dhcpcd: rework mtu handling on buggy nic's

some nic's loose the carrier after setting new mtu.

Signed-off-by: Arne Fitzenreiter <arne_f@ipfire.org>

config/dhcpc/dhcpcd-hooks/10-mtu

@@ -0,0 +1,31 @@
+# Configure the MTU for the interface
+
+eval $(/usr/local/bin/readhash /var/ipfire/ethernet/settings)
+
+set_mtu()
+{
+	local mtu=$1
+	ip link set "$interface" mtu "$mtu"
+
+	# test for buggy nic that lose link at mtu set...
+	carrier=`cat /sys/class/net/$interface/carrier`
+	if [ "$carrier" == "0" ]; then
+		syslog info "Warning! Carrier loss after MTU set. Reinit ..."
+		ip link set "$interface" down
+		ip link set "$interface" up
+	fi
+}
+if [ -n "$new_interface_mtu" ] && $if_up; then
+	if [ $RED_DHCP_FORCE_MTU -ge 576 ]; then
+		new_interface_mtu=$RED_DHCP_FORCE_MTU
+	fi
+	if [ ! "$new_interface_mtu" == "$ifmtu" ]; then
+		# The smalled MTU dhcpcd can work with is 576
+		if [ "$new_interface_mtu" -gt 576 ]; then
+			if set_mtu "$new_interface_mtu"; then
+				syslog info "MTU set to $new_interface_mtu"
+			fi
+		fi
+	fi
+fi
+