Back-to-origin port is the destination port number used for sending traffic back to the origin server in a load balancing or reverse proxy scenario. When a client sends a request to a load balancer or reverse proxy, the traffic is forwarded to one of the backend servers for processing. If the backend server needs to send a response back to the client, it needs to send the traffic through the load balancer or reverse proxy to ensure that the client's original request is correctly handled.
The back-to-origin port is the port number on the origin server where the traffic is sent back to. The load balancer or reverse proxy will replace the destination port number in the packet header with the back-to-origin port number, so that the traffic is correctly routed to the origin server.