A data center can share processing resources using virtual networks. A hosting program 9,10 hosts one or more virtual machines 11, 12. The program has a virtual interface VIF 1 14, to the virtual machines, a network interface 19 to enable communication between the virtual machines and other nodes of a network, and an infrastructure management interface 8, invisible to the virtual machines. The program has an intercept function 7 implemented as a comparator, switch or router, arranged to intercept a status message from one of the virtual machines, or applications run by that virtual machine. The status indication is sent to a status buffer 5 and is made available to the infrastructure management interface without providing a network path between the management interface and the virtual machine. This can discriminate between VM failure and communication failure, and the invisibility maintains isolation and helps avoid vulnerability to denial of service attack.