mod_brotli
Compress content via Brotli before it is delivered to the client
Extension
mod_brotli.c
brotli_module
Available in version 2.4.26 and later.

The mod_brotli module provides the `BROTLI_COMPRESS` output filter that allows output from your server to be compressed using the brotli compression format before being sent to the client over the network. This module uses the Brotli library found at https://github.com/google/brotli.

Filters

# Sample Configurations

## Compression and TLS

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

## Output Compression

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

SetOutputFilter BROTLI_COMPRESS SetEnvIfNoCase Request_URI \.(?:gif|jpe?g|png)$ no-brotli

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 BROTLI_COMPRESS text/html </Directory>

### Note

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

### Note

There is an environment variable `no-brotli`, set via SetEnv, which will disable brotli compression for a particular request, even if it is supported by the client.

# Dealing with proxy servers

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

<IfModule mod_headers.c> # Serve brotli compressed CSS and JS files if they exist # and the client accepts brotli. RewriteCond "%{HTTP:Accept-encoding}" "br" RewriteCond "%{REQUEST_FILENAME}\.br" "-s" RewriteRule "^(.*)\.(js|css)" "$1\.$2\.br" [QSA]

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

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

# Force proxies to cache brotli & # non-brotli css/js files separately. Header append Vary Accept-Encoding </FilesMatch> </IfModule>
BrotliFilterNote
Places the compression ratio in a note for logging
BrotliFilterNote [type] notename
server config
virtual host

The BrotliFilterNote 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

BrotliFilterNote ratio

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

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

BrotliFilterNote Input instream BrotliFilterNote Output outstream BrotliFilterNote Ratio ratio

LogFormat '"%r" %{outstream}n/%{instream}n (%{ratio}n%%)' brotli CustomLog "logs/brotli_log" brotli
mod_log_config
BrotliCompressionQuality
Compression quality
BrotliCompressionQuality value
BrotliCompressionQuality 5
server config
virtual host

The BrotliCompressionQuality directive specifies the compression quality (a value between 0 and 11). Higher quality values result in better, but also slower compression.

BrotliCompressionWindow
Brotli sliding compression window size
BrotliCompressionWindow value
BrotliCompressionWindow 18
server config
virtual host

The BrotliCompressionWindow directive specifies the brotli sliding compression window size (a value between 10 and 24). Larger window sizes can improve compression quality, but require more memory.

BrotliCompressionMaxInputBlock
Maximum input block size
BrotliCompressionMaxInputBlock value
(automatic)
server config
virtual host

The BrotliCompressionMaxInputBlock directive specifies the maximum input block size between 16 and 24, with the caveat that larger block sizes require more memory.

BrotliAlterETag
How the outgoing ETag header should be modified during compression
BrotliAlterETag AddSuffix|NoChange|Remove
BrotliAlterETag AddSuffix
server config
virtual host

The BrotliAlterETag directive specifies how the ETag hader 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. In another dynamic compression module, mod_deflate, this has been the default since 2.4.0. This setting prevents serving "HTTP Not Modified" (304) responses to conditional requests for compressed content.

NoChange

  Don't change the ETag on a compressed response. In another dynamic compression module, mod_deflate, this has been the default prior to 2.4.0. This setting 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.