Пункт 143. Apache Module mod_filter
| Description: | Context-sensitive smart filter configuration module |
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| Status: | Base |
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| Module Identifier: | filter_module |
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| Source File: | mod_filter.c |
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| Compatibility: | Version 2.1 and later |
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Summary
This module enables smart, context-sensitive configuration of
output content filters. For example, apache can be configured to
process different content-types through different filters, even
when the content-type is not known in advance (e.g. in a proxy).
mod_filter works by introducing indirection into
the filter chain. Instead of inserting filters in the chain, we insert
a filter harness which in turn dispatches conditionally
to a filter provider. Any content filter may be used as a provider
to mod_filter ; no change to existing filter modules is
required (although it may be possible to simplify them).
Smart Filtering
In the traditional filtering model, filters are inserted unconditionally
using AddOutputFilter and family.
Each filter then needs to determine whether to run, and there is little
flexibility available for server admins to allow the chain to be
configured dynamically.
mod_filter by contrast gives server administrators a
great deal of flexibility in configuring the filter chain. In fact,
filters can be inserted based on complex boolean
expressions This generalises the limited
flexibility offered by AddOutputFilterByType .
Filter Declarations, Providers and Chains
Figure 1: The traditional filter model
In the traditional model, output filters are a simple chain
from the content generator (handler) to the client. This works well
provided the filter chain can be correctly configured, but presents
problems when the filters need to be configured dynamically based on
the outcome of the handler.
Figure 2: The
mod_filter model
mod_filter works by introducing indirection into
the filter chain. Instead of inserting filters in the chain, we insert
a filter harness which in turn dispatches conditionally
to a filter provider. Any content filter may be used as a provider
to mod_filter ; no change to existing filter modules
is required (although it may be possible to simplify them). There can be
multiple providers for one filter, but no more than one provider will
run for any single request.
A filter chain comprises any number of instances of the filter
harness, each of which may have any number of providers. A special
case is that of a single provider with unconditional dispatch: this
is equivalent to inserting the provider filter directly into the chain.
Configuring the Chain
There are three stages to configuring a filter chain with
mod_filter . For details of the directives, see below.
- Declare Filters
- The
FilterDeclare directive
declares a filter, assigning it a name and filter type. Required
only if the filter is not the default type AP_FTYPE_RESOURCE.
- Register Providers
- The
FilterProvider
directive registers a provider with a filter. The filter may have
been declared with FilterDeclare ; if not, FilterProvider will implicitly
declare it with the default type AP_FTYPE_RESOURCE. The provider
must have been
registered with ap_register_output_filter by some module.
The final argument to FilterProvider is an expression: the provider will be
selected to run for a request if and only if the expression evaluates
to true. The expression may evaluate HTTP request or response
headers, environment variables, or the Handler used by this request.
Unlike earlier versions, mod_filter now supports complex expressions
involving multiple criteria with AND / OR logic (&& / ||)
and brackets. The details of the expression syntax are described in
the ap_expr documentation.
- Configure the Chain
- The above directives build components of a smart filter chain,
but do not configure it to run. The
FilterChain directive builds a filter chain from smart
filters declared, offering the flexibility to insert filters at the
beginning or end of the chain, remove a filter, or clear the chain.
Filtering and Response Status
mod_filter normally only runs filters on responses with
HTTP status 200 (OK). If you want to filter documents with
other response statuses, you can set the filter-errordocs
environment variable, and it will work on all responses
regardless of status. To refine this further, you can use
expression conditions with FilterProvider .
Upgrading from Apache HTTP Server 2.2 Configuration
The FilterProvider
directive has changed from httpd 2.2: the match and
dispatch arguments are replaced with a single but
more versatile expression. In general, you can convert
a match/dispatch pair to the two sides of an expression, using
something like:
"dispatch = 'match'"
The Request headers, Response headers and Environment variables
are now interpreted from syntax %{req:foo},
%{resp:foo} and %{env:foo} respectively.
The variables %{HANDLER} and %{CONTENT_TYPE}
are also supported.
Note that the match no longer support substring matches. They can be
replaced by regular expression matches.
