Apache Module mod_deflate

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Description:	Compress content before it is delivered to the -client
Status:	Extension
Module Identifier:	deflate_module
Source File:	mod_deflate.c

Summary

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The mod_deflate module provides - the DEFLATE output filter that allows output from - your server to be compressed before being sent to the client over - the network.

Topics

Sample Configurations
Enabling Compression
Dealing with proxy servers

Sample Configurations

This is a simple sample configuration for the impatient.

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Compress only a few types

- AddOutputFilterByType DEFLATE text/html text/plain text/xml -

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The following configuration, while resulting in more compressed content, - is also much more complicated. Do not use this unless you fully understand - all the configuration details.

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Compress everything except images

- <Location /> - - # Insert filter - SetOutputFilter DEFLATE - - # Netscape 4.x has some problems... - BrowserMatch ^Mozilla/4 gzip-only-text/html - - # Netscape 4.06-4.08 have some more problems - BrowserMatch ^Mozilla/4\.0[678] no-gzip - - # MSIE masquerades as Netscape, but it is fine - # BrowserMatch \bMSIE !no-gzip !gzip-only-text/html - - # NOTE: Due to a bug in mod_setenvif up to Apache 2.0.48 - # the above regex won't work. You can use the following - # workaround to get the desired effect: - BrowserMatch \bMSI[E] !no-gzip !gzip-only-text/html - - # Don't compress images - SetEnvIfNoCase Request_URI \ - - \.(?:gif|jpe?g|png)$ no-gzip dont-vary - - - # Make sure proxies don't deliver the wrong content - Header append Vary User-Agent env=!dont-vary - - </Location> -

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Enabling Compression

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Output Compression

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

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- SetOutputFilter DEFLATE -

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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.

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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:

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- <Directory "/your-server-root/manual"> - - AddOutputFilterByType DEFLATE text/html - - </Directory> -

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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:

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- BrowserMatch ^Mozilla/4 gzip-only-text/html - BrowserMatch ^Mozilla/4\.0[678] no-gzip - BrowserMatch \bMSIE !no-gzip !gzip-only-text/html -

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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.

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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.

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Note

- The DEFLATE filter is always inserted after RESOURCE - filters like PHP or SSI. It never touches internal subrequests. -

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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:

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- <Location /dav-area> - - SetInputFilter DEFLATE - - </Location> -

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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.

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Note on Content-Length

If you evaluate the request body yourself, don't trust - the Content-Length header! - The Content-Length header reflects the length of the - incoming data from the client and not the byte count of - the decompressed data stream.

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Dealing with proxy servers

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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.

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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:

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- Header append Vary User-Agent -

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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.

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Example

- Header set Vary * -

DeflateBufferSize Directive

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Description:	Fragment size to be compressed at one time by zlib
Syntax:	`DeflateBufferSize value`
Default:	`DeflateBufferSize 8096`
Context:	server config, virtual host
Status:	Extension
Module:	mod_deflate

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

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DeflateCompressionLevel Directive

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Description:	How much compression do we apply to the output
Syntax:	`DeflateCompressionLevel value`
Default:	`Zlib's default`
Context:	server config, virtual host
Status:	Extension
Module:	mod_deflate
Compatibility:	This directive is available since Apache 2.0.45

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).

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DeflateFilterNote Directive

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Description:	Places the compression ratio in a note for logging
Syntax:	`DeflateFilterNote [type] notename`
Context:	server config, virtual host
Status:	Extension
Module:	mod_deflate
Compatibility:	`type` is available since Apache 2.0.45

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.

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Example

- DeflateFilterNote ratio - - LogFormat '"%r" %b (%{ratio}n) "%{User-agent}i"' deflate - CustomLog logs/deflate_log deflate -

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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 note for logging. type can be one of:

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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.

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Thus you may log it this way:

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Accurate Logging

- DeflateFilterNote Input instream - DeflateFilterNote Output outstream - DeflateFilterNote Ratio ratio - - LogFormat '"%r" %{outstream}n/%{instream}n (%{ratio}n%%)' deflate - CustomLog logs/deflate_log deflate -

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DeflateMemLevel Directive

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Description:	How much memory should be used by zlib for compression
Syntax:	`DeflateMemLevel value`
Default:	`DeflateMemLevel 9`
Context:	server config, virtual host
Status:	Extension
Module:	mod_deflate

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

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DeflateWindowSize Directive

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Description:	Zlib compression window size
Syntax:	`DeflateWindowSize value`
Default:	`DeflateWindowSize 15`
Context:	server config, virtual host
Status:	Extension
Module:	mod_deflate

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.

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Apache Module mod_deflate

Summary

Directives

Topics

See also

Sample Configurations

Compress only a few types

Compress everything except images

Enabling Compression

Output Compression

Note

Input Decompression

Note on Content-Length

Dealing with proxy servers

Example

DeflateBufferSize Directive

DeflateCompressionLevel Directive

DeflateFilterNote Directive

Example

Accurate Logging

See also

DeflateMemLevel Directive

DeflateWindowSize Directive