Communication apparatus includes a host interface, which is configured to be connected to a peripheral component bus, and a network interface, which is configured to be connected to a network. Packet processing circuitry is coupled between the host interface and the network interface and is configured to receive from a first interface, selected from among the host interface and the network interface, a data packet comprising a header containing multiple fields having respective values, to identify, responsively to a value of at least one of the fields, a corresponding entry in a header modification table, and to modify the header in accordance with the identified entry. The data packet with the modified header is transmitted through a second interface selected from among the host interface and the network interface.