What does the command /ip firewall nat add chain=dstnat in-interface=ether1 protocol=tcp dst-port=3389 action=dst-nat toaddress=192.168.1.2 to-ports=81 do?

The command provided is designed to configure destination network address translation (dst-nat) on a MikroTik router. In this case, it specifies that any TCP traffic arriving on the router via the interface ether1 targeting destination port 3389 will be redirected (or forwarded) to the internal IP address 192.168.1.2 and mapped to port 81.

This means that when external clients send TCP packets to the router's public IP on port 3389, the router will intercept these packets and change the destination address to 192.168.1.2 and the destination port to 81. This is commonly used for services like Remote Desktop Protocol (RDP), which typically operates on port 3389, allowing internal devices to receive the traffic on a different port.

The action effectively creates a tunnel of sorts, directing specific incoming traffic to an internal server while allowing the server to operate on a port different from the one external clients use. This technique can enhance security by obscuring the internal service's actual operating port.

Other options do not describe this specific behavior, focusing instead on erroneous interpretations of how the command affects traffic flow or wrongly specifying the workings of the NAT configuration.