The debugging of data communications software in an active networking environment can be a tricky and quite tiresome affair, often so because intermittent disturbances occurring in productive systems cannot be reproduced reliably and consistently or cannot be traced thoroughly enough. Thus for any Linux-based networking appliances like routers, firewalls, intrusion detection systems, VPNs, VoIP gear, etc., the User-Mode-Linux (UML) framework presented in this paper might be the optimum solution for running and testing these systems in a virtual networking environment under near real-time conditions. E. g. when a user of the OpenSource Linux strongSwan VPN software reported an IPsec re-keying error occurring in conjunction with a NAT router, this rare problem could be reproduced in a virtual UML test setup within two hours of simulation and a bug fix was found, tested and released on the same day. UML-based networks are also a powerful didactic tool in education where students can gain practical experience with complex network setups without the need of heavy investments in hardware equipment. This paper shows how a UML network can be set up with relative ease either for interactive explorative use or for automated regression testing.