Caching of images and script in the _layouts directory

Question

I am trying the figure out why the images (or js, or css) in the _layouts directory are not cached by either IE or FF.

The authentication on the site collection is NTLM. The _layouts folder has anonymous access enabled in IIS.

If I debug with fiddler a page that has been loaded before, I can see the following requests:

GET /_layouts/MyApplication/Images/newProject.png HTTP/1.1 401 Unauthorized

GET /_layouts/MyApplication/Images/newProject.png HTTP/1.1 200 OK

...

Cache-Control: max-age=31536000

Last-Modified: Tue, 09 Feb 2010 14:40:01 GMT

ETag: "5945d7c295a9ca1:3d0416"

...

This happens for each subsequent access of the page. Do you have any idea why the image is not cached?

Thank you

Can you try turning off NTLM authentication for that directory to see if that changes the behaviour? — a'r
If I uncheck Integrated Windows Authentication in IIS Manager for this folder and check Anonymous Access with an administrator account the script and images in that folder are not loaded anymore. I just get 401 Unathorized for them. — user204906

Zeb Zeb · Accepted Answer · 2010-02-25T10:33:18

I don't think this has anything to do with Caching. Your images are simply not accessible.

Though in theory your setup should just work, I would check the following:

Application Pool - Make sure your sub application ('MyApplciation') is using the same App Pool as the main SharePoint site. I've encountered problem with this before, I couldn't find a solution for this so ended up assigning the same application pool.
IIS - If you have set up a virtual directory / application, check all of security settings. I would definately try to delete any virtual directory nodes and then seeing if I can access the images.
If you're using a CAS policy, try changing it to an out of the box configuration.
If all else fails, consider moving your images to _layouts/images/ (I know, not ideal but might be a work around)

Can't think of anything else at the moment.