defaultroute: Fix dual-stack and IPv6-only handling

Since IPv6-only setups are becoming more common, and IPv6
connectivity is often sufficient for tasks such as DNS
resolution and NTP time synchronization, update defaultroute
rc.d script to support IPv6-only environments.

Reviewed by:	pouria, ae
Differential Revision: https://reviews.freebsd.org/D56797

libexec/rc/rc.d/defaultroute

@@ -1,6 +1,6 @@
 #!/bin/sh
 #
-# Wait for the default route to be up if DHCP is in use
+# Wait for the default route to be up if DHCPv4 or IPv6 SLAAC is in use
 #
 #
 
@@ -22,7 +22,7 @@ defaultroute_carrier()
 	local carrier nocarrier
 
 	carrier=1
-	for _if in ${dhcp_interfaces}; do
+	for _if in ${dhcp_interfaces} ${autoconf_interfaces}; do
 		output=`/sbin/ifconfig ${_if}`
 		nocarrier=`expr "${output}" : '.*[[:blank:]]status: \(no carrier\)'`
 		[ -z "${nocarrier}" ] && carrier=0
@@ -32,22 +32,30 @@ defaultroute_carrier()
 
 defaultroute_start()
 {
-	local nl waited
+	local have_inet have_inet6 nl waited
 
-	afexists inet || return 0
+	# Check for IPv4/IPv6 support
+	have_inet=0
+	afexists inet  && have_inet=1
+	have_inet6=0
+	afexists inet6 && have_inet6=1
+	[ ${have_inet} -eq 0 ] && [ ${have_inet6} -eq 0 ] && return
 
-	# Return without waiting if we don't have dhcp interfaces or
+	# Return without waiting if we don't have dhcp or autoconf interfaces,
-	# if none of the dhcp interfaces is plugged in.
+	# or if none of them is plugged in.
-	dhcp_interfaces=`list_net_interfaces dhcp`
+	[ ${have_inet}  -ne 0 ] && dhcp_interfaces=`list_net_interfaces dhcp`
-	[ -z "${dhcp_interfaces}" ] && return
+	[ ${have_inet6} -ne 0 ] && autoconf_interfaces=`list_net_interfaces autoconf`
+	[ -z "${dhcp_interfaces}" -a -z "${autoconf_interfaces}" ] && return
 
 	# Wait for a default route
 	waited=0
 	while [ ${waited} -lt ${defaultroute_delay} ]; do
-		defif=`get_default_if -inet`
+		[ ${have_inet}  -ne 0 ] && defif=`get_default_if -inet`
-		if [ -n "${defif}" ]; then
+		[ ${have_inet6} -ne 0 ] && defif6=`get_default_if -inet6`
+		if [ -n "${defif}" ] || [ -n "${defif6}" ]; then
 			if [ ${waited} -ne 0 ]; then
-				echo -n "($defif)"
+				[ -n "${defif}"  ] && echo -n "($defif)=>inet "
+				[ -n "${defif6}" ] && echo -n "($defif6)=>inet6"
 				nl=1
 			fi
 			break