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IETF RFC 7830



Last modified on Wednesday, May 11th, 2016

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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/RFC 7830.

 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
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   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.









Mayrhofer                    Standards Track                 PAGE 1 top


RFC 7830 EDNS(0) Padding May 2016 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) [RFC 1035] 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 [RFC 6891]. 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 [RFC 2119]. Mayrhofer Standards Track PAGE 2 top

RFC 7830 EDNS(0) Padding May 2016 3. The "Padding" Option The EDNS(0) [RFC 6891] 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 [RFC 6891]). 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. Mayrhofer Standards Track PAGE 3 top

RFC 7830 EDNS(0) Padding May 2016 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 [RFC 7525]), 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 [RFC 1035] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, DOI 10.17487/RFC 1035, November 1987, <http://www.rfc-editor.org/info/RFC 1035>. [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC 2119, March 1997, <http://www.rfc-editor.org/info/RFC 2119>. [RFC 6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms for DNS (EDNS(0))", STD 75, RFC 6891, DOI 10.17487/RFC 6891, April 2013, <http://www.rfc-editor.org/info/RFC 6891>. Mayrhofer Standards Track PAGE 4 top

RFC 7830 EDNS(0) Padding May 2016 7.2. Informative References [RFC 7525] 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/RFC 7525, May 2015, <http://www.rfc-editor.org/info/RFC 7525>. 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 Mayrhofer Standards Track PAGE 5 top

RFC TOTAL SIZE: 9520 bytes PUBLICATION DATE: Wednesday, May 11th, 2016 LEGAL RIGHTS: The IETF Trust (see BCP 78)


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