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Checking references for intended status: Informational ---------------------------------------------------------------------------- == Unused Reference: 'PRA' is defined on line 213, but no explicit reference was found in the text -- Obsolete informational reference (is this intentional?): RFC 4408 (ref. 'SPF') (Obsoleted by RFC 7208) Summary: 0 errors (**), 0 flaws (~~), 2 warnings (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 SPFBIS Working Group M. Kucherawy 3 Internet-Draft Cloudmark 4 Intended status: Informational February 21, 2012 5 Expires: August 24, 2012 7 The SPF/Sender-ID Experiment 8 draft-kucherawy-spfbis-experiment-02 10 Abstract 12 In 2006 the IETF published a suite of protocol documents comprising 13 SPF and Sender-ID, two proposed email authentication protocols. 14 Because of interoperability concerns created by simultaneous use of 15 the two protocols by a receiver, and some concerns with Sender-ID and 16 compatibility with existing standards, the IESG required them to have 17 Experimental status and invited the community to observe their 18 deployments for a period of time, hoping convergence would be 19 possible later. 21 After six years, sufficient experience and evidence have been 22 collected that the experiment thus created can be considered 23 concluded, and a common path forward can be selected. This memo 24 presents those findings. 26 Status of this Memo 28 This Internet-Draft is submitted in full conformance with the 29 provisions of BCP 78 and BCP 79. 31 Internet-Drafts are working documents of the Internet Engineering 32 Task Force (IETF). Note that other groups may also distribute 33 working documents as Internet-Drafts. The list of current Internet- 34 Drafts is at http://datatracker.ietf.org/drafts/current/. 36 Internet-Drafts are draft documents valid for a maximum of six months 37 and may be updated, replaced, or obsoleted by other documents at any 38 time. It is inappropriate to use Internet-Drafts as reference 39 material or to cite them other than as "work in progress." 41 This Internet-Draft will expire on August 24, 2012. 43 Copyright Notice 45 Copyright (c) 2012 IETF Trust and the persons identified as the 46 document authors. All rights reserved. 48 This document is subject to BCP 78 and the IETF Trust's Legal 49 Provisions Relating to IETF Documents 50 (http://trustee.ietf.org/license-info) in effect on the date of 51 publication of this document. Please review these documents 52 carefully, as they describe your rights and restrictions with respect 53 to this document. Code Components extracted from this document must 54 include Simplified BSD License text as described in Section 4.e of 55 the Trust Legal Provisions and are provided without warranty as 56 described in the Simplified BSD License. 58 Table of Contents 60 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 61 2. The Need For Consensus . . . . . . . . . . . . . . . . . . . . 3 62 3. Evidence of Deployment . . . . . . . . . . . . . . . . . . . . 4 63 4. Evidence of Differences . . . . . . . . . . . . . . . . . . . . 4 64 5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . 5 65 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5 66 7. Security Considerations . . . . . . . . . . . . . . . . . . . . 5 67 8. Informative References . . . . . . . . . . . . . . . . . . . . 5 68 Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 6 69 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7 71 1. Introduction 73 In April, 2006, the IETF published the [SPF] and [SUBMITTER]/ 74 [SENDER-ID]/[PRA] email authentication protocols. Both of these 75 enabled one to publish via the Domain Name System a policy declaring 76 which mail servers were authorized to send email on behalf of a 77 specific domain name. The two protocols made use of this policy 78 statement and some specific (but different) logic to evaluate whether 79 or not the email client sending or relaying a message was authorized 80 to do so. 82 Because Sender-ID could use the same policy statement as SPF, the 83 IESG at the time was concerned that an implementation of Sender-ID 84 might erroneously apply that statement to a message and, depending on 85 selected recipient actions, could improperly interfere with message 86 delivery. As a result, the IESG required the publication of all of 87 these documents as Experimental, and requested that the community 88 observe deployment and operation of the protocols over a period of 89 two years from publication in order to determine a reasonable path 90 forward. (For further details about the IESG's concern, see the IESG 91 Note prepended to all of those documents.) 93 Accordingly, this working group has convened to resolve this 94 experiment and propose advancement of a single protocol going 95 forward. This memo presents evidence on both deployment and efficacy 96 of the two protocols, and further discusses the increasing need for 97 consensus. At the end it presents conclusions and recommends a path 98 forward, as the IESG requested. 100 2. The Need For Consensus 102 These two protocols fall into a family of protocols that provide 103 domain-level email authentication services. Another prominent one is 104 [DKIM]. Various efforts exist that use these as building blocks to 105 increased abuse filtering capabilties, and indeed this sort of work 106 has spawned another working group in the Applications area, with 107 still more of these incubating in associations and trade groups 108 outside of the IETF. 110 There is thus some palpable interest in having a path authorization 111 scheme, as well as a domain-level signing scheme, on the Standards 112 Track so that these newer technologies can develop with confidence. 