Apache Module mod_cgi

Summary

Any file that has the handler cgi-script will be treated as a CGI script, and run by the server, with its output being returned to the client. Files acquire this handler either by having a name containing an extension defined by the AddHandler directive, or by being in a ScriptAlias directory.

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.

For backward-compatibility, the cgi-script handler will also be activated for any file with the mime-type application/x-httpd-cgi. The use of the magic mime-type is deprecated.

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.

This module also leverages the core functions ap_add_common_vars and ap_add_cgi_vars to add environment variables like:

DOCUMENT_ROOT

Set with the content of the related DocumentRoot directive.

SERVER_NAME

The fully qualified domain name related to the request.

SERVER_ADDR

The IP address of the Virtual Host serving the request.

SERVER_ADMIN

Set with the content of the related ServerAdmin directive.

For an exhaustive list it is suggested to write a basic CGI script that dumps all the environment variables passed by Apache in a convenient format.

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 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).

ScriptLog file-path

ScriptLog logs/cgi_log

ScriptLogBuffer bytes

ScriptLogBuffer 1024

ScriptLogLength bytes

ScriptLogLength 10385760