Network Working Group M. Nottingham
Internet-Draft
Intended status: Standards Track February 13, 2015
Expires: August 17, 2015
The "safe" HTTP Preference
draft-nottingham-safe-hint-06
Abstract
This specification defines a "safe" preference for HTTP requests,
expressing a desire to avoid "objectionable" content.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. The "safe" Preference . . . . . . . . . . . . . . . . . . . . 3
3. Implementation Status . . . . . . . . . . . . . . . . . . . . 4
4. Security Considerations . . . . . . . . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 6
Appendix B. Setting "safe" from Web Browsers . . . . . . . . . . 6
Appendix C. Supporting "safe" on Web Sites . . . . . . . . . . . 6
1. Introduction
Many Web sites have a "safe" mode, to assist those who don't want to
be exposed (or have their children exposed) to "objectionable"
content.
However, that goal is often difficult to achieve, because of the need
to go to each Web site in turn, navigate to the appropriate page
(possibly creating an account along the way) to get a cookie
[RFC6265] set in the browser, for each browser on every device used.
If this desire is proactively advertised by the user agent, things
become much simpler. A user agent that supports doing so (whether it
be an individual browser, or through an Operating System HTTP
library) need only be configured once to assure that the preference
is advertised to a set of sites, or even all sites.
This specification defines how to declare this desire in requests as
a HTTP Preference [RFC7240].
Note that this specification does not precisely define what "safe"
is; rather, it is interpreted within the scope of each Web site that
chooses to act upon this information. Furthermore, sending "safe"
does not guarantee that the Web site will use it.
That said, the intent of "safe" is to allow end users (or those
acting on their behalf) to express a desire to avoid content that is
considered "objectionable" within the cultural context of that site;
usually (but not always) content that is unsuitable for minors. The
"safe" preference ought not be used for other purposes.
It is also important to note that the "safe" preference is not a
reliable indicator that the end user is a child; other users might
have a desire for unobjectionable content, and some children might
browse without the preference being set.
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Simply put, it is a statement by (or on behalf of) the end user to
the effect "If your site has a 'safe' setting, this user is hereby
opting into that, according to your definition of the term."
1.1. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. The "safe" Preference
When present in a request, the "safe" preference indicates that the
content which is not objectionable is preferred, according to the
server's definition of the concept.
For example, a request that includes the "safe" preference:
GET /foo.html HTTP/1.1
Host: www.example.org
User-Agent: ExampleBrowser/1.0
Prefer: safe
User agents SHOULD include the "safe" preference in all requests, to
ensure that the preference is available to the applicable resources.
Note that the resources which "safe" is sent to is potentially
configurable; see Appendix B for more information.
Additionally, other clients MAY insert it; e.g., an operating system
might choose to insert the preference in requests based upon system-
wide configuration.
Origin servers that utilize the "safe" preference ought to document
that they do so, along with the criteria that they use to denote
objectionable content. If a server has more fine-grained degrees of
"safety", it SHOULD select a reasonable default to use, and document
that; it MAY use additional mechanisms (e.g., cookies [RFC6265]) to
fine-tune.
A response corresponding to the request above might have headers that
look like this:
HTTP/1.1 200 OK
Transfer-Encoding: chunked
Content-Type: text/html
Server: ExampleServer/2.0
Vary: Prefer
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Note that the Vary response header needs to be sent if cacheable
responses associated with the resource might change depending on the
value of the "Prefer" header. This is not only true for those
responses that are "safe", but also the default "unsafe" response.
See [RFC7234] Section 4.1 for more information the interaction
between Vary and Web caching.
See Appendix C for additional advice specific to Web servers wishing
to use "safe".
3. Implementation Status
_Note to RFC Editor: Please remove this section before publication._
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft. The description of implementations in this section
is intended to assist the IETF in its decision processes in
progressing drafts to RFCs. Please note that the listing of any
individual implementation here does not imply endorsement by the
IETF. Furthermore, no effort has been spent to verify the
information presented here that was supplied by IETF contributors.
