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Apache HTTP Server Version 2.0

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Apache Module mod_cgi

Description: Execution of CGI scripts
Status: Base
Module Identifier: cgi_module
Source File: mod_cgi.c

Summary

Any file that has the mime type application/x-httpd-cgi or handler cgi-script (Apache 1.1 or later) will be treated as a CGI script, and run by the server, with its output being returned to the client. Files acquire this type either by having a name containing an extension defined by the AddType directive, or by being in a ScriptAlias directory.

When the server invokes a CGI script, it will add a variable called DOCUMENT_ROOT to the environment. This variable will contain the value of the DocumentRoot configuration variable.

For an introduction to using CGI scripts with Apache, see our tutorial on Dynamic Content With CGI.

When using a multi-threaded MPM under unix, the module mod_cgid should be used in place of this module. At the user level, the two modules are essentially identical.

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CGI Environment variables

The server will set the CGI environment variables as described in the CGI specification, with the following provisions:

PATH_INFO
This will not be available if the AcceptPathInfo directive is explicitly set to off. The default behavior, if AcceptPathInfo is not given, is that mod_cgi will accept path info (trailing /more/path/info following the script filename in the URI), while the core server will return a 404 NOT FOUND error for requests with additional path info. Omitting the AcceptPathInfo directive has the same effect as setting it on for mod_cgi requests.
REMOTE_HOST
This will only be set if HostnameLookups is set to on (it is off by default), and if a reverse DNS lookup of the accessing host's address indeed finds a host name.
REMOTE_IDENT
This will only be set if IdentityCheck is set to on and the accessing host supports the ident protocol. Note that the contents of this variable cannot be relied upon because it can easily be faked, and if there is a proxy between the client and the server, it is usually totally useless.
REMOTE_USER
This will only be set if the CGI script is subject to authentication.
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CGI Debugging

Debugging CGI scripts has traditionally been difficult, mainly because it has not been possible to study the output (standard output and error) for scripts which are failing to run properly. These directives, included in Apache 1.2 and later, provide more detailed logging of errors when they occur.

CGI Logfile Format

When configured, the CGI error log logs any CGI which does not execute properly. Each CGI script which fails to operate causes several lines of information to be logged. The first two lines are always of the format:

%% [time] request-line
%% HTTP-status CGI-script-filename

If the error is that CGI script cannot be run, the log file will contain an extra two lines:

%%error
error-message

Alternatively, if the error is the result of the script returning incorrect header information (often due to a bug in the script), the following information is logged:

%request
All HTTP request headers received
POST or PUT entity (if any)
%response
All headers output by the CGI script
%stdout
CGI standard output
%stderr
CGI standard error

(The %stdout and %stderr parts may be missing if the script did not output anything on standard output or standard error).

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ScriptLog Directive

Description: Location of the CGI script error logfile
Syntax: ScriptLog file-path
Context: server config, virtual host
Status: Base
Module: mod_cgi, mod_cgid

The ScriptLog directive sets the CGI script error logfile. If no ScriptLog is given, no error log is created. If given, any CGI errors are logged into the filename given as argument. If this is a relative file or path it is taken relative to the ServerRoot.

Example

ScriptLog logs/cgi_log

This log will be opened as the user the child processes run as, ie. the user specified in the main User directive. This means that either the directory the script log is in needs to be writable by that user or the file needs to be manually created and set to be writable by that user. If you place the script log in your main logs directory, do NOT change the directory permissions to make it writable by the user the child processes run as.

Note that sHTTP/1.1 200 OK Date: Mon, 07 Jul 2025 09:21:20 GMT Server: Apache/2.0.42 (Win32) PHP/5.2.10 Content-Location: mod_cgi.html.en Vary: negotiate TCN: choice Accept-Ranges: bytes Content-Length: 13885 Keep-Alive: timeout=15, max=100 Connection: Keep-Alive Content-Type: text/html; charset=ISO-8859-1 Content-Language: en mod_cgi - Apache HTTP Server

Modules | Directives | FAQ | Glossary | Sitemap

Apache HTTP Server Version 2.0

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Apache Module mod_cgi

Description: Execution of CGI scripts
Status: Base
Module Identifier: cgi_module
Source File: mod_cgi.c

Summary

Any file that has the mime type application/x-httpd-cgi or handler cgi-script (Apache 1.1 or later) will be treated as a CGI script, and run by the server, with its output being returned to the client. Files acquire this type either by having a name containing an extension defined by the AddType directive, or by being in a ScriptAlias directory.

When the server invokes a CGI script, it will add a variable called DOCUMENT_ROOT to the environment. This variable will contain the value of the DocumentRoot configuration variable.

For an introduction to using CGI scripts with Apache, see our tutorial on Dynamic Content With CGI.

When using a multi-threaded MPM under unix, the module mod_cgid should be used in place of this module. At t