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Nottingham 3 Internet-Draft November 5, 2018 4 Intended status: Informational 5 Expires: May 9, 2019 7 The "safe" HTTP Preference 8 draft-nottingham-safe-hint-07 10 Abstract 12 This specification defines a "safe" preference for HTTP requests that 13 expresses a desire to avoid objectionable content, according to the 14 definition of that term by the origin server. 16 Status of This Memo 18 This Internet-Draft is submitted in full conformance with the 19 provisions of BCP 78 and BCP 79. 21 Internet-Drafts are working documents of the Internet Engineering 22 Task Force (IETF). Note that other groups may also distribute 23 working documents as Internet-Drafts. The list of current Internet- 24 Drafts is at https://datatracker.ietf.org/drafts/current/. 26 Internet-Drafts are draft documents valid for a maximum of six months 27 and may be updated, replaced, or obsoleted by other documents at any 28 time. It is inappropriate to use Internet-Drafts as reference 29 material or to cite them other than as "work in progress." 31 This Internet-Draft will expire on May 9, 2019. 33 Copyright Notice 35 Copyright (c) 2018 IETF Trust and the persons identified as the 36 document authors. All rights reserved. 38 This document is subject to BCP 78 and the IETF Trust's Legal 39 Provisions Relating to IETF Documents 40 (https://trustee.ietf.org/license-info) in effect on the date of 41 publication of this document. Please review these documents 42 carefully, as they describe your rights and restrictions with respect 43 to this document. Code Components extracted from this document must 44 include Simplified BSD License text as described in Section 4.e of 45 the Trust Legal Provisions and are provided without warranty as 46 described in the Simplified BSD License. 48 Table of Contents 50 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 51 2. The "safe" Preference . . . . . . . . . . . . . . . . . . . . 3 52 3. Implementation Status . . . . . . . . . . . . . . . . . . . . 4 53 4. Security Considerations . . . . . . . . . . . . . . . . . . . 5 54 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 55 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 56 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 7 57 Appendix B. Sending "safe" from Web Browsers . . . . . . . . . . 7 58 Appendix C. Supporting "safe" on Web Sites . . . . . . . . . . . 7 59 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8 61 1. Introduction 63 Many Web sites have a "safe" mode, to assist those who don't want to 64 be exposed (or have their children exposed) to objectionable content. 66 However, that goal is often difficult to achieve, because of the need 67 to go to every Web site that might be used, navigate to the 68 appropriate page (possibly creating an account along the way) to get 69 a cookie [RFC6265] set in the browser, for each browser on every 70 device used. 72 A more manageable approach is for the browser to proactively indicate 73 a preference for safe content. A user agent that supports doing so 74 (whether it be an individual browser, or through an Operating System 75 HTTP library) need only be configured once to assure that the 76 preference is advertised to a set of sites, or even all sites. 78 This specification defines how to declare this desire in requests as 79 a HTTP Preference [RFC7240]. 81 Note that this specification does not precisely define what "safe" 82 is; rather, it is interpreted within the scope of each Web site that 83 chooses to act upon this information. 85 That said, the intent of "safe" is to allow end users (or those 86 acting on their behalf) to express a desire to avoid content that is 87 considered objectionable within the cultural context of that site; 88 usually (but not always) content that is unsuitable for minors. The 89 "safe" preference ought not be used for other purposes. 91 Furthermore, sending "safe" does not guarantee that the Web site will 92 use it. As such, its effect can be described as "best effort," but 93 not to be relied upon. In other words, sending the preference is no 94 more reliable than going to each Web site and manually selecting a 95 "safe" mode, but it is considerably easier. 