shimmer is a pair of small programs (a client and a server) that provide an alternative to port knocking programs such as tumbler and are used to hide a valuable port (such as a hidden web server or SSH) on a public IP address.

shimmer works by cryptographically changing a set of 16 ports (one of which forwards to the real service, and 15 others that lead to a trap to blacklist attackers). The 16 ports change every minute frustrating an attacker, but a legitimate user with access to a secret shared between the client and server can determine the real port, avoid blacklisting, and get a connection.

Since both client and server must be time synchronized to the nearest minute shimmer actual holds 48 ports open at a time (16 for the previous minute, 16 for the current minute and 16 for the next minute) to avoid problems due to small amounts of clock drift.