What content with cgi - how the web works: http and cgi explained

Introduction ¶

Related Modules
Related Directives

The CGI (Common Gateway Interface) defines a way for a web server to interact with external content-generating programs, which are often referred to as CGI programs or CGI scripts. It is a simple way lớn put dynamic content on your web site, using whatever programming language you"re most familiar with. This document will be an introduction lớn setting up CGI on your Apache website server, và getting started writing CGI programs.

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Configuring Apache to lớn permit CGI ¶

In order to lớn get your CGI programs khổng lồ work properly, you"ll need to lớn have Apache configured lớn permit CGI execution. There are several ways to vì chưng this.

Note: If Apache has been built with shared module tư vấn you need to lớn ensure that the module is loaded; in your httpd.conf you need to make sure the Load
Module directive has not been commented out. A correctly configured directive may look lượt thích this:

Load
Module cgid_module modules/mod_cgid.so On Windows, or using a non-threaded MPM like prefork, A correctly configured directive may look lượt thích this: Load
Module cgi_module modules/mod_cgi.so

Script
Alias

The Script
Alias directive tells Apache that a particular directory is mix aside for CGI programs. Apache will assume that every file in this directory is a CGI program, and will attempt khổng lồ execute it, when that particular resource is requested by a client.

The Script
Alias directive looks like:

Script
Alias "/cgi-bin/" "/usr/local/apache2/cgi-bin/" The example shown is from your mặc định httpd.conf configuration file, if you installed Apache in the mặc định location. The Script
Alias directive is much lượt thích the Alias directive, which defines a URL prefix that is to lớn mapped khổng lồ a particular directory. Alias and Script
Alias are usually used for directories that are outside of the Document
Root directory. The difference between Alias & Script
Alias is that Script
Alias has the added meaning that everything under that URL prefix will be considered a CGI program. So, the example above tells Apache that any request for a resource beginning with /cgi-bin/ should be served from the directory /usr/local/apache2/cgi-bin/, và should be treated as a CGI program.

For example, if the URL http://www.example.com/cgi-bin/test.pl is requested, Apache will attempt khổng lồ execute the file /usr/local/apache2/cgi-bin/test.pl & return the output. Of course, the file will have to lớn exist, and be executable, and return output đầu ra in a particular way, or Apache will return an error message.

CGI outside of Script
Alias directories

CGI programs are often restricted to Script
Alias"ed directories for security reasons. In this way, administrators can tightly control who is allowed khổng lồ use CGI programs. However, if the proper security precautions are taken, there is no reason why CGI programs cannot be run from arbitrary directories. For example, you may wish to let users have web nội dung in their trang chủ directories with the User
Dir directive. If they want lớn have their own CGI programs, but don"t have access khổng lồ the main cgi-bin directory, they will need to lớn be able to lớn run CGI programs elsewhere.

There are two steps to allowing CGI execution in an arbitrary directory. First, the cgi-script handler must be activated using the Add
Handler or Set
Handler directive. Second, Exec
CGI must be specified in the Options directive.

Explicitly using Options khổng lồ permit CGI execution

You could explicitly use the Options directive, inside your main vps configuration file, lớn specify that CGI execution was permitted in a particular directory:

Options +Exec
CGI The above directive tells Apache khổng lồ permit the execution of CGI files. You will also need khổng lồ tell the vps what files are CGI files. The following Add
Handler directive tells the server to treat all files with the cgi or pl extension as CGI programs:

Add
Handler cgi-script .cgi .pl

.htaccess files

The .htaccess tutorial shows how lớn activate CGI programs if you bởi vì not have access lớn httpd.conf.

User Directories

To allow CGI program execution for any tệp tin ending in .cgi in users" directories, you can use the following configuration.

Options +Exec
CGI Add
Handler cgi-script .cgi If you wish designate a cgi-bin subdirectory of a user"s directory where everything will be treated as a CGI program, you can use the following.

