cpp: Enable CGI Apache2 (Debian et al)

C++ Server Side Web Development

CGI, the Common Gateway Interface is a simple way to write web applications. It is by no means the fastest and some may say that:

"If you're planning to write your web application in C++, it would be total waste to then interface it as CGI.

My suggestion would be to build it asynchronous using ASIO (Asynchronous I/O). With that you can build blazing fast web service (combine with nginx as a reverse-proxy and statics server for best effects); Combine that with template library like Wt and you're ready to serve tens of thousands request per second from a single server.

Whether this is practical alternative to dynamic language web framework is another issue." vartec 2013

However, it is a simple way of demonstrating server side web development with a number of programming languages, including C++.

Install Apache2

To support websites built with C++ using the CGI requires webserver software capable of (which is all of them) and configured to enable (not all of them) execute code in a cgi directory.

A common webserver choice is Apache2 herein version 2.4 since 2012-02-21 latest 2019-08-14 (2.4.41)

$ sudo apt-get install apache2

Install cURL

cURL (client URL) aka and henceforth curl, is a command line tool for file transfer with a URL syntax. It supports a number of protocols including HTTP, HTTPS, FTP and assuming remote headless server hardware can be used to fetch web pages.

$ sudo apt-get install curl

Usage for to access localhost:

$ curl http://127.0.0.1/

Will print the default Apache2 index.html web page HTML on the screen:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
<html>
 <head>
  <title>Index of /</title>
 </head>
 <body>
<h1>Index of /</h1>
  <table>
   <tr><th valign="top"><img src="/icons/blank.gif" alt="[ICO]"></th><th><a href="?C=N;O=D">Name</a></th><th><a href="?C=M;O=A">Last modified</a></th><th><a href="?C=S;O=A">Size</a></th><th><a href="?C=D;O=A">Description</a></th></tr>
   <tr><th colspan="5"><hr></th></tr>
<tr><td valign="top"><img src="/icons/folder.gif" alt="[DIR]"></td><td><a href="cgi-bin/">cgi-bin/</a></td><td align="right">2019-12-08 13:15  </td><td align="right">  - </td><td>&nbsp;</td></tr>
<tr><td valign="top"><img src="/icons/folder.gif" alt="[DIR]"></td><td><a href="html/">html/</a></td><td align="right">2019-12-08 11:52  </td><td align="right">  - </td><td>&nbsp;</td></tr>
   <tr><th colspan="5"><hr></th></tr>
</table>
<address>Apache/2.4.29 (Ubuntu) Server at 127.0.0.1 Port 80</address>
</body></html>$ curl http://127.0.0.1/

Enable Apache2 CGI module

By default Apache2 does not have its cgi module enabled.

The collection of available modules can be found in:

/etc/apache2/mods-available

Whilst the enabled modules are present as symbolic links to the available modules in:

/etc/apache2/mods-enabled

Enable the cgi module by adding a symbolic link from mods-enabled to mods-available:

$ cd /etc/apache2/mods-enabled
$ sudo ln -s ../mods-available/cgi.load

Where to put CGI executables?

Examing the Apache2 configuration files reveals that, in a similar manner to modules, the available to enabled relationship is used:

/etc/apache2/conf-available/serve-cgi-bin.conf

file has a symbolic link from:

/etc/apache2/conf-enabled/serve-cgi-bin.conf

It has a section that maps the /cgi-bin path in the URLs to the /usr/lib/cgi-bin/ directory and enables CGI execution in this directory:

serve-cgi-bin.conf

<IfModule mod_alias.c>
 <IfModule mod_cgi.c>
  Define ENABLE_USR_LIB_CGI_BIN
 </IfModule>
 
 <IfModule mod_cgid.c>
  Define ENABLE_USR_LIB_CGI_BIN
 </IfModule>
 
 <IfDefine ENABLE_USR_LIB_CGI_BIN>
  ScriptAlias /cgi-bin/ /usr/lib/cgi-bin/
  <Directory "/usr/lib/cgi-bin">
   AllowOverride None
   Options +ExecCGI -MultiViews +SymLinksIfOwnerMatch
   Require all granted
  </Directory>
 </IfDefine>
</IfModule>

Reload Apache configuration

As the configuration of Apache has changed tell Apache to reload its configuration files:

$ sudo /etc/init.d/apache2 restart

Create the first CGI script

With CGI enabled and Apache2 restarted test with a simple Bash scrip:

helloworld.sh


#!/bin/bash
printf "Content-type: text/html\n\n"
printf "Hello World!\n"

Render it executable:

$ sudo chmod +x /usr/lib/cgi-bin/helloworld.sh

Then access it:

$ curl http://127.0.0.1/cgi-bin/hw.sh
Hello World!

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Sunday, 8 December 2019

Enable CGI Apache2 (Debian et al)