TweetFollow Us on Twitter

Enabling CGI Scripts

Volume Number: 19 (2003)
Issue Number: 9
Column Tag: Programming

Untangling the Web

Enabling CGI Scripts

by Kevin Hemenway

Prepare your server for more powerful dynamic capabilities

Last month, we started down the road of adding dynamic content to our web server with the use of Server Side Includes (SSI). Often regarded as "simple" compared to languages like Perl and PHP, they can provide some quick turnaround to minor features like small single-item databases, changing content based on user whim or URL, and so forth. Regardless of whether you plan on using SSIs or not, we laid some important groundwork with our understanding of how modules work, Apache's "block" configuration format, and a soft introduction to GET and POST.

All relatively Lego. Let's break out the Technix.

A Quick 'How do ya do?' To CGI

SSI's are built into Apache through the use of a module--when they're enabled, the web server handles the request of a resource, processes the SSI statements within, and then sends the final output back to the user. Since Apache is a web server and not a programming language, the capabilities of the built-in SSIs are minimal--they're not intended to be a full-fledged coding environment, and they never will be.

This is where CGI comes in. Meaning "Common Gateway Interface", it defines a way for a web server to interact with external programs: Apache handles the request, sends some relevant information to the requested resource, and trusts the resource to handle the rest (including sending back the final content). If the resource doesn't respond properly (either due to bad programming, incorrect permissions, etc.), Apache generates an error.

What does this really mean? Ultimately, it allows you to use any shell programming language to "do stuff" before showing the results to your visitors. Whether this means displaying content from a database, resetting passwords, saving a comment to a weblog, etc., as long as your program finishes properly, Apache doesn't give a darn.

Most commonly, these "CGI scripts" are written in Perl, but you can use PHP, C, Bash, Ruby, Python, and anything else you may be interested in. Likewise, you can use them all in tandem--you're not locked into any one programming language at any one time. Be forewarned, however: you are locked into security concerns: anything possible in your programming language of choice is doable within your CGI script, including deletion of files, exhaustion of memory, buffer overflows, poor security, and impolite use of resources. Add in the fact that all of these errors in judgment become publicly run-able by anyone who accesses your server, and you've got a whole mess of potential trouble.

Enabling The Gateway to Your Scripts

Learning about CGI follows my oft-repeated method of "search the httpd.conf for the feature you want". So, open up /etc/httpd/httpd.conf in your favorite text editor and search for the word "CGI"--our first two matches look vaguely familiar:

LoadModule cgi_module         libexec/httpd/mod_cgi.so
AddModule mod_cgi.c

If you recall from our previous articles, Apache contains most of its features broken up into modules--the equivalent of plugins. To enable a module, two lines must exist uncommented (not preceded by a # character) in the httpd.conf file, an AddModule and a LoadModule. As previously with SSI, so too with CGI: our two lines already exist, and are already enabled. Let's move on to our next search result, which is also similar to what we saw last month:

<Directory "/Library/WebServer/Documents">
    # This may also be "None", "All", or any combination of 
    # "Indexes", "Includes", "FollowSymLinks", "ExecCGI",   
    # or "MultiViews".
    Options Indexes FollowSymLinks MultiViews
    AllowOverride None
    Order allow,deny
    Allow from all
</Directory>

Whereas last article the above result was triggered by the word Includes, it's ExecCGI this time around. Unfortunately, explaining ExecCGI without some prior knowledge will cause a little bit of confusion, so let's skip ahead to our next few matches before we go much further. I've truncated them for relevance:

# ScriptAlias: This controls which directories
# contain server scripts. ScriptAliases are essentially
# the same as Aliases, except that documents in the 
# realname directory are treated as applications and
# run by the server when requested rather than as
# documents sent to the client. 
#
ScriptAlias /cgi-bin/ "/Library/WebServer/CGI-Executables/"
# "/Library/WebServer/CGI-Executables" should be changed
# to whatever your ScriptAliased CGI directory exists,
# if you have that configured.
#
<Directory "/Library/WebServer/CGI-Executables">
    AllowOverride None
    Options None
    Order allow,deny
    Allow from all
</Directory>

The most common CGI configuration is all about segregation: "only files in this directory should be executed as programs--everything else, anywhere else, are just plain old files that should be processed normally." From the standpoint of an overbearing system administrator, this makes sense. With only one directory that can execute code unconditionally, and only one person (you) who can put programs there, it can bring a little sanity to your day-to-day security paranoia.

