How do I know if image from my site is getting cached by proxy servers?

Question

Hi all,

The following is a http response header from a image on our company's website.

HTTP/1.1 200 OK
Content-Type: image/png
Last-Modified: Thu, 03 Dec 2009 15:51:57 GMT
Accept-Ranges: bytes
ETag: "1e61e38a3074ca1:0"
Date: Wed, 06 Jan 2010 22:06:23 GMT
Content-Length: 9140

Is there anyway to know if this image is publicly cacheable in some proxy server? The RFC definition seems to be ambiguous http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.9.1 and http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.4.

Answer 1

The headers you show appear to be cacheable.

If you would like to control the caching behavior of correctly configured proxies and web browsers, you might investigate using the Cache-Control and Expires headers to gain additional control.

Here is a webpage I had bookmarked that has one person's opinion of how to intepret the specifications you list (plus some other ones): http://www.web-caching.com/mnot_tutorial/how.html

If you need to guarantee that someone sees a completely new image each time (even with misconfigured devices between you and them), you may want to consider using a randomized or GUID value as part of the URL.

Answer 2

Run RED on your URL and it'll tell you whether the response is cacheable, among other information.

Answer 3

Here is a tutorial on setting headers for proxy caching. Be sure to read the part about setting cookies!