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Function cb_packetin_task

This callback is called by the RTIP-32 internal IP task before an incoming data packet is processed:

int cb_packetin_task(int interface, byte * data, int length, int buffer_size);

Parameters

interface

Interface number (as returned by xn_interface_open or xn_interface_open_config) on which the new packet came in.

data

Pointer to the packet's data.

length

Number of bytes pointed to by parameter data.

buffer_size

Number of bytes the buffer pointed to by data can hold. buffer_size will always be >= length.

return value

Number of bytes RTIP-32 should process at *data. If 0 is returned, RTIP-32 will discard this packet.

This callback can be used to intercept incoming packets. The callback function may modify the data pointed to by parameter data, if designed. It must return the number of bytes RTIP-32 should process. If the packet data is not modified and RTIP-32 should continue normal processing, return parameter length. To discard the data, return 0.

This callback can be used to implement a simple firewall. The example below filters out all packets addressed at the Microsoft RPC (Remote Procedure Call) service:

#define SWAPPED(w) ((word)((w >> 8) | (w << 8))) 

int cb_packetin_task(int iface, byte * data, int length, int buffer_size) 
{ 
   // poor man's firewall: 
   // filter out all RPC packets (TCP, port 135) 
   if (*(word*)(data+0x0C) == SWAPPED(0x0800) &&   // IP 
       data[0x17]          == 0x06            &&   // TCP 
       *(word*)(data+0x24) == SWAPPED(135))        // destination port 
   { 
      return 0; // discard 
   } 
   return length; 
} 

... 
int main(void) 
{ 
  ... 
  xn_callbacks()->cb_packetin_task  = cb_packetin_task; 
  Result = xn_rtip_init(); 
  ...

Extended RTIP-32 API

Function cb_packetin_isr

Function cb_packetout