multihomed

#!/bin/sh
# Install into /etc/dhcp and symlink to /etc/dhcp/dhclient-exit-hooks.d

# Copyright 2022, Daniel Gröber
#
# Copying and distribution of this file, with or without modification, are
# permitted in any medium without royalty, provided the copyright notice and
# this notice are preserved. This file is offered as-is, without any
# warranty.
#
# SPDX-License-Identifier: FSFAP

mh_calc_table_id () {
	IFS=. read -r a b c d <<-EOF
	      $1
	EOF
        TABLE_ID=$(( (a<<24) | (b<<16) | (c<<8) | d))
	[ "$TABLE_ID" -gt 255 ] || return 1
}

multihomed_old () {
	if [ -n "${old_ip_address:-}" ]; then
		mh_calc_table_id "$old_ip_address" || return
		while ip rule del table "$TABLE_ID" >/dev/null 2>&1; do :; done
		ip rule del from "$old_ip_address" suppress_prefixlength 0
		ip route flush table "$TABLE_ID"
	fi
	if [ -n "$fwmark" ]; then
		while ip rule del fwmark "$fwmark" suppress_prefixlength 0 >/dev/null 2>&1; do :; done
		while ip rule del fwmark "$fwmark" unreachable >/dev/null 2>&1; do :; done
	fi
}

multihomed_new () {
	mh_calc_table_id "$new_ip_address" || return
	ip rule add from "$new_ip_address" table "$TABLE_ID"
	ip rule add from "$new_ip_address" suppress_prefixlength 0
	#^ alternative: suppress_ifgroup wan

	if [ -n "$fwmark" ]; then
		ip rule add fwmark "$fwmark" unreachable
		ip rule add fwmark "$fwmark" table "$TABLE_ID"
		ip rule add fwmark "$fwmark" suppress_prefixlength 0
	fi

        for router in $new_routers; do
	        ip route add default via "$router" table "$TABLE_ID"
		break; # only one default router supported
        done

	IFACE=$interface FWMARK=$fwmark TABLE_ID=$TABLE_ID \
	      run-parts /etc/dhcp/multihomed.d || true
}

do_multihomed () {
	IFQ=$(ifquery "$interface") || unset IFQ
	if [ -n "$IFQ" ]; then
		fwmark=$(printf '%s\n' "$IFQ" \
				 | awk '/^multihomed-fwmark:/{ print $2 }')
	fi

        case "$reason" in
	        BOUND|REBOOT)
		        multihomed_old; multihomed_new ;;
		RENEW|REBIND) ;;
	        EXPIRE|FAIL|RELEASE|STOP)
                        multihomed_old;;
        esac
}

do_multihomed