This is how Apache is configured by default, and the above httpd.conf snippet shows these settings. Our first directive, ScriptAlias, lets us pick and choose which directory we'd like CGI scripts to live in. Anything within that directory, CGI script or not, will be executed by the web server as if it were a program. If something goes wrong, Apache will return an "Internal Server Error" to the requesting user-agent (ie. your visitor's browser).

ScriptAlias consists of two space-separated parts: the fake name and the real name. The fake name, /cgi-bin/, defines what URL will be used to access the real name, /Library/WebServer/CGI-Executables/. In the currently configured settings, whenever we access a URL like http://127.0.0.1/cgi-bin/printenv, we'll really be accessing the matching file stored at /Library/WebServer/CGI-Executables/printenv.

The second part of our configuration is a block directive focusing on that specific location of our hard drive. It merely ensures that said directory has no special privileges besides that ScriptAlias: it can't override anything in the Apache configuration (AllowOverride None), and there are no special capabilities (Options None) associated with it.

At this point, if you only care about the directory segregation of CGI, you can skip over to our next subheading, "Let's See One of These CGI Thingies In Action!" If, on the other hand, you'd like to hear me get high-and-mighty over the "right" way of doing things, read on.

CGI EVERYWHERE: "A Brilliant Psychological Thriller!"

In the very first "Untangling the Web", I professed a fondness and desire to teach you the "right" way to create URLs (MacTech Magazine, June 2003, "Untangling The Web", "Familiarity That Breeds An Uh-Oh!"). A good URL will remain in existence until the dusk of time, never needing to be changed, never needing to be refactored. It's not a snap-decision: if you find yourself changing your URLs around, you designed your site poorly (from an engineering, not visual, standpoint).

One of the maxims, which I'll reiterate here, is that "Your URLs Should Not Reveal Your Technical Capabilities". This has absolutely nothing to do with the oft-maligned "security through obscurity"--the belief that hiding information makes you less susceptible to risk. Instead, removing the technicalities of your URL will allow your site to grow with your needs.

Consider the default configuration of CGI: anytime you want to add some dynamic functionality to your site (with Perl, Python, etc.), you've got to stick your script in a directory and point to it with a URL like http://127.0.0.1/cgi-bin/add_user.pl (where .pl denotes a Perl script) or http://127.0.0.1/cgi-bin/del_user.py (.py denoting Python). What happens, though, when you no longer need CGI scripts, but have moved to an embedded language like PHP? Suddenly, all of your pages that point to that ugly /cgi-bin/ URL are broken, useless, and inaccurate. You find yourself refactoring pages to point to new locations.

The solution? Remove /cgi-bin/ and the file extension from the URL. By clearing this needless technical cruft, you get stronger addresses with less need for change. http://127.0.0.1/admin/add_user and http://127.0.0.1/admin/del_user contain no indication of the technology behind them, merely their purpose in the grande scheme of things.

We're already halfway through removing /cgi-bin/ from our URLs (file extensions I keep putting off, but I'll get to 'em eventually, I promise!). Our very first match for our "CGI" search was for an ExecCGI addition to the Options line of our root web directory (/Library/WebServer/Documents/). ExecCGI works similarly to Includes, which helped get our SSIs running last month. By adding it to the Options line of any Directory we, like SSIs, are telling Apache to allow CGI scripts at that location:

Options Indexes FollowSymLinks MultiViews ExecCGI

To make the comparison even more apt, there's one other thing we need to do before we can use CGIs wherever we need them: we need to add a handler that says "Apache! Listen up! Whenever we request a file with this extension, we want you to treat it as an executable program!" This handler was also needed last month--we told Apache that anytime it ran across a file with an shtml extension, it should process it for SSIs statements.

Thankfully, there's not much brainwork involved here, as our final relevant search match for "CGI" brings us to the below. Simply uncomment the AddHandler (by removing the #), restart Apache (sudo apachectl restart in your Terminal) and you're ready to go:

# If you want to use server side includes, or CGI outside
# ScriptAliased directories, uncomment the following lines.
#
# To use CGI scripts:
#
#AddHandler cgi-script .cgi

Two caveats that may be running through head: "what about the cgi extension?" and "what about that security brouhaha you cautioned us about?" Concerning the file extension, yes, we're swapping /cgi-bin/ for cgi, but this will never be an issue once we remove the need for them in our URLs. As for security, there's not much I can say besides "be careful"--you'll no longer have the niceties of a single and central directory for your code execution, so double and triple check the scripts you'll be programming or using. I'll talk a little more about script security in next month's column.