Examples
- Server side Includes (SSI)
- A simple case of replacing
AddOutputFilterByType
FilterDeclare SSI
FilterProvider SSI INCLUDES "%{CONTENT_TYPE} =~ m|^text/html|"
FilterChain SSI
- Server side Includes (SSI)
- The same as the above but dispatching on handler (classic
SSI behaviour; .shtml files get processed).
FilterProvider SSI INCLUDES "%{HANDLER} = 'server-parsed'"
FilterChain SSI
- Emulating mod_gzip with mod_deflate
- Insert INFLATE filter only if "gzip" is NOT in the
Accept-Encoding header. This filter runs with ftype CONTENT_SET.
FilterDeclare gzip CONTENT_SET
FilterProvider gzip inflate "%{req:Accept-Encoding} !~ /gzip/"
FilterChain gzip
- Image Downsampling
- Suppose we want to downsample all web images, and have filters
for GIF, JPEG and PNG.
FilterProvider unpack jpeg_unpack "%{CONTENT_TYPE} = 'image/jpeg'"
FilterProvider unpack gif_unpack "%{CONTENT_TYPE} = 'image/gif'"
FilterProvider unpack png_unpack "%{CONTENT_TYPE} = 'image/png'"
FilterProvider downsample downsample_filter "%{CONTENT_TYPE} = m|^image/(jpeg|gif|png)|"
FilterProtocol downsample "change=yes"
FilterProvider repack jpeg_pack "%{CONTENT_TYPE} = 'image/jpeg'"
FilterProvider repack gif_pack "%{CONTENT_TYPE} = 'image/gif'"
FilterProvider repack png_pack "%{CONTENT_TYPE} = 'image/png'"
<Location "/image-filter">
FilterChain unpack downsample repack
</Location>
Protocol Handling
Historically, each filter is responsible for ensuring that whatever
changes it makes are correctly represented in the HTTP response headers,
and that it does not run when it would make an illegal change. This
imposes a burden on filter authors to re-implement some common
functionality in every filter:
- Many filters will change the content, invalidating existing content
tags, checksums, hashes, and lengths.
- Filters that require an entire, unbroken response in input need to
ensure they don't get byteranges from a backend.
- Filters that transform output in a filter need to ensure they don't
violate a
Cache-Control: no-transform header from the
backend.
- Filters may make responses uncacheable.
mod_filter aims to offer generic handling of these
details of filter implementation, reducing the complexity required of
content filter modules. This is work-in-progress; the
FilterProtocol implements
some of this functionality for back-compatibility with Apache 2.0
modules. For httpd 2.1 and later, the
ap_register_output_filter_protocol and
ap_filter_protocol API enables filter modules to
declare their own behaviour.
At the same time, mod_filter should not interfere
with a filter that wants to handle all aspects of the protocol. By
default (i.e. in the absence of any FilterProtocol directives), mod_filter
will leave the headers untouched.
At the time of writing, this feature is largely untested,
as modules in common use are designed to work with 2.0.
Modules using it should test it carefully.
AddOutputFilterByType Directive
| Description: | assigns an output filter to a particular media-type |
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| Syntax: | AddOutputFilterByType filter[;filter...]
media-type [media-type] ... |
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| Context: | server config, virtual host, directory, .htaccess |
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| Override: | FileInfo |
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| Status: | Base |
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| Module: | mod_filter |
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| Compatibility: | Had severe limitations before
being moved to mod_filter in version 2.3.7 |
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This directive activates a particular output filter for a request depending on the
response media-type.
The following example uses the DEFLATE filter, which
is provided by mod_deflate . It will compress all
output (either static or dynamic) which is labeled as
text/html or text/plain before it is sent
to the client.
AddOutputFilterByType DEFLATE text/html text/plain
If you want the content to be processed by more than one filter, their
names have to be separated by semicolons. It's also possible to use one
AddOutputFilterByType directive for each of
these filters.
The configuration below causes all script output labeled as
text/html to be processed at first by the
INCLUDES filter and then by the DEFLATE
filter.
