mod_isapi - Apache HTTP Server Version 2.4









Modules | Directives | FAQ | Glossary | Sitemap
Apache HTTP Server Version 2.4



Apache > HTTP Server > Documentation > Version 2.4 > Modules

Apache Module mod_isapi

Available Languages:  en  |
 fr  |
 ko 

Description:ISAPI Extensions within Apache for Windows
Status:Base
Module Identifier:isapi_module
Source File:mod_isapi.c
Compatibility:Win32 only
Summary

    This module implements the Internet Server extension API. It
    allows Internet Server extensions (e.g. ISAPI .dll
    modules) to be served by Apache for Windows, subject to the
    noted restrictions.

    ISAPI extension modules (.dll files) are written by third
    parties. The Apache Group does not author these modules, so we
    provide no support for them. Please contact the ISAPI's author
    directly if you are experiencing problems running their ISAPI
    extension. Please do not post such problems to
    Apache's lists or bug reporting pages.

Topics

 Usage
 Additional Notes
 Programmer's Journal
Directives

 ISAPIAppendLogToErrors
 ISAPIAppendLogToQuery
 ISAPICacheFile
 ISAPIFakeAsync
 ISAPILogNotSupported
 ISAPIReadAheadBuffer

Bugfix checklisthttpd changelogKnown issuesReport a bugSee also

Comments


Usage

    In the server configuration file, use
    the AddHandler directive to
    associate ISAPI files with the isapi-handler handler, and map
    it to them with their file extensions. To enable any .dll file to be
    processed as an ISAPI extension, edit the httpd.conf file and add the
    following line:
    AddHandler isapi-handler .dll


    In older versions of the Apache server,
    isapi-isa was the proper handler name, rather than
    isapi-handler. As of 2.3 development versions of the Apache
    server, isapi-isa is no longer valid. You will need to
    change your configuration to use isapi-handler
    instead.

    There is no capability within the Apache server to leave a
    requested module loaded. However, you may preload and keep a
    specific module loaded by using the following syntax in your
    httpd.conf:
    ISAPICacheFile c:/WebWork/Scripts/ISAPI/mytest.dll


    Whether or not you have preloaded an ISAPI extension, all
    ISAPI extensions are governed by the same permissions and
    restrictions as CGI scripts. That is, Options ExecCGI must be set for the
    directory that contains the ISAPI .dll file.

    Review the Additional Notes and the Programmer's Journal for additional details
    and clarification of the specific ISAPI support offered by
    mod_isapi.


Additional Notes

    Apache's ISAPI implementation conforms to all of the ISAPI
    2.0 specification, except for some "Microsoft-specific"
    extensions dealing with asynchronous I/O. Apache's I/O model
    does not allow asynchronous reading and writing in a manner
    that the ISAPI could access. If an ISA tries to access
    unsupported features, including async I/O, a message is placed
    in the error log to help with debugging. Since these messages
    can become a flood, the directive ISAPILogNotSupported
    Off exists to quiet this noise.

    Some servers, like Microsoft IIS, load the ISAPI extension
    into the server and keep it loaded until memory usage is too
    high, or unless configuration options are specified. Apache
    currently loads and unloads the ISAPI extension each time it is
    requested, unless the ISAPICacheFile directive is specified.
    This is inefficient, but Apache's memory model makes this the
    most effective method. Many ISAPI modules are subtly
    incompatible with the Apache server, and unloading these
    modules helps to ensure the stability of the server.

    Also, remember that while Apache supports ISAPI Extensions,
    it does not support ISAPI Filters. Support for
    filters may be added at a later date, but no support is planned
    at this time.


Programmer's Journal

    If you are programming Apache 2.0 mod_isapi
    modules, you must limit your calls to ServerSupportFunction
    to the following directives:

    
      HSE_REQ_SEND_URL_REDIRECT_RESP
      Redirect the user to another location.
      This must be a fully qualified URL (e.g.
      http://server/location).

      HSE_REQ_SEND_URL
      Redirect the user to another location.
      This cannot be a fully qualified URL, you are not allowed to
      pass the protocol or a server name (e.g. simply
      /location).
      This redirection is handled by the server, not the
      browser.
      Warning
        In their recent documentation, Microsoft appears to have
        abandoned the distinction between the two
        HSE_REQ_SEND_URL functions. Apache continues to treat
        them as two distinct functions with different requirements
        and behaviors.
      

      HSE_REQ_SEND_RESPONSE_HEADER
      Apache accepts a response body following the header if it
      follows the blank line (two consecutive newlines) in the
      headers string argument. This body cannot contain NULLs,
      since the headers argument is NULL terminated.

      HSE_REQ_DONE_WITH_SESSION
      Apache considers this a no-op, since the session will be
      finished when the ISAPI returns from processing.

      HSE_REQ_MAP_URL_TO_PATH
      Apache will translate a virtual name to a physical
      name.

