A communication apparatus includes a host interface, connected to a peripheral component bus so as to communicate with a CPU and a memory of a host computer. A network interface is connected to a network. Packet processing circuitry is configured to receive from a first interface a data packet including a set of one or more headers that include header fields having respective values, to identify, responsively to at least one of the header fields, a corresponding entry in a header modification table that specifies a header modification operation, to modify the set of headers in accordance with the header modification operation, to check whether the entry specifies an additional header modification operation, to output the modified set of headers if the entry does not specify an additional header modification operation, and, if the entry specifies an additional header modification operation, to feed-back the modified set of headers.