An approach for securing a DHCP server against unauthorized client attacks in a SDN environment is presented. In an embodiment, a method comprises: determining a count of sub-interfaces implemented on an interface card of a virtual machine; setting a count of unique client identifiers for the virtual machine to zero; determining whether a dynamic host configuration protocol (DHCP) request has been received from the virtual machine; in response to determining that a DHCP request has been received from the virtual machine, incrementing the count of unique client identifiers; determining whether the count of unique client identifiers exceeds the count of sub-interfaces implemented on the interface card of the virtual machine; and in response to determining that the count of unique client identifiers does not exceed the count of sub-interfaces implemented on the interface card of the virtual machine, forwarding the DHCP request to an uplink port.