nixpkgs/modules/services/networking/firewall.nix

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{ config, pkgs, ... }:
with pkgs.lib;
let
iptables = "${pkgs.iptables}/sbin/iptables";
cfg = config.networking.firewall;
in
{
###### interface
options = {
networking.firewall.enable = mkOption {
default = false;
description =
''
Whether to enable the firewall.
'';
};
networking.firewall.logRefusedConnections = mkOption {
default = true;
description =
''
Whether to log rejected or dropped incoming connections.
'';
};
networking.firewall.logRefusedPackets = mkOption {
default = false;
description =
''
Whether to log all rejected or dropped incoming packets.
This tends to give a lot of log messages, so it's mostly
useful for debugging.
'';
};
networking.firewall.rejectPackets = mkOption {
default = false;
description =
''
If set, forbidden packets are rejected rather than dropped
(ignored). This means that a ICMP "port unreachable" error
message is sent back to the client. Rejecting packets makes
port scanning somewhat easier.
'';
};
networking.firewall.allowedTCPPorts = mkOption {
default = [];
example = [22 80];
type = types.list types.int;
description =
''
List of TCP ports on which incoming connections are
accepted.
'';
};
};
###### implementation
# !!! Maybe if `enable' is false, the firewall should still be built
# but not started by default. However, currently nixos-rebuild
# doesn't deal with such Upstart jobs properly (it starts them if
# they are changed, regardless of whether the start condition
# holds).
config = mkIf config.networking.firewall.enable {
environment.systemPackages = [pkgs.iptables];
jobs.firewall =
{ startOn = "network-interfaces/started";
preStart =
''
${iptables} -F
# Accept all traffic on the loopback interface.
${iptables} -A INPUT -i lo -j ACCEPT
# Accept packets from established or related connections.
${iptables} -A INPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
# Accept connections to the allowed TCP ports.
${concatMapStrings (port:
''
${iptables} -A INPUT -p tcp --dport ${toString port} -j ACCEPT
''
) config.networking.firewall.allowedTCPPorts
}
# Accept multicast. Not a big security risk since
# probably nobody is listening anyway.
${iptables} -A INPUT -d 224.0.0.0/4 -j ACCEPT
# Drop everything else.
${optionalString cfg.logRefusedConnections ''
${iptables} -A INPUT -p tcp --syn -j LOG --log-level info --log-prefix "rejected connection: "
''}
${optionalString cfg.logRefusedPackets ''
${iptables} -A INPUT -j LOG --log-level info --log-prefix "rejected packet: "
''}
${iptables} -A INPUT -j ${if cfg.rejectPackets then "REJECT" else "DROP"}
'';
postStop =
''
${iptables} -F
'';
};
};
}