So I think that there are couple of ways to address this.
1) If your fowarding system is non-splunk: By writing a small proxy. You could spawn a small multi threaded TCP server (in python for ease), and then have some form of authentication of forwarders, as a handshake step after connection is established. After handshake is done you can just blindly start forwarding data to the tcpinput port.
2) If you forwarding system is Splunk based: There is a mechanism of setting up shared secret keys between forwarding an receiving side. You can do this by https://docs.splunk.com/Documentation/Forwarder/6.4.2/Forwarder/Controlforwarderaccess#Configure_the_indexer_with_the_token
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