Improved Port Knocking with Strong Authentication Rennie deGraaf, John Aycock, and Michael Jacobson, Jr.∗ Department of Computer Science University of Calgary Calgary, Alberta, Canada fdegraaf,aycock,
[email protected] Abstract is no way for users to authenticate themselves. A com- mon solution to this is to limit the hosts from which It is sometimes desirable to allow access to open users can connect. ports on a firewall only to authorized external users and The usual method of limiting the sources of network present closed ports to all others. We examine ways connections is to use a firewall. A firewall works by se- to construct an authentication service to achieve this lectively accepting or rejecting network packets based goal, and then examine one such method, “port knock- on their source addresses or other characteristics. Un- ing”, and its existing implementations, in detail. We fortunately, source addresses on packets tell little about improve upon these existing implementations by pre- the person who sent those packets; determined attack- senting a novel port knocking architecture that provides ers are quite capable of disguising the source of packets strong authentication while addressing the weaknesses that they send. Once a port on a firewall is open to one of existing port knocking systems. host, any attacker could potentially (ab)use that open- ing. Also, not all authorized users have predictable IP addresses, so granting them access through a firewall re- 1. Introduction quires opening the firewall to much or all of the Internet. Thus, while firewalls are useful and effective defenses Thirty-five years after the birth of the Internet, it against many attacks, they are not complete solutions.