NAT definition of already used port does not fail

[archfrog]

I just tried to set up a Linux guest as an HTTP server on port 80 and therefore created a NAT port forwarding rule of TCP 80 -> TCP 80. I then verified that the server did run on Linux, using lynx on the guest, and then continued on to try to open ​http://localhost. I got a blank page in return. Digging around a bit, I finally realized that something might be using port 80 on my host computer. And sure enough, it turned out that Skype was using that port. But I got no warning or error from the NAT configuration window (or VirtualBox) when I allocated an already used port for NAT forwarding.

[Frank Mehnert]

How did you setup the port forwarding rule, using the GUI or using VBoxManage? And are you using 'NAT' or 'NAT network' as network attachment type?

[Valery Ushakov]

NAT logs a warning the VM log file that it failed to setup port-forwarding if the specified host port is already bound by some other program.

The situation is quite normal (and often intentional - e.g. interchangeably using different programs to provide the same service). VirtualBox is not different from any other program that may be configured to use a port that some other program is also configured to use. At configuration time it's impossible to predict whether the port will or will not be in use at run time. Managing this is user's responsibility.