/* This module enables a simple firewall. The firewall can be customised in arbitrary ways by setting ‘networking.firewall.extraCommands’. For modularity, the firewall uses several chains: - ‘nixos-fw’ is the main chain for input packet processing. - ‘nixos-fw-accept’ is called for accepted packets. If you want additional logging, or want to reject certain packets anyway, you can insert rules at the start of this chain. - ‘nixos-fw-log-refuse’ and ‘nixos-fw-refuse’ are called for refused packets. (The former jumps to the latter after logging the packet.) If you want additional logging, or want to accept certain packets anyway, you can insert rules at the start of this chain. - ‘nixos-fw-rpfilter’ is used as the main chain in the mangle table, called from the built-in ‘PREROUTING’ chain. If the kernel supports it and `cfg.checkReversePath` is set this chain will perform a reverse path filter test. - ‘nixos-drop’ is used while reloading the firewall in order to drop all traffic. Since reloading isn't implemented in an atomic way this'll prevent any traffic from leaking through while reloading the firewall. However, if the reloading fails, the ‘firewall-stop’ script will be called which in return will effectively disable the complete firewall (in the default configuration). */ { config, lib, pkgs, ... }: with lib; let cfg = config.networking.firewall; inherit (config.boot.kernelPackages) kernel; kernelHasRPFilter = ((kernel.config.isEnabled or (x: false)) "IP_NF_MATCH_RPFILTER") || (kernel.features.netfilterRPFilter or false); helpers = import ./helpers.nix { inherit config lib; }; writeShScript = name: text: let dir = pkgs.writeScriptBin name '' #! ${pkgs.runtimeShell} -e ${text} ''; in "${dir}/bin/${name}"; startScript = writeShScript "firewall-start" '' ${helpers} # Flush the old firewall rules. !!! Ideally, updating the # firewall would be atomic. Apparently that's possible # with iptables-restore. ip46tables -D INPUT -j nixos-fw 2> /dev/null || true for chain in nixos-fw nixos-fw-accept nixos-fw-log-refuse nixos-fw-refuse; do ip46tables -F "$chain" 2> /dev/null || true ip46tables -X "$chain" 2> /dev/null || true done # The "nixos-fw-accept" chain just accepts packets. ip46tables -N nixos-fw-accept ip46tables -A nixos-fw-accept -j ACCEPT # The "nixos-fw-refuse" chain rejects or drops packets. ip46tables -N nixos-fw-refuse ${if cfg.rejectPackets then '' # Send a reset for existing TCP connections that we've # somehow forgotten about. Send ICMP "port unreachable" # for everything else. ip46tables -A nixos-fw-refuse -p tcp ! --syn -j REJECT --reject-with tcp-reset ip46tables -A nixos-fw-refuse -j REJECT '' else '' ip46tables -A nixos-fw-refuse -j DROP ''} # The "nixos-fw-log-refuse" chain performs logging, then # jumps to the "nixos-fw-refuse" chain. ip46tables -N nixos-fw-log-refuse ${optionalString cfg.logRefusedConnections '' ip46tables -A nixos-fw-log-refuse -p tcp --syn -j LOG --log-level info --log-prefix "refused connection: " ''} ${optionalString (cfg.logRefusedPackets && !cfg.logRefusedUnicastsOnly) '' ip46tables -A nixos-fw-log-refuse -m pkttype --pkt-type broadcast \ -j LOG --log-level info --log-prefix "refused broadcast: " ip46tables -A nixos-fw-log-refuse -m pkttype --pkt-type multicast \ -j LOG --log-level info --log-prefix "refused multicast: " ''} ip46tables -A nixos-fw-log-refuse -m pkttype ! --pkt-type unicast -j nixos-fw-refuse ${optionalString