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Make IE and Firefox Stop Revalidating Images

Images are set to be cached but browser keeps Checking with Last-Modified

10:06 am on Aug 16, 2005 (gmt 0)

Junior Member

10+ Year Member

joined:Apr 6, 2004
votes: 0


On each pageload on my site, Firefox and IE send a new request (with Last-Modified headers) for all the images on my site (the ones being used on the menubar).

But the button images are set to expire after 2 hours, so I would like a situation where the browser will not ask the server on each pageload if the images have been modified since the last time. I want to know how to make Firefox and IE stop checking the images on each pageload!


GET /Themes/Optimal/style-2.css HTTP/1.1
Accept: text/css,*/*;q=0.1
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive: 300
Connection: keep-alive
Cookie: ==
If-Modified-Since: Wed, 03 Aug 2005 20:32:01 GMT
If-None-Match: "bf1134-12c4-5b5d3240"

HTTP/1.x 304 Not Modified
Etag: "bf1134-12c4-5b5d3240"
Date: Mon, 15 Aug 2005 06:56:08 GMT
Warning: 113 speed "Heuristic expiration"

1:02 am on Aug 17, 2005 (gmt 0)

Senior Member from CA 

WebmasterWorld Senior Member encyclo is a WebmasterWorld Top Contributor of All Time 10+ Year Member

joined:Aug 31, 2003
votes: 6

Warning: 113 speed "Heuristic expiration"

It looks like a server issue - your server is not setting an explicit expiration time for the files, so the cache is rechecking with the server rather than just serving the files. That means that the cache (browser or other) is dependent on what is called "Heuristic Expiration" - in other words, it uses an algorithm based on calculations from other header values such as Last-Modified. The algorithm varies with each cache.

See section 13.2.2 "Heuristic Expiration" in the HTTP 1.1 specification:



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