[people/stevee/network.git] / src / functions / functions.firewall

#!/bin/bash
###############################################################################
#                                                                             #
# IPFire.org - A linux based firewall                                         #
# Copyright (C) 2012  IPFire Network Development Team                         #
#                                                                             #
# This program is free software: you can redistribute it and/or modify        #
# it under the terms of the GNU General Public License as published by        #
# the Free Software Foundation, either version 3 of the License, or           #
# (at your option) any later version.                                         #
#                                                                             #
# This program is distributed in the hope that it will be useful,             #
# but WITHOUT ANY WARRANTY; without even the implied warranty of              #
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the               #
# GNU General Public License for more details.                                #
#                                                                             #
# You should have received a copy of the GNU General Public License           #
# along with this program.  If not, see <http://www.gnu.org/licenses/>.       #
#                                                                             #
###############################################################################

# This function initializes all kernel parameters that need to be adjusted
# to run this firewall properly.
firewall_kernel_init() {
	log INFO "Configuring kernel parameters..."
	local option

	# Enable conntrack accounting
	conntrack_set_accounting "true"

	# Adjust max. amount of simultaneous connections
	conntrack_set_max_connections "${CONNTRACK_MAX_CONNECTIONS}"

	# Increase UDP connection timeout (fixes DNS)
	conntrack_set_udp_timeout "${CONNTRACK_UDP_TIMEOUT}"

	# Disable sending redirects
	log INFO "Disabling sending redirects"
	sysctl_set_recursively "net.ipv6.conf" "send_redirects" 0
	sysctl_set_recursively "net.ipv4.conf" "send_redirects" 0

	# Enable source route protection
	log INFO "Enabling source route protection"
	sysctl_set_recursively "net.ipv6.conf" "accept_source_route" 0
	sysctl_set_recursively "net.ipv4.conf" "accept_source_route" 0

	# ICMP broadcast protection (smurf amplifier protection)
	log INFO "Enabling ICMP broadcast protection (smurf amplifier protection)"
	sysctl_set "net.ipv4.icmp_echo_ignore_broadcasts" 1

	# ICMP Dead Error Message protection
	log INFO "Enabling ICMP dead error message protection"
	sysctl_set "net.ipv4.icmp_ignore_bogus_error_responses" 0

	# Enable packet forwarding
	log INFO "Enabling packet forwarding"
	sysctl_set_recursively "net.ipv6.conf" "forwarding" 1
	sysctl_set_recursively "net.ipv4.conf" "forwarding" 1

	# Setting some kernel performance options
	log INFO "Setting some kernel performance options"
	for option in window_scaling timestamps sack dsack fack; do
		sysctl_set "net.ipv4.tcp_${option}" 1
	done
	sysctl_set "net.ipv4.tcp_low_latency" 0

	# Reduce DoS ability by reducing timeouts
	log INFO "Reducing DoS ability"
	sysctl_set "net.ipv4.tcp_fin_timeout" 30
	sysctl_set "net.ipv4.tcp_keepalive_time" 1800

	# Set number of times to retry SYN in a new connection
	sysctl_set "net.ipv4.tcp_syn_retries" 3

	# Set number of times to retry a SYN-ACK in a half-open new connection
	sysctl_set "net.ipv4.tcp_synack_retries" 2

	# Enable a fix for RFC1337 - time-wait assassination hazards in TCP
	sysctl_set "net.ipv4.tcp_rfc1337" 1

	# SYN-flood protection
	if enabled FIREWALL_SYN_COOKIES; then
		log INFO "Enabling SYN-flood protection via SYN-cookies"
		sysctl_set_bool "net.ipv4.tcp_syncookies" 1
	else
		log INFO "Disabling SYN-flood protection via SYN-cookies"
		sysctl_set_bool "net.ipv4.tcp_syncookies" 0
	fi

	# rp_filter
	if enabled FIREWALL_RP_FILTER; then
		log INFO "Enabling anti-spoof from non-routable IP addresses"
		sysctl_set_recursively "net.ipv4.conf" "rp_filter" 1
	else
		log INFO "Disabling anti-spoof from non-routable IP addresses"
		sysctl_set_recursively "net.ipv4.conf" "rp_filter" 0
	fi

