590 lines
20 KiB
Nix
590 lines
20 KiB
Nix
/* This module enables a simple firewall.
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The firewall can be customised in arbitrary ways by setting
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‘networking.firewall.extraCommands’. For modularity, the firewall
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uses several chains:
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- ‘nixos-fw’ is the main chain for input packet processing.
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- ‘nixos-fw-accept’ is called for accepted packets. If you want
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additional logging, or want to reject certain packets anyway, you
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can insert rules at the start of this chain.
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- ‘nixos-fw-log-refuse’ and ‘nixos-fw-refuse’ are called for
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refused packets. (The former jumps to the latter after logging
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the packet.) If you want additional logging, or want to accept
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certain packets anyway, you can insert rules at the start of
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this chain.
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- ‘nixos-fw-rpfilter’ is used as the main chain in the raw table,
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called from the built-in ‘PREROUTING’ chain. If the kernel
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supports it and `cfg.checkReversePath` is set this chain will
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perform a reverse path filter test.
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- ‘nixos-drop’ is used while reloading the firewall in order to drop
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all traffic. Since reloading isn't implemented in an atomic way
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this'll prevent any traffic from leaking through while reloading
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the firewall. However, if the reloading fails, the ‘firewall-stop’
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script will be called which in return will effectively disable the
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complete firewall (in the default configuration).
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*/
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{ config, lib, pkgs, ... }:
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with lib;
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let
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cfg = config.networking.firewall;
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inherit (config.boot.kernelPackages) kernel;
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kernelHasRPFilter = ((kernel.config.isEnabled or (x: false)) "IP_NF_MATCH_RPFILTER") || (kernel.features.netfilterRPFilter or false);
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helpers = import ./helpers.nix { inherit config lib; };
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writeShScript = name: text: let dir = pkgs.writeScriptBin name ''
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#! ${pkgs.runtimeShell} -e
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${text}
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''; in "${dir}/bin/${name}";
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defaultInterface = { default = mapAttrs (name: value: cfg.${name}) commonOptions; };
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allInterfaces = defaultInterface // cfg.interfaces;
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startScript = writeShScript "firewall-start" ''
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${helpers}
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# Flush the old firewall rules. !!! Ideally, updating the
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# firewall would be atomic. Apparently that's possible
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# with iptables-restore.
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ip46tables -D INPUT -j nixos-fw 2> /dev/null || true
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for chain in nixos-fw nixos-fw-accept nixos-fw-log-refuse nixos-fw-refuse; do
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ip46tables -F "$chain" 2> /dev/null || true
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ip46tables -X "$chain" 2> /dev/null || true
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done
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# The "nixos-fw-accept" chain just accepts packets.
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ip46tables -N nixos-fw-accept
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ip46tables -A nixos-fw-accept -j ACCEPT
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# The "nixos-fw-refuse" chain rejects or drops packets.
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ip46tables -N nixos-fw-refuse
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${if cfg.rejectPackets then ''
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# Send a reset for existing TCP connections that we've
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# somehow forgotten about. Send ICMP "port unreachable"
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# for everything else.
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ip46tables -A nixos-fw-refuse -p tcp ! --syn -j REJECT --reject-with tcp-reset
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ip46tables -A nixos-fw-refuse -j REJECT
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'' else ''
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ip46tables -A nixos-fw-refuse -j DROP
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''}
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# The "nixos-fw-log-refuse" chain performs logging, then
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# jumps to the "nixos-fw-refuse" chain.
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ip46tables -N nixos-fw-log-refuse
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${optionalString cfg.logRefusedConnections ''
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ip46tables -A nixos-fw-log-refuse -p tcp --syn -j LOG --log-level info --log-prefix "refused connection: "
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''}
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${optionalString (cfg.logRefusedPackets && !cfg.logRefusedUnicastsOnly) ''
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ip46tables -A nixos-fw-log-refuse -m pkttype --pkt-type broadcast \
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-j LOG --log-level info --log-prefix "refused broadcast: "
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ip46tables -A nixos-fw-log-refuse -m pkttype --pkt-type multicast \
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-j LOG --log-level info --log-prefix "refused multicast: "
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''}
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ip46tables -A nixos-fw-log-refuse -m pkttype ! --pkt-type unicast -j nixos-fw-refuse
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${optionalString cfg.logRefusedPackets ''
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ip46tables -A nixos-fw-log-refuse \
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-j LOG --log-level info --log-prefix "refused packet: "
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''}
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ip46tables -A nixos-fw-log-refuse -j nixos-fw-refuse
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# The "nixos-fw" chain does the actual work.
