Roughly described, a network interface device for connection between a host data processing device and a network, comprising: a controller for supporting communication with a host data processing device over a data bus interface; and a packet capture unit between the controller and the network and comprising: a packet inspector configured to parse incoming data packets to identify data packets of a first data flow; a duplication engine to generate a duplicate data flow from the first data flow; and a packet capture engine to process said duplicate data flow into a packet capture data stream having a predefined format. The network interface device is configured to cause: the first data flow to be made available to a consumer process of a host data processing device to which the network interface device is connected; and the processed packet capture data stream to be stored at a packet capture buffer.