	# Log martians
	if enabled FIREWALL_LOG_MARTIANS; then
		log INFO "Enabling the logging of martians"
		sysctl_set_recursively "net.ipv4.conf" "log_martians" 1
	else
		log INFO "Disabling the logging of martians"
		sysctl_set_recursively "net.ipv4.conf" "log_martians" 0
	fi

	# ICMP redirect messages
	if enabled FIREWALL_ACCEPT_ICMP_REDIRECTS; then
		log INFO "Enabling accepting ICMP-redirect messages"
		sysctl_set_recursively "net.ipv6.conf" "accept_redirects" 1
		sysctl_set_recursively "net.ipv4.conf" "accept_redirects" 1
	else
		log INFO "Disabling accepting ICMP-redirect messages"
		sysctl_set_recursively "net.ipv6.conf" "accept_redirects" 0
		sysctl_set_recursively "net.ipv4.conf" "accept_redirects" 0
	fi

	# Explicit Congestion Notification
	if enabled FIREWALL_USE_ECN; then
		log INFO "Enabling ECN (Explicit Congestion Notification)"
		sysctl_set "net.ipv4.tcp_ecn" 1
		sysctl_set "net.ipv4.tcp_ecn_fallback" 1
	else
		log INFO "Disabling ECN (Explicit Congestion Notification)"
		sysctl_set "net.ipv4.tcp_ecn" 2
	fi

	# Dynamic IP address hacking
	log INFO "Enabling kernel support for dynamic IP addresses"
	sysctl_set "net.ipv4.ip_dynaddr" 1

	if enabled FIREWALL_PMTU_DISCOVERY; then
		log INFO "Enabling PMTU discovery"
		sysctl_set "net.ipv4.ip_no_pmtu_disc" 0
	else
		log INFO "Disabling PMTU discovery"
		sysctl_set "net.ipv4.ip_no_pmtu_disc" 1
	fi

	# TTL
	if ipv4_ttl_valid "${FIREWALL_DEFAULT_TTL}"; then
		log INFO "Setting default TTL to ${FIREWALL_DEFAULT_TTL}"
		sysctl_set "net.ipv4.ip_default_ttl" "${FIREWALL_DEFAULT_TTL}"
	else
		log ERROR "Invalid value for default TTL '${FIREWALL_DEFAULT_TTL}'"
		log ERROR "  Must be between 10 and 255!"
	fi

	return ${EXIT_OK}
}

# High-level function which will create a ruleset for the current firewall
# configuration and load it into the kernel.
firewall_start() {
	local protocol="${1}"
	assert isset protocol
	shift

	# Test mode.
	local test="false"

	while [ $# -gt 0 ]; do
		case "${1}" in
			--test)
				test="true"
				;;
			*)
				error "Unrecognized argument: ${1}"
				return ${EXIT_ERROR}
				;;
		esac
		shift
	done

	if enabled test; then
		log INFO "Test mode enabled."
		log INFO "The firewall ruleset will not be loaded."
	fi

	firewall_lock_acquire

	# Initialize an empty iptables ruleset.
	iptables_init "${protocol}" "DROP"

	# Add default chains.
	firewall_filter_rh0_headers "${protocol}"
	firewall_filter_icmp "${protocol}"
	firewall_filter_invalid_packets "${protocol}"
	firewall_custom_chains "${protocol}"
	firewall_connection_tracking "${protocol}"
	firewall_tcp_clamp_mss "${protocol}"

	# Add policies for every zone.
	firewall_localhost_create_chains "${protocol}"

	local zone
	for zone in $(zones_get_all); do
		# Create all needed chains for the zone.
		firewall_zone_create_chains "${protocol}" "${zone}"

		# After the chains that are always available have been
		# created, we will add a custom policy to every single
		# zone.

		policy_zone_add "${protocol}" "${zone}"
	done

	# Load the new ruleset.
	local args
	if enabled testmode; then
		list_append args "--test"
	fi
	iptables_commit "${protocol}" ${args}

	firewall_lock_release
}

firewall_stop() {
	local protocol="${1}"
	assert isset protocol

	firewall_lock_acquire

	# Initialize an empty firewall ruleset
	# with default policy ACCEPT.
	iptables_init "${protocol}" ACCEPT

	# Load it.
	ipables_load "${protocol}"

	firewall_lock_release
}

firewall_show() {
	local protocol="${1}"
	assert isset protocol

	# Shows the ruleset that is currently loaded.
	iptables_status "${protocol}"

	return ${EXIT_OK}
}

firewall_panic() {
	local protocol="${1}"
	assert isset protocol
	shift

	local admin_hosts="$@"

	firewall_lock_acquire "${protocol}"

