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This is to be used together with encrypted DNS 15 transports in order to impede message-size based correlation attacks 16 on the confidentiality of messages. 18 Status of This Memo 20 This Internet-Draft is submitted in full conformance with the 21 provisions of BCP 78 and BCP 79. 23 Internet-Drafts are working documents of the Internet Engineering 24 Task Force (IETF). Note that other groups may also distribute 25 working documents as Internet-Drafts. The list of current Internet- 26 Drafts is at http://datatracker.ietf.org/drafts/current/. 28 Internet-Drafts are draft documents valid for a maximum of six months 29 and may be updated, replaced, or obsoleted by other documents at any 30 time. It is inappropriate to use Internet-Drafts as reference 31 material or to cite them other than as "work in progress." 33 This Internet-Draft will expire on January 24, 2016. 35 Copyright Notice 37 Copyright (c) 2015 IETF Trust and the persons identified as the 38 document authors. All rights reserved. 40 This document is subject to BCP 78 and the IETF Trust's Legal 41 Provisions Relating to IETF Documents 42 (http://trustee.ietf.org/license-info) in effect on the date of 43 publication of this document. Please review these documents 44 carefully, as they describe your rights and restrictions with respect 45 to this document. Code Components extracted from this document must 46 include Simplified BSD License text as described in Section 4.e of 47 the Trust Legal Provisions and are provided without warranty as 48 described in the Simplified BSD License. 50 Table of Contents 52 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 53 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2 54 3. The 'Padding' Option . . . . . . . . . . . . . . . . . . . . 2 55 4. Client Considerations . . . . . . . . . . . . . . . . . . . . 3 56 5. Server Considerations . . . . . . . . . . . . . . . . . . . . 3 57 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 58 7. Security Considerations . . . . . . . . . . . . . . . . . . . 4 59 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 4 60 9. Normative References . . . . . . . . . . . . . . . . . . . . 4 61 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 4 63 1. Introduction 65 The Domain Name System (DNS) [RFC1035] was specified to transport DNS 66 packets in clear text form. Since this can expose significant 67 amounts of information about the internet activities of an end user, 68 the IETF has undertaken work to provide confidentiality to DNS 69 transactions (see the DPRIVE WG). Encrypting the DNS transport is 70 considered as one of the options to improve the current situation. 72 However, even if both DNS query and response packets were encrypted, 73 meta data of these packets could be used to correlate such packets 74 with well known unencrypted packets, and hence jeopardizing some of 75 the confidentiality gained by encryption. One such property is the 76 message size. 78 Size-based correlation of encrypted packets can be avoided by padding 79 application messages with additional data. This document specifies 80 the Extensions Mechanisms for DNS (EDNS(0)) "Padding" Option, which 81 allows to artificially increase the size of a DNS packet by a 82 variable number of bytes, in order to prevent size-based correlation 83 once the packet is encrypted. 85 2. Terminology 87 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 88 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 89 "OPTIONAL" in this document are to be interpreted as described in 90 [RFC2119]. 92 3. The 'Padding' Option 94 The EDNS0 specification [RFC6891] specifies a way to include new 95 options for DNS packets, contained in the RDATA of the OPT meta-RR. 96 This document specifies one such new option in order to allow clients 97 and servers pad DNS packets by a variable number of bytes. The 98 'Padding' option MUST occur at most once per OPT meta-RR. 100 The figure below specifies the structure of the option in the RDATA 101 of the OPT RR: 103 0 8 16 104 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 105 | OPTION-CODE | 106 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 107 | OPTION-LENGTH | 108 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 109 | PADDING | (PADDING) ... / 110 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 112 Figure 1 114 The OPTION-CODE for the 'Padding' option is [[TODO-IANA]]. 116 The OPTION-LENGTH for the 'Padding' option is the size (in octects) 117 of the PADDING. The minimum number of padding octects is 1. 119 The PADDING octects SHOULD be set to 0x00 (TODO: Discuss - together 120 with compression in the encrypted transport, this could weaken the 121 padding). 123 4. Client Considerations 125 A client SHOULD use the 'Padding' option in a DNS query (QR=0) only 126 when transport of the DNS packets is encrypted. Note that there 127 might be situations (such as bump-in-the-wire encryption) where a 128 client is unable to identify whether or not encryption is being 129 performed. 131 This document is silent on the length of the padding a client should 132 use, since this is believed to be subject of the specification of an 133 actual encrypted DNS transport (and might depend on its properties). 135 5. Server Considerations 137 A server MUST use the 'Padding' option in a DNS response (QR=1) only 138 when that response correlates to a query that contained the 'Padding' 139 option. 141 This document is silent on the length of the padding a server should 142 use, since this is believed to be subject of the specification of an 143 actual encrypted DNS transport. 145 6. IANA Considerations 147 IANA is requested to assign an EDNS Option Code (as described in 148 Section 9 of [RFC6891]) for the 'Padding' option specified in this 149 document. 151 7. Security Considerations 153 Padding DNS packets obviously increases their size, and will 154 therefore lead to increased traffic, and can lead to increased number 155 of truncated packets when used over UDP-based transport, or trigger 156 similar operational issues. 158 The use of the EDNS(0) Padding provides only a benefit when DNS 159 packets are not transported in clear text. Implementations therefore 160 SHOULD avoid using this option if the DNS transport is not encrypted. 162 8. Acknowledgements 164 This document was inspired by a discussion with Daniel Kahn Gillmor 165 during IETF93, as an alternative to the proposed padding on the TLS 166 layer. 168 9. Normative References 170 [RFC1035] Mockapetris, P., "Domain names - implementation and 171 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 172 November 1987, . 174 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 175 Requirement Levels", BCP 14, RFC 2119, 176 DOI 10.17487/RFC2119, March 1997, 177 . 179 [RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms 180 for DNS (EDNS(0))", STD 75, RFC 6891, 181 DOI 10.17487/RFC6891, April 2013, 182 . 184 Author's Address 186 Alexander Mayrhofer 187 nic.at GmbH 188 Karlsplatz 1/2/9 189 Vienna 1010 190 Austria 192 Email: alexander.mayrhofer@nic.at