Converting a Mac Mini to dual (or more) ethernet is simple, just add a USB to Ethernet connector.
Most outward-facing connections are slower than Fast Ethernet, so there is little chance of overwhelming the USB controller.
Dedicate the onboard ethernet connector to the internal network, and OS X Server is plenty happy to firewall from USB ethernet to internal, and it will even let you set up a DMZ, if you wanted to buy 2 USB to ethernet connectors.
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Converting a Mac Mini to dual (or more) ethernet is simple, just add a USB to Ethernet connector. Most outward-facing connections are slower than Fast Ethernet, so there is little chance of overwhelming the USB controller. Dedicate the onboard ethernet connector to the internal network, and OS X Server is plenty happy to firewall from USB ethernet to internal, and it will even let you set up a DMZ, if you wanted to buy 2 USB to ethernet connectors.