	# Drop all communications.
	iptables_init "${protocol}" DROP

	# If an admin host is provided, some administrative
	# things will be allowed from there.
	local admin_host
	for admin_host in ${admin_hosts}; do
		iptables "${protocol}" -A INPUT  -s "${admin_host}" -j ACCEPT
		iptables "${protocol}" -A OUTPUT -d "${admin_host}" -j ACCEPT
	done

	# Load it.
	iptables_commit "${protocol}"

	firewall_lock_release
}

firewall_lock_acquire() {
	# XXX DEPRECATED
	#lock_acquire ${RUN_DIR}/.firewall_lock

	# Make sure the lock is released after the firewall
	# script has crashed or exited early.
	trap firewall_lock_release EXIT TERM KILL

	# Create a directory where we can put our
	# temporary data in the most secure way as possible.
	IPTABLES_TMPDIR=$(mktemp -d)
}

firewall_lock_release() {
	if isset IPTABLES_TMPDIR; then
		# Remove all temporary data.
		rm -rf ${IPTABLES_TMPDIR}

		# Reset the tempdir variable.
		IPTABLES_TMPDIR=
	fi

	# Reset the trap.
	trap true EXIT TERM KILL

	lock_release ${RUN_DIR}/.firewall_lock
}

firewall_custom_chains() {
	local protocol="${1}"
	assert isset protocol

	log INFO "Creating CUSTOM* chains..."

	# These chains are intened to be filled with
	# rules by the user. They are processed at the very
	# beginning so it is possible to overwrite everything.

	iptables_chain_create "${protocol}" CUSTOMINPUT
	iptables "${protocol}" -A INPUT -j CUSTOMINPUT

	iptables_chain_create "${protocol}" CUSTOMFORWARD
	iptables "${protocol}" -A FORWARD -j CUSTOMFORWARD

	iptables_chain_create "${protocol}" CUSTOMOUTPUT
	iptables "${protocol}" -A OUTPUT -j CUSTOMOUTPUT

	iptables_chain_create "${protocol}" -t nat CUSTOMPREROUTING
	iptables "${protocol}" -t nat -A PREROUTING -j CUSTOMPREROUTING

	iptables_chain_create "${protocol}" -t nat CUSTOMPOSTROUTING
	iptables "${protocol}" -t nat -A POSTROUTING -j CUSTOMPOSTROUTING

	iptables_chain_create "${protocol}" -t nat CUSTOMOUTPUT
	iptables "${protocol}" -t nat -A OUTPUT -j CUSTOMOUTPUT
}

firewall_filter_invalid_packets() {
	local protocol="${1}"
	assert isset protocol

	local log_limit="-m limit --limit 5/m --limit-burst 10"

	# Create a chain
	iptables_chain_create "${protocol}" FILTER_INVALID
	iptables "${protocol}" -A INPUT   -j FILTER_INVALID
	iptables "${protocol}" -A OUTPUT  -j FILTER_INVALID
	iptables "${protocol}" -A FORWARD -j FILTER_INVALID

	# Create a chain where only TCP packets go
	iptables_chain_create "${protocol}" FILTER_INVALID_TCP
	iptables "${protocol}" -A FILTER_INVALID -p tcp -j FILTER_INVALID_TCP

	# Create a chain where only UDP packets go
	iptables_chain_create "${protocol}" FILTER_INVALID_UDP
	iptables "${protocol}" -A FILTER_INVALID -p udp -j FILTER_INVALID_UDP

	# Create a chain where only ICMP packets go
	iptables_chain_create "${protocol}" FILTER_INVALID_ICMP
	iptables "${protocol}" -A FILTER_INVALID -p icmp -j FILTER_INVALID_ICMP


	# Optionally log all port scans

	if enabled FIREWALL_LOG_STEALTH_SCANS; then
		log INFO "Logging of stealth scans enabled"

