PATH_INFO Changes in the CGI Environment

Overview

As implemented in Apache 1.1.1 and earlier versions, the method Apache used to create PATH_INFO in the CGI environment was counterintiutive, and could result in crashes in certain cases. In Apache 1.2 and beyond, this behavior has changed. Although this results in some compatibility problems with certain legacy CGI applications, the Apache 1.2 behavior is still compatible with the CGI/1.1 specification, and CGI scripts can be easily modified (see below).

The Problem

Apache 1.1.1 and earlier implemented the PATH_INFO and SCRIPT_NAME environment variables by looking at the filename, not the URL. While this resulted in the correct values in many cases, when the filesystem path was overloaded to contain path information, it could result in errant behavior. For example, if the following appeared in a config file:

     Alias /cgi-ralph /usr/local/httpd/cgi-bin/user.cgi/ralph

In this case, user.cgi is the CGI script, the "/ralph" is information to be passed onto the CGI. If this configuration was in place, and a request came for "/cgi-ralph/script/", the code would set PATH_INFO to "/ralph/script", and SCRIPT_NAME to "/cgi-". Obviously, the latter is incorrect. In certain cases, this could even cause the server to crash.

The Solution

Apache 1.2 and later now determine SCRIPT_NAME and PATH_INFO by looking directly at the URL, and determining how much of the URL is client-modifiable, and setting PATH_INFO to it. To use the above example, PATH_INFO would be set to "/script", and SCRIPT_NAME to "/cgi-ralph". This makes sense and results in no server behavior problems. It also permits the script to be gauranteed that "http://$SERVER_NAME:$SERVER_PORT$SCRIPT_NAME$PATH_INFO" will always be an accessable URL that points to the current script, something which was not neccessarily true with previous versions of Apache.

However, the "/ralph" information from the Alias directive is lost. This is unfortunate, but we feel that using the filesystem to pass along this sort of information is not a recommended method, and a script making use of it "deserves" not to work. Apache 1.2b3 and later, however, do provide a workaround.

Compatibility with Previous Servers

It may be neccessary for a script that was designed for earlier versions of Apache or other servers to need the information that the old PATH_INFO variable provided. For this purpose, Apache 1.2 (1.2b3 and later) sets an additional variable, FILEPATH_INFO. This environment variable contains the value that PATH_INFO would have had with Apache 1.1.1.

A script that wishes to work with both Apache 1.2 and earlier versions can simply test for the existance of FILEPATH_INFO, and use it if available. Otherwise, it can use PATH_INFO. For example, in Perl, one might use:

    $path_info = $ENV{'FILEPATH_INFO'} || $ENV{'PATH_INFO'};

By doing this, a script can work with all servers supporting the CGI/1.1 specification, including all versions of Apache.