This document shows the back.gif
The server's access log should now show a reference to this page,
but should show no new GET for these images,
because they should be in your browser's cache.
There are several important things to notice about this preloading:
this page should have loaded with little or no delay.
The images were loaded into your browser's cache
while you read the previous page, and don't need to be loaded again.
Why this is useful.
If a web site has a lot of related pages that share graphic images,
The reason is that the common images are downloaded while you read the main page,
which should have relatively few graphics.
The network load is the same (or possibly higher),
but the images are downloaded during otherwise idle time,
so from the user's viewpoint it seems faster.
How this can be abused.
You probably didn't notice, but there were actually three
images loaded by the previous page. The third isn't shown
here. Can you find it? (Hint: It is in your browser's cache.)
Think about this a bit.
What a site can do to you.
While this feature was intended to speed up web accesses for
sets of related pages, it can also be used to embarrass people
by loading things into their browser's cache. For example, suppose
a coworker gets angry with you and decides to cause you some trouble.
They can copy this demo code to a page of their own,
replace the hidden image's URL with a pointer to a pornographic web site,
and invite you to look at their page. You won't see anything suspicious,
but the porn site's page is now in your browser's cache.
If anyone is monitoring your Web usage,
they will see that you just accessed a pornographic site.
If someone examines your browser's cache,
they will find a pornographic image there.
In some companies, this can get you fired.
How can you protect yourself from this sort of misuse of preloading?
other sort of remote programming or scripting that your browser has.
They all let remote sites do this sort of preloading.
If you do this, preloading will no longer work,
and web sites that use it will not download as fast.
The pages will look the same on your screen,
unless they require scripts to work,
but you won't get the hidden image.
Only images that are actually used will be downloaded.
As more and more companies install software that monitors their
employees' Web usage, and more Web-site developers learn the tricks
of the trade, this sort of thing will be come much more common.
When used to speed up Web access, it is useful. But it is a real
source of potentially embarrassing files on your disk. Your only
defense is to disable anything that allows a remote site to run code
on your machine.
to your browser that is run automatically. Allowing strangers to run
code on your machine is never a good idea. If you value the contents of
your disk, or are worried about embarrassing files appearing there without
your knowledge, you should make sure that all such programming tools are