Apache Module mod_deflate
Description: | Compress content before it is delivered to the client |
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Status: | Extension |
ModuleIdentifier: | deflate_module |
SourceFile: | 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.
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 types
AddOutputFilterByType 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
TheDEFLATE
filter is always inserted after RESOURCE filters like PHP or SSI. It never touches internal subrequests. Note
There is an environment variableforce-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.
Example
Header 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 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 |
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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 |
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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 |
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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
DeflateFilterNote 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 Logging
DeflateFilterNote Input instream DeflateFilterNote Output outstream DeflateFilterNote Ratio ratio LogFormat '"%r" %{outstream}n/%{instream}n (%{ratio}n%%)' deflate CustomLog "logs/deflate_log" deflate
See also
DeflateInflateLimitRequestBody Directive
Description: | Maximum size of inflated request bodies |
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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 |
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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 |
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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 |
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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 |
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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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