The EDNS(0) Padding Option
RFC 7830

Internet Engineering Task Force (IETF)                      A. Mayrhofer
Request for Comments: 7830                                   nic.at GmbH
Category: Standards Track                                       May 2016
ISSN: 2070-1721

                       The EDNS(0) Padding Option

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

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7830.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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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.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   The Domain Name System (DNS) [RFC1035] was specified to transport DNS
   messages in cleartext 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 working group).  Encrypting the DNS
   transport is considered one of the options to improve the situation.

   However, even if both DNS query and response messages were encrypted,
   metadata 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 DNS clients and servers 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" and "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 to 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 the 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 Sections 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 and the "Padding" option
   was not included.

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   Responders MUST NOT pad DNS responses when the respective DNS query
   did not indicate EDNS(0) support.

5.  IANA Considerations

   IANA has assigned Option Code 12 for "Padding" in the "DNS EDNS0
   Option Codes (OPT)" registry.

   IANA has updated the respective registration record by changing the
   Reference field to RFC 7830 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 cleartext.  Further, it is possible
   that EDNS(0) padding may make DNS amplification attacks easier.
   Therefore, implementations 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 compression at the Transport Layer
   Security (TLS) level.

   The payload of the "Padding" option could (like many other fields in
   the DNS protocol) be used as a covert channel.

7.  References

7.1.  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>.

   [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>.

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7.2.  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>.

Acknowledgements

   This document was inspired by a discussion with Daniel Kahn Gillmor
   during IETF 93, 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.

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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