In fact, the Norwegian Supreme Court was recently asked to rule on whether or not such scanning was illegal. The court decided that it was not, because it was similar to a knock on the door, not forced entry. This is equivalent to calling a list of sequential telephone numbers and seeing who answers. Kerby at the Naval Surface Warfare Center said that most military sites routinely block out ping requests. We just note that they came up and rang the door bell, but we had everything secured before they got here.
Some hackers use more sophisticated probes.
It is possible, for instance, to see if a computer accepts electronic mail by sending a trial message. This information can be exploited, in some cases, because older versions of the popular electronic mail program known as Sendmail have numerous security holes that could give a hacker access to a system. In the case of the latest probes, the hackers tried to conceal the scale of their effort by sending requests from a number of different computers. It's not detected by most commercial detection systems. What made this significant is that it was low and slow.