Among other things, systems and techniques are described for authenticating one or more client devices. A system includes one or more client devices and a server to communicate with the one or more client devices over a network. The server receives a request for network connection from at least one of the client devices. In response to the received request, the server performs authentication of the requesting client device based on at least two factors. The at least two factors includes a transmission control protocol (TCP) header verification to identify a media access control (MAC) address of the requesting client device as an authorized or unauthorized MAC address; and a challenge-reply verification performed based on the TCP header verification. The challenge-reply verification includes sending a challenge message sent to the requesting client device; receiving a reply message from the requesting client device; and identifying the received reply message as a correct or incorrect reply.