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.

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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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