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ip46tables -N nixos-fw
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# Clean up rpfilter rules
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ip46tables -t raw -D PREROUTING -j nixos-fw-rpfilter 2> /dev/null || true
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ip46tables -t raw -F nixos-fw-rpfilter 2> /dev/null || true
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ip46tables -t raw -X nixos-fw-rpfilter 2> /dev/null || true
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${optionalString (kernelHasRPFilter && (cfg.checkReversePath != false)) ''
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# Perform a reverse-path test to refuse spoofers
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# For now, we just drop, as the raw table doesn't have a log-refuse yet
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ip46tables -t raw -N nixos-fw-rpfilter 2> /dev/null || true
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ip46tables -t raw -A nixos-fw-rpfilter -m rpfilter --validmark ${optionalString (cfg.checkReversePath == "loose") "--loose"} -j RETURN
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# Allows this host to act as a DHCP4 client without first having to use APIPA
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iptables -t raw -A nixos-fw-rpfilter -p udp --sport 67 --dport 68 -j RETURN
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# Allows this host to act as a DHCPv4 server
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iptables -t raw -A nixos-fw-rpfilter -s 0.0.0.0 -d 255.255.255.255 -p udp --sport 68 --dport 67 -j RETURN
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${optionalString cfg.logReversePathDrops ''
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ip46tables -t raw -A nixos-fw-rpfilter -j LOG --log-level info --log-prefix "rpfilter drop: "
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''}
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ip46tables -t raw -A nixos-fw-rpfilter -j DROP
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ip46tables -t raw -A PREROUTING -j nixos-fw-rpfilter
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''}
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# Accept all traffic on the trusted interfaces.
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${flip concatMapStrings cfg.trustedInterfaces (iface: ''
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ip46tables -A nixos-fw -i ${iface} -j nixos-fw-accept
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'')}
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# Accept packets from established or related connections.
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ip46tables -A nixos-fw -m conntrack --ctstate ESTABLISHED,RELATED -j nixos-fw-accept
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# Accept connections to the allowed TCP ports.
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${concatStrings (mapAttrsToList (iface: cfg:
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concatMapStrings (port:
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''
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ip46tables -A nixos-fw -p tcp --dport ${toString port} -j nixos-fw-accept ${optionalString (iface != "default") "-i ${iface}"}
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''
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) cfg.allowedTCPPorts
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) allInterfaces)}
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# Accept connections to the allowed TCP port ranges.
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${concatStrings (mapAttrsToList (iface: cfg:
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concatMapStrings (rangeAttr:
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let range = toString rangeAttr.from + ":" + toString rangeAttr.to; in
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''
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ip46tables -A nixos-fw -p tcp --dport ${range} -j nixos-fw-accept ${optionalString (iface != "default") "-i ${iface}"}
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''
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) cfg.allowedTCPPortRanges
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) allInterfaces)}
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# Accept packets on the allowed UDP ports.
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${concatStrings (mapAttrsToList (iface: cfg:
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concatMapStrings (port:
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''
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ip46tables -A nixos-fw -p udp --dport ${toString port} -j nixos-fw-accept ${optionalString (iface != "default") "-i ${iface}"}
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''
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) cfg.allowedUDPPorts
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) allInterfaces)}
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# Accept packets on the allowed UDP port ranges.