		# NMAP FIN/URG/PSH - XMAS scan
		iptables "${protocol}" -A FILTER_INVALID_TCP -p tcp --tcp-flags ALL FIN,URG,PSH \
			"${log_limit}" -j "$(iptables_LOG "Stealth XMAS scan")"

		# SYN/RST/ACK/FIN/URG
		iptables "${protocol}" -A FILTER_INVALID_TCP -p tcp --tcp-flags ALL SYN,RST,ACK,FIN,URG \
			"${log_limit}" -j "$(iptables_LOG "Stealth XMAS-PSH scan")"

		# ALL/ALL
		iptables "${protocol}" -A FILTER_INVALID_TCP -p tcp --tcp-flags ALL ALL \
			"${log_limit}" -j "$(iptables_LOG "Stealth XMAS-ALL scan")"

		# NMAP FIN Stealth
		iptables "${protocol}" -A FILTER_INVALID_TCP -p tcp --tcp-flags ALL FIN \
			"${log_limit}" -j "$(iptables_LOG "Stealth FIN scan")"

		# SYN/RST
		iptables "${protocol}" -A FILTER_INVALID_TCP -p tcp --tcp-flags SYN,RST SYN,RST \
			"${log_limit}" -j "$(iptables_LOG "Stealth SYN/RST scan")"

		# SYN/FIN
		iptables "${protocol}" -A FILTER_INVALID_TCP -p tcp --tcp-flags SYN,FIN SYN,FIN \
			"${log_limit}" -j "$(iptables_LOG "Stealth SYN/FIN scan")"

		# Null scan
		iptables "${protocol}" -A FILTER_INVALID_TCP -p tcp --tcp-flags ALL NONE \
			"${log_limit}" -j "$(iptables_LOG "Stealth NULL scan")"
	else
		log INFO "Logging of stealth scans disabled"
	fi


	# Drop scan packets

	# NMAP FIN/URG/PSH
	iptables "${protocol}" -A FILTER_INVALID_TCP -p tcp --tcp-flags ALL FIN,URG,PSH -j DROP

	# SYN/RST/ACK/FIN/URG
	iptables "${protocol}" -A FILTER_INVALID_TCP -p tcp --tcp-flags ALL SYN,RST,ACK,FIN,URG -j DROP

	# ALL/ALL
	iptables "${protocol}" -A FILTER_INVALID_TCP -p tcp --tcp-flags ALL ALL -j DROP

	# NMAP FIN Stealth
	iptables "${protocol}" -A FILTER_INVALID_TCP -p tcp --tcp-flags ALL FIN -j DROP

	# SYN/RST
	iptables "${protocol}" -A FILTER_INVALID_TCP -p tcp --tcp-flags SYN,RST SYN,RST -j DROP

	# SYN/FIN
	iptables "${protocol}" -A FILTER_INVALID_TCP -p tcp --tcp-flags SYN,FIN SYN,FIN -j DROP

	# Null scan
	iptables "${protocol}" -A FILTER_INVALID_TCP -p tcp --tcp-flags ALL NONE -j DROP


	# Log packets with bad flags

	if enabled FIREWALL_LOG_BAD_TCP_FLAGS; then
		log INFO "Logging of packets with bad TCP flags enabled"

		# Option 64
		iptables "${protocol}" -A FILTER_INVALID_TCP -p tcp --tcp-option 64 \
			"${log_limit}" -j "$(iptables_LOG "Bad TCP flag(64)")"

		# Option 128
		iptables "${protocol}" -A FILTER_INVALID_TCP -p tcp --tcp-option 128 \
			"${log_limit}" -j "$(iptables_LOG "Bad TCP flag(128)")"
	else
		log INFO "Logging of packets with bad TCP flags disabled"
	fi

	# Drop packets with bad flags

	iptables "${protocol}" -A FILTER_INVALID_TCP -p tcp --tcp-option 64 -j DROP
	iptables "${protocol}" -A FILTER_INVALID_TCP -p tcp --tcp-option 128 -j DROP


