Ran into a situation recently where I needed a Webform sitting out on the internet to hit a server in a DMZ and then be relayed from that server to a coreBOS server behind a firewall. The DMZ server was the only one allowed to talk to the coreBOS instance and it wasn't the server hosting the webform. Here's what I did in case anyone has a similar need.
Set the form post on the internet webform to go to "themiddlemanserver/relay.php"
Enabled cURL in PHP on the DMZ server. Contents of relay.php is:
<?php
$postParams = file_get_contents("php://input");
$ch = curl_init('http://vtigerbehindfirewall/modules/Webforms/capture.php');
curl_setopt ($ch, CURLOPT_POST, 1);
curl_setopt ($ch, CURLOPT_POSTFIELDS, $postParams);
curl_exec ($ch);
curl_close ($ch);
?>Webform hits the DMZ relay and is sent on to the coreBOS instance behind the firewall. Passes all values correctly.