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 output filter 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.

DeflateBufferSize Directive

The DeflateBufferSize directive specifies the size in bytes of the fragments that zlib should compress at one time.

DeflateCompressionLevel Directive

The DeflateCompressionLevel directive specifies what level of compression should be used, the higher the value, the better the compression, but the more CPU time is required to achieve this.

The value must between 1 (less compression) and 9 (more compression).

DeflateFilterNote Directive

The DeflateFilterNote directive specifies that a note about compression ratios should be attached to the request. The name of the note is the value specified for the directive. You can use that note for statistical purposes by adding the value to your access log.

If you want to extract more accurate values from your logs, you can use the type argument to specify the type of data left as a note for logging. type can be one of:

Input

Store the byte count of the filter's input stream in the note.

Output

Store the byte count of the filter's output stream in the note.

Ratio

Store the compression ratio (output/input * 100) in the note. This is the default, if the type argument is omitted.

Thus you may log it this way:

See also

• mod_log_config

DeflateInflateLimitRequestBody Directive

The DeflateInflateLimitRequestBody directive specifies the maximum size of an inflated request body. If it is unset, LimitRequestBody is applied to the inflated body.

DeflateInflateRatioBurst Directive

The DeflateInflateRatioBurst directive specifies the maximum number of times the DeflateInflateRatioLimit can be crossed before terminating the request.

DeflateInflateRatioLimit Directive

The DeflateInflateRatioLimit directive specifies the maximum ratio of deflated to inflated size of an inflated request body. This ratio is checked as the body is streamed in, and if crossed more than DeflateInflateRatioBurst times, the request will be terminated.

DeflateMemLevel Directive

The DeflateMemLevel directive specifies how much memory should be used by zlib for compression (a value between 1 and 9).

DeflateWindowSize Directive

The DeflateWindowSize directive specifies the zlib compression window size (a value between 1 and 15). Generally, the higher the window size, the higher can the compression ratio be expected.