One way id to create two logical interfaces (trusted and untrusted) on your firewall, using 802.1q trunking. You will configure your switch port to be a trunk with two VLANs. Logically, your Linux box will have two ports.

Linux configuration is off-topic here. You can get details on configuring 802.1q on Server Fault.

One way is to create two logical interfaces (trusted and untrusted) on your firewall, using 802.1q trunking. You will configure your switch port to be a trunk with two VLANs. Logically, your Linux box will have two ports.

Linux configuration is off-topic here. You can get details on configuring 802.1q on Server Fault.