-export MIRROR_UBUNTU_URL := $(shell ./f_isoroot/f_repobuild/select_ubuntu_repo.sh --url)
-export MIRROR_UBUNTU := $(shell echo "$(MIRROR_UBUNTU_URL)" | cut -d'/' -f3 )
-export MIRROR_UBUNTU_ROOT := $(shell echo -n '/' ; echo "$(MIRROR_UBUNTU_URL)" | cut -d'/' -f4-)
+export MIRROR_UBUNTU_URL ?= $(shell ./f_isoroot/f_repobuild/select_ubuntu_repo.sh)
+export MIRROR_UBUNTU ?= $(shell echo "$(MIRROR_UBUNTU_URL)" | cut -d'/' -f3 )
+export MIRROR_UBUNTU_ROOT ?= $(shell echo -n '/' ; echo "$(MIRROR_UBUNTU_URL)" | cut -d'/' -f4-)
+ifeq (,$(MIRROR_UBUNTU_URL))
+$(warning $(shell ./f_isoroot/f_repobuild/select_ubuntu_repo.sh -d))
+$(error No sane Ubuntu mirror available)
+endif