W3C home > Mailing lists > Public > www-tag@w3.org > November 2011

Re: Web browsers should preserve the file system Last-Modified time of downloaded files

From: Paul Libbrecht <paul@hoplahup.net>
Date: Sat, 5 Nov 2011 18:29:13 +0100
Cc: "www-tag@w3.org List" <www-tag@w3.org>
Message-Id: <5DBDBD40-25C6-49DC-BCCB-0BEF1AF05E44@hoplahup.net>
To: Brolin Empey <brolin@brolin.be>

it's funny that this request comes now because it's a fairly natural request that could have been made about ten years ago for practical reasons. Now the reasons become more operationalized...

I would warn, however, that precise modification dates are dangerous for tracking:

As reported a few months ago in this mailing list, it is possible to build a business that enables tracking far beyond cookies so that a user cannot get rid of such a tracking.

As I recall, the exploit is to use more than cookies (which get erased and well segregated) and that includes:
- Etags of cached entries: if your browser is referenced to an image and my server says this is Etag BLOP, the browser will tell the server the same Etag next time: you are the one  I know.
- Modification dates: if the browser is referenced to an image, it stores it in cache with a modification date. The next time this image is referenced, a header "If-Modified-Since" is sent with, currently, the exact same date: you are the one I know again.

I would say it is likely that web browsers will not want to be too constrained in this modification date setting of cached entries so that they can still offer some way of not being tracked (e.g. not send etags, change the modification date a bit to the future).

For downloading it is likely to be the same or?


PS: personnally, I want my "Downloads" folder to be organized by arrival date, and that's the modification date.

Le 5 nov. 2011 à 00:18, Brolin Empey a écrit :

> Hello,
> I have already contacted timbl and amy (in CC above) about this subject because I wanted timbl’s personal opinion, but timbl referred me to this list.
> Most GUI Web browsers (maybe most Web browsers, including non-GUI ones?) do not preserve the file system last modified time of downloaded files:  even if the HTTP server sends the Last-Modified response header, the Web browser does not set the last modified time (mtime) field of the downloaded file in the local file system to the Last-Modified time served with the file.  This behaviour effectively *changes* the last modified time of the downloaded file even if the file has not actually been modified.
> The Web browser should preserve the file system last modified time by default because this time cannot easily be recovered after it is discarded.
> I believe implementing this basic file copy functionality should be prioritised above advanced features such as WebGL and Typed Arrays [1]:
> current releases of both Mozilla Firefox [2] and Google Chrome [3] can run Quake II [4] [5] and boot and run a Linux v2.6.20 kernel and BusyBox userland on an x86 computer emulated in JavaScript [6] without requiring any extensions or plug-ins but still cannot even preserve the file system last modified time of a downloaded file without an optional extension (for Mozilla only, not Google Chrome).
> Mozilla Firefox >= v5.0 can preserve the file system last modified time if the optional Preserve Download Modification Timestamp (PDMTS) extension is installed:
> <https://addons.mozilla.org/en-US/firefox/addon/preserve-download-modification/>
> This extension probably would not have been created if I had not acted to fix Mozilla:
> <http://forums.mozillazine.org/viewtopic.php?p=10608005#p10608005>
> I used to believe my priorities in life were inverted, but my priority inversion seems to have been outdone by the developers of major GUI Web browsers.
> I do not want to start Yet Another™ debate over the last modified time preservation behaviour but, if necessary, I can fully support my argument, as I have already done elsewhere multiple times.
> I want the W3C to convince the developers of (at least major GUI) Web browsers to finally fix this bug by preserving the file system last modified time of downloaded files by default.  The behaviour could be a policy with an easily discoverable GUI to change it because I know some confused users will resist change by wanting the broken behaviour.
> At the least, I want Mozilla to include PDMTS in the base installation of both Firefox and SeaMonkey with file system last modified time preservation enabled by default.
> Thank you,
> Brolin Empey
> [1] <http://www.khronos.org/registry/typedarray/specs/latest/>
> [2] Tested with v5.0 on Windows 7 Professional Service Pack 1 IA-32
> [3] Tested v15 on on Windows 7 Professional Service Pack 1 IA-32
> [4] <http://code.google.com/p/quake2-gwt-port/>
> [5] <http://playwebgl.com/games/quake-2-webgl/
> [6] <http://bellard.org/jslinux/>
Received on Saturday, 5 November 2011 17:29:45 UTC

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