Network Working Group M. Nottingham
Internet-Draft March 15, 2014
Intended status: Informational
Expires: September 16, 2014

The "safe" HTTP Preference


This specification defines a “safe” preference for HTTP, expressing a user preference to avoid “objectionable” content.

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Table of Contents

1. Introduction

Many Web sites have a “safe” mode, to assist those who don’t want to be exposed to “objectionable” content, or who don’t want their children to be exposed to such content. YouTube [youtube], Yahoo! Search [yahoo], Google Search [google], Bing Search [bing], and many other services have such a setting.

However, a user that wishes to have this preference honoured would 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. They would need to do this for each browser on every device they use. As has been widely noted, this is difficult [age-privacy].

This can be onerous to nearly impossible to achieve effectively, because there are too many permutations of sites, user agents and devices.

If instead this preference is proactively advertised by the user agent, things become much simpler. A user agent that supports this (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 all sites that understand and choose to act upon it. It’s no longer necessary to go to each site that has potentially “unsafe” content and configure a “safe” mode.

Furthermore, a proxy (for example, at a school) can be used to ensure that the preference is associated with all (unencrypted) requests flowing through it, helping to assure that clients behind it are not exposed to “objectionable” content.

This specification defines how to associate this preference with a request, as a HTTP Preference [I-D.snell-http-prefer].

Note that this approach does not define what “safe” is; rather, it is interpreted within the scope of each Web site that chooses to act upon this information (or not). As such, it does not require agreement upon what “safe” is, nor does it require application of policy in the user agent or an intermediary (which can be problematic for many reasons).

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 user prefers content which is not objectionable, according to the server’s definition of the concept.

For example a request that includes the “safe” preference:

GET /foo.html HTTP/1.1
User-Agent: ExampleBrowser/1.0
Prefer: safe

When configured to do so, user agents SHOULD include the “safe” preference in every request, to ensure that the preference is applied (where possible) to all resources.

For example, a Web browser might have a “Request Safe Browsing” option. additionally, other clients MAY insert it; e.g., an operating system might choose to insert the preference in requests based upon system-wide configuration, or a proxy might do so based upon its configuration.

Servers that utilise the “safe” preference SHOULD document that they do so, along with the criteria that they use to denote objectionable content. If a site 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) 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

Note that the Vary response header needs to be sent if responses associated with the resource might change depending on the value of the “Prefer” header; this is not only true for those responses that have changed, but also the “default” unchanged responses.

NOTE: currently, the safe preference doesn’t have a payload, but one could be used to indicate a “level” of safety desired; e.g., “safe=hi” or “safe=lo”. Feedback appreciated.

3. Security Considerations

The “safe” preference is not a secure mechanism; it can be inserted or removed by intermediaries with access to the data stream. Its presence reveals information about the user, which may be of small assistance in “fingerprinting” the user (1 bit of information, to be precise).

Due to its nature, including it in requests does not assure that all content will actually be safe; it is only when servers elect to honour it that it might change content.

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; users 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.

4. IANA Considerations

This specification registers the “safe” preference [I-D.snell-http-prefer]:

5. References

5.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[I-D.snell-http-prefer] Snell, J., "Prefer Header for HTTP", Internet-Draft draft-snell-http-prefer-18, January 2013.

5.2. Informative References

[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265, April 2011.
[yahoo] Yahoo! Inc., "Yahoo! Search Preferences", 2013.
[bing] Microsoft, "Bing Help: Block Explicit Web Sites", 2013.
[google] Google, "SafeSearch: turn on or off", 2013.
[youtube] Google, "How to access and turn on Safety Mode?", 2013.
[age-privacy] Moses, A., "Privacy concern as apps share data from kids left to their own devices", 2012.

Appendix A. Acknowledgements

Thanks to Alissa Cooper, Ilya Grigorik, Emma Llanso and Jeff Hughes for their comments.

Appendix B. Using “safe” on Your Web Site

Web sites that allow configuration of a “safe” mode can add support for the “safe” preference incrementally; since it will not be supported by all clients immediately, it is necessary to still have a “manual” safety configuration option.

When honouring the safe preference, it is important that it not be possible to disable it through the Web UI, since “safe” may be inserted by an intermediary (e.g., at a school) or configured and locked down by an administrator (e.g., a parent).

The safe preference is designed to make as much of the Web a “safe” experience as possible; it is not intended to be configured site-by-site. Therefore, 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.

Author's Address

Mark Nottingham EMail: URI: