18.12.5. Automatic IP Address Detection and DHCP Snooping

The detection of IP addresses used on a virtual machine's interface is automatically activated if the variable IP is referenced but no value has been assigned to it. The variable CTRL_IP_LEARNING can be used to specify the IP address learning method to use. Valid values include: any, dhcp, or none.

The value any instructs libvirt to use any packet to determine the address in use by a virtual machine, which is the default setting if the variable TRL_IP_LEARNING is not set. This method will only detect a single IP address per interface. Once a guest virtual machine's IP address has been detected, its IP network traffic will be locked to that address, if for example, IP address spoofing is prevented by one of its filters. In that case, the user of the VM will not be able to change the IP address on the interface inside the guest virtual machine, which would be considered IP address spoofing. When a guest virtual machine is migrated to another host physical machine or resumed after a suspend operation, the first packet sent by the guest virtual machine will again determine the IP address that the guest virtual machine can use on a particular interface.

The value of dhcp instructs libvirt to only honor DHCP server-assigned addresses with valid leases. This method supports the detection and usage of multiple IP address per interface. When a guest virtual machine resumes after a suspend operation, any valid IP address leases are applied to its filters. Otherwise the guest virtual machine is expected to use DHCP to obtain a new IP addresses. When a guest virtual machine migrates to another physical host physical machine, the guest virtual machine is required to re-run the DHCP protocol.

If CTRL_IP_LEARNING is set to none, libvirt does not do IP address learning and referencing IP without assigning it an explicit value is an error.