	# Log invalid packets

	if enabled FIREWALL_LOG_INVALID_TCP; then
		log INFO "Logging of INVALID TCP packets enabled"

		iptables "${protocol}" -A FILTER_INVALID_TCP -p tcp -m conntrack --ctstate INVALID \
			"${log_limit}" -j "$(iptables_LOG "INVALID TCP")"
	else
		log INFO "Logging of INVALID TCP packets disabled"
	fi

	if enabled FIREWALL_LOG_INVALID_UDP; then
		log INFO "Logging of INVALID UDP packets enabled"

		iptables "${protocol}" -A FILTER_INVALID_UDP -p udp -m conntrack --ctstate INVALID \
			"${log_limit}" -j "$(iptables_LOG "INVALID UDP")"
	else
		log INFO "Logging of INVALID UDP packets disabled"
	fi

	if enabled FIREWALL_LOG_INVALID_ICMP; then
		log INFO "Logging of INVALID ICMP packets enabled"

		iptables "${protocol}" -A FILTER_INVALID_ICMP -p icmp -m conntrack --ctstate INVALID \
			"${log_limit}" -j "$(iptables_LOG "INVALID ICMP")"
	else
		log INFO "Logging of INVALID ICMP packets disabled"
	fi

	# Drop all INVALID packets
	iptables "${protocol}" -A FILTER_INVALID -m conntrack --ctstate INVALID -j DROP
}

firewall_tcp_clamp_mss() {
	# Do nothing if this has been disabled.
	enabled FIREWALL_CLAMP_PATH_MTU || return ${EXIT_OK}

	local protocol="${1}"
	assert isset protocol

	log DEBUG "Adding rules to clamp MSS to path MTU..."

	iptables "${protocol}" -t mangle -A FORWARD \
		-p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu
}

firewall_connection_tracking() {
	local protocol="${1}"
	assert isset protocol

	log INFO "Creating Connection Tracking chain..."

	iptables_chain_create "${protocol}" CONNTRACK
	iptables "${protocol}" -A CONNTRACK -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
	iptables "${protocol}" -A CONNTRACK -m conntrack --ctstate INVALID -j "$(iptables_LOG "INVALID packet: ")"
	iptables "${protocol}" -A CONNTRACK -m conntrack --ctstate INVALID -j DROP

	iptables "${protocol}" -A INPUT   -j CONNTRACK
	iptables "${protocol}" -A OUTPUT  -j CONNTRACK
	iptables "${protocol}" -A FORWARD -j CONNTRACK
}

firewall_localhost_create_chains() {
	local protocol="${1}"
	assert isset protocol

	log DEBUG "Creating firewall chains for localhost..."

	# Accept everything on lo
	iptables "${protocol}" -A INPUT  -i lo -j ACCEPT
	iptables "${protocol}" -A OUTPUT -o lo -j ACCEPT
}

firewall_filter_rh0_headers() {
	local protocol="${1}"
	assert isset protocol

	# Only IPv6.
	[ "${protocol}" = "ipv6" ] || return ${EXIT_OK}

	# Filter all packets that have RH0 headers
	# http://www.ietf.org/rfc/rfc5095.txt
	iptables_chain_create "${protocol}" FILTER_RH0
	iptables "${protocol}" -A FILTER_RH0 -m rt --rt-type 0 -j DROP

	iptables "${protocol}" -A INPUT   -j FILTER_RH0
	iptables "${protocol}" -A FORWARD -j FILTER_RH0
	iptables "${protocol}" -A OUTPUT  -j FILTER_RH0
}

firewall_filter_icmp() {
	local protocol="${1}"
	assert isset protocol

	# Only IPv6.
	[ "${protocol}" = "ipv6" ] || return ${EXIT_OK}

	local chain="FILTER_ICMPV6"

	# Create an extra chain for handling ICMP packets.
	iptables_chain_create "${protocol}" "${chain}_COMMON"

	local suffix
	for suffix in INC FWD OUT; do
		iptables_chain_create "${protocol}" "${chain}_${suffix}"
		iptables "${protocol}" -A "${chain}_${suffix}" -j "${chain}_COMMON"
	done
	iptables "${protocol}" -A INPUT   -p icmpv6 -j "${chain}_INC"
	iptables "${protocol}" -A FORWARD -p icmpv6 -j "${chain}_FWD"
	iptables "${protocol}" -A OUTPUT  -p icmpv6 -j "${chain}_OUT"