      HSE_APPEND_LOG_PARAMETER
      
        This logged message may be captured in any of the following
        logs:

        
          in the \"%{isapi-parameter}n\" component in a
          CustomLog
          directive

          in the %q log component with the
          ISAPIAppendLogToQuery
          On directive

          in the error log with the ISAPIAppendLogToErrors On directive
        

        The first option, the %{isapi-parameter}n component,
        is always available and preferred.
      

      HSE_REQ_IS_KEEP_CONN
      Will return the negotiated Keep-Alive status.

      HSE_REQ_SEND_RESPONSE_HEADER_EX
      Will behave as documented, although the fKeepConn
      flag is ignored.

      HSE_REQ_IS_CONNECTED
      Will report false if the request has been aborted.
    

    Apache returns FALSE to any unsupported call to
    ServerSupportFunction, and sets the
    GetLastError value to
    ERROR_INVALID_PARAMETER.

    ReadClient retrieves the request body exceeding the
    initial buffer (defined by ISAPIReadAheadBuffer). Based on the
    ISAPIReadAheadBuffer setting (number of bytes
    to buffer prior to calling the ISAPI handler) shorter requests are sent
    complete to the extension when it is invoked. If the request is
    longer, the ISAPI extension must use ReadClient to
    retrieve the remaining request body.

    WriteClient is supported, but only with the
    HSE_IO_SYNC flag or no option flag (value of
    0). Any other WriteClient request
    will be rejected with a return value of FALSE, and a
    GetLastError value of
    ERROR_INVALID_PARAMETER.

    GetServerVariable is supported, although extended server
    variables do not exist (as defined by other servers.) All the
    usual Apache CGI environment variables are available from
    GetServerVariable, as well as the ALL_HTTP
    and ALL_RAW values.

    Since httpd 2.0, mod_isapi supports additional
    features introduced in later versions of the ISAPI specification,
    as well as limited emulation of async I/O and the
    TransmitFile semantics. Apache httpd also supports preloading
    ISAPI .dlls for performance.


ISAPIAppendLogToErrors Directive

Description:Record HSE_APPEND_LOG_PARAMETER requests from
ISAPI extensions to the error log
Syntax:ISAPIAppendLogToErrors on|off
Default:ISAPIAppendLogToErrors off
Context:server config, virtual host, directory, .htaccess
Override:FileInfo
Status:Base
Module:mod_isapi

    Record HSE_APPEND_LOG_PARAMETER requests from ISAPI
    extensions to the server error log.



ISAPIAppendLogToQuery Directive

Description:Record HSE_APPEND_LOG_PARAMETER requests from
ISAPI extensions to the query field
Syntax:ISAPIAppendLogToQuery on|off
Default:ISAPIAppendLogToQuery on
Context:server config, virtual host, directory, .htaccess
Override:FileInfo
Status:Base
Module:mod_isapi

    Record HSE_APPEND_LOG_PARAMETER requests from ISAPI
    extensions to the query field (appended to the CustomLog %q
    component).



ISAPICacheFile Directive

Description:ISAPI .dll files to be loaded at startup
Syntax:ISAPICacheFile file-path [file-path]
...
Context:server config, virtual host
Status:Base
Module:mod_isapi

    Specifies a space-separated list of file names to be loaded
    when the Apache server is launched, and remain loaded until the
    server is shut down. This directive may be repeated for every
    ISAPI .dll file desired. The full path name of each file should
    be specified. If the path name is not absolute, it will be treated
    relative to ServerRoot.



ISAPIFakeAsync Directive

Description:Fake asynchronous support for ISAPI callbacks
Syntax:ISAPIFakeAsync on|off
Default:ISAPIFakeAsync off
Context:server config, virtual host, directory, .htaccess
Override:FileInfo
Status:Base
Module:mod_isapi

    While set to on, asynchronous support for ISAPI callbacks is
    simulated.



ISAPILogNotSupported Directive

Description:Log unsupported feature requests from ISAPI
extensions
Syntax:ISAPILogNotSupported on|off
Default:ISAPILogNotSupported off
Context:server config, virtual host, directory, .htaccess
Override:FileInfo
Status:Base
Module:mod_isapi

    Logs all requests for unsupported features from ISAPI
    extensions in the server error log. This may help administrators
    to track down problems. Once set to on and all desired ISAPI modules
    are functioning, it should be set back to off.



ISAPIReadAheadBuffer Directive

Description:Size of the Read Ahead Buffer sent to ISAPI
extensions
Syntax:ISAPIReadAheadBuffer size
Default:ISAPIReadAheadBuffer 49152
Context:server config, virtual host, directory, .htaccess
Override:FileInfo
Status:Base
Module:mod_isapi

    Defines the maximum size of the Read Ahead Buffer sent to
    ISAPI extensions when they are initially invoked. All remaining
    data must be retrieved using the ReadClient callback; some
    ISAPI extensions may not support the ReadClient function.
    Refer questions to the ISAPI extension's author.




Available Languages:  en  |
 fr  |
 ko 

Copyright 2026 The Apache Software Foundation.Licensed under the Apache License, Version 2.0.
Modules | Directives | FAQ | Glossary | Sitemap