The "safe" HTTP Preference
draft-nottingham-safe-hint-09
The information below is for an old version of the document.
draft-nottingham-safe-hint-09
Network Working Group M. Nottingham
Internet-Draft April 17, 2019
Intended status: Informational
Expires: October 19, 2019
The "safe" HTTP Preference
draft-nottingham-safe-hint-09
Abstract
This specification defines a "safe" preference for HTTP requests that
expresses a desire to avoid objectionable content, according to the
definition of that term by the origin server.
Support for this preference by clients and servers is optional.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on October 19, 2019.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. The "safe" Preference . . . . . . . . . . . . . . . . . . . . 3
3. Implementation Status . . . . . . . . . . . . . . . . . . . . 4
4. Security Considerations . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 7
Appendix B. Sending "safe" from Web Browsers . . . . . . . . . . 7
Appendix C. Supporting "safe" on Web Sites . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
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 content to which they
might object.
However, that goal is often difficult to achieve, because of the need
to go to every Web site that might be used, 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.
A more manageable approach is for the browser to proactively indicate
a preference for safe content. 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 define what content might be
considered objectionable, and so the concept of "safe" is also not
precisely defined. Rather, the term is interpreted by the server and
within the scope of each Web site that chooses to act upon this
information.
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 is not intended to be used for other purposes.
Furthermore, sending "safe" does not guarantee that the Web site will
use it, nor that it will apply a concept of "objectionable" that is
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consistent with the requester's views. As such, its effect can be
described as "best effort," and not to be relied upon. In other
words, sending the preference is no more reliable than going to each
Web site and manually selecting a "safe" mode, but it is considerably
easier.
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.
Note also that the cultural context applies to the hosting location
of a site, the content provider, and the source of the content. It
cannot be guaranteed that a user-agent and origin server will have
the same view of the concept of what is objectionable.
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", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. The "safe" Preference
When present in a request, the "safe" preference indicates that the
user prefers that the origin server to not respond with content which
is designated as objectionable, according to the origin 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
Typically, user agents that emit the "safe" preference will include
it in all requests with the "https" URI scheme, although some might
expose finer-grained controls over when it is sent; this ensures that
the preference is available to the applicable resources. User agents
MUST NOT emit the "safe" preference on requests with the "http" URI
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scheme (see Section 4). See Appendix B for more information about
configuring the set of resources "safe" is sent to.
Safe MAY be implemented in common HTTP libraries (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
Preference-Applied: safe
Server: ExampleServer/2.0
Vary: Prefer
Here, the Preference-Applied response header ([RFC7240]) indicates
that the site has applied the preference. Servers are not required
to send Preference-Applied (even when they have applied the
preference), but are encouraged to where possible.
Note that the Vary response header needs to be sent if the response
is cacheable and 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. Please note that the listing of any individual
implementation here does not imply endorsement by the IETF.
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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 https://support.microsoft.com/
en-hk/help/2980016/
o Microsoft Bing - see https://developer.microsoft.com/en-us/
microsoft-edge/testdrive/demos/familysearch/
o Mozilla Firefox - see https://support.mozilla.org/en-US/kb/block-
and-unblock-websites-parental-controls-firef
o Cisco - see http://blogs.cisco.com/security/filtering-explicit-
content
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). Therefore, it is prohibited from being included in
requests with the "http" scheme.
Its presence reveals limited information about the user, which may be
of small assistance in "fingerprinting" the user by sites.
Therefore, user agents SHOULD NOT include it in requests when the
user has expressed a desire to avoid such attacks (e.g., some forms
of "private mode" browsing).
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 as well as over time might
(and probably will) change.
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5. IANA Considerations
This specification registers the following entry in the "HTTP
Preferences" registry [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,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
RFC 7234, DOI 10.17487/RFC7234, June 2014,
<https://www.rfc-editor.org/info/rfc7234>.
[RFC7240] Snell, J., "Prefer Header for HTTP", RFC 7240,
DOI 10.17487/RFC7240, June 2014,
<https://www.rfc-editor.org/info/rfc7240>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
6.2. Informative References
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011,
<https://www.rfc-editor.org/info/rfc6265>.
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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. Sending "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.
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 require browsers to send "safe"
on all requests, although that is one possible implementation; e.g.,
alternate implementation strategies include blacklists and
whitelists.
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 or computer's
administrator (e.g., a parent). If the site has such a means of
configuration (e.g., stored user preferences) and the safe preference
is received in a request, the "safer" interpretation ought to be
used.
The appropriate level of "safety" is a site-specific decision. When
selecting it, sites ought to bear in mind that disabling the
preference might be considerably more onerous than through other
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means, especially if the preference is generated based upon Operating
System configuration.
Sites might offer different levels of "safeness" through Web
configuration, they will need to either inform their users of what
level the "safe" hint corresponds to, or provide them with some means
of adjusting it.
If the user expresses a wish to disable "safe" mode, the site can
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).
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: https://www.mnot.net/
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