title: connect two clients together by a server without port forwarding
body: <p>I’m trying to write an app that performs the following actions:</p>
  <ol>
  <li>client (1) connects to a server and picks another client (2) from a list</li>
  <li>server connects clients (1) and (2) together (there is not port forwarding involved)</li>
  </ol>
  <p><strong>How to connect two clients to each other directly, using a server which knows their addresses, ports and has an opened connection to both?</strong></p>
  <p>In many posts about p2p there are statements, that it is possible and a common scenario, but I’m struggling to find an explanation how its done.</p>