mod_deflate - Apache HTTP Server Version 2.4









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Apache HTTP Server Version 2.4



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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

    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

 Supported Encodings
 Sample Configurations
 Enabling Compression
 Dealing with proxy servers
 Serving pre-compressed
content
Directives

 DeflateAlterETag
 DeflateBufferSize
 DeflateCompressionLevel
 DeflateFilterNote
 DeflateInflateLimitRequestBody
 DeflateInflateRatioBurst
 DeflateInflateRatioLimit
 DeflateMemLevel
 DeflateWindowSize

Bugfix checklisthttpd changelogKnown issuesReport a bugSee also

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

    Compress only a few typesAddOutputFilterByType DEFLATE text/html text/plain text/xml text/css text/javascript application/javascript




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


      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:

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


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

    Header append Vary User-Agent


    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.

    ExampleHeader set Vary *



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 and JS files if they exist
    # and the client accepts gzip.
    RewriteCond "%{HTTP:Accept-encoding}" "gzip"
    RewriteCond "%{REQUEST_FILENAME}\.gz" -s
    RewriteRule "^(.*)\.(css|js)"         "$1\.$2\.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>




DeflateAlterETag Directive

Description:How the outgoing ETag header should be modified during compression
Syntax:DeflateAlterETag AddSuffix|NoChange|Remove
Default:DeflateAlterETag AddSuffix
Context:server config, virtual host
Status:Extension
Module:mod_deflate
Compatibility:Available in Apache 2.4.58 and later

    The DeflateAlterETag directive specifies
    how the ETag header should be altered when a response is compressed.
    
    AddSuffix
    Append the compression method onto the end of the ETag, causing
        compressed and uncompressed representations to have unique ETags.
        This has been the default since 2.4.0, but prevents serving
        "HTTP Not Modified" (304) responses to conditional requests for
        compressed content.
    NoChange
    Don't change the ETag on a compressed response. This was the default
        prior to 2.4.0, but does not satisfy the HTTP/1.1 property that all
        representations of the same resource have unique ETags.
    Remove
    Remove the ETag header from compressed responses. This prevents
        some conditional requests from being possible, but avoids the
        shortcomings of the preceding options.
    



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.

    ExampleDeflateFilterNote ratio

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


    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:

    Accurate LoggingDeflateFilterNote Input instream
DeflateFilterNote Output outstream
DeflateFilterNote Ratio ratio

LogFormat '"%r" %{outstream}n/%{instream}n (%{ratio}n%%)' deflate
CustomLog "logs/deflate_log" deflate


See also

mod_log_config



DeflateInflateLimitRequestBody Directive

Description:Maximum size of inflated request bodies
Syntax:DeflateInflateLimitRequestBody value
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:DeflateInflateRatioBurst 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:DeflateInflateRatioLimit 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.




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