cfg.logRefusedPackets '' ip46tables -A nixos-fw-log-refuse \ -j LOG --log-level info --log-prefix "refused packet: " ''} ip46tables -A nixos-fw-log-refuse -j nixos-fw-refuse # The "nixos-fw" chain does the actual work. ip46tables -N nixos-fw # Clean up rpfilter rules ip46tables -t mangle -D PREROUTING -j nixos-fw-rpfilter 2> /dev/null || true ip46tables -t mangle -F nixos-fw-rpfilter 2> /dev/null || true ip46tables -t mangle -X nixos-fw-rpfilter 2> /dev/null || true ${optionalString (kernelHasRPFilter && (cfg.checkReversePath != false)) '' # Perform a reverse-path test to refuse spoofers # For now, we just drop, as the mangle table doesn't have a log-refuse yet ip46tables -t mangle -N nixos-fw-rpfilter 2> /dev/null || true ip46tables -t mangle -A nixos-fw-rpfilter -m rpfilter --validmark ${optionalString (cfg.checkReversePath == "loose") "--loose"} -j RETURN # Allows this host to act as a DHCP4 client without first having to use APIPA iptables -t mangle -A nixos-fw-rpfilter -p udp --sport 67 --dport 68 -j RETURN # Allows this host to act as a DHCPv4 server iptables -t mangle -A nixos-fw-rpfilter -s 0.0.0.0 -d 255.255.255.255 -p udp --sport 68 --dport 67 -j RETURN ${optionalString cfg.logReversePathDrops '' ip46tables -t mangle -A nixos-fw-rpfilter -j LOG --log-level info --log-prefix "rpfilter drop: " ''} ip46tables -t mangle -A nixos-fw-rpfilter -j DROP ip46tables -t mangle -A PREROUTING -j nixos-fw-rpfilter ''} # Accept all traffic on the trusted interfaces. ${flip concatMapStrings cfg.trustedInterfaces (iface: '' ip46tables -A nixos-fw -i ${iface} -j nixos-fw-accept '')} # Accept packets from established or related connections. ip46tables -A nixos-fw -m conntrack --ctstate ESTABLISHED,RELATED -j nixos-fw-accept # Accept connections to the allowed TCP ports. ${concatStrings (mapAttrsToList (iface: cfg: concatMapStrings (port: '' ip46tables -A nixos-fw -p tcp --dport ${toString port} -j nixos-fw-accept ${optionalString (iface != "default") "-i ${iface}"} '' ) cfg.allowedTCPPorts ) cfg.allInterfaces)} # Accept connections to the allowed TCP port ranges. ${concatStrings (mapAttrsToList (iface: cfg: concatMapStrings (rangeAttr: let range = toString rangeAttr.from + ":" + toString rangeAttr.to; in '' ip46tables -A nixos-fw -p tcp --dport ${range} -j nixos-fw-accept ${optionalString (iface != "default") "-i ${iface}"} '' ) cfg.allowedTCPPortRanges ) cfg.allInterfaces)} # Accept packets on the allowed UDP ports. ${concatStrings (mapAttrsToList (iface: cfg: concatMapStrings (port: '' ip46tables -A nixos-fw -p udp --dport ${toString port} -j nixos-fw-accept ${optionalString (iface != "default") "-i ${iface}"} '' ) cfg.allowedUDPPorts ) cfg.allInterfaces)} # Accept packets on the allowed UDP port ranges. ${concatStrings (mapAttrsToList (iface: cfg: concatMapStrings (rangeAttr: let range = toString rangeAttr.from + ":" + toString rangeAttr.to; in '' ip46tables -A nixos-fw -p udp --dport ${range} -j nixos-fw-accept ${optionalString (iface != "default") "-i ${iface}"} '' ) cfg.allowedUDPPortRanges ) cfg.allInterfaces)} # Optionally respond to ICMPv4 pings. ${optionalString cfg.allowPing '' iptables -w -A nixos-fw -p icmp --icmp-type echo-request ${optionalString (cfg.pingLimit != null) "-m limit ${cfg.pingLimit} " }-j nixos-fw-accept ''} ${optionalString config.networking.enableIPv6 '' # Accept all ICMPv6 messages except redirects and node # information queries (type 