extern void tcp_gen(char *packet,unsigned short sport,                    unsigned short dport,unsigned long seq,                    unsigned long ack);extern void udp_gen(char *packet,unsigned short sport,                    unsigned short dport,unsigned short length);extern void ip_gen(char *packet,unsigned char protocol,struct in_addr saddr,            struct in_addr daddr,unsigned short length);extern unsigned short trans_check(unsigned char proto,                                  char *packet,                                  int length,                                  struct in_addr source_address,                                  struct in_addr dest_address);