CGI Programming Guide - [Chapter 3] 3.7 Status Codes

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: https://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.