370 lines
12 KiB
Nix
370 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-input’ is the main chain for input packet processing.
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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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these chain.
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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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*/
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{ config, pkgs, ... }:
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with pkgs.lib;
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let
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cfg = config.networking.firewall;
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helpers =
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''
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# Helper command to manipulate both the IPv4 and IPv6 tables.
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ip46tables() {
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iptables "$@"
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${optionalString config.networking.enableIPv6 ''
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ip6tables "$@"
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''}
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}
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'';
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kernelPackages = config.boot.kernelPackages;
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kernelHasRPFilter = kernelPackages.kernel ? features
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&& kernelPackages.kernel.features ? netfilterRPFilter
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&& kernelPackages.kernel.features.netfilterRPFilter;
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kernelCanDisableHelpers = kernelPackages.kernel ? features
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&& kernelPackages.kernel.features ? canDisableNetfilterConntrackHelpers
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&& kernelPackages.kernel.features.canDisableNetfilterConntrackHelpers;
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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.enable = mkOption {
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default = false;
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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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networking.firewall.logRefusedConnections = mkOption {
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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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'';
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};
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networking.firewall.logRefusedPackets = mkOption {
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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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'';
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};
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networking.firewall.logRefusedUnicastsOnly = mkOption {
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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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networking.firewall.rejectPackets = mkOption {
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default = false;
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description =
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''
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If set, forbidden packets are rejected rather than dropped
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(ignored). This means that a ICMP "port unreachable" error
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message is sent back to the client. Rejecting packets makes
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port scanning somewhat easier.
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'';
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};
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networking.firewall.trustedInterfaces = mkOption {
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type = types.list types.string;
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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.
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'';
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};
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networking.firewall.allowedTCPPorts = mkOption {
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default = [];
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example = [ 22 80 ];
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type = types.list types.int;
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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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networking.firewall.allowedUDPPorts = mkOption {
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default = [];
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example = [ 53 ];
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type = types.list types.int;
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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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networking.firewall.allowPing = mkOption {
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default = false;
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type = types.bool;
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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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networking.firewall.checkReversePath = mkOption {
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default = kernelHasRPFilter;
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type = types.bool;
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description =
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''
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Performs a reverse path filter test on a packet.
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If a reply to the packet would not be sent via the same interface
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that the packet arrived on, it is refused.
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If using asymmetric routing or other complicated routing,
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disable this setting and setup your own counter-measures.
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(needs kernel 3.3+)
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'';
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};
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networking.firewall.connectionTrackingModules = mkOption {
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default = [ "ftp" ];
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example = [ "ftp" "irc" "sane" "sip" "tftp" "amanda" "h323" "netbios_sn" "pptp" "snmp" ];
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type = types.list types.string;
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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 adviced to
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set this to an empty list and disable the setting
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networking.firewall.autoLoadConntrackHelpers
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Loading of helpers is recommended to be done through the new
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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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networking.firewall.autoLoadConntrackHelpers = mkOption {
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default = true;
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type = types.bool;
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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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networking.firewall.extraCommands = mkOption {
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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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};
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###### implementation
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# !!! Maybe if `enable' is false, the firewall should still be built
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# but not started by default. However, currently nixos-rebuild
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# doesn't deal with such Upstart jobs properly (it starts them if
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# they are changed, regardless of whether the start condition
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# holds).
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config = mkIf cfg.enable {
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networking.firewall.trustedInterfaces = [ "lo" ];
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environment.systemPackages = [ pkgs.iptables ];
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boot.kernelModules = map (x: "nf_conntrack_${x}") cfg.connectionTrackingModules;
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boot.extraModprobeConfig = optionalString (!cfg.autoLoadConntrackHelpers) ''
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options nf_conntrack nf_conntrack_helper=0
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'';
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assertions = [ { assertion = ! cfg.checkReversePath || kernelHasRPFilter;
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message = "This kernel does not support rpfilter"; }
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{ assertion = cfg.autoLoadConntrackHelpers || kernelCanDisableHelpers;
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message = "This kernel does not support disabling conntrack helpers"; }
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];
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jobs.firewall =
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{ description = "Firewall";
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startOn = "started network-interfaces";
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path = [ pkgs.iptables ];
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preStart =
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''
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${helpers}
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# Flush the old firewall rules. !!! Ideally, updating the
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# firewall would be atomic. Apparently that's possible
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# with iptables-restore.
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ip46tables -D INPUT -j nixos-fw || true
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for chain in nixos-fw nixos-fw-accept nixos-fw-log-refuse nixos-fw-refuse FW_REFUSE; do
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ip46tables -F "$chain" || true
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ip46tables -X "$chain" || true
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done
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# The "nixos-fw-accept" chain just accepts packets.
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ip46tables -N nixos-fw-accept
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ip46tables -A nixos-fw-accept -j ACCEPT
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# The "nixos-fw-refuse" chain rejects or drops packets.
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ip46tables -N nixos-fw-refuse
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${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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${optionalString cfg.logRefusedConnections ''
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ip46tables -A nixos-fw-log-refuse -p tcp --syn -j LOG --log-level info --log-prefix "rejected connection: "
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''}
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${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 "rejected 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 "rejected 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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${optionalString cfg.logRefusedPackets ''
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ip46tables -A nixos-fw-log-refuse \
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-j LOG --log-level info --log-prefix "rejected packet: "
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''}
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ip46tables -A nixos-fw-log-refuse -j nixos-fw-refuse
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# The "nixos-fw" chain does the actual work.
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ip46tables -N nixos-fw
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# Perform a reverse-path test to refuse spoofers
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# For now, we just drop, as the raw table doesn't have a log-refuse yet
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${optionalString (kernelHasRPFilter && cfg.checkReversePath) ''
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ip46tables -A PREROUTING -t raw -m rpfilter --invert -j DROP
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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 ${iface} -j nixos-fw-accept
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'')}
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# Accept packets from established or related connections.
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ip46tables -A nixos-fw -m conntrack --ctstate ESTABLISHED,RELATED -j nixos-fw-accept
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# Accept connections to the allowed TCP ports.
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${concatMapStrings (port:
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''
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ip46tables -A nixos-fw -p tcp --dport ${toString port} -j nixos-fw-accept
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''
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) cfg.allowedTCPPorts
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}
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# Accept packets on the allowed UDP ports.
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${concatMapStrings (port:
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''
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ip46tables -A nixos-fw -p udp --dport ${toString port} -j nixos-fw-accept
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''
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) cfg.allowedUDPPorts
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}
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# Accept IPv4 multicast. Not a big security risk since
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# probably nobody is listening anyway.
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#iptables -A nixos-fw -d 224.0.0.0/4 -j nixos-fw-accept
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# Optionally respond to ICMPv4 pings.
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${optionalString cfg.allowPing ''
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iptables -A nixos-fw -p icmp --icmp-type echo-request -j nixos-fw-accept
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''}
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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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${optionalString config.networking.enableIPv6 ''
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ip6tables -A nixos-fw -p icmpv6 --icmpv6-type redirect -j DROP
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ip6tables -A nixos-fw -p icmpv6 --icmpv6-type 139 -j DROP
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ip6tables -A nixos-fw -p icmpv6 -j nixos-fw-accept
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''}
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${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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postStop =
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''
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${helpers}
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ip46tables -D INPUT -j nixos-fw || true
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#ip46tables -P INPUT ACCEPT
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'';
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};
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};
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}
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