Let's See One of These CGI Thingies In Action!

Browse to your /Library/WebServer/CGI-Executables/ directory, the default location that Apache has been configured for CGI scripts. You should see two files, printenv and test-cgi, there already. These are some default Apache CGI scripts that will help you quickly test if things are a-ok. Since CGI was already pre-configured in the httpd.conf, we should be able to just load one of these files in the browser and see some fireworks, right? Load http://127.0.0.1/cgi-bin/test-cgi, and we'll receive the response in Figure 1.


Figure 1: An error after loading our test-cgi, but why?

If CGI scripts were already configured, why this rather rude error message? Let's check Apache's error_log, which will always contain some sort of light bulb enlightening response. After running tail /var/log/httpd/error_log on the command line, we'll see something similar to the below as the last line of output:

[Sun Aug 17 10:41:16 2003] [error] [client 127.0.0.1] file 
permissions deny server execution: /Library/WebServer/CGI-
Executables/test-cgi

This is one of the more common errors you'll experience with CGI scripts: the test-cgi file doesn't have the proper credentials to be considered an executable program. I won't get into the vagaries of file ownership or permissions, but follow through the bolded commands below to give both of the default CGI files "execute" access for all users:

> cd /Library/WebServer/CGI-Executables/
> ls -l
total 24
-rw-r--r--  1 root  admin  5398 Jul 27  2002 printenv
-rw-r--r--  1 root  admin   757 Jul 27  2002 test-cgi
> sudo chmod 755 *
Password: **********
> ls -l
total 24
-rwxr-xr-x  1 root  admin  5398 Jul 27  2002 printenv
-rwxr-xr-x  1 root  admin   757 Jul 27  2002 test-cgi

Now, let's try our URL again. Figure 2 shows our new output:


Figure 2: The correct output of our test-cgi script.

Figure 3 shows the output from the other script, http://127.0.0.1/cgi-bin/printenv:


Figure 3: Similar output from the default printenv script.

If you followed the instructions under the "CGI EVERYWHERE!" subheading, you can also move either of the printenv or test-cgi scripts out of their current location and stick them in /Library/WebServer/Documents/ with a cgi file extension. Once you do so, you should then be able to run them under the non-/cgi-bin/ URL and receive the same output (if not, what's your first course of action? Check the error_log!).

Assuming you get similar output as Figures 2 and 3, what does all this gibberish mean? What exactly did we just do? Both scripts show a number of environment variables, which is just a fancy term for data that exists "invisibly" and floats around waiting to be used or read. Some of the variables were set by the browser (like HTTP_USER_AGENT), and some have been set by the server (like SERVER_PROTOCOL). When I first introduced CGI, I mentioned that Apache "sends some relevant information to the requested resource". This "relevant information" is stored in environment variables.

Environment variables can exist outside of your web server too. For instance, if you run printenv in your Terminal, you'll see a listing of variables similar to Figure 4:


Figure 4: Environment variables in your Terminal.

Believe it or not, we've dealt with environment variables already in this series. In our article on Server Side Includes, we checked the $QUERY_STRING variable to see which quote should be displayed at the user's request. If you check back to Figure 3, you'll see an environment variable named QUERY_STRING, which is currently empty. Try adding ?q02 to the end of our URL (http://127.0.0.1/cgi-bin/printenv?q02), and see how that changes the output (Figure 5):


Figure 5: Our QUERY_STRING now has a value.

With the above two scripts working correctly, our CGI capabilities are ready for use.

Homework Malignments

In our next column, we'll chat further about CGI: how to begin creating our own, the most common errors during development and deployment, security issues we should be aware of, and how to find the "good ones" from the immense ocean of crap. If we have time, we'll show you how to install a few that have stood the test of time. For now, students may contact the teacher at morbus@disobey.com.

  • Take a look inside printenv and test-cgi in a text editor.

  • View the printenv script from various browsers and machines. Determine how the output changes, and see if you can figure out what each setting actually means.

  • Forgive me for not including anything amusing in this article.


