StarTech.com Professional RJ45 Network Cable Tester with 4 Remote Loopback Plugs - LAN Cable Tester Professional - Network testing device - Token Ring - Test several cable runs simultaneously - lan cable
Physical loopback?
RJ45 Loopback Tester | Computer Cable Store
Source RJ45 Ethernet Loopback connector for network on m.alibaba.com
Make your own Ethernet Loopback connector – TikkiWeb