Apache Module mod_deflate

Sample Configurations

This is a simple configuration that compresses common text-based content types.

AddOutputFilterByType DEFLATE text/html text/plain text/xml text/css text/javascript application/javascript

Enabling Compression

Output Compression

Compression is implemented by the DEFLATE filter. The following directive will enable compression for documents in the container where it is placed:

Some popular browsers cannot handle compression of all content so you may want to set the gzip-only-text/html note to 1 to only allow html files to be compressed (see below). If you set this to anything but 1 it will be ignored.

If you want to restrict the compression to particular MIME types in general, you may use the AddOutputFilterByType directive. Here is an example of enabling compression only for the html files of the Apache documentation:

For browsers that have problems even with compression of all file types, use the BrowserMatch directive to set the no-gzip note for that particular browser so that no compression will be performed. You may combine no-gzip with gzip-only-text/html to get the best results. In that case the former overrides the latter. Take a look at the following excerpt from the configuration example defined in the section above:

At first we probe for a User-Agent string that indicates a Netscape Navigator version of 4.x. These versions cannot handle compression of types other than text/html. The versions 4.06, 4.07 and 4.08 also have problems with decompressing html files. Thus, we completely turn off the deflate filter for them.

The third BrowserMatch directive fixes the guessed identity of the user agent, because the Microsoft Internet Explorer identifies itself also as "Mozilla/4" but is actually able to handle requested compression. Therefore we match against the additional string "MSIE" (\b means "word boundary") in the User-Agent Header and turn off the restrictions defined before.

Output Decompression

The mod_deflate module also provides a filter for inflating/uncompressing a gzip compressed response body. In order to activate this feature you have to insert the INFLATE filter into the outputfilter chain using SetOutputFilter or AddOutputFilter, for example:

This Example will uncompress gzip'ed output from example.com, so other filters can do further processing with it.

Input Decompression

The mod_deflate module also provides a filter for decompressing a gzip compressed request body . In order to activate this feature you have to insert the DEFLATE filter into the input filter chain using SetInputFilter or AddInputFilter, for example:

Now if a request contains a Content-Encoding: gzip header, the body will be automatically decompressed. Few browsers have the ability to gzip request bodies. However, some special applications actually do support request compression, for instance some WebDAV clients.

Dealing with proxy servers

The mod_deflate module sends a Vary: Accept-Encoding HTTP response header to alert proxies that a cached response should be sent only to clients that send the appropriate Accept-Encoding request header. This prevents compressed content from being sent to a client that will not understand it.

If you use some special exclusions dependent on, for example, the User-Agent header, you must manually configure an addition to the Vary header to alert proxies of the additional restrictions. For example, in a typical configuration where the addition of the DEFLATE filter depends on the User-Agent, you should add:

If your decision about compression depends on other information than request headers (e.g. HTTP version), you have to set the Vary header to the value *. This prevents compliant proxies from caching entirely.