113 This is, in part, why the community has decided to expend the effort 114 to bring this experiment to a conclusion and document the results, 115 and then advance a single path authorization technology. 117 3. Evidence of Deployment 119 Two different sources (see Appendix A) report that over 239,000 120 domains have been observed to be publishing SPF records of some kind. 121 This includes either TXT or SPF resource record types, with the 122 prefix "v=spf1" found. The same two reporting sources indicated far 123 fewer that published "spf2.0" records that would indicate the 124 publisher requested Sender-ID handling only; one source projected 125 approximately 1,200 such domains while the other observed fewer than 126 100. 128 It is likely impossible to determine from a survey which MTAs have 129 SPF or Sender-ID checking enabled at message ingress since it does 130 not appear, for example, in the reply to the EHLO command from 131 extended [SMTP]. 133 A survey of over 20 MTAs in current or recent use includes only two 134 implementations of the SMTP SUBMITTER extension that could act as an 135 enabler to [SENDER-ID]. An unknown number of clients implement it; 136 although there is substantial activity showing its use in logs, it is 137 unclear whether these are separate implementations by legitimate 138 senders, or merely instances of distributed automated malware seeking 139 to improve their odds of reaching the end user. 141 The [OPENSPF] web site maintains a list of known implementations of 142 SPF. At the time of this memo's writing it listed six libraries, 22 143 MTAs with built-in SPF implementations, and numerous patches for MTAs 144 and mail clients. No such summary about Sender-ID support in 145 software could be found; two specific product implementations for 146 inbound checking were discovered. 148 [pending: passive DNS query report from John Levine] 150 [pending: active DNS query report from TDP] 152 [other data TBD] 154 4. Evidence of Differences 156 It is plain from inspection of the two protocols that they have much 157 in common: For a single message, both require the same number of DNS 158 queries, and both require the same code to parse the result. The PRA 159 algorithm applied by Sender-ID is, however, more expensive than 160 simply extracting the domain name from the omnipresent 161 RFC5321.MailFrom. Thus, SPF is cheaper to apply to a message. 163 One set of specific data collected by a working group contributor 164 shows that in more than 95.5% of cases, Sender-ID and SPF reach the 165 same conclusion about a message, meaning either both protocols return 166 a "pass" result or both return a "fail" result. The data set 167 yielding this response could not further characterize the cases in 168 which the answers differed. 170 Another data set from a different (much larger) source showed that in 171 fewer than 5% of cases did the RFC5321.MailFrom domain and the 172 RFC5322.From domain differ. This is somewhat meaningful when 173 determining the difference between the two protocols, but is more 174 important when considering the fact that most Mail User Agents 175 (MUAs), which actually present mail to the end user, typically only 176 render the contents of the RFC5322.From field. 178 [pending: updated field comparison report from Hotmail] 180 [other data TBD] 182 5. Conclusions 184 It is standard procedure within the IETF to document as standard 185 those protocols and practices that have come into sufficient common 186 use as to become part of the basic infrastructure. 188 Given the evidence above, the working group feels that the experiment 189 allows the following conclusions: 191 1. [WG conclusions here] 193 6. IANA Considerations 195 This memo presents no actions for IANA. [RFC Editor: Please remove 196 this section prior to publication.] 198 7. Security Considerations 200 This memo contains information for the community only, akin to an 201 implementation report, and does not introduce any new security 202 concerns. Its implications could, in fact, resolve some. 204 8. Informative References 206 [DKIM] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed., 207 "DomainKeys Identified Mail (DKIM) Signatures", RFC 6376, 208 September 2011. 210 [OPENSPF] "Sender Policy Framework: Project Overview", 211 . 213 [PRA] Lyon, J., "Purported Responsible Address in E-Mail 214 Messages", RFC 4407, April 2006. 216 [SENDER-ID] 217 Lyon, J. and M. Wong, "Sender ID: Authenticating E-Mail", 218 RFC 4406, April 2006. 220 [SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 221 October 2008. 223 [SPF] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF) 224 for Authorizing Use of Domains in E-Mail, Version 1", 225 RFC 4408, April 2006. 227 [SUBMITTER] 228 Allman, E. and H. Katz, "SMTP Service Extension for 229 Indicating the Responsible Submitter of an E-Mail 230 Message", RFC 4405, April 2006. 232 Appendix A. Acknowledgments 234 The following provided operational data that contributed to the 235 findings presented above: 237 Cisco: contributed data about observed Sender-ID and SPF records in 238 the DNS for a large number of domains 240 Hotmail: contributed data about the difference between 241 RFC5321.MailFrom and RFC5322.From domains across large mail 242 volumes 244 Santronics: contributed data about the use of the SUBMITTER 245 extension in aggregate SMTP client traffic 247 The Trusted Domain Project: contributed data about the difference 248 between Sender-ID and SPF results, and counts of unique domains 249 appearing to publish different kinds of SPF and Sender-ID records 251 The author would also like to thank the following for their 252 contributions to the development of this memo: Dave Crocker, Scott 253 Kitterman 255 Author's Address 257 Murray S. Kucherawy 258 Cloudmark 259 128 King St., 2nd Floor 260 San Francisco, CA 94107 261 USA 263 Phone: +1 415 946 3800 264 Email: msk@cloudmark.com