# Module mod_cgi

This module provides for execution of CGI scripts.

Status: Base
Source File: mod_cgi.c
Module Identifier: cgi_module

## 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 intruction 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.

## Directives

- ScriptLog
- ScriptLogLength
- ScriptLogBuffer

See also: Options, ScriptAlias, AddType and AddHandler.

## CGI Environment variables

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

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.

## 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).

### ScriptLog directive

Syntax: ScriptLog filename
Default: none
Context: server config
Status: mod_cgi

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 server root.

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 script logging is meant to be a debugging feature when writing CGI scripts, and is not meant to be activated continuously on running servers. It is not optimized for speed or efficiency, and may have security problems if used in a manner other than that for which it was designed.

### ScriptLogLength directive

Syntax: ScriptLogLength bytes
Default: 10385760
Context: server config
Status: mod_cgi

`ScriptLogLength` can be used to limit the size of the CGI script logfile. Since the logfile logs a lot of information per CGI error (all request headers, all script output) it can grow to be a big file. To prevent problems due to unbounded growth, this directive can be used to set an maximum file-size for the CGI logfile. If the file exceeds this size, no more information will be written to it.

### ScriptLogBuffer

Syntax: ScriptLogBuffer bytes
Default: 1024
Context: server config
Status: mod_cgi

The size of any PUT or POST entity body that is logged to the file is limited, to prevent the log file growing too big too quickly if large bodies are being received. By default, up to 1024 bytes are logged, but this can be changed with this directive.