pfSense

	$ifdescrs = get_configured_interface_list();

	foreach ($ifdescrs as $ifdescr => $ifname) {

		/* check IPv4 */

		if(is_ipaddrv4($carp_ip)) {

			$interfaceip = get_interface_ip($ifname);

			$subnet_bits = get_interface_subnet($ifname);

			$subnet_ip = gen_subnet("{$interfaceip}", "{$subnet_bits}");

			if(ip_in_subnet($carp_ip, "{$subnet_ip}/{$subnet_bits}"))

				return $ifname;

		}

		/* Check IPv6 */

		if(is_ipaddrv6($carp_ipv6)) {

			$interfaceipv6 = get_interface_ipv6($ifname);

			$prefixlen = get_interface_subnetv6($ifname);

			if(ip_in_subnet($carp_ipv6, "{$interfaceipv6}/{$prefixlen}"))

				return $ifname;

		}

	}