PHP: symlink - Manual

symlink

anything at the domain williamfrantz.com
14-Feb-2006 05:15


Olszewski_marek makes a good suggestion, but the readfile() function can also be used to obscure file downloads from end users.



/* Setup the file that will be sent */

$downloadDir = "some/secret/directory/";

$file = "theFileName.dat";



/* Required for IE, otherwise Content-disposition is ignored */

if(ini_get('zlib.output_compression')) ini_set('zlib.output_compression', 'Off');



/* Output HTTP headers that force "Save As" dialog */

header("Pragma: public");

header("Expires: 0");

header("Cache-Control: must-revalidate, post-check=0, pre-check=0");

header("Cache-Control: private",false);

header("Content-Type: application/octet-stream");

header("Content-Disposition: attachment; filename=\\"$file\\";");

header("Content-Transfer-Encoding: binary");

header("Content-Length: ".@filesize($downloadDir.$file));



/* Prevent the script from timing out for large files */

set_time_limit(0);



/* Send the entire file using @ to ignore all errors */

@readfile($downloadDir.$file);



/* Exit immediately so no garbage follows the file contents */

exit;

29-Dec-2005 07:21


Olszewski seems pretty good, but just to boost the security a bit, the fifth line of his script should read

<?

...

    $q = rand(1,26); 

...

?>



instead of rand(1,24).

contact at tcknetwork dot com
11-Sep-2005 08:04


To make your code portable on unix AND win32, do the following

1. Download http://www.dynawell.com/reskit/microsoft/win2000/linkd.zip

2. Unzip, put linkd.exe in C:\Windows\System32 or C:\WINNT\System32

3. Include in your code the following

<?

function _syslink($t /*target*/ ,$l /*link*/ ) {

 if ($_SERVER["WINDIR"]) {

  $p=dirname($_SERVER["SCRIPT_FILENAME"])."/";

  exec("linkd ".$p.$t." ".$p.$l);

 } else syslink($t,$l);

}

function _unlink($l /*link*/ ) {

 if ($_SERVER["WINDIR"]) {

  $p=dirname($_SERVER["SCRIPT_FILENAME"])."/";

  exec("linkd ".$p.$l." /D");

 } else unlink($l);

}

?>

4. Enjoy

_symlink(TARGET,LINK) works like symlink() on *nix

_unlink(LINK) to delete properly the link created

aircha at cix dot co dot uk
15-Mar-2005 08:49


Olszewski's method of creating downloadable links on the fly is pretty good.



I use a different technique under Apache where if you want a file, you might use a url like:



mydomain.com/files/mysecretfile.doc



But in fact Apache redirects this to a script with a url like:



mydomain.com/utilities/downloadfile.php?filename=mysecretfile.doc



(The browser address bar will still show the first url).



The script downloadfile.php can then handle all the mucky stuff like checking session variables to see if someone is logged on, whether they have access to mysecretfile.doc, and if you want to encrypt before download.



An advantage of this is that the php code to achieve the equivalent of Olszewski's is shorter, so might execute faster. But more importantly, a casual hacker might think all he has to do is find myverysecurefile.doc under the /home/user/public_html/files/mysecretfile.doc, or copy the url - but there's nothing there! He won't find it or get an Apache error page.

veronique at talafone dot com
29-May-2003 02:23


This might sound obvious, but sometimes these details slip from our minds...

Make sure the directory in which you are trying to create the symbolic link (AND the directory you are trying to link to in the case of directory linking) has the proper permissions, or you will get a "permission denied" from PHP. If you use for example, 

echo shell_exec("ln -sv $target $name")

you might get the correct "link created" message, but without the link being actually created.



It's probably the same thing with hard links, I just haven't tried it.

stephen at opido dot com
07-Feb-2002 12:38


Faking Symlinks on Windows (Win2K + NTFS)



Every once in a while I'll bang my head against something really difficult. I needed to make symlinks, Windows won't let me. The workaround is a couple files in the Win2K Resource Kit called linkd.exe and delrp.exe



http://support.microsoft.com/default.aspx?scid=kb;EN-US;q205524



It only works on folders as far as I can tell, there is another program from SysInternals called Junction which will do both (although I only had success with files, not folders).



http://www.sysinternals.com/ntw2k/source/misc.shtml#junction



I hope this helps those who have experienced this same problem. One step closer to portable applications :)

olszewski_marek at yahoo dot com
04-Dec-2000 08:54


Here is a simple way to control who downloads your files...





You will have to set: $filename, $downloaddir, $safedir and $downloadURL.





Basically $filename is the name of a file, $downloaddir is any dir on your server, $safedir is a dir that is not accessible by a browser that contains a file named $filename and $downloadURL is the URL equivalent of your $downloaddir.





The way this works is when a user wants to download a file, a randomly named dir is created in the $downloaddir, and a symbolic link is created to the file being requested.  The browser is then redirected to the new link and the download begins.





The code also deletes any past symbolic links created by any past users before creating one for itself.  This in effect leaves only one symbolic link at a time and prevents past users from downloading the file again without going through this script.  There appears to be no problem if a symbolic link is deleted while another person is downloading from that link.





This is not too great if not many people download the file since the symbolic link will not be deleted until another person downloads the same file. 





Anyway enjoy:





$letters = 'abcdefghijklmnopqrstuvwxyz';


srand((double) microtime() * 1000000);


$string = '';


for ($i = 1; $i <= rand(4,12); $i++) {


   $q = rand(1,24);


   $string = $string . $letters[$q];


}


$handle = opendir($downloaddir);


while ($dir = readdir($handle)) {


   if (is_dir($downloaddir . $dir)){


      if ($dir != "." && $dir != ".."){


         @unlink($downloaddir . $dir . "/" . $filename);


         @rmdir($downloaddir . $dir);


      }


   }


}


closedir($handle);


mkdir($downloaddir . $string, 0777);


symlink($safedir . $filename, $downloaddir . $string . "/" . $filename);


Header("Location: " . $downloadURL . $string . "/" . $filename);

symlink

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