Apache Module mod_isapi

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.Pleasedo 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 checklist

• httpd changelog
• Known issues
• Report a bug

See also

• Comments

Usage

In the server configuration file, use theAddHandler directive to associate ISAPI files with theisapi-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

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,OptionsExecCGI must be set for the directory that contains the ISAPI .dll file.

Review theAdditional Notes and theProgrammer's Journal for additional details and clarification of the specific ISAPI support offered bymod_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 directiveISAPILogNotSupported 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 theISAPICacheFile 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, itdoes 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 2.0mod_isapi modules, you must limit your calls toServerSupportFunction 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 twoHSE_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 aCustomLog directive
• in the%q log component with theISAPIAppendLogToQueryOn directive
• in the error log with theISAPIAppendLogToErrorsOn 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 thefKeepConn flag is ignored.

HSE_REQ_IS_CONNECTED

Will report false if the request has been aborted.

Apache returnsFALSE to any unsupported call toServerSupportFunction, and sets theGetLastError value toERROR_INVALID_PARAMETER.

ReadClient retrieves the request body exceeding the initial buffer (defined byISAPIReadAheadBuffer). Based on theISAPIReadAheadBuffer 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 useReadClient to retrieve the remaining request body.

WriteClient is supported, but only with theHSE_IO_SYNC flag or no option flag (value of0). Any otherWriteClient request will be rejected with a return value ofFALSE, and aGetLastError value ofERROR_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 fromGetServerVariable, as well as theALL_HTTP andALL_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 theTransmitFile semantics. Apache httpd also supports preloading ISAPI .dlls for performance.

ISAPIAppendLogToErrorsDirective

RecordHSE_APPEND_LOG_PARAMETER requests from ISAPI extensions to the server error log.

ISAPIAppendLogToQueryDirective

RecordHSE_APPEND_LOG_PARAMETER requests from ISAPI extensions to the query field (appended to theCustomLog%q component).

ISAPICacheFileDirective

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 toServerRoot.

ISAPIFakeAsyncDirective

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

ISAPILogNotSupportedDirective

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.

ISAPIReadAheadBufferDirective

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 theReadClient callback; some ISAPI extensions may not support theReadClient function. Refer questions to the ISAPI extension's author.