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335 lines
12 KiB
Nix
335 lines
12 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 mangle 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:
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let
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dir = pkgs.writeScriptBin name ''
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#! ${pkgs.runtimeShell} -e
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${text}
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'';
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in
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"${dir}/bin/${name}";
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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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${optionalString cfg.logRefusedPackets ''
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ip46tables -A nixos-fw-log-refuse \
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-j LOG --log-level info --log-prefix "refused packet: "
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''}
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ip46tables -A nixos-fw-log-refuse -j nixos-fw-refuse
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# The "nixos-fw" chain does the actual work.
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ip46tables -N nixos-fw
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# Clean up rpfilter rules
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ip46tables -t mangle -D PREROUTING -j nixos-fw-rpfilter 2> /dev/null || true
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ip46tables -t mangle -F nixos-fw-rpfilter 2> /dev/null || true
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ip46tables -t mangle -X nixos-fw-rpfilter 2> /dev/null || true
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${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 mangle table doesn't have a log-refuse yet
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ip46tables -t mangle -N nixos-fw-rpfilter 2> /dev/null || true
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ip46tables -t mangle -A nixos-fw-rpfilter -m rpfilter --validmark ${optionalString (cfg.checkReversePath == "loose") "--loose"} -j RETURN
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# Allows this host to act as a DHCP4 client without first having to use APIPA
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iptables -t mangle -A nixos-fw-rpfilter -p udp --sport 67 --dport 68 -j RETURN
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# Allows this host to act as a DHCPv4 server
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iptables -t mangle -A nixos-fw-rpfilter -s 0.0.0.0 -d 255.255.255.255 -p udp --sport 68 --dport 67 -j RETURN
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${optionalString cfg.logReversePathDrops ''
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ip46tables -t mangle -A nixos-fw-rpfilter -j LOG --log-level info --log-prefix "rpfilter drop: "
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''}
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ip46tables -t mangle -A nixos-fw-rpfilter -j DROP
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ip46tables -t mangle -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 ${iface}"}
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''
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) cfg.allowedTCPPorts
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) cfg.allInterfaces)}
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# Accept connections to the allowed TCP port ranges.
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${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 ${iface}"}
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''
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) cfg.allowedTCPPortRanges
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) cfg.allInterfaces)}
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# Accept packets on the allowed UDP ports.
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${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 ${iface}"}
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''
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) cfg.allowedUDPPorts
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) cfg.allInterfaces)}
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# Accept packets on the allowed UDP port ranges.
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${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 ${iface}"}
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''
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) cfg.allowedUDPPortRanges
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) cfg.allInterfaces)}
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# Optionally respond to ICMPv4 pings.
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${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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${optionalString (kernelHasRPFilter && (cfg.checkReversePath != false)) ''
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ip46tables -t mangle -D PREROUTING -j nixos-fw-rpfilter 2>/dev/null || true
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''}
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${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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in
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{
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options = {
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networking.firewall = {
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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 = lib.mdDoc ''
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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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This option only works with the iptables based firewall.
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'';
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};
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extraStopCommands = mkOption {
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type = types.lines;
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default = "";
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example = "iptables -P INPUT ACCEPT";
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description = lib.mdDoc ''
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Additional shell commands executed as part of the firewall
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shutdown script. These are executed just after the removal
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of the NixOS input rule, or if the service enters a failed
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state.
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This option only works with the iptables based firewall.
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'';
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};
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};
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};
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# FIXME: Maybe if `enable' is false, the firewall should still be
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# built but not started by default?
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config = mkIf (cfg.enable && config.networking.nftables.enable == false) {
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assertions = [
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# This is approximately "checkReversePath -> kernelHasRPFilter",
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# but the checkReversePath option can include non-boolean
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# values.
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{
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assertion = cfg.checkReversePath == false || kernelHasRPFilter;
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message = "This kernel does not support rpfilter";
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}
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];
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networking.firewall.checkReversePath = mkIf (!kernelHasRPFilter) (mkDefault false);
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systemd.services.firewall = {
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description = "Firewall";
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wantedBy = [ "sysinit.target" ];
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wants = [ "network-pre.target" ];
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before = [ "network-pre.target" ];
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after = [ "systemd-modules-load.service" ];
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path = [ cfg.package ] ++ cfg.extraPackages;
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# FIXME: this module may also try to load kernel modules, but
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# containers don't have CAP_SYS_MODULE. So the host system had
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# better have all necessary modules already loaded.
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unitConfig.ConditionCapability = "CAP_NET_ADMIN";
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unitConfig.DefaultDependencies = false;
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reloadIfChanged = true;
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serviceConfig = {
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Type = "oneshot";
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RemainAfterExit = true;
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ExecStart = "@${startScript} firewall-start";
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ExecReload = "@${reloadScript} firewall-reload";
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ExecStop = "@${stopScript} firewall-stop";
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};
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};
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};
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}
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