Website image caching with Apache

Question

How can I get static content on Apache to be {cached by browser} and not {checked for freshness {with every request}}?

I'm working on a website hosted on Apache webserver. Recently, I was testing something with headers (Content-Type for different types of content) and saw a lot of conditional requests for images. Example:

200 /index.php?page=1234&action=list
304 /favicon.ico
304 /img/logo.png
304 /img/arrow.png
(etc.)

Although the image files are static content and are cached by the browser, every time an user opens a page that links to them, they are conditionally requested, to which they send "304 Not Modified". That's good (less data transferred), but it means 20+ more requests with every page load (longer page load due to all those round-trips, even with Keep-Alive and pipelining enabled).

How do I tell the browser to keep the existing file and not check for newer version?

EDIT: the mod_expires method works, even with the favicon.

Answer 1

I'm experimenting with mod_expires in Apache, loading in and using it in .htaccess.

With an Expires header, the resource is only requested the first time. Before it hits the expiration date, subsequent requests are fulfilled from cache. After the specified time expires and the resource is needed, it is requested again. The only reliable way to clear it from the cache before it expires is manually, or by forcing a refresh (Ctrl-F5). (This could be an issue if it changes in the meantime, but statical images don't change very often.)

For favicon.ico, a bit more work is needed (Apache normally sends this as text/plain).

ExpiresActive on
ExpiresByType image/png "access plus 1 month"
ExpiresByType image/gif "access plus 1 month"
ExpiresByType image/jpeg "access plus 1 month"
# css may change a bit sometimes
ExpiresByType text/css "access plus 1 days"

# special MIME type for icons - see http://www.iana.org/assignments/media-types/image/vnd.microsoft.icon
AddType image/vnd.microsoft.icon .ico
# now we have icon MIME type, we can use it
# my favicon doesn't change much
ExpiresByType image/vnd.microsoft.icon "access plus 3 months"

And voila, It Works™!

Answer 2

If you set the Expires header on your http response for your static images, your server won't be checked again for that image after first download until the time specified has passed, e.g. if I download a file from your server now that gives it's Expires header as

Expires: Fri, 1 Jan 2010 00:00:01 GMT

then my browser won't look for it from your server again until 2010, unless I clear my cache/do a force refresh (Ctrl+F5 on windows).

There's a simple introduction to setting this up here, and a list of other possibly helpful responses over at wikipedia