This is a guide to use CGI (Common Gateway Interface) module in Apache httpd server.

1) In httpd.conf, Apache configuration file located in 'conf' directory,

  first remove the comment character, # at the line of mod_cgid.so in LoadModule part,

  so as to use Apache CGI module.

LoadModule cgid_module modules/mod_cgid.so

2) In 'alias_module' configuration part, set the path of '/cgi-bin/' like below.

  This module configuration part is already in the 'httpd.conf' file, so find it and then write it.

  In Apache configuration, the sharp, '#' character is comment directive.

<IfModule alias_module>
    #ScriptAlias /cgi-bin/ "/usr/local/apache2/cgi-bin/"
    ScriptAlias /cgi-bin/ "/home/ubuntu/cgi-bin/"
</IfModule>

3) For the directory configured above, do directory configuration, too.

  There should be 'ExecCGI' property in 'Options' part.

  The directory setting part like '/usr/local/apache2/cgi-bin/' is already there in 'httpd.conf' file.

<Directory "/home/ubuntu/cgi-bin">
    AllowOverride All
    Options ExecCGI
    SetHandler cgi-script
    Require all granted
    Order allow,deny
    Allow from all
</Directory>

4) In 'mime_module' configuration, designate the extension for 'cgi-script' file.

  This configures the handler for what extension 'cgi-script' does operate.

  Search 'mime_module' configuration part in the 'httpd.conf' file.

<IfModule mime_module>
    AddHandler cgi-script .cgi .pl .py
</IfModule>

5) In case of the Tomcat connection with Apache,

  map the 'cgi-bin' path like below to be processed by Apache httpd server, not by Tomcat in URI mapping.

  (! : exclude by exclamation mark)

!/cgi-bin/*=worker1

6) Write CGI script. This is bash shell script.

  Write the file in the directory configured in 2).

#!/bin/bash

echo "Content-type: text/html"
echo ""
echo "Hello.<br/>"

7) Give execute permission to the file above.

  Set executable by all, owner, group and other.

chmod a+x test.cgi

8) At browser address, write this '/cgi-bin/' file url.

  It outputs 'Hello.'

https://www.jisblee.me/cgi-bin/test.cgi

 9) This cgi program can be written with C language.

  Write the source code below (hello.c) to the '/cgi-bin/' directory.

  Then compile and make executable by 'gcc -o hello.cgi hello.c'.

#include <stdio.h>
int main()
{
        printf("Content-type: text/html%c%c", 10, 10);
        printf("Hello, World!");
        return 0;
}

10) CGI vulnerability

  By manipulating the input parameter, system command could be executed.

  This can be abused by malicious user.

  CGI must be used in safe manner if necessary.