Options Exec
CGI Set
Handler cgi-script

Secondly, your đầu ra needs to be in HTML, or some other format that a browser will be able to display. Most of the time, this will be HTML, but occasionally you might write a CGI program that outputs a gif image, or other non-HTML content.

Apart from those two things, writing a CGI program will look a lot like any other program that you might write.

Your first CGI program

The following is an example CGI program that prints one line to your browser. Type in the following, save it lớn a file called first.pl, & put it in your cgi-bin directory.

#!/usr/bin/perlprint "Content-type: text/html ";print "Hello, World."; Even if you are not familiar with Perl, you should be able to lớn see what is happening here. The first line tells Apache (or whatever shell you happen khổng lồ be running under) that this program can be executed by feeding the file to the interpreter found at the location /usr/bin/perl. The second line prints the content-type declaration we talked about, followed by two carriage-return newline pairs. This puts a blank line after the header, khổng lồ indicate the over of the HTTP headers, and the beginning of the body. The third line prints the string "Hello, World.". Và that"s the end of it.

If you mở cửa your favorite browser & tell it khổng lồ get the address

or wherever you put your file, you will see the one line Hello, World. Appear in your browser window. It"s not very exciting, but once you get that working, you"ll have a good chance of getting just about anything working.

But it"s still not working! ¶

There are four basic things that you may see in your browser when you try to lớn access your CGI program from the web:

The output of your CGI program Great! That means everything worked fine. If the đầu ra is correct, but the browser is not processing it correctly, make sure you have the correct Content-Type set in your CGI program. The source code of your CGI program or a "POST Method Not Allowed" message That means that you have not properly configured Apache to lớn process your CGI program. Reread the section on configuring Apache and try lớn find what you missed. A message starting with "Forbidden" That means that there is a permissions problem. Kiểm tra the Apache error log và the section below on file permissions. A message saying "Internal hệ thống Error" If you check the Apache error log, you will probably find that it says "Premature kết thúc of script headers", possibly along with an error message generated by your CGI program. In this case, you will want to kiểm tra each of the below sections khổng lồ see what might be preventing your CGI program from emitting the proper HTTP headers.

File permissions

Remember that the hệ thống does not run as you. That is, when the server starts up, it is running with the permissions of an unprivileged user - usually nobody, or www - and so it will need extra permissions lớn execute files that are owned by you. Usually, the way khổng lồ give a file sufficient permissions lớn be executed by nobody is khổng lồ give everyone execute permission on the file:

Also, if your program reads from, or writes to, any other files, those files will need khổng lồ have the correct permissions to lớn permit this.

Path information & environment

When you run a program from your command line, you have certain information that is passed lớn the shell without you thinking about it. For example, you have a PATH, which tells the shell where it can look for files that you reference.

When a program runs through the web hệ thống as a CGI program, it may not have the same PATH. Any programs that you invoke in your CGI program (like sendmail, for example) will need to be specified by a full path, so that the shell can find them when it attempts to execute your CGI program.

A common manifestation of this is the path to lớn the script interpreter (often perl) indicated in the first line of your CGI program, which will look something like:

#!/usr/bin/perl Make sure that this is in fact the path to the interpreter.

When editing CGI scripts on Windows, end-of-line characters may be appended khổng lồ the interpreter path. Ensure that files are then transferred to lớn the hệ thống in ASCII mode. Failure to do so may result in "Command not found" warnings from the OS, due to the unrecognized end-of-line character being interpreted as a part of the interpreter filename.

Missing environment variables

If your CGI program depends on non-standard environment variables, you will need lớn assure that those variables are passed by Apache.

When you miss HTTP headers from the environment, make sure they are formatted according to RFC 2616, section 4.2: Header names must start with a letter, followed only by letters, numbers or hyphen. Any header violating this rule will be dropped silently.

