430 lines
14 KiB
Nix
430 lines
14 KiB
Nix
/* 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, lib, pkgs, ... }:
|
||
|
||
with lib;
|
||
|
||
let
|
||
|
||
cfg = config.networking.firewall;
|
||
|
||
helpers =
|
||
''
|
||
# Helper command to manipulate both the IPv4 and IPv6 tables.
|
||
ip46tables() {
|
||
iptables -w "$@"
|
||
${optionalString config.networking.enableIPv6 ''
|
||
ip6tables -w "$@"
|
||
''}
|
||
}
|
||
'';
|
||
|
||
kernelPackages = config.boot.kernelPackages;
|
||
|
||
kernelHasRPFilter = kernelPackages.kernel.features.netfilterRPFilter or false;
|
||
kernelCanDisableHelpers = kernelPackages.kernel.features.canDisableNetfilterConntrackHelpers or false;
|
||
|
||
in
|
||
|
||
{
|
||
|
||
###### interface
|
||
|
||
options = {
|
||
|
||
networking.firewall.enable = mkOption {
|
||
type = types.bool;
|
||
default = true;
|
||
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 {
|
||
type = types.bool;
|
||
default = true;
|
||
description =
|
||
''
|
||
Whether to log rejected or dropped incoming connections.
|
||
'';
|
||
};
|
||
|
||
networking.firewall.logRefusedPackets = mkOption {
|
||
type = types.bool;
|
||
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 {
|
||
type = types.bool;
|
||
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 {
|
||
type = types.bool;
|
||
default = false;
|
||
description =
|
||
''
|
||
If set, forbidden packets are rejected rather than dropped
|
||
(ignored). This means that an 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.allowedTCPPortRanges = mkOption {
|
||
default = [];
|
||
example = [ { from = 8999; to = 9003; } ];
|
||
type = types.listOf (types.attrsOf types.int);
|
||
description =
|
||
''
|
||
A range 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.allowedUDPPortRanges = mkOption {
|
||
default = [];
|
||
example = [ { from = 60000; to = 61000; } ];
|
||
type = types.listOf (types.attrsOf types.int);
|
||
description =
|
||
''
|
||
Range 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.pingLimit = mkOption {
|
||
default = null;
|
||
type = types.nullOr (types.separatedString " ");
|
||
description =
|
||
''
|
||
If pings are allowed, this allows setting rate limits
|
||
on them. If non-null, this option should be in the form
|
||
of flags like "-limit 1/minute -limit-burst 5"
|
||
'';
|
||
};
|
||
|
||
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 advised 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 {
|
||
type = types.lines;
|
||
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
|
||
|
||
# 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 = [ 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"; }
|
||
];
|
||
|
||
systemd.services.firewall =
|
||
{ description = "Firewall";
|
||
|
||
wantedBy = [ "network.target" ];
|
||
after = [ "network-interfaces.target" "systemd-modules-load.service" ];
|
||
|
||
path = [ pkgs.iptables ];
|
||
|
||
# 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";
|
||
|
||
serviceConfig.Type = "oneshot";
|
||
serviceConfig.RemainAfterExit = true;
|
||
|
||
script =
|
||
''
|
||
${helpers}
|
||
|
||
# Flush the old firewall rules. !!! Ideally, updating the
|
||
# firewall would be atomic. Apparently that's possible
|
||
# with iptables-restore.
|
||
ip46tables -D INPUT -j nixos-fw 2> /dev/null || true
|
||
for chain in nixos-fw nixos-fw-accept nixos-fw-log-refuse nixos-fw-refuse FW_REFUSE; do
|
||
ip46tables -F "$chain" 2> /dev/null || true
|
||
ip46tables -X "$chain" 2> /dev/null || 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) ''
|
||
if ! ip46tables -A PREROUTING -t raw -m rpfilter --invert -j DROP; then
|
||
echo "<2>failed to initialise rpfilter support" >&2
|
||
fi
|
||
''}
|
||
|
||
# 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 connections to the allowed TCP port ranges.
|
||
${concatMapStrings (rangeAttr:
|
||
let range = toString rangeAttr.from + ":" + toString rangeAttr.to; in
|
||
''
|
||
ip46tables -A nixos-fw -p tcp --dport ${range} -j nixos-fw-accept
|
||
''
|
||
) cfg.allowedTCPPortRanges
|
||
}
|
||
|
||
# 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 packets on the allowed UDP port ranges.
|
||
${concatMapStrings (rangeAttr:
|
||
let range = toString rangeAttr.from + ":" + toString rangeAttr.to; in
|
||
''
|
||
ip46tables -A nixos-fw -p udp --dport ${range} -j nixos-fw-accept
|
||
''
|
||
) cfg.allowedUDPPortRanges
|
||
}
|
||
|
||
# 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 -w -A nixos-fw -p icmp --icmp-type echo-request ${optionalString (cfg.pingLimit != null)
|
||
"-m limit ${cfg.pingLimit} "
|
||
}-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
|
||
'';
|
||
};
|
||
|
||
};
|
||
|
||
}
|