check_udp and icmp unreachable



I’ve been reading the Barth book “Nagios”. About check_udp it says “If you find an unoccupied port, the requested host normally sends back an “ICMP port unreachable” message, which evaluates the plugin.” I don’t understand what “evaluates the plugin” means. Does it mean that check_udp can distinquish between the host returning ICMP port unreachable and the host not responding? What would I test for in the output from check_udp? In both cases no data comes back. It’d be nice if the output from check_udp could say “ICMP port unreachable” or “timed out”.