97 It is also important to note that the "safe" preference is not a 98 reliable indicator that the end user is a child; other users might 99 have a desire for unobjectionable content, and some children might 100 browse without the preference being set. 102 Simply put, it is a statement by (or on behalf of) the end user to 103 the effect "If your site has a 'safe' setting, this user is hereby 104 opting into that, according to your definition of the term." 106 1.1. Notational Conventions 108 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 109 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 110 "OPTIONAL" in this document are to be interpreted as described in BCP 111 14 [RFC2119] [RFC8174] when, and only when, they appear in all 112 capitals, as shown here. 114 2. The "safe" Preference 116 When present in a request, the "safe" preference indicates that the 117 content which is not objectionable is preferred, according to the 118 origin server's definition of the concept. 120 For example, a request that includes the "safe" preference: 122 GET /foo.html HTTP/1.1 123 Host: www.example.org 124 User-Agent: ExampleBrowser/1.0 125 Prefer: safe 127 User agents SHOULD include the "safe" preference in all HTTPS 128 requests unless otherwise configured, to ensure that the preference 129 is available to the applicable resources. See Appendix B for more 130 information about configuring the set of resources "safe" is sent to. 132 Safe MAY be implemented in common HTTP libraries (e.g., an operating 133 system might choose to insert the preference in requests based upon 134 system-wide configuration). 136 Origin servers that utilize the "safe" preference ought to document 137 that they do so, along with the criteria that they use to denote 138 objectionable content. If a server has more fine-grained degrees of 139 "safety", it SHOULD select a reasonable default to use, and document 140 that; it MAY use additional mechanisms (e.g., cookies [RFC6265]) to 141 fine-tune. 143 A response corresponding to the request above might have headers that 144 look like this: 146 HTTP/1.1 200 OK 147 Transfer-Encoding: chunked 148 Content-Type: text/html 149 Preference-Applied: safe 150 Server: ExampleServer/2.0 151 Vary: Prefer 153 Here, the Preference-Applied response header ([RFC7240]) indicates 154 that the site has applied the preference. Servers are not required 155 to send Preference-Applied, but are encouraged to where possible. 157 Note that the Vary response header needs to be sent if the response 158 is cacheable and might change depending on the value of the "Prefer" 159 header. This is not only true for those responses that are "safe", 160 but also the default "unsafe" response. 162 See [RFC7234] Section 4.1 for more information the interaction 163 between Vary and Web caching. 165 See Appendix C for additional advice specific to Web servers wishing 166 to use "safe". 168 3. Implementation Status 170 _Note to RFC Editor: Please remove this section before publication._ 172 This section records the status of known implementations of the 173 protocol defined by this specification at the time of posting of this 174 Internet-Draft. Please note that the listing of any individual 175 implementation here does not imply endorsement by the IETF. 176 Furthermore, no effort has been spent to verify the information 177 presented here that was supplied by IETF contributors. This is not 178 intended as, and must not be construed to be, a catalog of available 179 implementations or their features. Readers are advised to note that 180 other implementations may exist. 182 o Microsoft Internet Explorer - see https://support.microsoft.com/ 183 en-hk/help/2980016/ 185 o Microsoft Bing - see https://developer.microsoft.com/en-us/ 186 microsoft-edge/testdrive/demos/familysearch/ 188 o Mozilla Firefox - see https://support.mozilla.org/en-US/kb/block- 189 and-unblock-websites-parental-controls-firef 191 o Cisco - see http://blogs.cisco.com/security/filtering-explicit- 192 content 194 4. Security Considerations 196 The "safe" preference is not a secure mechanism; it can be inserted 197 or removed by intermediaries with access to the request stream (e.g. 198 for "http" URLs). Therefore, it SHOULD NOT be included in requests 199 with the "http" scheme. 201 Its presence reveals limited information about the user, which may be 202 of small assistance in "fingerprinting" the user by sites. 203 Therefore, user agents SHOULD NOT include it in requests when the 204 user has expressed a desire to avoid such attacks (e.g., some forms 205 of "private mode" browsing). 207 By its nature, including "safe" in requests does not assure that all 208 content will actually be safe; it is only when servers elect to honor 209 it that content might be "safe". 