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${concatStrings (mapAttrsToList (iface: cfg:
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concatMapStrings (rangeAttr:
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let range = toString rangeAttr.from + ":" + toString rangeAttr.to; in
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''
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ip46tables -A nixos-fw -p udp --dport ${range} -j nixos-fw-accept ${optionalString (iface != "default") "-i ${iface}"}
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''
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) cfg.allowedUDPPortRanges
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) allInterfaces)}
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# Accept IPv4 multicast. Not a big security risk since
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# probably nobody is listening anyway.
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#iptables -A nixos-fw -d 224.0.0.0/4 -j nixos-fw-accept
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# Optionally respond to ICMPv4 pings.
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${optionalString cfg.allowPing ''
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iptables -w -A nixos-fw -p icmp --icmp-type echo-request ${optionalString (cfg.pingLimit != null)
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"-m limit ${cfg.pingLimit} "
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}-j nixos-fw-accept
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''}
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${optionalString config.networking.enableIPv6 ''
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# Accept all ICMPv6 messages except redirects and node
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# information queries (type 139). See RFC 4890, section
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# 4.4.
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ip6tables -A nixos-fw -p icmpv6 --icmpv6-type redirect -j DROP
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ip6tables -A nixos-fw -p icmpv6 --icmpv6-type 139 -j DROP
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ip6tables -A nixos-fw -p icmpv6 -j nixos-fw-accept
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# Allow this host to act as a DHCPv6 client
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ip6tables -A nixos-fw -d fe80::/64 -p udp --dport 546 -j nixos-fw-accept
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''}
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${cfg.extraCommands}
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# Reject/drop everything else.
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ip46tables -A nixos-fw -j nixos-fw-log-refuse
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# Enable the firewall.
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ip46tables -A INPUT -j nixos-fw
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'';
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stopScript = writeShScript "firewall-stop" ''
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${helpers}
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# Clean up in case reload fails
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ip46tables -D INPUT -j nixos-drop 2>/dev/null || true
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# Clean up after added ruleset
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ip46tables -D INPUT -j nixos-fw 2>/dev/null || true
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${optionalString (kernelHasRPFilter && (cfg.checkReversePath != false)) ''
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ip46tables -t raw -D PREROUTING -j nixos-fw-rpfilter 2>/dev/null || true
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''}
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${cfg.extraStopCommands}
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'';
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reloadScript = writeShScript "firewall-reload" ''
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${helpers}
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# Create a unique drop rule
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ip46tables -D INPUT -j nixos-drop 2>/dev/null || true
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ip46tables -F nixos-drop 2>/dev/null || true
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ip46tables -X nixos-drop 2>/dev/null || true
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ip46tables -N nixos-drop
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ip46tables -A nixos-drop -j DROP
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# Don't allow traffic to leak out until the script has completed
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ip46tables -A INPUT -j nixos-drop
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${cfg.extraStopCommands}
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if ${startScript}; then
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ip46tables -D INPUT -j nixos-drop 2>/dev/null || true
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else
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echo "Failed to reload firewall... Stopping"
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${stopScript}
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exit 1
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fi
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'';
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canonicalizePortList =
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ports: lib.unique (builtins.sort builtins.lessThan ports);
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commonOptions = {
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allowedTCPPorts = mkOption {
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type = types.listOf types.port;
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default = [ ];
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apply = canonicalizePortList;
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example = [ 22 80 ];
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description =
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''
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List of TCP ports on which incoming connections are
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accepted.
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'';
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};
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allowedTCPPortRanges = mkOption {
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type = types.listOf (types.attrsOf types.port);
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default = [ ];
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example = [ { from = 8999; to = 9003; } ];
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description =
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''
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A range of TCP ports on which incoming connections are
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accepted.
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'';
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};
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allowedUDPPorts = mkOption {
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type = types.listOf types.port;
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default = [ ];
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apply = canonicalizePortList;
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example = [ 53 ];
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description =
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''
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List of open UDP ports.
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'';
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};
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allowedUDPPortRanges = mkOption {
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type = types.listOf (types.attrsOf types.port);
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default = [ ];
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example = [ { from = 60000; to = 61000; } ];
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description =
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''
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Range of open UDP ports.
