The other file that affects the use of communication facilities is the remote.unknown file. This file is a binary program that executes when a machine that is not found when any of the Systems files starts a conversation. This program logs the conversation attempt and drops the connection.
If you change the permissions of the remote.unknown file so that the file cannot execute, your system accepts connections from any system.
This program executes when a machine that is not in any of the Systems starts a conversation. The program logs the conversation attempt but fails to make a connection. If you change the permissions of this file so that the file cannot execute (chmod 000 remote.unknown), your system accepts any conversation requests. This change is not trivial. You should have good reasons for making this change.