1. For a managed forwarding element that forwards packets according to flow entries, a method comprising: receiving a packet that does not match any flow entry in a flow cache; for a new flow entry to be stored in the flow cache for processing the packet, initializing a plurality of match fields of the new flow entry as wildcard fields; dynamically generating the new flow entry by un-wildcarding each match field of the new flow entry that corresponds to a match field that is consulted in matching the packet against a set of flow entries in a set of flow tables, wherein the new flow entry has at least one wildcard field remaining; performing a set of actions on the packet according to the new flow entry; and storing the new flow entry in the flow cache to process any subsequent packet that matches each non-wildcard field of the new flow entry.