CGI Programming on the World Wide Web By Shishir Gundavaram 1st Edition March 1996 This book is out of print, but it has been made available online through the O'Reilly Open Books Project.

Chapter 3 Output from the Common Gateway Interface

 

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3.7 Status Codes

Status codes are used by the HTTP protocol to communicate the status of a request. For example, if a document does not exist, the server returns a "404" status code to the browser. If a document has been moved, a "301" status code is returned.

CGI programs can send status information as part of a virtual document. Here's an arbitrary example that returns success if the remote host name is bu.edu, and failure otherwise:

#!/usr/local/bin/perl
$remote_host = $ENV{'REMOTE_HOST'};
print "Content-type: text/plain", "\n";
if ($remote_host eq "bu.edu") {
        print "Status: 200 OK", "\n\n";
        print "Great! You are from Boston University!", "\n";
} else {
        print "Status: 400 Bad Request", "\n\n";
        print "Sorry! You need to access this from Boston University!", "\n";
}
exit (0);

The Status header consists of a three-digit numerical status code, followed by a string representing the code. A status value of 200 indicates success, while a value of 400 constitutes a bad request. In addition to these two, there are numerous other status codes you can use for a variety of situations, ranging from an unauthorized or forbidden request to internal system errors. Table 3.3 shows a list of some of commonly used status codes.

Table 3.3: HTTP Status Codes

Status Code	Message
200	Success
204	No Response
301	Document Moved
401	Unauthorized
403	Forbidden
404	Not Found
500	Internal Server Error
501	Not Implemented

For a complete listing of status codes, see: http://www.w3.org/hypertext/WWW/Protocols/HTTP/HTRESP.html

Unfortunately, most browsers do not support all of them.

The "No Response" Code

One status code that deserves special attention is status code 204, which produces a "no response." In other words, the browser will not load a new page if your CGI program returns a status code of 204:

#!/usr/local/bin/perl
print "Content-type: text/plain", "\n";
print "Status: 204 No Response", "\n\n";
print "You should not see this message. If you do, your browser does", "\n";
print "not implement status codes correctly.", "\n";
exit (0);

The "no response" status code can be used when dealing with forms or imagemaps. For example, if the user enters an invalid value in one of the fields in a form or clicks in an unassigned section of an imagemap, you can return this status code to instruct the client to not load a new page.

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The "Expires" and "Pragma" Headers		Complete (Non-Parsed) Headers

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