[network-manager] Duplicated IP addresses appear on service re-start

[Alfonso Sanchez-Beato]

When re-starting the network-manager service, in some cases duplicated IP addresses appear in the managed interfaces. This happens because apparently NM is ignoring the old DHCP leases, and sends always a "DHCP discover" package. Some DHCP servers assign a new IP when that happens, and NM does not remove the old address.

The right thing to do would be to send a "DHCP request" containing the old lease. This is actually what I see happening on bionic desktop. In that case, the DHCP server can either acknowledge the old address or assign a new one.

The specified behaviour when a discover is received in the DHCP server is to consider that the leases previously assigned to the sending MAC are not in use anymore. This can lead to problems if the server decides to assign those addresses to other devices. Also, we have the additional problem of NM setting the old lease as the primary address.