This will let a request proceed if the
envariable is set (with any value whatsoever). Any and all other
requests will be denied because they don’t meet the
conditions and the default is to deny access.
Putting all these pieces together, we end up with a stanza of
directives that looks something like this:
SetEnvIfNoCase Referer "^http://my.apache.org/" local_ref=1 Order Allow,Deny Allow from env=local_ref
These may all appear in your server-wide configuration files
httpd.conf), or you can put the
container in one or more
files. The effect is the same: Within the scope of these directives,
images can only be fetched if they were linked to from one of your
I mentioned earlier that you can’t fully prevent image theft. That’s
because of two things, which apply pretty much to the two different
types of poaching respectively:
Someone who really wants your artwork can always
request it using a faked-up
happens to meet your criteria. In other words, by jiggering up
the request so it looks like it’s a reference from your site.
If someone legitimately views your artwork by going through
your pages, the image files are almost certainly in his client’s
cache somewhere. So he can pull it out of a cached valid request
rather than making another one just to pick up the image.
Though it’s essentially impossible to foil someone who’s really desperate
to snitch your artwork, the steps described in this article should
make it too difficult for the casual poacher.
Another thing you can do, depending upon how protective you are of
your art, is to watermark the images. Watermarking a digital
image consists of encoding a special ‘signature’ into the graphic
so that it can be detected later. Digital watermarking doesn’t
degrade the quality of the image, and can be done in such a way
that even a cropped subsection of the image contains the mark, and
it’s detectable even if the image has been otherwise edited since the
mark was inserted.
It’s even possible to detect a watermark in an image that was
printed and then scanned in, having left the digital realm
altogether! If you watermark your images, there’s an excellent chance
you’ll be able to prove snitching if you ever find a suspicious image
on another site somewhere.