3.6.2 From an Ethernet Probe

The section Observing Network Traffic explained how to use an Ethernet probe to listen to the incoming and outgoing traffic of a device connected on an Ethernet link. A probe can also be used to modify this traffic by dropping and changing existing packets, or injecting new ones.

typedef enum {
        Eth_Probe_Port_A = 0,
        Eth_Probe_Port_B = 1
} eth_probe_side_t;

typedef void (*ethernet_probe_snoop_t)(lang_void *user_data,
                                       conf_object_t *probe,
                                       eth_probe_side_t to_side,
                                       const frags_t *frame,
                                       eth_frame_crc_status_t crc_status);

SIM_INTERFACE(ethernet_probe) {
        void (*attach_snooper)(conf_object_t *NOTNULL probe,
                               ethernet_probe_snoop_t snoop_fun,
                               lang_void *user_data);
        void (*attach_probe)(conf_object_t *NOTNULL probe,
                             ethernet_probe_snoop_t snoop_fun,
                             lang_void *user_data);
        void (*detach)(conf_object_t *NOTNULL probe);
        void (*send_frame)(conf_object_t *NOTNULL probe,
                           eth_probe_side_t to_side,
                           const frags_t *frame,
                           eth_frame_crc_status_t crc_status);
};

#define ETHERNET_PROBE_INTERFACE "ethernet_probe"

A complete description of this interface is provided in the Simics Reference Manual. What we are interested in now is to register a probe callback that will be allowed to modify the traffic:

# a callback that drops all outgoing packets
simics> @def callback(user_data, probe, to_side, packet, crc_status):
            if to_side == Eth_Probe_Port_A:
                print('dropping incoming packet')
            else:
                print('forwarding outgoing packet')
                probe.iface.ethernet_probe.send_frame(to_side, packet, crc_status)

........
simics> @conf.probe0.iface.ethernet_probe.attach_probe(callback, None)
simics> c 
forwarding outgoing packet
forwarding outgoing packet
[...]

When a callback is registered as a probe, it takes responsibility for forwarding packets it receives. It is also allowed to drop them, modify them, modify their CRC status or inject new packets. For example, it could duplicate outgoing packets, inject errors, etc. Note that if your callback modifies the simulation in this manner, you may need to create an object representing the changes' state engine to make sure the simulation can be checkpointed and stays deterministic.