Firewall: Trusted Interfaces
Desktop Training - Fedora

Trusted interfaces represent network connections to a network that you trust. In reality this may be a misnomer as trust can often get you into trouble.  When you trust an interface it is usually pointing to an internal network of computers you are very familiar with.  A trusted interface should never face the Internet. Basically a trusted interface will not have an active firewall protecting that connection.  That is why you should be very careful about using this option.

Lesson 6 / Lesson 8

You can see that a trusted interface may be an Ethernet card, modems, wireless connections or VPN connection.  Just be very careful in using thisoption.