Another excellent site, which provides step by step chapters is: http://www.cgi101.com/class/

Understanding File Permissions:

There are a number of file permissions, which can be used for a variety of different purposes, however we'll limit this tutorial to the ones most commonly used. To begin with, it's important you understand the three categories of permissions, which are:

Owner Permissions:

The owner is you. In most cases, this is not so much of a concern, as you can only obtain owner permissions in one of two ways. 1. FTP into your account using your Username and Password. 2. Login via Telnet with the same information.

Group Permissions:

The represents a group of users who have access to a particular directory. For example, a password protected directory, whereas only members can access it upon providing the correct Username and Password. In this case, any permissions you assign to "Group" would be applicable to users with access to that particular directory upon logging in with their username and password.

Public Permissions:

This is the most important one of all. Public permissions determine what your world wide visitors can and cannot do with your files. ALWAYS make sure you understand what a particular permission does before assigning it to a file. If not, you may wakeup to find your website demolished by some clown who was snooping about and gained access to your files.

Setting File Permissions:

To set file permissions:

1. Login with your FTP client
2. Open the directory where the file you wish to set permissions on resides
3. Right click on the file and select CHMOD
A box similar to the one above will appear

Observe how you can "select" the individual permissions you want, or simply enter the 3 digit number if you know what it is. Most instructions included with downloaded scripts will tell indicate this to you.

By default, all files uploaded to the server automatically have permissions set to 644. The setting 644 is relatively safe, as it provides "Read" and "Write" access to the owner, while limiting the rest of the public to "Read Only" access.

When setting permissions for cgi scripts, the most common permissions setting is 755.   755 allows the owner "Read and Write" access, while allowing the Group and Public "Read and Execute" permissions. So what are we actually saying? In short, when users access your cgi script, the server has been instructed to grant them permissions to "Read and Execute" it. Sound scary? It's not actually…

Remember that a script is a program that must be processed by the server. As long as the script is written properly, you can safely allow users to execute it, and thus providing the desired results. For example, if they wanted to post a message to your wwwboard discussion forum, then they would need these permissions to execute wwwboard.pl, which would write their new message to an html file, which is displayed on the main forum.   The new message would reside in a directory on your site so other users could view it. Most cgi, perl and other scripts you'll be installing come complete with instructions telling you which permissions you'll need to set them to.

WARNING!

Setting permissions on files is a relatively simple task, however MAKE SURE you fully understand what it is you're allowing the public to do with your files. For example, some less experienced users often make the fatal mistake of simply setting ALL of their files to 777. While 777 will automatically allow executing privileges, it also allows full "READ, WRITE, and EXECUTION ability to the entire world!!!!

This is how web sites get hacked! While most visitors have good intentions, all it takes is one person whom snoops about your files seeking an "Open Back Door." This could result is them gaining full access to your directories, which means they can do anything from deleting your entire site, to defacing it with obscenities.


New to cgi? Here is a page with questions and answers to numerous questions evolving around the inns and outs of using cgi within your scripts: http://www.w3.org/Security/Faq/www-security-faq.html  

Using Server Side Includes - SSI

SSI works in conjunction with a web page usually with the .shtml extension.  The .shtml extension tells the server to do something different with the web page. When you append the .html or .htm extension, this tells the server to "read" the page only. The .shtml extension tells the server to "Execute" the page, in addition to just reading it.

So, why would you want to execute the page? There are various commands you can program into a web page, which the server will look for and parse when the file is called as .shtml. In many cases, this mode is used in conjunction with Server Side Include (SSI) tags, to call a CGI script. For example, you have a visitor counter script, and we'll call it count.cgi. Every time someone visits your website, you want the script to be called, so that it logs the visitor into a file.

To do this, you would place an SSI tag into your web page. The tag in this case, would look something like:

<!--#exec cgi="/cgi-bin/count.cgi" -->

This small tag, which is hidden in the html coding of your page is telling the server to:

1. Go to the cgi-bin
2. Execute count.cgi

That's it! The information has been captured and processed by the count.cgi script. Of course, that's the short version of what happens. The long version would no doubt, would take us far beyond the scope of this document.

PLEASE do not use the .shtml extension on "all" of your web pages unless it's absolutely necessary. With a busy web site, this means that every page must be executed, as opposed to just read. This as you can appreciate, can add considerable memory and CPU load to the system. As always, read the instructions that came with your script carefully.   They should provide specific instructions on how to configure the script, as well as the SSI tag.