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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 INTERNET-DRAFT David Conrad 2 draft-ietf-dnsext-dnssec-okbit-00.txt Nominum Inc. 3 August, 2000 5 Indicating Resolver Support of DNSSEC 7 Status of this Memo 9 This document is an Internet-Draft and is in full conformance with 10 all provisions of Section 10 of RFC2026. 12 Internet-Drafts are working documents of the Internet Engineering 13 Task Force (IETF), its areas, and its working groups. Note that 14 other groups may also distribute working documents as Internet- 15 Drafts. 17 Internet-Drafts are draft documents valid for a maximum of six months 18 and may be updated, replaced, or obsoleted by other documents at any 19 time. It is inappropriate to use Internet-Drafts as reference 20 material or to cite them other than as "work in progress." 22 The list of current Internet-Drafts can be accessed at 23 http://www.ietf.org/ietf/1id-abstracts.txt 25 The list of Internet-Draft Shadow Directories can be accessed at 26 http://www.ietf.org/shadow.html. 28 Abstract 30 In order to deploy DNSSEC operationally, DNSSEC aware servers should 31 only respond with DNSSEC RRs when there is an explicit indication 32 that the resolver can understand those RRs. This document proposes 33 the use of a bit in the EDNS0 header to provide that explicit 34 indication and the necessary protocol changes to implement that 35 notification. 37 1. Introduction 39 DNSSEC [RFC2535] has been specified to provide data integrity and 40 authentication to security aware resolvers and applications through 41 the use of cryptographic digital signatures. However, as DNSSEC is 42 deployed, non-DNSSEC-aware clients will likely query DNSSEC-aware 43 servers. In such situations, the DNSSEC-aware server (responding to 44 a request for data in a signed zone) will respond with SIG, KEY, 45 and/or NXT records. For reasons described in the subsequent section, 46 such responses can have significant negative operational impacts for 47 the DNS infrastructure. 49 This document discusses a method to avoid these negative impacts, 50 namely DNSSEC-aware servers should only respond with SIG, KEY, and/or 51 NXT RRs when there is an explicit indication from the resolver that 52 it can understand those RRs. 54 For the purposes of this document, "DNSSEC security RRs" are 55 considered RRs of type SIG, KEY, or NXT. 57 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 58 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 59 document are to be interpreted as described in [RFC2119]. 61 2. Rationale 63 As DNSSEC is deployed, the vast majority of queries will be from 64 resolvers that are not DNSSEC aware and thus do not understand or 65 support the DNSSEC security RRs. When a query from such a resolver 66 is received for a DNSSEC signed zone, the DNSSEC specification 67 indicates the nameserver must respond with the appropriate DNSSEC 68 security RRs. As DNS UDP datagrams are limited to 512 bytes 69 [RFC1035], responses including DNSSEC security RRs have a high 70 probability of resulting in a truncated response being returned and 71 the resolver retrying the query using TCP. 73 TCP DNS queries result in significant overhead due to connection 74 setup and teardown. Operationally, the impact of these TCP queries 75 will likely be quite detrimental in terms of increased network 76 traffic (typically five packets for a single query/response instead 77 of two), increased latency resulting from the additional round trip 78 times, increased incidences of queries failing due to timeouts, and 79 significantly increased load on nameservers. 81 In addition, in preliminary and experimental deployment of DNSSEC, 82 there have been reports of non-DNSSEC aware resolvers being unable to 83 handle responses which contain DNSSEC security RRs, resulting in the 84 resolver failing (in the worst case) or entire responses being 85 ignored (in the better case). 87 Given these operational implications, explicitly notifying the 88 nameserver that the client is prepared to receive (if not understand) 89 DNSSEC security RRs would be prudent. 91 Client-side support of DNSSEC is assumed to be binary -- either the 92 client is willing to receive all DNSSEC security RRs or it is not 93 willing to accept any. As such, a single bit is sufficient to 94 indicate client-side DNSSEC support. As effective use of DNSSEC 95 implies the need of EDNS0 [RFC2671], bits in the "classic" (non-EDNS 96 enhanced DNS header) are scarce, and there may be situations in which 97 non-compliant caching or forwarding servers inappropriately copy data 98 from classic headers as queries are passed on to authoritative 99 servers, the use of a bit from the EDNS0 header is proposed. 101 An alternative approach would be to use the existance of an EDNS0 102 header as an implicit indication of client-side support of DNSSEC. 103 This approach was not chosen as there may be applications in which 104 EDNS0 is supported but in which the use of DNSSEC is inappropriate. 