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Apache HTTP Server Version 2.4

Apache > HTTP Server > Documentation > Version 2.4 > Modules

Apache Module mod_deflate

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


Summary

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


Topics

Directives


Bugfix checklist


See also


Supported Encodings

The gzip encoding is the only one supported to ensure complete compatibility with old browser implementations. The deflate encoding is not supported, please check the zlib's documentation for a complete explanation.


Sample Configurations

Compress only a few types


Enabling Compression

Compression and TLS

Some web applications are vulnerable to an information disclosure attack when a TLS connection carries deflate compressed data. For more information, review the details of the "BREACH" family of attacks.


Output Compression

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

SetOutputFilter DEFLATE
SetEnvIfNoCase Request_URI "\.(?:gif|jpe?g|png)$" no-gzip

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

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

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

There is an environment variable force-gzip, set via SetEnv, which will ignore the accept-encoding setting of your browser and will send compressed output.


Output Decompression

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

<Location "/dav-area">
    ProxyPass "http://example.com/"
    SetOutputFilter INFLATE
</Location>

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 we have to insert the DEFLATE filter into the input filter chain using SetInputFilter or AddInputFilter, for example:

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

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.


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.


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 we use some special exclusions dependent on, for example, the User-Agent header, we 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, we should add:

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


Example


Serving pre-compressed content

Since mod_deflate re-compresses content each time a request is made, some performance benefit can be derived by pre-compressing the content and telling mod_deflate to serve them without re-compressing them. This may be accomplished using a configuration like the following:

<IfModule mod_headers.c>
    # Serve gzip compressed CSS files if they exist 
    # and the client accepts gzip.
    RewriteCond "%{HTTP:Accept-encoding}" "gzip"
    RewriteCond "%{REQUEST_FILENAME}\.gz" -s
    RewriteRule "^(.*)\.css" "$1\.css\.gz" [QSA]

    # Serve gzip compressed JS files if they exist 
    # and the client accepts gzip.
    RewriteCond "%{HTTP:Accept-encoding}" "gzip"
    RewriteCond "%{REQUEST_FILENAME}\.gz" -s
    RewriteRule "^(.*)\.js" "$1\.js\.gz" [QSA]


    # Serve correct content types, and prevent mod_deflate double gzip.
    RewriteRule "\.css\.gz$" "-" [T=text/css,E=no-gzip:1]
    RewriteRule "\.js\.gz$" "-" [T=text/javascript,E=no-gzip:1]


    <FilesMatch "(\.js\.gz|\.css\.gz)$">
      # Serve correct encoding type.
      Header append Content-Encoding gzip

      # Force proxies to cache gzipped & 
      # non-gzipped css/js files separately.
      Header append Vary Accept-Encoding
    </FilesMatch>
</IfModule>



DeflateBufferSize Directive

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. If the compressed response size is bigger than the one specified by this directive then httpd will switch to chunked encoding (HTTP header Transfer-Encoding set to Chunked), with the side effect of not setting any Content-Length HTTP header. This is particularly important when httpd works behind reverse caching proxies or when httpd is configured with mod_cache and mod_cache_disk because HTTP responses without any Content-Length header might not be cached.



DeflateCompressionLevel Directive

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

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

Description:Places the compression ratio in a note for logging
Syntax:DeflateFilterNote [type] notename
Context:server config, virtual host
Status:Extension
Module:mod_deflate

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.


Example

To extract more accurate values from the 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 we may log it this way:


Accurate Logging


See also



DeflateInflateLimitRequestBody Directive

Description:Maximum size of inflated request bodies
Syntax:DeflateInflateLimitRequestBodyvalue
Default:None, but LimitRequestBody applies after deflation
Context:server config, virtual host, directory, .htaccess
Status:Extension
Module:mod_deflate
Compatibility:2.4.10 and later

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

Description:Maximum number of times the inflation ratio for request bodies can be crossed
Syntax:DeflateInflateRatioBurst value
Default:3
Context:server config, virtual host, directory, .htaccess
Status:Extension
Module:mod_deflate
Compatibility:2.4.10 and later

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



DeflateInflateRatioLimit Directive

Description:Maximum inflation ratio for request bodies
Syntax:DeflateInflateRatioLimit value
Default:200
Context:server config, virtual host, directory, .htaccess
Status:Extension
Module:mod_deflate
Compatibility:2.4.10 and later

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

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).



DeflateWindowSize Directive

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.