mod_filter - Apache HTTP Server Version 2.4

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Apache Module mod_filter

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Description:Context-sensitive smart filter configuration module
Compatibility:Version 2.1 and later

    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
 Filter Declarations, Providers and Chains
 Configuring the Chain
 Filtering and Response Status
 Upgrading from Apache HTTP Server 2.2 Configuration
 Protocol Handling


Bugfix checklisthttpd changelogKnown issuesReport a bugSee also


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.

    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


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

    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
Syntax:AddOutputFilterByType filter[;filter...]
media-type [media-type] ...
Context:server config, virtual host, directory, .htaccess
Compatibility:Had severe limitations before
being moved to mod_filter in version 2.3.7

    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

    <Location "/cgi-bin/">
    Options Includes
    AddOutputFilterByType INCLUDES;DEFLATE text/html

See also


FilterChain Directive

Description:Configure the filter chain
Syntax:FilterChain [+=-@!]filter-name ...
Context:server config, virtual host, directory, .htaccess

    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:

    Add filter-name to the end of the filter chain

    Insert filter-name at the start of the filter chain

    Remove filter-name from the filter chain

    Empty the filter chain and insert filter-name

    Empty the filter chain

    Equivalent to +filter-name

FilterDeclare Directive

Description:Declare a smart filter
Syntax:FilterDeclare filter-name [type]
Context:server config, virtual host, directory, .htaccess

    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,
    or NETWORK.

FilterProtocol Directive

Description:Deal with correct HTTP protocol handling
Syntax:FilterProtocol filter-name [provider-name]
Context:server config, virtual host, directory, .htaccess

    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

    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

    Specifies whether the filter changes the content, including possibly 
    the content length.  The "no" argument is supported in 2.4.7 and later.

    The filter changes the content, but will not change the content

    The filter cannot work on byteranges and requires complete input

    The filter should not run in a proxy context

    The filter transforms the response in a manner incompatible with
    the HTTP Cache-Control: no-transform header.

    The filter renders the output uncacheable (eg by introducing randomised
    content changes)

FilterProvider Directive

Description:Register a content filter
Syntax:FilterProvider filter-name provider-name
Context:server config, virtual host, directory, .htaccess

    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

    expression is an

See also

Expressions in Apache HTTP Server,
for a complete reference and examples.

FilterTrace Directive

Description:Get debug/diagnostic information from
Syntax:FilterTrace filter-name level
Context:server config, virtual host, directory

    This directive generates debug information from
    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.

    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

    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.

Available Languages:  en  |
CommentsNotice:This is not a Q&A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed again by our moderators if they are either implemented or considered invalid/off-topic. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Freenode, or sent to our mailing lists.

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