106 3. Protocol Changes 108 The mechanism chosen for the explicit notification of the ability of 109 the client to accept (if not understand) DNSSEC security RRs is using 110 the most significant bit of the Z field on the EDNS0 OPT header in 111 the query. This bit is referred to as the "DNSSEC OK" (DO) bit. In 112 the context of the EDNS0 OPT meta-RR, the DO bit is the first bit of 113 the the third and fourth bytes of the "extended RCODE and flags" 114 portion of the EDNS0 OPT meta-RR, structured as follows: 116 +0 (MSB) +1 (LSB) 117 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 118 0: | EXTENDED-RCODE | VERSION | 119 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 120 2: |DO| Z | 121 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 123 Setting the DO bit to one in a query indicates to the server that the 124 resolver is able to accept DNSSEC security RRs. The DO bit cleared 125 (set to zero) indicates the resolver is unprepared to handle DNSSEC 126 security RRs and those RRs MUST NOT be returned in the response 127 (unless DNSSEC security RRs are explicitly queried for). 129 More explicitly, in order to explicitly indicate DNSSEC security RRs 130 are acceptible to the resolver, DNSSEC-aware nameservers (both BASIC 131 and FULL according to [RFC2535] definitions) MUST NOT add DNSSEC 132 security RRs to any section of a response unless at least one of the 133 following is true: 135 1) The DO bit of the query EDNS0 header was set on the request, 136 indicating that the client would like DNSSEC security RRs. 138 2) The query type is SIG, KEY, or NXT and the RRs added match the 139 query name and query type. 141 In case 1), response generation is as indicated in [RFC2535]. 143 In case 2), only those RRs which match the query name and query type 144 are added. 146 Recursive DNSSEC-aware server MUST set the DO bit on recursive 147 requests, regardless of the status of the DO bit on the initiating 148 resolver request. If the initiating resolver request does not have 149 the DO bit set, the recursive DNSSEC-aware server MUST remove DNSSEC 150 security RRs before returning the data to the client, however cached 151 data MUST NOT be modified. 153 In the event a server returns a NOTIMPL, FORMERR or SERVFAIL response 154 to a query that has the DO bit set, the resolver SHOULD NOT expect 155 DNSSEC security RRs and SHOULD retry the query without the EDNS0 in 156 accordance with section 5.3 of [RFC2671]. 158 Security Considerations 160 The absence of DNSSEC data in response to a query with the DO bit set 161 MUST NOT be taken to mean no security information is available for 162 that zone as the response may be forged or a non-forged response of 163 an altered (DO bit cleared) query. 165 IANA Considerations 167 Allocation of the most significant bit of the Z field in the EDNS0 168 OPT meta-RR is required. 170 Acknowledgements 172 This document is based on a rough draft by Bob Halley with input from 173 Olafur Gudmundsson, Andreas Gustafsson, Brian Wellington, Randy Bush, 174 Rob Austein, and Steve Bellovin. 176 References 178 [RFC1034] Mockapetris, P., "Domain Names - Concepts and Facilities", 179 RFC 1034, November 1987. 181 [RFC1035] Mockapetris, P., "Domain Names - Implementation and 182 Specifications", RFC 1035, November 1987. 184 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 185 Requirement Levels", BCP 14, RFC 2119, March 1997. 187 [RFC2535] Eastlake, D., "Domain Name System Security Extensions", RFC 188 2535, March 1999. 190 [RFC2671] Vixie, P., Extension Mechanisms for DNS (EDNS0)", RFC 2671, 191 August 1999 193 Author's Address 194 David Conrad 195 Nominum Inc. 196 950 Charter Street 197 Redwood City, CA 94063 198 USA 200 Phone: +1 650 779 6003 202 Email: david.conrad@nominum.com 204 Full Copyright Statement 206 Copyright (C) The Internet Society (2000). All Rights Reserved. 208 This document and translations of it may be copied and furnished to 209 others, and derivative works that comment on or otherwise explain it 210 or assist in its implmentation may be prepared, copied, published and 211 distributed, in whole or in part, without restriction of any kind, 212 provided that the above copyright notice and this paragraph are 213 included on all such copies and derivative works. However, this 214 document itself may not be modified in any way, such as by removing 215 the copyright notice or references to the Internet Society or other 216 Internet organizations, except as needed for the purpose of 217 developing Internet standards in which case the procedures for 218 copyrights defined in the Internet Standards process must be 219 followed, or as required to translate it into languages other than 220 English. 222 The limited permissions granted above are perpetual and will not be 223 revoked by the Internet Society or its successors or assigns. 225 This document and the information contained herein is provided on an 226 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING 227 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING 228 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION 229 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF 230 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."