This is not intended as, and must not be construed to be, a catalog
of available implementations or their features. Readers are advised
to note that other implementations may exist.
o Microsoft Internet Explorer - see http://support.microsoft.com/
kb/2980016
o Microsoft Bing
o Mozilla Firefox - see https://bugzilla.mozilla.org/
show_bug.cgi?id=995070
o Cisco - see http://blogs.cisco.com/security/filtering-explicit-
content
o YouTube - based upon testing the live site (not formally
announced)
4. Security Considerations
The "safe" preference is not a secure mechanism; it can be inserted
or removed by intermediaries with access to the request stream (e.g.
for "http://" URLs). Its presence reveals limited information about
the user, which may be of small assistance in "fingerprinting" the
user.
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By its nature, including "safe" in requests does not assure that all
content will actually be safe; it is only when servers elect to honor
it that content might be "safe".
Even then, a malicious server might adapt content so that it is even
less "safe" (by some definition of the word). As such, this
mechanism on its own is not enough to assure that only "safe" content
is seen; those who wish to ensure that will need to combine its use
with other techniques (e.g., content filtering).
Furthermore, the server and user may have differing ideas regarding
the semantics of "safe." As such, the "safety" of the user's
experience when browsing from site to site might (and probably will)
change.
5. IANA Considerations
This specification registers the "safe" preference [RFC7240]:
o Preference: safe
o Value: (no value)
o Description: Indicates that "safe" / "unobjectionable" content is
preferred.
o Reference: (this document)
o Notes:
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC7240] Snell, J., "Prefer Header for HTTP", RFC 7240, June 2014.
6.2. Informative References
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
April 2011.
[RFC7234] Fielding, R., Nottingham, M., and J. Reschke, "Hypertext
Transfer Protocol (HTTP/1.1): Caching", RFC 7234, June
2014.
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Appendix A. Acknowledgements
Thanks to Alissa Cooper, Ilya Grigorik, Emma Llanso, Jeff Hughes,
Lorrie Cranor, Doug Turner and Dave Crocker for their comments.
Appendix B. Setting "safe" from Web Browsers
As discussed in Section 2, there are many possible ways for the
"safe" preference to be generated. One possibility is for a Web
browser to allow its users to configure the preference to be sent.
When doing so, it is important not to misrepresent the preference as
binding to Web sites. For example, an appropriate setting might be a
checkbox with wording such as:
[] Request "safe" content from Web sites
... along with further information available upon request (e.g., from
a "help" system).
Browsers might also allow the "safe" preference to be "locked" - that
is, prevent modification without administrative access, or a
passcode.
Note that this specification does not constrain browsers to send
"safe" on all requests, although that is one possible implementation;
e.g., an alternate implementation strategy would be to allow a
blacklist (of sites that "safe" is not sent to).
Appendix C. Supporting "safe" on Web Sites
Web sites that allow configuration of a "safe" mode (for example,
using a cookie) can add support for the "safe" preference
incrementally; since the preference will not be supported by all
clients immediately, it is necessary to have another way to configure
it.
When honoring the safe preference, it is important that it not be
possible to disable it through the Web site's interface, since "safe"
may be configured and locked down by the browser's administrator
(e.g., a parent). If the site has configuration (e.g., stored user
preferences) and the safe preference is received in a request, the
"safer" interpretation is always used.
If the user expresses a wish to disable "safe" mode, the site should
remind them that the safe preference is being sent, and ask them to
consult their administrator (since "safe" might be set by a locked-
down Operating System configuration).
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As explained in Section 2, responses that change based upon the
presence of the "safe" preference need to either carry the "Vary:
Prefer" response header field, or be uncacheable by shared caches
(e.g., with a "Cache-Control: private" response header field). This
is to avoid an unsafe cached response being served to a client that
prefers safe content (or vice versa).
Author's Address
Mark Nottingham
EMail: mnot@mnot.net
URI: http://www.mnot.net/
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