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nixpkgs/modules/services/networking/firewall.nix

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/* 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 "$@"
${optionalString config.networking.enableIPv6 ''
ip6tables "$@"
''}
}
'';
kernelPackages = config.boot.kernelPackages;
kernelHasRPFilter = kernelPackages.kernel ? features
&& kernelPackages.kernel.features ? netfilterRPFilter
&& kernelPackages.kernel.features.netfilterRPFilter;
kernelCanDisableHelpers = kernelPackages.kernel ? features
&& kernelPackages.kernel.features ? canDisableNetfilterConntrackHelpers
&& kernelPackages.kernel.features.canDisableNetfilterConntrackHelpers;
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 <option>networking.firewall.logRefusedPackets</option>
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.listOf types.string;
description =
''
Traffic coming in from these interfaces will be accepted
unconditionally.
'';
};
networking.firewall.allowedTCPPorts = mkOption {
default = [];
example = [ 22 80 ];
type = types.listOf types.int;
description =
''
List of TCP ports on which incoming connections are
accepted.
'';
};
networking.firewall.allowedUDPPorts = mkOption {
default = [];
example = [ 53 ];
type = types.listOf 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.checkReversePath = mkOption {
default = kernelHasRPFilter;
type = types.bool;
description =
''
Performs a reverse path filter test on a packet.
If a reply to the packet would not be sent via the same interface
that the packet arrived on, it is refused.
If using asymmetric routing or other complicated routing,
disable this setting and setup your own counter-measures.
(needs kernel 3.3+)
'';
};
networking.firewall.connectionTrackingModules = mkOption {
default = [ "ftp" ];
example = [ "ftp" "irc" "sane" "sip" "tftp" "amanda" "h323" "netbios_sn" "pptp" "snmp" ];
type = types.listOf types.string;
description =
''
List of connection-tracking helpers that are auto-loaded.
The complete list of possible values is given in the example.
As helpers can pose as a security risk, it is adviced to
set this to an empty list and disable the setting
networking.firewall.autoLoadConntrackHelpers
Loading of helpers is recommended to be done through the new
CT target. More info:
https://home.regit.org/netfilter-en/secure-use-of-helpers/
'';
};
networking.firewall.autoLoadConntrackHelpers = mkOption {
default = true;
type = types.bool;
description =
''
Whether to auto-load connection-tracking helpers.
See the description at networking.firewall.connectionTrackingModules
(needs kernel 3.5+)
'';
};
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 = map (x: "nf_conntrack_${x}") cfg.connectionTrackingModules;
boot.extraModprobeConfig = optionalString (!cfg.autoLoadConntrackHelpers) ''
options nf_conntrack nf_conntrack_helper=0
'';
assertions = [ { assertion = ! cfg.checkReversePath || kernelHasRPFilter;
message = "This kernel does not support rpfilter"; }
{ assertion = cfg.autoLoadConntrackHelpers || kernelCanDisableHelpers;
message = "This kernel does not support disabling conntrack helpers"; }
];
jobs.firewall =
{ description = "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
# Perform a reverse-path test to refuse spoofers
# For now, we just drop, as the raw table doesn't have a log-refuse yet
${optionalString (kernelHasRPFilter && cfg.checkReversePath) ''
ip46tables -A PREROUTING -t raw -m rpfilter --invert -j DROP
''}
# 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
'';
};
};
}