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'';
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};
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};
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in
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{
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###### interface
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options = {
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networking.firewall = {
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enable = mkOption {
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type = types.bool;
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default = true;
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description =
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''
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Whether to enable the firewall. This is a simple stateful
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firewall that blocks connection attempts to unauthorised TCP
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or UDP ports on this machine. It does not affect packet
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forwarding.
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'';
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};
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package = mkOption {
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type = types.package;
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default = pkgs.iptables;
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defaultText = "pkgs.iptables";
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example = literalExample "pkgs.iptables-nftables-compat";
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description =
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''
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The iptables package to use for running the firewall service."
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'';
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};
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logRefusedConnections = mkOption {
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type = types.bool;
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default = true;
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description =
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''
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Whether to log rejected or dropped incoming connections.
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Note: The logs are found in the kernel logs, i.e. dmesg
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or journalctl -k.
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'';
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};
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logRefusedPackets = mkOption {
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type = types.bool;
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default = false;
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description =
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''
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Whether to log all rejected or dropped incoming packets.
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This tends to give a lot of log messages, so it's mostly
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useful for debugging.
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Note: The logs are found in the kernel logs, i.e. dmesg
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or journalctl -k.
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'';
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};
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logRefusedUnicastsOnly = mkOption {
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type = types.bool;
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default = true;
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description =
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''
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If <option>networking.firewall.logRefusedPackets</option>
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and this option are enabled, then only log packets
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specifically directed at this machine, i.e., not broadcasts
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or multicasts.
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'';
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};
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rejectPackets = mkOption {
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type = types.bool;
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default = false;
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description =
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''
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If set, refused packets are rejected rather than dropped
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(ignored). This means that an ICMP "port unreachable" error
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message is sent back to the client (or a TCP RST packet in
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case of an existing connection). Rejecting packets makes
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port scanning somewhat easier.
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'';
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};
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trustedInterfaces = mkOption {
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type = types.listOf types.str;
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default = [ ];
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example = [ "enp0s2" ];
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description =
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''
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Traffic coming in from these interfaces will be accepted
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unconditionally. Traffic from the loopback (lo) interface
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will always be accepted.
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'';
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};
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allowPing = mkOption {
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type = types.bool;
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default = true;
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description =
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''
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Whether to respond to incoming ICMPv4 echo requests
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("pings"). ICMPv6 pings are always allowed because the
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larger address space of IPv6 makes network scanning much
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less effective.
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'';
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};
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pingLimit = mkOption {
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type = types.nullOr (types.separatedString " ");
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default = null;
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example = "--limit 1/minute --limit-burst 5";
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description =
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''
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If pings are allowed, this allows setting rate limits
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on them. If non-null, this option should be in the form of
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flags like "--limit 1/minute --limit-burst 5"
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'';
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};
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checkReversePath = mkOption {
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type = types.either types.bool (types.enum ["strict" "loose"]);
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default = kernelHasRPFilter;
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example = "loose";
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description =
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''
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Performs a reverse path filter test on a packet. If a reply
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to the packet would not be sent via the same interface that
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the packet arrived on, it is refused.
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If using asymmetric routing or other complicated routing, set
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this option to loose mode or disable it and setup your own
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counter-measures.
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This option can be either true (or "strict"), "loose" (only
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drop the packet if the source address is not reachable via any
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interface) or false. Defaults to the value of
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kernelHasRPFilter.
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(needs kernel 3.3+)
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'';
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};
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logReversePathDrops = mkOption {
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type = types.bool;
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default = false;
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description =
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''
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Logs dropped packets failing the reverse path filter test if
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the option networking.firewall.checkReversePath is enabled.
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'';
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};
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connectionTrackingModules = mkOption {
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type = types.listOf types.str;
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default = [ ];
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example = [ "ftp" "irc" "sane" "sip" "tftp" "amanda" "h323" "netbios_sn" "pptp" "snmp" ];
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description =
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''
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List of connection-tracking helpers that are auto-loaded.
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The complete list of possible values is given in the example.