Kevin Hemenway, coauthor of Mac OS X Hacks, is better known as Morbus Iff, the creator of disobey.com, which bills itself as "content for the discontented." Publisher and developer of more home cooking than you could ever imagine (like the popular open-sourced aggregator AmphetaDesk, the best-kept gaming secret Gamegrene.com, articles for Apple's Internet Developer and the O'Reilly Network, etc.), he's been spending the last two months listening to every song in his iTunes library twice. Contact him at morbus@disobey.com.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

calibre 5.0.1 - Complete e-book library...
Calibre is a complete e-book library manager. Organize your collection, convert your books to multiple formats, and sync with all of your devices. Let Calibre be your multi-tasking digital librarian... Read more
Compressor 4.4.8 - Adds power and flexib...
Compressor adds power and flexibility to Final Cut Pro X export. Customize output settings, work faster with distributed encoding, and tap into a comprehensive set of delivery features. Features:... Read more
Adobe Acrobat Reader 20.012.20048 - View...
Adobe Acrobat Reader allows users to view PDF documents. You may not know what a PDF file is, but you've probably come across one at some point. PDF files are used by companies and even the IRS to... Read more
Adobe Acrobat DC 20.012.20048 - Powerful...
Acrobat DC is available only as a part of Adobe Creative Cloud, and can only be installed and/or updated through Adobe's Creative Cloud app. Adobe Acrobat DC with Adobe Document Cloud services is... Read more
Box Sync 4.0.8009 - Online synchronizati...
Box Sync gives you a hard-drive in the Cloud for online storage. Note: You must first sign up to use Box. What if the files you need are on your laptop -- but you're on the road with your iPhone? No... Read more
Daylite 2020.36.1 - Dynamic business org...
Daylite helps businesses organize themselves with tools such as shared calendars, contacts, tasks, projects, notes, and more. Enable easy collaboration with features such as task and project... Read more
Catalina Cache Cleaner 15.0.6 - Clear ca...
Catalina Cache Cleaner is an award-winning general-purpose tool for macOS X. CCC makes system maintenance simple with an easy point-and-click interface to many macOS X functions. Novice and expert... Read more
Final Cut Pro X 10.4.10 - Professional v...
Final Cut Pro X is a professional video editing solution. Completely redesigned from the ground up, Final Cut Pro adds extraordinary speed, quality, and flexibility to every part of the post-... Read more
Civilization VI 1.3.4 - Next iteration o...
Civilization® VI is the award-winning experience. Expand your empire across the map, advance your culture, and compete against history’s greatest leaders to build a civilization that will stand the... Read more
iTubeDownloader 6.5.23 - Easily download...
iTubeDownloader is a powerful-yet-simple YouTube downloader for the masses. Because it contains a proprietary browser, you can browse YouTube like you normally would. When you see something you want... Read more

Latest Forum Discussions

See All

Undercrawl is a procedurally generated r...
Undercrawl is a roguelike dungeon crawler from indie developer Monster Shop Games. It's a genre that's popular in gaming in general but features even more frequently on mobile devices since the shorter, 'run' style of playthrough suits playing in... | Read more »
Distract Yourself With These Great Mobil...
There’s a lot going on right now, and I don’t really feel like trying to write some kind of pithy intro for it. All I’ll say is lots of people have been coming together and helping each other in small ways, and I’m choosing to focus on that as I... | Read more »
BTS Universe Story, the social game that...
Netmarble's highly anticipated social game, BTS Universe Story, is available now for iOS and Android. It's the second collaboration between the hugely successful mobile developer and the K-pop superstars following BTS World. [Read more] | Read more »
The 5 Best Mobile Games Like Hades
Supergiant Games finally released Hades upon the world this week, and we’re loving it. The game plays to all of the studio’s strengths while still retaining a strong sense of identity. It also just so happens to play rather well using the Steam... | Read more »
A Year of Apple Arcade: The Good, The Ba...
Apple Arcade has persisted for just over a year at this point, and although that means I've been busy ranking and re-ranking every game on the service for just about as long, I haven't done much reflection on the service as a whole. [Read more] | Read more »
Animal Restaurant anniversary event team...
Animal idle simulator Animal Restaurant is celebrating its first-year anniversary with a crossover event with popular YouTube series Aaron’s Animals. [Read more] | Read more »
Raziel: Dungeon Arena is a hack 'n...
Raziel: Dungeon Arena is available now on mobile and will appeal to fans of both comic books and old school dungeon crawlers. Not only will you hack 'n' slash your way through mobs of enemies but there's also fully-narrated animated comic to enjoy... | Read more »
Steam Link Spotlight - Hades
Steam Link Spotlight is a feature where we look at PC games that play exceptionally well using the Steam Link app. Our last entry was on Disco Elysium. Read about how it plays using Steam Link over here. | Read more »
Microsoft has acquired ZeniMax Media and...
In the latest of a series of blockbuster moves, Microsoft has now acquired Zenimax Media and its subsidiary, Bethesda Softworks, for $7.5 billion. [Read more] | Read more »
Infinity Mechs is an upcoming idle game...
Indie developer SkullStar studio has announced an upcoming idle mech game called Infinity Mechs. It draws inspiration from the mobile game Iron Saga and has been officially licensed by Game Duchy. It's set to launch for both iOS and Android on... | Read more »

