way to include new
options for DNS packets, contained in the RDATA of the OPT meta-RR.
This document specifies one such new option in order to allow clients
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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 1.
The PADDING octects SHOULD be set to 0x00 (TODO: Discuss - together
with compression in the encrypted transport, this could weaken the
padding).
4. Client Considerations
A client SHOULD use the 'Padding' option in a DNS query (QR=0) only
when transport of the DNS packets is encrypted. Note that there
might be situations (such as bump-in-the-wire encryption) where a
client is unable to identify whether or not encryption is being
performed.
This document is silent on the length of the padding a client should
use, since this is believed to be subject of the specification of an
actual encrypted DNS transport (and might depend on its properties).
5. Server Considerations
A server MUST use the 'Padding' option in a DNS response (QR=1) only
when that response correlates to a query that contained the 'Padding'
option.
This document is silent on the length of the padding a server should
use, since this is believed to be subject of the specification of an
actual encrypted DNS transport.
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6. 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.
7. Security Considerations
Padding DNS packets obviously increases their size, and will
therefore lead to increased traffic, and can lead to increased number
of truncated packets when used over UDP-based transport, or trigger
similar operational issues.
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.
8. 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.
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>.
[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: alexander.mayrhofer@nic.at
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