Apache HTTP Server Version 1.3

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Module mod_isapi

This module supports ISAPI Extensions within Apache for Windows.

Status: Base
Source File: mod_isapi.c
Module Identifier: isapi_module
Compatibility: WIN32 only


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 extention. Please do not post such problems to Apache's lists or bug reporting pages.



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

    AddHandler isapi-isa .dll

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 the "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, a new directive; ISAPILogNotSupported Off, is introduced in Apache 1.3.13.

Some servers, like Microsoft IIS, load the ISA into the server, and keep it loaded until memory usage is too high, or specific configuration options are used. Apache currently loads and unloads the ISA for each request. This is inefficient, but Apache's request model makes this method the only method that currently works. Apache 2.0 is expected to support more effective loading and caching methods, with more precise control over individual ISAPI modules and directories.

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 1.3 mod_isapi modules, you must limit your calls to ServerSupportFunction to the following directives:

Redirect the user to another location.
This must be a fully qualified URL (e.g. http://server/location).
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.
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.
Apache considers this a no-op, since the session will be finished when the ISAPI returns from processing.
Apache will translate a virtual name to a physical name.
HSE_APPEND_LOG_PARAMETER Apache 1.3.13 and later
This logged message may be captured in any of the following logs: The first option, the %{isapi-parameter}n component, is always available and prefered.
HSE_REQ_IS_KEEP_CONN Apache 1.3.13 and later
Will return the negotiated Keep-Alive status.
HSE_REQ_SEND_RESPONSE_HEADER_EX Apache 1.3.13 and later
Will behave as documented, although the fKeepConn flag is ignored.
HSE_REQ_IS_CONNECTED Apache 1.3.13 and later
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.

Prior to Apache 1.3.13, ReadClient was a noop, and any request with a request body greater than 48kb was rejected by mod_isapi. As of Apache 1.3.13, ReadClient now 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 of Apache 1.3.13, the ALL_HTTP and ALL_RAW and variables are now available.

Apache 2.0 mod_isapi may support additional features introduced in later versions of the ISAPI specification, as well as limited emulation of async I/O and the TransmitFile semantics. Apache 2.0 may also support caching of ISAPI .dlls for performance. No further enhancements to the Apache 1.3 mod_isapi features are anticipated.

ISAPIReadAheadBuffer directive

Syntax: ISAPIReadAheadBuffer size
Default: 49152
Context: server config
Override: None
Status: Base
Module: mod_isapi
Compatibility: Apache 1.3.13 and later, Win32 only

Defines the maximum size of the Read Ahead Buffer sent to ISAPI extentions when they are initally 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 extention's author.

ISAPILogNotSupported directive

Syntax: ISAPILogNotSupported on|off
Default: on
Context: server config
Override: None
Status: Base
Module: mod_isapi
Compatibility: Apache 1.3.13 and later, Win32 only

Logs all requests for unsupported features from ISAPI extentions in the server error log. While this should be turned off once all desired ISAPI modules are functioning, it defaults to on to help administrators track down problems.

ISAPIAppendLogToErrors directive

Syntax: ISAPIAppendLogToErrors on|off
Default: off
Context: server config
Override: None
Status: Base
Module: mod_isapi
Compatibility: Apache 1.3.13 and later, Win32 only

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

ISAPIAppendLogToQuery directive

Syntax: ISAPIAppendLogToQuery on|off
Default: off
Context: server config
Override: None
Status: Base
Module: mod_isapi
Compatibility: Apache 1.3.13 and later, Win32 only

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

Apache HTTP Server Version 1.3

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