NixOS has a simple stateful firewall that blocks incoming connections and other unexpected packets. The firewall applies to both IPv4 and IPv6 traffic. It is enabled by default. It can be disabled as follows: networking.firewall.enable = false; If the firewall is enabled, you can open specific TCP ports to the outside world: networking.firewall.allowedTCPPorts = [ 80 443 ]; Note that TCP port 22 (ssh) is opened automatically if the SSH daemon is enabled (services.openssh.enable = true). UDP ports can be opened through networking.firewall.allowedUDPPorts. Also of interest is networking.firewall.allowPing = true; to allow the machine to respond to ping requests. (ICMPv6 pings are always allowed.)