In an era of continuous networking and permanent accessibility of computers, network security has become a serious issue. News about data theft in companies has become a common headline. Sensitive data is sent unencrypted over the wire. This culpable negligence of certain network administrators makes the life for data thieves easier. All you need is a Linux-LiveCD and a common PC to start snooping around for sensitive data. The Software need is a network analyser or sniffer. Both of them are passive elements and theoretically not detectable from outside. As a matter of fact, accurate software does not exist. This makes it possible to exploit weakness of implementation and detect the unwanted listener. Up until now there was no open source tool that was able to detect a sniffer. Within the scope of our diploma thesis, existing methods should be collected, analysed and evaluated. Furthermore a tool had to be developed, which implements the auspicious methods and closes the gap. The document you?re holding in your hands provides you with a tool which is able to detect sniffers from outside. The testing of nine out of ten operating systems with all implemented methods was successful. SunOS 5.7 though was able to defy three of four methods. During testing we discovered that some embedded systems, e.g. network printers, print servers and ADSL routers have a really bad network stack. Using such stacks can lead to false suspicion. However, administrators in a corporate network should have all the necessary information about such devices so they should not pose a challenge anymore. We successfully finished our diploma thesis. Network administrators are provided with a useful and powerful tool which will make the life for script-kiddies hard.