139). See RFC 4890, section # 4.4. ip6tables -A nixos-fw -p icmpv6 --icmpv6-type redirect -j DROP ip6tables -A nixos-fw -p icmpv6 --icmpv6-type 139 -j DROP ip6tables -A nixos-fw -p icmpv6 -j nixos-fw-accept # Allow this host to act as a DHCPv6 client ip6tables -A nixos-fw -d fe80::/64 -p udp --dport 546 -j nixos-fw-accept ''} ${cfg.extraCommands} # Reject/drop everything else. ip46tables -A nixos-fw -j nixos-fw-log-refuse # Enable the firewall. ip46tables -A INPUT -j nixos-fw ''; stopScript = writeShScript "firewall-stop" '' ${helpers} # Clean up in case reload fails ip46tables -D INPUT -j nixos-drop 2>/dev/null || true # Clean up after added ruleset ip46tables -D INPUT -j nixos-fw 2>/dev/null || true ${optionalString (kernelHasRPFilter && (cfg.checkReversePath != false)) '' ip46tables -t mangle -D PREROUTING -j nixos-fw-rpfilter 2>/dev/null || true ''} ${cfg.extraStopCommands} ''; reloadScript = writeShScript "firewall-reload" '' ${helpers} # Create a unique drop rule ip46tables -D INPUT -j nixos-drop 2>/dev/null || true ip46tables -F nixos-drop 2>/dev/null || true ip46tables -X nixos-drop 2>/dev/null || true ip46tables -N nixos-drop ip46tables -A nixos-drop -j DROP # Don't allow traffic to leak out until the script has completed ip46tables -A INPUT -j nixos-drop ${cfg.extraStopCommands} if ${startScript}; then ip46tables -D INPUT -j nixos-drop 2>/dev/null || true else echo "Failed to reload firewall... Stopping" ${stopScript} exit 1 fi ''; in { options = { networking.firewall = { extraCommands = mkOption { type = types.lines; default = ""; example = "iptables -A INPUT -p icmp -j ACCEPT"; description = lib.mdDoc '' 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. This option only works with the iptables based firewall. ''; }; extraStopCommands = mkOption { type = types.lines; default = ""; example = "iptables -P INPUT ACCEPT"; description = lib.mdDoc '' Additional shell commands executed as part of the firewall shutdown script. These are executed just after the removal of the NixOS input rule, or if the service enters a failed state. This option only works with the iptables based firewall. ''; }; }; }; # FIXME: Maybe if `enable' is false, the firewall should still be # built but not started by default? config = mkIf (cfg.enable && config.networking.nftables.enable == false) { assertions = [ # This is approximately "checkReversePath -> kernelHasRPFilter", # but the checkReversePath option can include non-boolean # values. { assertion = cfg.checkReversePath == false || kernelHasRPFilter; message = "This kernel does not support rpfilter"; } ]; networking.firewall.checkReversePath = mkIf (!kernelHasRPFilter) (mkDefault false); systemd.services.firewall = { description = "Firewall"; wantedBy = [ "sysinit.target" ]; wants = [ "network-pre.target" ]; before = [ "network-pre.target" ]; after = [ "systemd-modules-load.service" ]; path = [ cfg.package ] ++ cfg.extraPackages; # FIXME: this module may also try to load kernel modules, but # containers don't have CAP_SYS_MODULE. So the host system had # better have all necessary modules already loaded. unitConfig.ConditionCapability = "CAP_NET_ADMIN"; unitConfig.DefaultDependencies = false; reloadIfChanged = true; serviceConfig = { Type = "oneshot"; RemainAfterExit = true; ExecStart = "@${startScript} firewall-start"; ExecReload = "@${reloadScript} firewall-reload"; ExecStop = "@${stopScript} firewall-stop"; }; }; }; }