Question

Many users of my site have reported problems downloading a large file (80 MB). I am using a forced download using headers. I can provide additional php settings if necessary. I am using the CakePHP framework, but this code is all regular php. I am using php 5.2 with apache on a dedicated virtual server from media temple, CentOS Linux. Do you see any problems with the following code:

        set_time_limit(1500);
        header("Content-Type: application/octet-stream");
        header("Content-Disposition: attachment; filename=\"" . basename($file_path) . "\"");
        header("Content-Length: ".$content_length);
        header("Content-Transfer-Encoding: binary");
        header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
        header('Cache-Control: private', false);
        header('Pragma: public');
        header('Expires: 0');

        //Change this part
        $handle = fopen($file_path, 'rb');
        while (!feof($handle))
        {
            echo fread($handle, 4096);
            ob_flush();
            flush();
        }
        fclose($handle);
        exit;

Basically, the problem being reported is that the download starts and then stops in the middle. I was thinking it was a problem with the time limit, so I add the set_time_limit code. I was using the php readfile function before, but that also did not work smoothly.

Answer 1

The problem with PHP-initiated http transfers is that they seldomly support partial requests:

GET /yourfile HTTP/1.1
Range: bytes=31489531-79837582

Whenever a browser encounters a transmission problem, it will try to resume the download. Your php script does not accomodate for that (it’s not trivial, so nobody does).

So really avoid that. Redirect users to a static file and let your webserver handle it. If you need to handle authorization, use tricks like symlinks or rewriterules that check for session cookies or even a static permission file (./allowed/178.224.2.55-file-1). Any required extra HTTP headers can be injected likewise, or with a .meta file.