Program errors

Most of the time when a CGI program fails, it"s because of a problem with the program itself. This is particularly true once you get the hang of this CGI stuff, & no longer make the above two mistakes. The first thing to vày is to make sure that your program runs from the command line before testing it via the website server. For example, try:

(Do not gọi the perl interpreter. The shell and Apache should find the interpreter using the path information on the first line of the script.)

The first thing you see written by your program should be a phối of HTTP headers, including the Content-Type, followed by a blank line. If you see anything else, Apache will return the Premature kết thúc of script headers error if you try khổng lồ run it through the server. See Writing a CGI program above for more details.

Error logs

The error logs are your friend. Anything that goes wrong generates message in the error log. You should always look there first. If the place where you are hosting your website site does not permit you access to the error log, you should probably host your site somewhere else. Learn to read the error logs, và you"ll find that almost all of your problems are quickly identified, và quickly solved.

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Suexec

The suexec tư vấn program allows CGI programs to lớn be run under different user permissions, depending on which virtual host or user trang chủ directory they are located in. Suexec has very strict permission checking, & any failure in that checking will result in your CGI programs failing with Premature over of script headers.

To kiểm tra if you are using suexec, run apachectl -V and kiểm tra for the location of SUEXEC_BIN. If Apache finds an suexec binary there on startup, suexec will be activated.

Unless you fully understand suexec, you should not be using it. Khổng lồ disable suexec, simply remove (or rename) the suexec binary pointed to lớn by SUEXEC_BIN và then restart the server. If, after reading about suexec, you still wish khổng lồ use it, then run suexec -V to find the location of the suexec log file, và use that log file to find what policy you are violating.

What"s going on behind the scenes? ¶

As you become more advanced in CGI programming, it will become useful to understand more about what"s happening behind the scenes. Specifically, how the browser & server communicate with one another. Because although it"s all very well to write a program that prints "Hello, World.", it"s not particularly useful.

Environment variables

Environment variables are values that float around you as you use your computer. They are useful things like your path (where the computer searches for the actual tệp tin implementing a command when you type it), your username, your terminal type, and so on. For a full danh sách of your normal, every day environment variables, type env at a command prompt.

During the CGI transaction, the server và the browser also set environment variables, so that they can communicate with one another. These are things lượt thích the browser type (Netscape, IE, Lynx), the server type (Apache, IIS, Web
Site), the name of the CGI program that is being run, and so on.

These variables are available lớn the CGI programmer, và are half of the story of the client-server communication. The complete menu of required variables is at Common Gateway Interface RFC.

This simple Perl CGI program will display all of the environment variables that are being passed around. Two similar programs are included in the cgi-bin directory of the Apache distribution. Chú ý that some variables are required, while others are optional, so you may see some variables listed that were not in the official list. In addition, Apache provides many different ways for you to add your own environment variables khổng lồ the basic ones provided by default.

#!/usr/bin/perluse strict;use warnings;print "Content-type: text/html ";foreach my $key (keys %ENV) print "$key --> $ENV$key";

STDIN and STDOUT

Other communication between the server & the client happens over standard input đầu vào (STDIN) & standard output đầu ra (STDOUT). In normal everyday context, STDIN means the keyboard, or a tệp tin that a program is given to lớn act on, & STDOUT usually means the console or screen.

When you POST a web form to a CGI program, the data in that khung is bundled up into a special format and gets delivered to your CGI program over STDIN. The program then can process that data as though it was coming in from the keyboard, or from a file

The "special format" is very simple. A field name and its value are joined together with an equals (=) sign, và pairs of values are joined together with an ampersand (&). Inconvenient characters like spaces, ampersands, and equals signs, are converted into their hex equivalent so that they don"t gum up the works. The whole data string might look something like:

You"ll sometimes also see this type of string appended to lớn a URL. When that is done, the server puts that string into the environment variable called QUERY_STRING. That"s called a GET request. Your HTML khung specifies whether a GET or a POST is used khổng lồ deliver the data, by setting the METHOD attribute in the form tag.

