I have a server that I use to send emails. The emails have an image, that is used to track their being opened, that is in the form of
<img src="http://tracker.site.com/index.php?e=test@site.com&c=somemd5ishcode">
The script that serves up the image is as follows:
<?php
// turn off errors
ini_set('display_errors', 0);
ini_set('log_errors', 0);
// database settings
define('DB_HOST','localhost');
define('DB_USER','user');
define('DB_PASSWORD','pass');
define('DB_NAME','db');
// connecting to the db
$dbh = mysql_connect(DB_HOST,DB_USER,DB_PASSWORD,true);
mysql_select_db(DB_NAME, $dbh);
// clean the vars
$code = mysql_real_escape_string( $_GET['c'] );
$email = mysql_real_escape_string( $_GET['e'] );
// insert a record if need be
if( $code <> '' && $email <> '' )
{
// quick debug
$dump = '';
foreach( $_SERVER as $k=>$v )
{
$dump.= '"'. $k .'" => "'. $v .'",\n';
}
mysql_query( "INSERT INTO `tracker_hits`
(`CODE_TEXT`,`HIT_EMAIL`,`HIT_IP`,`HIT_TIMESTAMP`,`SERVER_DUMP`)
VALUES
('$code','$email','{$_SERVER['REMOTE_ADDR']}','" .time(). "','$dump')", $dbh );
}
// disables caching of the image
header("Expires: Sat, 26 Jul 1997 05:00:00 GMT");
header('Cache-Control: no-cache');
header('Pragma: no-cache');
// outputs a 1x1 transparent PNG
header('Content-type: image/png');
echo gzinflate(base64_decode('6wzwc+flkuJiYGDg9fRwCQLSjCDMwQQkJ5QH3wNSbCVBfsEMYJC3jH0ikOLxdHEMqZiTnJCQAOSxMDB+E7cIBcl7uvq5rHNKaAIA'));
die();
When I send the email out though, I'm getting hundreds of hits within the exact same second, from the exact same IP address, which varies, but whois says it belongs to Mediacom / Verizon / Frontier / etc. I looked around, and it seems like my issue may be with an ISP cache server. I used this website: http://redbot.org/ to check the 'cacheability' of my file, and this was what they had to say:
HTTP/1.1 200 OK
Date: Thu, 17 Mar 2011 17:05:20 GMT
Server: Apache/2.2.10 (Win32) mod_ssl/2.2.10 OpenSSL/0.9.8i PHP/5.2.6
X-Powered-By: PHP/5.2.6
Expires: Sat, 26 Jul 1997 05:00:00 GMT
Cache-Control: no-cache
Pragma: no-cache
Content-Length: 87
Keep-Alive: timeout=5, max=500
Connection: Keep-Alive
Content-Type: image/png
With these notes:
General
- The Content-Length header is correct.
Caching
- Pragma: no-cache is a request directive, not a response directive.
- This response allows all caches to store it.
- This response cannot be served from cache without validation.
So, I looked around, and found that I could supposedly use Apache's expires_module to get the job done, but when setting it up with the following settings, I'm still getting the same results from REDbot:
LoadModule expires_module modules/mod_expires.so
<IfModule expires_module>
ExpiresActive On
ExpiresByType text/php A0
ExpiresByType image/png A0
ExpiresByType image/jpg A0
ExpiresByType image/gif A0
</IfModule>
What am I missing? I looked around, and to my understanding the PHP header()s and the <meta> headers are used only for the users browser, not the ISP cache. One thought that I had was since the group IP hits are all within a few seconds of the email going out, in my log script I could check to see if at least 10 seconds had passed before allowing a hit to be valid. The issue I figured I may encounter with that though is that later on when a person actually physically reads the email, won't the ISP catch the request and short circuit the one that would have logged a hit to my server, and just serve them the cache version?
