Ensure $SERVER_ADDR is free on the server.

vpnerator.sh

@@ -59,6 +59,11 @@ cleanup() {
 	EXIST_INTERFACE=\$(ip addr show to ${SERVER_ADDR} | head -1 | awk '{ print \$2 }' | cut -d':' -f1)
 
 	if [ ! -z "\${EXIST_INTERFACE}" ] ; then
+		if [ -z "\$(ip link show "\${EXIST_INTERFACE}" type wireguard)" ] ; then
+			# bail if the existing interface isn't of type wireguard
+			return 2
+		fi
+
 		remove_interface \${EXIST_INTERFACE}
 		return 0
 	elif ip link show ${INTERFACE_NAME} type wireguard > /dev/null 2>&1 ; then
@@ -81,6 +86,11 @@ deps || exit 1
 
 # Cleanup previous tunnel
 cleanup
+if [ "\$?" -eq 2 ] ; then
+	echo "$SERVER_ADDR is already being used on the server."
+	echo "Please choose another address."
+	exit 1
+fi
 
 umask 0177