Add expires headers to minimize load-time for repeat visitors

“Add Expires headers” is a ySlow component that’s missing from most blogs. It’s overlooked because the rules for these expires headers must be buried deep within the bowels of the .htaccess file. Generally speaking, you want to avoid these errors, so most people avoid the .htaccess file. Spelunking in that file is only problematic when you don’t know what you’re doing, though. If you start adding random shit, you’re likely to have a problem.

So, rule #1: Don’t add shit to the .htaccess file if you don’t know what the shit does.

If you violate rule #1, you’ll end up with a website that looks like this.

Rule #2: plan for incompatibility. I can easily drop in new Apache rules, as my server supports just about all of them. Your server might not. If it doesn’t, and you don’t plan for incompatibility, you’re going to end up with a pretty Internal Server Error.

With two two rules in mind, here’s what you need to add. If you want to know what it means, keep scrolling. Otherwise, just copy it, paste it into your .htaccess file and have a nice day.

<IfModule mod_expires.c>
AddType image/x-icon .ico
ExpiresActive on
ExpiresByType image/jpeg A2592000
ExpiresByType image/gif A2592000
ExpiresByType image/png A2592000
ExpiresByType image/x-icon A2592000
ExpiresByType application/x-javascript A2592000
ExpiresByType text/css M604800
ExpiresDefault M604800
</IfModule>

And here’s the breakdown.

1. <IfModule mod_expires.c> If Apache allows expires headers, then process the rules.
2. AddType {mime-type} {file extension} ICOs (favicons) aren’t included in the Apache mime-type by default. This command assigns a mime-type (determined by you) to a specific file extension so other rules can be applied to it.
3. ExpiresActive on Turns on the Expires Headers
4. ExpiresByType {mime-type} {code}{seconds} Here, we’re letting Apache know how long we want a specific type of file to stay cached on your reader’s computer. Or, more accurately, how long we want the reader’s computer to wait before asking for the file again.
   1. {mime-type} This portion of the command uses the predetermined mime-types, most of which you don’t get to set yourself. Webmaster Toolkit has a nice list of common mime-types.
   2. {code} Either A or M.
      1. A tells the browser to not request the file for {seconds} from when it was last requested.
      2. M tells the browser to not request the file for {seconds} from when the file was last updated.
   3. {seconds} Time, in seconds.
      1. 1 hour = 3,600 seconds
      2. 1 day = 86,400 seconds
      3. 1 week = 604,800 seconds
      4. 1 month = [about]  2,629,743 seconds
      5. 1 year = 31,556,926 seconds
5. ExpiresDefault {code}{seconds} Same as ExpiresByType, except it applies to all files that aren’t specifically addressed.