Techniques are disclosed for a network device to preserve packet flow information across bump-in-the-wire (BITW) firewalls. For example, a method comprises receiving, by a network device, a packet. The method also comprises determining, by the network device, that the packet matches a packet flow that is associated with an action to redirect the packet to a firewall configured as a bump-in-the-wire. The method further comprises, in response to the determination: modifying, by the network device, a Media Access Control (MAC) address field of a layer 2 (L2) packet header with a flow identifier of the packet flow; sending, by the network device, the packet to the firewall; receiving, by the network device, the packet from the firewall; and recovering, by the network device, the packet flow by modifying the packet according to the flow identifier in the packet to restore the L2 packet header of the packet.