How to protect a file on sub-domain of main site from download without auth in wordpress

For example my main domain www.example.com which is a file sharing website where we keep files in sub domain like server1.example.com and server2.example.com to share with visitor. My question is how to protect a file on sub-domain (sub-domain might point to different server or host) direct from download without authentication in wordpress/mainsite.

My current code credit goes to http://wordpress.stackexchange.com/a/37743/12438

.htaccess file

RewriteCond %{REQUEST_FILENAME} -s
RewriteRule ^wp-content/uploads/(.*)$ dl-file.php?file=$1 [QSA,L]

SO my entire .htaccess code looks like

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]

# BEGIN THIS DL-FILE.PHP ADDITION
RewriteCond %{REQUEST_URI} ^.*wp-content/uploads/.*
RewriteRule ^wp-content/uploads/(.*)$ dl-file.php?file=$1 [QSA,L]
# END THIS DL-FILE.PHP ADDITION

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

# END WordPress

Options -Indexes

but it can Protect only a file inside wp-content/uploads/*

<?php
ob_start();
require_once('wp-load.php');
require_once ABSPATH . WPINC . '/formatting.php';
require_once ABSPATH . WPINC . '/capabilities.php';
require_once ABSPATH . WPINC . '/user.php';
require_once ABSPATH . WPINC . '/meta.php';
require_once ABSPATH . WPINC . '/post.php';
require_once ABSPATH . WPINC . '/pluggable.php';
wp_cookie_constants();
$discard = ob_get_clean();

is_user_logged_in() ||  auth_redirect();


list($basedir) = array_values(array_intersect_key(wp_upload_dir(), array('basedir' => 1)))+array(NULL);

$file = rtrim($basedir, '/') . '/' . (isset($_GET['file']) ? $_GET['file'] : '');
$file = realpath($file);

if ($file === FALSE || !$basedir || !is_file($file)) {
    status_header(404);
    die('404 &#8212; File not found.');
}

if (strpos($file, $basedir) !== 0) {
    status_header(403);
    die('403 &#8212; Access denied.');
}


$mime = wp_check_filetype($file);
if( false === $mime[ 'type' ] && function_exists( 'mime_content_type' ) )
    $mime[ 'type' ] = mime_content_type( $file );
if( $mime[ 'type' ] )
    $mimetype = $mime[ 'type' ];
else
    $mimetype = 'image/' . substr( $file, strrpos( $file, '.' ) + 1 );
header( 'Content-Type: ' . $mimetype ); // always send this
if ( false === strpos( $_SERVER['SERVER_SOFTWARE'], 'Microsoft-IIS' ) )
    header( 'Content-Length: ' . filesize( $file ) );
$last_modified = gmdate( 'D, d M Y H:i:s', filemtime( $file ) );
$etag = '"' . md5( $last_modified ) . '"';
header( "Last-Modified: $last_modified GMT" );
header( 'ETag: ' . $etag );
header( 'Expires: ' . gmdate( 'D, d M Y H:i:s', time() + 100000000 ) . ' GMT' );
// Support for Conditional GET
$client_etag = isset( $_SERVER['HTTP_IF_NONE_MATCH'] ) ? stripslashes( $_SERVER['HTTP_IF_NONE_MATCH'] ) : false;
if( ! isset( $_SERVER['HTTP_IF_MODIFIED_SINCE'] ) )
    $_SERVER['HTTP_IF_MODIFIED_SINCE'] = false;
$client_last_modified = trim( $_SERVER['HTTP_IF_MODIFIED_SINCE'] );
// If string is empty, return 0. If not, attempt to parse into a timestamp
$client_modified_timestamp = $client_last_modified ? strtotime( $client_last_modified ) : 0;
// Make a timestamp for our most recent modification...
$modified_timestamp = strtotime($last_modified);
if ( ( $client_last_modified && $client_etag )
    ? ( ( $client_modified_timestamp >= $modified_timestamp) && ( $client_etag == $etag ) )
    : ( ( $client_modified_timestamp >= $modified_timestamp) || ( $client_etag == $etag ) )
    ) {
    status_header( 304 );
    exit;
}
// If we made it this far, just serve the file
readfile( $file );

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