A Note About the HTTP

HTTP 1.1 compliance is mentioned several times in this document. As of the time it was written, the protocol is a work in progress. Because of this, it is virtually impossible for an application (whether a server, proxy or client) to be truly compliant. However, the protocol has been openly discussed for some time, and feature-frozen for enough time to allow developers to use the ideas contained in it, like Cache-Control and ETags. When HTTP 1.1 is final, expect more vendors to openly state that their applications are compliant.

Implementation Notes - Web Servers

Generally speaking, it's best to use the latest version of whatever Web server you've chosen to deploy. Not only will they likely contain more cache-friendly features, new versions also usually have important security and performance improvements.

Apache 1.3

Apache (http://www.apache.org/) uses optional modules to include headers, including both Expires and Cache-Control. Both modules are available in the 1.2 or greater distribution.

The modules need to be built into Apache; although they are included in the distribution, they are not turned on by default. To find out if the modules are enabled in your server, find the httpd binary and run httpd -l; this should print a list of the available modules. The modules we're looking for are mod_expires and mod_headers.

• If they aren't available, and you have administrative access, you can recompile Apache to include them. This can be done either by uncommenting the appropriate lines in the Configuration file, or using the -enable-module=expires and -enable-module=headers arguments to configure (1.3 or greater). Consult the INSTALL file found with the Apache distribution.

Once you have an Apache with the appropriate modules, you can use mod_expires to specify when objects should expire, either in .htaccess files or in the server's access.conf file. You can specify expiry from either access or modification time, and apply it to a file type or as a default. See http://www.apache.org/docs/mod/mod_expires.html for more information, and speak with your local Apache guru if you have trouble.

To apply Cache-Control headers, you'll need to use the mod_headers module, which allows you to specify arbitrary HTTP headers for a resource. See http://www.apache.org/docs/mod/mod_headers.html

Here's an example .htaccess file that demonstrates the use of some headers.

• .htaccess files allow web publishers to use commands normally only found in configuration files. They affect the content of the directory they're in and their subdirectories. Talk to your server administrator to find out if they're enabled.

### activate mod_expires
ExpiresActive On
### Expire .gif's 1 month from when they're accessed
ExpiresByType image/gif A2592000
### Expire everything else 1 day from when it's last modified
### (this uses the Alternative syntax)
ExpiresDefault "modification plus 1 day"
### Apply a Cache-Control header to index.html
<Files index.html>
Header append Cache-Control "public, must-revalidate"
</Files>

• Note that mod_expires automatically calculates and inserts a Cache-Control:max-age header as appropriate.

Netscape Enterprise 3.6

Netscape Enterprise Server (http://www.netscape.com/) does not provide any obvious way to set Expires headers. However, it has supported HTTP 1.1 features since version 3.0. This means that HTTP 1.1 caches (proxy and browser) will be able to take advantage of Cache-Control settings you make.

To use Cache-Control headers, choose Content Management | Cache Control Directives in the administration server. Then, using the Resource Picker, choose the directory where you want to set the headers. After setting the headers, click 'OK'. For more information, see http://developer.netscape.com/docs/manuals/enterprise/admnunix/content.htm#1006282

MS IIS 4.0

Microsoft's Internet Information Server (http://www.microsoft.com/) makes it very easy to set headers in a somewhat flexible way. Note that this is only possible in version 4 of the server, which will run only on NT Server.

To specify headers for an area of a site, select it in the Administration Tools interface, and bring up its properties. After selecting the HTTP Headers tab, you should see two interesting areas; Enable Content Expiration and Custom HTTP headers. The first should be self-explanatory, and the second can be used to apply Cache-Control headers.

See the ASP section below for information about setting headers in Active Server Pages. It is also possible to set headers from ISAPI modules; refer to MSDN for details.

Lotus Domino R5

Lotus' (http://www.lotus.com/) servers are notoriously difficult to cache; they don't provide any validators, so both browser and proxy caches can only use default mechanisms (i.e., once per session, and a few minutes of 'fresh' time, usually) to cache any content from them.

Even if this limitation is overcome, Notes' habit of referring to the same object by different URLs (depending on a variety of factors) bars any measurable gains. There is also no documented way to set an Expires, Cache-Control or other arbitrary HTTP header.

Frequently Asked Questions
Caching Tutorial for Web Authors and Webmasters
Implementation Notes - Server-Side Scripting