Your program is then responsible for splitting that string up into useful information. Fortunately, there are libraries và modules available to help you process this data, as well as handle other of the aspects of your CGI program.

CGI modules/libraries ¶

When you write CGI programs, you should consider using a code library, or module, to vị most of the grunt work for you. This leads to lớn fewer errors, và faster development.

If you"re writing CGI programs in Perl, modules are available on CPAN. The most popular module for this purpose is CGI.pm. You might also consider CGI::Lite, which implements a minimal mix of functionality, which is all you need in most programs.

If you"re writing CGI programs in C, there are a variety of options. One of these is the CGIC library, from https://web.mit.edu/wwwdev/www/cgic.html.

For more information ¶

The current CGI specification is available in the Common Gateway Interface RFC.

When you post a question about a CGI problem that you"re having, whether lớn a mailing list, or khổng lồ a newsgroup, make sure you provide enough information about what happened, what you expected to lớn happen, và how what actually happened was different, what hệ thống you"re running, what language your CGI program was in, and, if possible, the offending code. This will make finding your problem much simpler.

Note that questions about CGI problems should never be posted lớn the Apache bug database unless you are sure you have found a problem in the Apache source code.

L> 3.2 CGI và Response Headers


	CGI Programming on the World Wide Web By Shishir Gundavaram1st Edition March 1996 This book is out of print, but it has been made available online through the O"Reilly open Books Project.

Chapter 3Output from the Common Gateway Interface

3.2 CGI và Response Headers

By now, you should bereasonably comfortable designing CGI programs that create simplevirtual documents, lượt thích this one:#!/usr/local/bin/perlprint "Content-type: text/html", " ";print "", " ";print "Simple Virtual HTML Document", " ";print "", " ";print "", "Virtual HTML", "", "", " ";print "Hey look, I just created a virtual (yep, virtual) HTML document!", " ";print "", " ";exit (0);
Up khổng lồ this point, we have taken the line that outputs "Content-type"for granted. But this is only one type of header that CGI programscan use. "Content-type" is an HTTP header that contains a MIME contenttype describing the format of the data that follows. Other headerscan describe:The form size of the data
Another document that the server should return (thatis, instead of returning a virtual document created by the scriptitself)HTTP status codes
This chapter will discuss how HTTP headers can be used tofine-tune your CGI documents. First, however, Table 3.1 providesa quick listing of all the HTTP headers you might find useful.Table 3.1: Valid HTTP Headers
Header
Description
Content-length
The length (in bytes) of the output đầu ra stream.Implies binary data.Content-type
The MIME nội dung type of the đầu ra stream.Expires
Date và time when the document is nolonger valid and should be reloaded by the browser.Location
Server redirection (cannot be sent aspart of a complete header).Pragma
Turns document caching on & off.Status
Status of the request (cannot be sentas part of a complete header).
The following headers are "understood" only by Netscape-compatiblebrowsers (i.e., Netscape Navigator và Microsoft mạng internet Explorer).Table 3.2: Netscape-Compatible Headers
Header
Description
Refresh
Client reloads specified document.Set-Cookie
Client stores specified data. Usefulfor keeping track of data between requests.
Also, there are a couple of things you should know about headersyntax:Header lines don"t haveto be in any special order.In general, the headers you generate from a CGIprogram can be output in any order you like.The header block has to kết thúc with ablank line. HTTP is a very simple protocol. Theway the server knows that you"re done with your header informationis that it looks for a blank line. Everything before the blank lineis taken as header information; everything after the blank lineis assumed to lớn be data. In Perl, the blank line is generated by twonewline characters ( ) that are output đầu ra after the last line ofthe header. If you don"t include the blank line after the header,the vps will assume incorrectly that the entire information streamis an HTTP header, & will generate a hệ thống error.