The internal interface is used by the ITP/Document Processor Manager to communicate with the ITP/Document Processors in the ITP/Server setup. This is a TCP/IP interface and can use similar security settings to the external interface. By default the security is set to public. This might be a problem although requests must have a certain format to be processed. If all of the ITP/Server setup runs on one machine you might consider setting the security to Local or when the servers running ITP/Document Processors are known, you can set an access list.