Dynamic image resizing and caching

I’m working on an image system that does rescaling and effects on
demand, rather than on upload. The uploads are stored as master files,
and resized images are generated, cached and returned by a controller,
using a request similar to this:

/images/21/150x100/filename.jpg (where 21 is the image id)

I also have various filters implemented, adding the concept of filter
sets (which is a series of filters, identified by a string). For
example:

/images/blog-thumbnails/21/150x100/filename.jpg

blog-thumbnails is the name of a filter set that could apply unsharp
mask on the resized images, tint the colours to match the design, etc
etc.

This works incredibly well. It enables us to play around with effects
and image resizing at whim in views and stylesheets, through external
API to Flash etc.

There is one catch though: This is begging for DOS-attacks. Results are
cached, but processing images is CPU-intensive. I could easily cause a
server meltdown armed with nothing more than a poor connection and a
small script, by sending loads of requests with unique image sizes.

I can think of a few solutions of the top of my head:

  • I could limit the allowed image sizes to a set defined somewhere (in
    the config perhaps). Kind of defeats the purpose, though, and leads to
    more configuration. Maybe a constraint for production mode, while
    allowing any image size in development?

  • I could make a random hash token and put it in a table along with
    image id, image size, filters etc, and use this in the request:

/images/blog-thumbnails/21/150x100/filename.jpg?token=ab1a436y1b
or /images/blog-thumbnails/21/150x100/ab1a436y1b/filename.jpg
or /images/blog-thumbnails/21/150x100/ab1a436y1b.jpg

Drawbacks: rails would have to generate the URL for me every time i need
a new image variation. And it destroys the pretty and user-friendly
URLs.

  • I could detect the attack somehow, and drop the connection or return
    error codes instead of processing the image if there’s a possible siege
    going on. This sounds hacky to me, I’d have to keep record of the
    requests for the last few minutes. And it might result in occationally
    broken images for some users.

  • I could check the referrer header, but that could be faked.

Any tips? Is there anything in apache or lighthttpd that might save me
alot of work?

Inge Jørgensen
web developer / designer
[email protected]
[email protected]

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