Price Scanner via MacPrices.net

Clearance 8-core iMac Pro available for $3819...
Apple has Certified Refurbished, clearance, 27″ 3.2GHz 8-Core iMac Pros available $3819 including free shipping. Their price is $1180 off the original MSRP of new models. A standard Apple one-year... Read more
How The Upcoming Mac Transition To Apple Sili...
FEATURE: 09.25.20 – Apple’s plan to transition all of its desktop and notebook computers away from Intel processors to Apple silicon, chips designed by the company itself, has been eclipsed by the... Read more
New low price! Apple Watch SE for only $269
B&H Photo is reporting limited stock of Apple’s new Apple Watch SE GPS models for $10 off MSRP and including free shipping. Their $269 price for the 40mm model is the lowest price we’ve seen so... Read more
Lowest price anywhere: New 13″ 2.0GHz MacBook...
Amazon has new 2020 13″ 2.0GHz/512GB MacBook Pros with 10th generation Intel processors back in stock on sale today for $200 off Apple’s MSRP. Shipping is free. Be sure to purchase the MacBook Pro... Read more
Apple Pro Display XDR with Nano-Texture Glass...
Amazon Apple Premier Partner GatorTec has the Apple Pro Display XDR with Nano-Texture Glass on sale for $5599 shipped, on Amazon. Their price is $400 off Apple’s MSRP, and it’s the cheapest price... Read more
Get a 2019 13″ MacBook Air for only $779 toda...
Apple has clearance, Certified Refurbished, 2019 13″ 1.6GHz/128GB MacBook Airs available again for $779. Each MacBook features a new outer case, comes with a standard Apple one-year warranty, and is... Read more
2020 11″ iPad Pros on sale today for $50-$75...
Apple reseller Expercom has new 2020 11″ Apple iPad Pros on sale for $50-$75 off MSRP, with prices starting at $749. These are the same iPad Pros sold by Apple in their retail and online stores: – 11... Read more
Apple has restocked 2020 13″ MacBook Airs sta...
Apple has restocked Certified Refurbished 2020 13″ MacBook Airs starting at only $849 and up to $200 off the cost of new Airs. Each MacBook features a new outer case, comes with a standard Apple one-... Read more
Apple’s new 8th generation 10.2″ iPads are on...
Amazon is discounting new 2020 8th generation 10.2″ Apple iPads by up to $35 off MSRP with prices starting at only $299. Shipping is free. These are the same iPads sold by Apple in their retail and... Read more
Today on Woot: Apple refurbished 16″ MacBook...
Amazon-owned Woot has Apple refurbished 16″ MacBook Pros available today for up to $605 off the cost of new models. Shipping is free for Prime members: – 16″ 6-Core MacBook Pros: $1874.99 $525 off... Read more

Jobs Board

Freelance *Apple* Technology Journalist - V...
…freelance basis. Valnet Inc. is looking for journalists with strong knowledge of Apple technology for our website MakeUseOf.com MakeUseOf is one of the largest Read more
*Apple* Certified Macintosh Technician - Exc...
Apple Certified Macintosh Technician Summary Title: Apple Certified Macintosh Technician ID:350 Department:All Location:Bethesda, MD Description Apple Read more
Security Officer ($23.00/Hourly) - *Apple*...
**Security Officer \($23\.00/Hourly\) \- Apple Store** **Description** About NMS Built on a culture of safety and integrity, NMSdelivers award\-winning, integrated Read more
Security Officer ($23.00/Hourly) - *Apple*...
**Security Officer \($23\.00/Hourly\) \- Apple Store** **Description** About NMS Built on a culture of safety and integrity, NMSdelivers award\-winning, integrated Read more
*Apple* Certified Macintosh Technician - Exc...
Apple Certified Macintosh Technician Summary Title: Apple Certified Macintosh Technician ID:350 Department:All Location:Falls Church, VA Description Apple Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.