#include <uapi/linux/ptrace.h>
#include <net/sock.h>
#include <bcc/proto.h>

#define IP_TCP 	6
#define IP_UDP 17
#define IP_ICMP 1
/* 
  In 802.3, both the source and destination addresses are 48 bits (4 bytes) MAC address.
  6 bytes (src) + 6 bytes (dst) + 2 bytes (type) = 14 bytes 
*/
#define ETH_HLEN 14

/*eBPF program.
  Filter TCP/UDP/ICMP packets, having payload not empty
  if the program is loaded as PROG_TYPE_SOCKET_FILTER
  and attached to a socket
  return  0 -> DROP the packet
  return -1 -> KEEP the packet and return it to user space (userspace can read it from the socket_fd )
*/
int vlan_filter(struct __sk_buff *skb) { 
	u8 *cursor = 0;	

	struct ethernet_t *ethernet = cursor_advance(cursor, sizeof(*ethernet));
	
	//filter IP packets (ethernet type = 0x0800) 0x0800 is IPv4 packet
	switch(ethernet->type){
		case 0x0800: goto IP;
	    	default: goto DROP;
	}

	
	IP: ;
		struct ip_t *ip = cursor_advance(cursor, sizeof(*ip));  // IP header (datagram)
	        switch (ip->nextp){
			case 17: goto UDP;
			default: goto DROP;
		}

	UDP: ;
		struct udp_t *udp = cursor_advance(cursor, sizeof(*udp));
		switch (udp->dport) {
    			case 4789: goto KEEP;
    			default: goto DROP;
  		}

	//keep the packet and send it to userspace returning -1
	KEEP:
		return -1;

	//drop the packet returning 0
	DROP:
		return 0;
}