Reconfigurable systems are increasingly employed in many application fields, including aerospace. The long term exposure to radiation of space electronics can cause permanent faults, that may lead to the failure of the mission. In this paper we present a novel technique for on-line on-demand testing of permanent faults in the routing structure of SRAM-based FPGAs, that are employed in reconfigurable systems. The basic idea is to place testing circuits on the resources of the FPGA which are unused at the moment to test them before using those resources when a functional module of the reconfigurable system has to be placed. The proposed technique has been implemented and the achieved fault coverage has been assessed on a real-world reconfigurable system. This experiment demonstrated that all the faults in the routing resources under test can be detected.