211 Even then, a malicious server might adapt content so that it is even 212 less "safe" (by some definition of the word). As such, this 213 mechanism on its own is not enough to assure that only "safe" content 214 is seen; those who wish to ensure that will need to combine its use 215 with other techniques (e.g., content filtering). 217 Furthermore, the server and user may have differing ideas regarding 218 the semantics of "safe." As such, the "safety" of the user's 219 experience when browsing from site to site as well as over time might 220 (and probably will) change. 222 5. IANA Considerations 224 This specification registers the "safe" preference [RFC7240]: 226 o Preference: safe 228 o Value: (no value) 230 o Description: Indicates that "safe" / "unobjectionable" content is 231 preferred. 233 o Reference: (this document) 235 o Notes: 237 6. References 238 6.1. Normative References 240 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 241 Requirement Levels", BCP 14, RFC 2119, 242 DOI 10.17487/RFC2119, March 1997, 243 . 245 [RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, 246 Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching", 247 RFC 7234, DOI 10.17487/RFC7234, June 2014, 248 . 250 [RFC7240] Snell, J., "Prefer Header for HTTP", RFC 7240, 251 DOI 10.17487/RFC7240, June 2014, 252 . 254 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 255 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 256 May 2017, . 258 6.2. Informative References 260 [RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265, 261 DOI 10.17487/RFC6265, April 2011, 262 . 264 Appendix A. Acknowledgements 266 Thanks to Alissa Cooper, Ilya Grigorik, Emma Llanso, Jeff Hughes, 267 Lorrie Cranor, Doug Turner and Dave Crocker for their comments. 269 Appendix B. Sending "safe" from Web Browsers 271 As discussed in Section 2, there are many possible ways for the 272 "safe" preference to be generated. One possibility is for a Web 273 browser to allow its users to configure the preference to be sent. 275 When doing so, it is important not to misrepresent the preference as 276 binding to Web sites. For example, an appropriate setting might be a 277 checkbox with wording such as: 279 [] Request "safe" content from Web sites 281 ... along with further information available upon request. 283 Browsers might also allow the "safe" preference to be "locked" - that 284 is, prevent modification without administrative access, or a 285 passcode. 287 Note that this specification does not require browsers to send "safe" 288 on all requests, although that is one possible implementation; e.g., 289 alternate implementation strategies include blacklists and 290 whitelists. 292 Appendix C. Supporting "safe" on Web Sites 294 Web sites that allow configuration of a "safe" mode (for example, 295 using a cookie) can add support for the "safe" preference 296 incrementally; since the preference will not be supported by all 297 clients immediately, it is necessary to have another way to configure 298 it. 300 When honoring the safe preference, it is important that it not be 301 possible to disable it through the Web site's interface, since "safe" 302 may be configured and locked down by the browser or computer's 303 administrator (e.g., a parent). If the site has such a means of 304 configuration (e.g., stored user preferences) and the safe preference 305 is received in a request, the "safer" interpretation ought to be 306 used. 308 The appropriate level of "safety" is a site-specific decision. When 309 selecting it, sites ought to bear in mind that disabling the 310 preference might be considerably more onerous than through other 311 means, especially if the preference is generated based upon Operating 312 System configuration. 314 Sites might offer different levels of "safeness" through Web 315 configuration, they will need to either inform their users of what 316 level the "safe" hint corresponds to, or provide them with some means 317 of adjusting it. 319 If the user expresses a wish to disable "safe" mode, the site can 320 remind them that the safe preference is being sent, and ask them to 321 consult their administrator (since "safe" might be set by a locked- 322 down Operating System configuration). 324 As explained in Section 2, responses that change based upon the 325 presence of the "safe" preference need to either carry the "Vary: 326 Prefer" response header field, or be uncacheable by shared caches 327 (e.g., with a "Cache-Control: private" response header field). This 328 is to avoid an unsafe cached response being served to a client that 329 prefers safe content (or vice versa). 331 Author's Address 333 Mark Nottingham 335 EMail: mnot@mnot.net 336 URI: https://www.mnot.net/