Some embodiments provide a method for a managed forwarding element (MFE) executing on a data compute node (DCN) that operates on a host computer in a public datacenter. The MFE implements a logical network that connects multiple DCNs within the public datacenter. The method receives a packet, directed to the DCN, that (i) has a first logical network source address and (ii) is encapsulated with a second source address associated with an underlying public datacenter network. The method determines whether the first logical network source address is a valid source address for the packet based on a mapping table that maps logical network addresses to underlying public datacenter network addresses. When the first source address is not a valid source address for the packet, the method drops the packet.