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As helpers can pose as a security risk, it is advised to
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set this to an empty list and disable the setting
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networking.firewall.autoLoadConntrackHelpers unless you
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know what you are doing. Connection tracking is disabled
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by default.
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Loading of helpers is recommended to be done through the
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CT target. More info:
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https://home.regit.org/netfilter-en/secure-use-of-helpers/
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'';
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};
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autoLoadConntrackHelpers = mkOption {
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type = types.bool;
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default = false;
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description =
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''
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Whether to auto-load connection-tracking helpers.
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See the description at networking.firewall.connectionTrackingModules
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(needs kernel 3.5+)
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'';
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};
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extraCommands = mkOption {
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type = types.lines;
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default = "";
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example = "iptables -A INPUT -p icmp -j ACCEPT";
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description =
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''
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Additional shell commands executed as part of the firewall
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initialisation script. These are executed just before the
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final "reject" firewall rule is added, so they can be used
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to allow packets that would otherwise be refused.
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'';
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};
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extraPackages = mkOption {
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type = types.listOf types.package;
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default = [ ];
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example = literalExample "[ pkgs.ipset ]";
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description =
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''
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Additional packages to be included in the environment of the system
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as well as the path of networking.firewall.extraCommands.
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'';
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};
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extraStopCommands = mkOption {
|
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type = types.lines;
|
||
default = "";
|
||
example = "iptables -P INPUT ACCEPT";
|
||
description =
|
||
''
|
||
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.
|
||
'';
|
||
};
|
||
|
||
interfaces = mkOption {
|
||
default = { };
|
||
type = with types; attrsOf (submodule [ { options = commonOptions; } ]);
|
||
description =
|
||
''
|
||
Interface-specific open ports.
|
||
'';
|
||
};
|
||
} // commonOptions;
|
||
|
||
};
|
||
|
||
|
||
###### implementation
|
||
|
||
# FIXME: Maybe if `enable' is false, the firewall should still be
|
||
# built but not started by default?
|
||
config = mkIf cfg.enable {
|
||
|
||
networking.firewall.trustedInterfaces = [ "lo" ];
|
||
|
||
environment.systemPackages = [ cfg.package ] ++ cfg.extraPackages;
|
||
|
||
boot.kernelModules = (optional cfg.autoLoadConntrackHelpers "nf_conntrack")
|
||
++ map (x: "nf_conntrack_${x}") cfg.connectionTrackingModules;
|
||
boot.extraModprobeConfig = optionalString cfg.autoLoadConntrackHelpers ''
|
||
options nf_conntrack nf_conntrack_helper=1
|
||
'';
|
||
|
||
assertions = [
|
||
# This is approximately "checkReversePath -> kernelHasRPFilter",
|
||
# but the checkReversePath option can include non-boolean
|
||
# values.
|
||
{ assertion = cfg.checkReversePath == false || kernelHasRPFilter;
|
||
message = "This kernel does not support rpfilter"; }
|
||
];
|
||
|
||
systemd.services.firewall = {
|
||
description = "Firewall";
|
||
wantedBy = [ "sysinit.target" ];
|
||
wants = [ "network-pre.target" ];
|
||
before = [ "network-pre.target" ];
|
||
after = [ "systemd-modules-load.service" ];
|
||
|
||
path = [ cfg.package ] ++ cfg.extraPackages;
|
||
|
||
# FIXME: this module may also try to load kernel modules, but
|
||
# containers don't have CAP_SYS_MODULE. So the host system had
|
||
# better have all necessary modules already loaded.
|
||
unitConfig.ConditionCapability = "CAP_NET_ADMIN";
|
||
unitConfig.DefaultDependencies = false;
|
||
|
||
reloadIfChanged = true;
|
||
|
||
serviceConfig = {
|
||
Type = "oneshot";
|
||
RemainAfterExit = true;
|
||
ExecStart = "@${startScript} firewall-start";
|
||
ExecReload = "@${reloadScript} firewall-reload";
|
||
ExecStop = "@${stopScript} firewall-stop";
|
||
};
|
||
};
|
||
|
||
};
|
||
|
||
}
|