	# Packets that must always pass the firewall.
	# Type 4: Parameter Problem
	local type
	for type in ttl-zero-during-reassembly bad-header; do
		iptables "${protocol}" -A "${chain}_COMMON" \
			-p icmpv6 --icmpv6-type "${type}" -j ACCEPT
	done

	# Packets that are accepted if they belong to an existing connection.
	for type in echo-reply destination-unreachable packet-too-big \
			unknown-header-type unknown-option; do
		iptables "${protocol}" -A "${chain}_COMMON" \
			-m conntrack --ctstate ESTABLISHED,RELATED \
			-p icmpv6 --icmpv6-type "${type}" -j ACCEPT
	done

	# Packets that are always discarded.
	# Type 100, 101, 200, 201: Private Experimentation
	for type in 100 101 200 201; do
		iptables "${protocol}" -A "${chain}_COMMON" \
			-p icmpv6 --icmpv6-type "${type}" -j DROP
	done

	# Discard packets from local networks with hop limit smaller than $hoplimit.
	# Type 148: Path solicitation
	# Type 149: Path advertisement
	local hoplimit=255
	for type in {router,neighbour}-{advertisement,solicitation} 148 149; do
		iptables "${protocol}" -A "${chain}_INC" \
			-p icmpv6 --icmpv6-type "${type}" \
			-m hl --hl-lt "${hoplimit}" -j DROP
	done

	# The firewall is always allowed to send ICMP echo requests.
	iptables "${protocol}" -A "${chain}_OUT" \
		-p icmpv6 --icmpv6-type echo-request -j ACCEPT

	return ${EXIT_OK}
}

firewall_zone_create_chains() {
	local protocol="${1}"
	assert isset protocol

	local zone="${2}"
	assert isset zone

	log DEBUG "Creating firewall chains for zone '${zone}'."

	local chain_prefix="ZONE_${zone^^}"

	# Create filter chains.
	iptables_chain_create "${protocol}" "${chain_prefix}_INPUT"
	iptables "${protocol}" -A INPUT   -i ${zone} -j "${chain_prefix}_INPUT"

	iptables_chain_create "${protocol}" "${chain_prefix}_OUTPUT"
	iptables "${protocol}" -A OUTPUT  -o ${zone} -j "${chain_prefix}_OUTPUT"

	# Custom rules.
	iptables_chain_create "${protocol}" "${chain_prefix}_CUSTOM"

	# Intrusion Prevention System.
	iptables_chain_create "${protocol}" "${chain_prefix}_IPS"

	# Create a chain for each other zone.
	# This leaves us with n^2 chains. Duh.

	local other_zone other_chain_prefix
	for other_zone in $(zones_get_all); do
		other_chain_prefix="${chain_prefix}_${other_zone^^}"
		iptables_chain_create "${protocol}" "${other_chain_prefix}"

		# Connect the chain with the FORWARD chain.
		iptables "${protocol}" -A FORWARD -i "${zone}" -o "${other_zone}" \
			-j "${other_chain_prefix}"

		# Handle custom rules.
		iptables "${protocol}" -A "${other_chain_prefix}" -j "${chain_prefix}_CUSTOM"

		# Link IPS.
		iptables "${protocol}" -A "${other_chain_prefix}" -j "${chain_prefix}_IPS"

		# Rules.
		iptables_chain_create "${protocol}" "${other_chain_prefix}_RULES"
		iptables "${protocol}" -A "${other_chain_prefix}" -j "${other_chain_prefix}_RULES"

		# Policy.
		iptables_chain_create "${protocol}" "${other_chain_prefix}_POLICY"
		iptables "${protocol}" -A "${other_chain_prefix}" -j "${other_chain_prefix}_POLICY"
	done

	## Create mangle chain.
	#iptables_chain_create "${protocol}" -t mangle "${chain_prefix}"
	#iptables "${protocol}" -t mangle -A PREROUTING  -i "${zone}" -j "${chain_prefix}"
	#iptables "${protocol}" -t mangle -A POSTROUTING -o "${zone}" -j "${chain_prefix}"

	## Quality of Service
	#iptables_chain_create "${protocol}" -t mangle "${chain_prefix}_QOS_INC"
	#iptables "${protocol}" -t mangle -A "${chain_prefix}" -i "${zone}" -j "${chain_prefix}_QOS_INC"
	#iptables_chain_create "${protocol}" -t mangle "${chain_prefix}_QOS_OUT"
	#iptables "${protocol}" -t mangle -A "${chain_prefix}" -o "${zone}" -j "${chain_prefix}_QOS_OUT"

