About a year ago I wrote about how use mod_header with Apache 1.3x to send Cache-control headers. It worked so well that I want to configure my Apache 2.x servers to send the same headers. It's even simpler with Apache 2.x since mod_expires is included in most default installs. Here's what I did.
I added a configuration directive for the main server configuration (inside the Directory block) which sends the Cache-control header for common graphics.
# turn on the module for this directory
# cache common graphics for 3 days
ExpiresByType image/jpg "access plus 3 days"
ExpiresByType image/gif "access plus 3 days"
ExpiresByType image/jpeg "access plus 3 days"
ExpiresByType image/png "access plus 3 days"
# cache CSS for 24 hours
ExpiresByType text/css "access plus 24 hours"
# set the default to 24 hours
ExpiresDefault "access plus 24 hours"
All that was left was to restart Apache and test everything.
/<your path>/apache2/bin/apachectl configtest
/<your path>/apache2/bin/apachectl restart
Testing was simple. I tailed the access log and made two requests for the same page. The first request resulted with a flurry of files being requested (all the graphics being downloaded). The second request resulted in a single file request (no graphics) and the graphics where logged as HTTP status 304 (unmodified), success.