Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

I can access most of the environment variables, but for some reason I cannot access the HTTP_REFERER environment variable. Is it possible that some of these environment variables are locked out by the server? Here is the very simple test script: 
#!/usr/bin/perl
print ("Content-type: text/html\n\n");
print "<HTML><HEAD><TITLE>Environment Variables</TITLE></HEAD><BODY>";
print "<p><b>$ENV{'HTTP_USER_AGENT'} $ENV{'HTTP_REFERER'}</b></p>";
print "</BODY></HTML>";
open(FILE,">>testdata");
print FILE "\nHere";
close (FILE);

[download]
The HTTP_USER_AGENT is displayed but not the REFERER. Thanks in advance.

Replies are listed 'Best First'.
Re: HTTP_REFERER
by merlyn (Sage) on Aug 15, 2000 at 23:56 UTC
    It's probably a good thing you can't access it. {grin}

    REFERER is not to be trusted. Fine to use it as a hint. Fine to log it. But do not do anything with it other than look at it.

    • Some security firewalls strip it routinely
    • Some browsers send incorrect values
    • It's trivial to forge
    If you find yourself uttering "control" or "security" or "verify" in the same sentence as REFERER, please stop yourself now. Don't make me come over there! {grin}

    -- Randal L. Schwartz, Perl hacker

[reply]
      I am not using this for any security measures and am aware of the risks involved. I am only using the script to get a rough idea of where people who come to a site (low hit count) are entering from. I view the file every once and awhile. I was also testing the concept of a web bug. Those annoying pixel sized images that some companies are using to get data on a user. The concept is very simple and my script helped me learn how it is done. Thanks for the help and concern for others privacy.
[reply]
Re: HTTP_REFERER
by turnstep (Parson) on Aug 15, 2000 at 23:27 UTC

    Do this for a quick list of what is and is not there:

    print "<UL>\n";
for (sort keys %ENV) {
  print "<LI>$_ <STRONG>$ENV{$_}</STRONG>\n";
}
print "</UL>\n";

    [download]

    Be aware that this can be easily changed/faked, and that some browsers do not send anything at all (which could be what is happening here)

[reply]
[d/l]
RE: HTTP_REFERER
by steveAZ98 (Monk) on Aug 15, 2000 at 23:55 UTC
    It's also possible that there was no referer. If the first page accessed after the browser was opened was your script then their will not be a referer.
[reply]

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