letmein – authenticated port/firewall knocking

Letmein is a simple port knocker with a simple and secure authentication mechanism. It can be used to harden against pre-authentication attacks on services like SSH, VPN, IMAP and many more.

Letmein hides services on a server behind a knock authentication barrier to reduce the attack surface of a service. The service will not be accessible unless a knock authentication is successful. In case of a successful knock, the letmeind server will only open the knocked port for the client IP address that performed the knocking. Machines with different IP addresses still won’t have access to the protected service.

Machines that can’t successfully authenticate the knock sequence won’t be able to access the protected service. They will receive a ICMP reject on the protected service port with the provided example nftables.conf. (You can also decide to drop the packets in your nftables.conf instead).

Letmein requires an nftables based firewall. It will not work with iptables. If you use an iptables based firewall, please convert to nftables before installing letmein.

Website: github.com/mbuesch/letmein
Support:
Developer: Michael Büsch
License: MIT License or Apache License 2.0

Letmein is written in Rust. Learn Rust with our recommended free books and free tutorials.

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