Smallish script to prevent smallish A2S attacks with source engine servers, specifically tested with TF2. Supports servers running on docker (using the DOCKER-USER chain) and on bare metal/tmux/etc (using the INPUT chain). Vaguely adapted from https://forums.alliedmods.net/showthread.php?t=151551.
Under /etc/hosts.trusted, to allow a list of hosts unbridled access to your servers, you can create a file called /etc/hosts.trusted and add a list of IPs you trust separated by a newline.
Example of such a file:
192.168.0.1/24
10.0.12.1/24