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<title>PATH_INFO Changes in the CGI Environment</title>
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<h1>PATH_INFO Changes in the CGI Environment</h1>

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<h2><a name="over">Overview</a></h2>

<p>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 (<a
href="#compat">see below</a>).

<h2><a name="prob">The Problem</a></h2>

<p>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:
<pre>
     Alias /cgi-ralph /usr/local/httpd/cgi-bin/user.cgi/ralph
</pre>
<p>In this case, <code>user.cgi</code> 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 "<code>/cgi-ralph/script/</code>", the
code would set PATH_INFO to "<code>/ralph/script</code>", and
SCRIPT_NAME to "<code>/cgi-</code>". Obviously, the latter is
incorrect. In certain cases, this could even cause the server to
crash.</p>

<h2><a name="solution">The Solution</a></h2>

<p>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 "<code>/script</code>", and
SCRIPT_NAME to "<code>/cgi-ralph</code>". This makes sense and results
in no server behavior problems. It also permits the script to be
gauranteed that
"<code>http://$SERVER_NAME:$SERVER_PORT$SCRIPT_NAME$PATH_INFO</code>"
will always be an accessable URL that points to the current script,
something which was not neccessarily true with previous versions of
Apache.

<p>However, the "<code>/ralph</code>"
information from the <code>Alias</code> 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 href="#compat">a workaround.</a>

<h2><a name="compat">Compatibility with Previous Servers</a></h2>

<p>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.</p>

<p>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:
<pre>
    $path_info = $ENV{'FILEPATH_INFO'} || $ENV{'PATH_INFO'};
</pre>

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

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