HTTP Expires header not respected by browser

cachinghttp

I have a situation where my (embedded) web server is sending Expires header, but the browser does not seem to respect the header setting, i.e., if I refresh the page, the browser requests the resources that are supposed to be cached. Following are the headers that are getting exchanged:

https://192.168.1.180/scgi-bin/ajax/ajax.cgi

GET /scgi-bin/ajax/ajax.cgi HTTP/1.1
Host: 192.168.1.180
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US; rv:1.9.0.11) Gecko/2009060215 Firefox/3.0.11 (.NET CLR 3.5.30729)
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
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
Cache-Control: max-age=0

HTTP/1.x 200 OK
Date: Wed, 24 Jun 2009 20:26:47 GMT
Server: Embedded HTTP Server.
Connection: close
Content-Type: text/html
----------------------------------------------------------
https://192.168.1.180/scgi-bin/ajax/static.cgi?fn=images/logo.jpg&ts=20090624201057

GET /scgi-bin/ajax/static.cgi?fn=images/logo.jpg&ts=20090624201057 HTTP/1.1
Host: 192.168.1.180
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US; rv:1.9.0.11) Gecko/2009060215 Firefox/3.0.11 (.NET CLR 3.5.30729)
Accept: image/png,image/*;q=0.8,*/*;q=0.5
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
Referer: https://192.168.1.180/scgi-bin/ajax/ajax.cgi
Cache-Control: max-age=0

HTTP/1.x 200 OK
Date: Wed, 24 Jun 2009 20:26:47 GMT
Server: Embedded HTTP Server.
Connection: close
Expires: Wed, 1 Jun 2011 20:00:00 GMT
Content-Type: image/jpg
----------------------------------------------------------

The ajax.cgi returns an html page with a logo graphic (via the static.cgi script), which I'd like cached, but the browser is asking for the logo on every refresh.

Best Answer

The browser ignores the Expires header if you refresh the page. It always checks whether the cache entry is still valid by contacting the web server. Ideally, it will use the If-Modified-Since request header so that the server can return '304 Not modified' if the cache entry is still valid.

You're not setting the Last-Modified header, so the browser has to perform an unconditional GET of the content to ensure that it is up to date.

Some rules of thumb for setting Expires and Last-Modified are described in this blog post:

http://blog.httpwatch.com/2007/12/10/two-simple-rules-for-http-caching/

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