nixpkgs/modules/services/networking/firewall.nix

# This module enables a simple firewall.

{ config, pkgs, ... }:

with pkgs.lib;

let

  cfg = config.networking.firewall;

in

{

  ###### interface

  options = {
  
    networking.firewall.enable = mkOption {
      default = false;
      description =
        ''
          Whether to enable the firewall.  This is a simple stateful
          firewall that blocks connection attempts to unauthorised TCP
          or UDP ports on this machine.  It does not affect packet
          forwarding.
        '';
    };
  
    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.
        '';
    };
  
    networking.firewall.allowedUDPPorts = mkOption {
      default = [];
      example = [ 53 ];
      type = types.list types.int;
      description =
        ''
          List of open UDP ports.
        '';
    };
  
    networking.firewall.allowPing = mkOption {
      default = false;
      type = types.bool;
      description =
        ''
          Whether to respond to incoming ICMPv4 echo requests
          ("pings").  ICMPv6 pings are always allowed because the
          larger address space of IPv6 makes network scanning much
          less effective.
        '';
    };
  
    networking.firewall.extraCommands = mkOption {
      default = "";
      example = "iptables -A INPUT -p icmp -j ACCEPT";
      description =
        ''
          Additional shell commands executed as part of the firewall
          initialisation script.  These are executed just before the
          final "reject" firewall rule is added, so they can be used
          to allow packets that would otherwise be refused.
        '';
    };
  
  };


  ###### 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 cfg.enable {

    environment.systemPackages = [ pkgs.iptables ];

    boot.kernelModules = [ "nf_conntrack_ftp" ];

    jobs.firewall =
      { startOn = "started network-interfaces";

        path = [ pkgs.iptables ];

        preStart =
          ''
            # Helper command to manipulate both the IPv4 and IPv6 tables.
            ip46tables() {
              iptables "$@"
              ip6tables "$@"
            }

            ip46tables -F INPUT
            ip46tables -F FW_REFUSE || true
            ip46tables -X # flush unused chains
            ip46tables -P INPUT ACCEPT


            # The "FW_REFUSE" chain performs logging and
            # rejecting/dropping of packets.
            ip46tables -N FW_REFUSE

            ${optionalString cfg.logRefusedConnections ''
              ip46tables -A FW_REFUSE -p tcp --syn -j LOG --log-level info --log-prefix "rejected connection: "
            ''}
            ${optionalString cfg.logRefusedPackets ''
              ip46tables -A FW_REFUSE -j LOG --log-level info --log-prefix "rejected packet: "
            ''}

            ip46tables -A FW_REFUSE -j ${if cfg.rejectPackets then "REJECT" else "DROP"}


            # Accept all traffic on the loopback interface.
            ip46tables -A INPUT -i lo -j ACCEPT

            # Accept packets from established or related connections.
            ip46tables -A INPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT

            # Accept connections to the allowed TCP ports.
            ${concatMapStrings (port:
                ''
                  ip46tables -A INPUT -p tcp --dport ${toString port} -j ACCEPT
                ''
              ) cfg.allowedTCPPorts
            }

            # Accept packets on the allowed UDP ports.
            ${concatMapStrings (port:
                ''
                  ip46tables -A INPUT -p udp --dport ${toString port} -j ACCEPT
                ''
              ) cfg.allowedUDPPorts
            }

            # Accept IPv4 multicast.  Not a big security risk since
            # probably nobody is listening anyway.
            iptables -A INPUT -d 224.0.0.0/4 -j ACCEPT

            # Optionally respond to ICMPv4 pings.
            ${optionalString cfg.allowPing ''
              iptables -A INPUT -p icmp --icmp-type echo-request -j ACCEPT
            ''}

            # Accept all ICMPv6 messages except redirects and node
            # information queries (type 139).  See RFC 4890, section
            # 4.4.
            ip6tables -A INPUT -p icmpv6 --icmpv6-type redirect -j DROP
            ip6tables -A INPUT -p icmpv6 --icmpv6-type 139 -j DROP
            ip6tables -A INPUT -p icmpv6 -j ACCEPT

            ${cfg.extraCommands}

            # Reject/drop everything else.
            ip46tables -A INPUT -j FW_REFUSE
          '';

        postStop =
          ''
            iptables -F INPUT
            iptables -P INPUT ACCEPT
            ip6tables -F INPUT
            ip6tables -P INPUT ACCEPT
          '';
      };

  };

}