<Location "/cgi-bin/">
Options Includes
AddOutputFilterByType INCLUDES;DEFLATE text/html
</Location>
See also
-
AddOutputFilter
-
SetOutputFilter
- filters
FilterChain Directive
| Description: | Configure the filter chain |
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| Syntax: | FilterChain [+=-@!]filter-name ... |
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| Context: | server config, virtual host, directory, .htaccess |
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| Override: | Options |
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| Status: | Base |
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| Module: | mod_filter |
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This configures an actual filter chain, from declared filters.
FilterChain takes any number of arguments,
each optionally preceded with a single-character control that
determines what to do:
-
+filter-name
- Add filter-name to the end of the filter chain
-
@filter-name
- Insert filter-name at the start of the filter chain
-
-filter-name
- Remove filter-name from the filter chain
-
=filter-name
- Empty the filter chain and insert filter-name
-
!
- Empty the filter chain
-
filter-name
- Equivalent to
+filter-name
FilterDeclare Directive
| Description: | Declare a smart filter |
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| Syntax: | FilterDeclare filter-name [type] |
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| Context: | server config, virtual host, directory, .htaccess |
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| Override: | Options |
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| Status: | Base |
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| Module: | mod_filter |
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This directive declares an output filter together with a
header or environment variable that will determine runtime
configuration. The first argument is a filter-name
for use in FilterProvider ,
FilterChain and
FilterProtocol directives.
The final (optional) argument
is the type of filter, and takes values of ap_filter_type
- namely RESOURCE (the default), CONTENT_SET ,
PROTOCOL , TRANSCODE , CONNECTION
or NETWORK .
FilterProtocol Directive
| Description: | Deal with correct HTTP protocol handling |
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| Syntax: | FilterProtocol filter-name [provider-name]
proto-flags |
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| Context: | server config, virtual host, directory, .htaccess |
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| Override: | Options |
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| Status: | Base |
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| Module: | mod_filter |
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This directs mod_filter to deal with ensuring the
filter doesn't run when it shouldn't, and that the HTTP response
headers are correctly set taking into account the effects of the
filter.
There are two forms of this directive. With three arguments, it
applies specifically to a filter-name and a
provider-name for that filter.
With two arguments it applies to a filter-name whenever the
filter runs any provider.
Flags specified with this directive are merged with the flags
that underlying providers may have registerd with
mod_filter . For example, a filter may internally specify
the equivalent of change=yes , but a particular
configuration of the module can override with change=no .
proto-flags is one or more of
-
change=yes|no
- Specifies whether the filter changes the content, including possibly
the content length. The "no" argument is supported in 2.4.7 and later.
-
change=1:1
- The filter changes the content, but will not change the content
length
-
byteranges=no
- The filter cannot work on byteranges and requires complete input
-
proxy=no
- The filter should not run in a proxy context
-
proxy=transform
- The filter transforms the response in a manner incompatible with
the HTTP
Cache-Control: no-transform header.
-
cache=no
- The filter renders the output uncacheable (eg by introducing randomised
content changes)
FilterProvider Directive
| Description: | Register a content filter |
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| Syntax: | FilterProvider filter-name provider-name
expression |
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| Context: | server config, virtual host, directory, .htaccess |
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| Override: | Options |
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| Status: | Base |
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| Module: | mod_filter |
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This directive registers a provider for the smart filter.
The provider will be called if and only if the expression
declared evaluates to true when the harness is first called.
provider-name must have been registered by loading
a module that registers the name with
ap_register_output_filter .
expression is an
ap_expr.
See also
- Expressions in Apache HTTP Server,
for a complete reference and examples.
-
mod_include
FilterTrace Directive
| Description: | Get debug/diagnostic information from
mod_filter |
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| Syntax: | FilterTrace filter-name level |
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| Context: | server config, virtual host, directory |
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| Status: | Base |
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| Module: | mod_filter |
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This directive generates debug information from
mod_filter .
It is designed to help test and debug providers (filter modules), although
it may also help with mod_filter itself.
The debug output depends on the level set:
-
0 (default)
- No debug information is generated.
-
1
-
mod_filter will record buckets and brigades
passing through the filter to the error log, before the provider has
processed them. This is similar to the information generated by
mod_diagnostics.
-
2 (not yet implemented)
- Will dump the full data passing through to a tempfile before the
provider. For single-user debug only; this will not
support concurrent hits.
