Cause: When libvirt starts a NATed virtual network (that is with dnsmasq), it configures dnsmasq so that so called leases helper is ran on DHCP traffic. This helper binary updates a JSON file corresponding to the network, e.g. on new lease it adds IP address into the file, on lease timeout it removes it. The file is then used by 'virsh net-dhcp-leases' to report assigned IP addresses. However, to avoid two helpers writing the same file at once (which will certainly result in corrupted JSON) a PID file was used - the leases helper will try to acquire it (note that only one process can lock a file = pid file at once). Consequence: Due to a bug in implementation, the acquisition of the PID file was a single attempt without any wait only => if two dnsmasq-s from two different network decided to run the binary at once, the first process will acquire the PID file and starts updating a JSON file, while the other will fail to acquire it and exits immediately without updating the JSON file => the assigned IP address is not recorded and thus not reported later in virsh (or corresponding C API). Fix: The fix consists of making the PID file acquisition wait until the other process releases it. In code it's passing true instead of false to a function. Result: In the end, since the PID file acquisition waits, lease helpers will mutually exclude with each other, and wait for the data to be updated in the JSON file and thus virsh reports IP addresses correctly, without missing one.