1. Log in to the VM through VNC and analyze the system log of the VM. It is found that the log contains an IPSec error record (as shown in Figure 2). The error shows that the IPSec driver enters the Block mode and discards all packets that are not permitted by the IPSec policy. As a result, the network for the VM is disconnected.
Figure 2 IPSec error record
2. Check the IPSec service. The service is in disabled state. Enabling the service manually returns the error "The system cannot find the file specified."
To resolve this issue, we need to change the key value in HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\IPSec\Policy. However, the VM in question does not have the key value of the IPSec module (as shown in Figure 3) although a normal VM should have such a key value (as shown in Figure 4).
Figure 3 Registry of the VM in question
Figure 4 Registry of a normal VM
It can be concluded that the network unavailability occurs because of a sequence of issues: the key value of the IPSec module in the VM registry is deleted, the IPSec service fails to start normally, and the IPSec driver enters the Block mode.
3. Export the key value of the IPSec module in a normal VM registry, import it into the registry of the VM in question, and restart the IPSec service of the VM in question. Then the network becomes available for the VM in question.