/* 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-input’ is the main chain for input packet processing. - ‘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 these chain. - ‘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. */ { config, pkgs, ... }: with pkgs.lib; let cfg = config.networking.firewall; helpers = '' # Helper command to manipulate both the IPv4 and IPv6 tables. ip46tables() { iptables "$@" ${optionalString config.networking.enableIPv6 '' ip6tables "$@" ''} } ''; 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.logRefusedUnicastsOnly = mkOption { default = true; description = '' If and this option are enabled, then only log packets specifically directed at this machine, i.e., not broadcasts or multicasts. ''; }; 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.trustedInterfaces = mkOption { type = types.list types.string; description = '' Traffic coming in from these interfaces will be accepted unconditionally. ''; }; 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 { networking.firewall.trustedInterfaces = [ "lo" ]; environment.systemPackages = [ pkgs.iptables ]; boot.kernelModules = [ "nf_conntrack_ftp" ]; jobs.firewall = { startOn = "started network-interfaces"; path = [ pkgs.iptables ]; preStart = '' ${helpers} # Flush the old firewall rules. !!! Ideally, updating the # firewall would be atomic. Apparently that's possible # with iptables-restore. ip46tables -D INPUT -j nixos-fw || true for chain in nixos-fw nixos-fw-accept nixos-fw-log-refuse nixos-fw-refuse FW_REFUSE; do ip46tables -F "$chain" || true ip46tables -X "$chain" || 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 "rejected connection: " ''} ${optionalString (cfg.logRefusedPackets && !cfg.logRefusedUnicastsOnly) '' ip46tables -A nixos-fw-log-refuse -m pkttype --pkt-type broadcast \ -j LOG --log-level info --log-prefix "rejected broadcast: " ip46tables -A nixos-fw-log-refuse -m pkttype --pkt-type multicast \ -j LOG --log-level info --log-prefix "rejected 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 "rejected packet: " ''} ip46tables -A nixos-fw-log-refuse -j nixos-fw-refuse # The "nixos-fw" chain does the actual work. ip46tables -N nixos-fw # 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. ${concatMapStrings (port: '' ip46tables -A nixos-fw -p tcp --dport ${toString port} -j nixos-fw-accept '' ) cfg.allowedTCPPorts } # Accept packets on the allowed UDP ports. ${concatMapStrings (port: '' ip46tables -A nixos-fw -p udp --dport ${toString port} -j nixos-fw-accept '' ) cfg.allowedUDPPorts } # Accept IPv4 multicast. Not a big security risk since # probably nobody is listening anyway. #iptables -A nixos-fw -d 224.0.0.0/4 -j nixos-fw-accept # Optionally respond to ICMPv4 pings. ${optionalString cfg.allowPing '' iptables -A nixos-fw -p icmp --icmp-type echo-request -j nixos-fw-accept ''} # Accept all ICMPv6 messages except redirects and node # information queries (type 139). See RFC 4890, section # 4.4. ${optionalString config.networking.enableIPv6 '' 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 ''} ${cfg.extraCommands} # Reject/drop everything else. ip46tables -A nixos-fw -j nixos-fw-log-refuse # Enable the firewall. ip46tables -A INPUT -j nixos-fw ''; postStop = '' ${helpers} ip46tables -D INPUT -j nixos-fw || true #ip46tables -P INPUT ACCEPT ''; }; }; }