	# Create NAT chain.
	iptables_chain_create "${protocol}" -t nat "${chain_prefix}"
	iptables "${protocol}" -t nat -A PREROUTING  -i "${zone}" -j "${chain_prefix}"
	iptables "${protocol}" -t nat -A POSTROUTING -o "${zone}" -j "${chain_prefix}"

	# Network Address Translation
	iptables_chain_create "${protocol}" -t nat "${chain_prefix}_DNAT"
	iptables "${protocol}" -t nat -A PREROUTING  -i "${zone}" -j "${chain_prefix}_DNAT"
	iptables_chain_create "${protocol}" -t nat "${chain_prefix}_SNAT"
	iptables "${protocol}" -t nat -A POSTROUTING -o "${zone}" -j "${chain_prefix}_SNAT"

	# UPnP
	iptables_chain_create "${protocol}" -t nat "${chain_prefix}_UPNP"
	iptables "${protocol}" -t nat -A "${chain_prefix}" -j "${chain_prefix}_UPNP"

	return ${EXIT_OK}
}

firewall_parse_rules() {
	local file=${1}
	assert isset file
	shift

	# End if no rule file exists.
	[ -r "${file}" ] || return ${EXIT_OK}

	local cmd

	local ${FIREWALL_RULES_CONFIG_PARAMS}
	local line
	while read -r line; do
		# Skip empty lines.
		[ -n "${line}" ] || continue

		# Skip commented lines.
		[ "${line:0:1}" = "#" ] && continue

		# Parse the rule.
		_firewall_parse_rule_line ${line}
		if [ $? -ne ${EXIT_OK} ]; then
			log WARNING "Skipping invalid line: ${line}"
			continue
		fi

		cmd="iptables $@"

		# Source IP address/net.
		if isset src; then
			list_append cmd "-s ${src}"
		fi

		# Destination IP address/net.
		if isset dst; then
			list_append cmd "-d ${dst}"
		fi

		# Protocol.
		if isset proto; then
			list_append cmd "-p ${proto}"

			if list_match ${proto} ${FIREWALL_PROTOCOLS_SUPPORTING_PORTS}; then
				if isset sport; then
					list_append cmd "--sport ${sport}"
				fi

				if isset dport; then
					list_append cmd "--dport ${dport}"
				fi
			fi
		fi

		# Always append the action.
		list_append cmd "-j ${action}"

		# Execute command.
		${cmd}
	done < ${file}
}

_firewall_parse_rule_line() {
	local arg

	# Clear all values.
	for arg in ${FIREWALL_RULES_CONFIG_PARAMS}; do
		assign "${arg}" ""
	done

	local key val
	while read -r arg; do
		key=$(cli_get_key ${arg})

		if ! list_match "${key}" ${FIREWALL_RULES_CONFIG_PARAMS}; then
			log WARNING "Unrecognized argument: ${arg}"
			return ${EXIT_ERROR}
		fi

		val=$(cli_get_val "${arg}")
		assign "${key}" "${val}"
	done <<< "$(args "$@")"

	# action must always be set.
	if ! isset action; then
		log WARNING "'action' is not set: $@"
		return ${EXIT_ERROR}
	fi

	for arg in src dst; do
		isset ${arg} || continue

		# Check for valid IP addresses.
		if ! ip_is_valid ${!arg}; then
			log WARNING "Invalid IP address for '${arg}=${!arg}': $@"
			return ${EXIT_ERROR}
		fi
	done

	if isset proto; then
		# Make lowercase.
		proto=${proto,,}

		if ! list_match "${proto}" ${FIREWALL_SUPPORTED_PROTOCOLS}; then
			log WARNING "Unsupported protocol type 'proto=${proto}': $@"
			return ${EXIT_ERROR}
		fi
	fi

	for arg in sport dport; do
		isset ${arg} || continue

		# Check if port is valid.
		if ! isinteger ${arg}; then
			log WARNING "Invalid port '${arg}=${!arg}': $@"
			return ${EXIT_ERROR}
		fi
	done

	return ${EXIT_OK}
}