The variables related to the web server that I consider important
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Written by Robert on March 31, 2008 – 10:04 am
The CGI specification defines the data that is to be transferred through environment variables. Some relate to the web server itself; for example:
SERVER_SOFTWARE This should identify the server (e.g. Apache, IIS) and its version number.
SERVER_PORT The port number used by the server (usually port 80; alternatives include 8080, 8000, and other rarer choices). These server data are really just for completeness; it is quite unusual for a CGI script or program to need details of the web server that launched it. Another group of environment variables hold data about the client:
REMOTE_HOST If the web server is configured to use DNS to lookup hostnames, this will be the hostname of the client’s machine.
REMOTE_ADDR The value of this environment variable is the IP address of the client’s machine.
REMOTE_IDENT Usually this variable is not set. It could contain the client’s actual login identifier (as used on the client’s machine). The client’s machine has to be running the identd daemon to provide the information (this process is rarely active), and the web server must be configured to request the information. (Apart from breaching users’ privacy, use of the ‘identify’ feature significantly slows the processing of requests.)
REMOTE_USER If a website is password-protected using HTTP authentication, then this variable will be set and will contain the username supplied to the authentication system. (Often this is not the same as the user’s personal login identifier.)
USER_AGENT This variable contains a string identifying the web browser used to submit the request. A web server normally logs details of all requests, so most of these data have already been recorded. Most CGI programs and scripts would not make any further use of these data; a few might check the USER_AGENT value so as to generate different responses in styles that are optimal for Netscape or IE browsers. The environment data characterizing a request will include:
REQUEST_METHOD This specifies ‘get’ or ‘post’ and is checked by the CGI program so as to determine where the name=value data are to be found.
PATH_INFO This is the pathname for the CGI program as it appeared in the request. Sometimes extra data are added to the path.
PATH_TRANSLATED This is the pathname for CGI program as actually used. A web server can be configured to ignore extra data added to paths, or to change directories specified in the requests to other directories where the files are actually held.
HTTP_REFERER If the user submitted the query by clicking on an <a href=…> link, then this variable should have the URL of the page containing that link.
HTTP_ACCEPT The value of this variable is a string with details of theMIME types that the user’s browser can accept. (This information might be used if the CGI program generated an image as its response rather than a normal web page, and was capable of generating the image in different formats.)
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