//Mac头部，总长度14字节
typedef struct _eth_hdr
{
unsigned char dstmac[6]; //目标mac地址
unsigned char srcmac[6]; //源mac地址
unsigned short eth_type; //以太网类型
}eth_hdr;
 
//IP头部，总长度20字节
typedef struct _ip_hdr
{
#if LITTLE_ENDIAN
unsigned char ihl:4;   //首部长度
unsigned char version:4, //版本 
#else  www.2cto.com  
unsigned char version:4, //版本
unsigned char ihl:4;   //首部长度
#endif
unsigned char tos;   //服务类型
unsigned short tot_len; //总长度
unsigned short id;    //标志
unsigned short frag_off; //分片偏移
unsigned char ttl;   //生存时间
unsigned char protocol; //协议
unsigned short chk_sum; //检验和
struct in_addr srcaddr; //源IP地址
struct in_addr dstaddr; //目的IP地址
}ip_hdr;
 
//TCP头部，总长度20字节
typedef struct _tcp_hdr
{
unsigned short src_port;   //源端口号
unsigned short dst_port;   //目的端口号
unsigned int seq_no;    //序列号
unsigned int ack_no;    //确认号
#if LITTLE_ENDIAN
unsigned char reserved_1:4; //保留6位中的4位首部长度
unsigned char thl:4;    //tcp头部长度
unsigned char flag:6;    //6位标志
unsigned char reseverd_2:2; //保留6位中的2位
#else
unsigned char thl:4;    //tcp头部长度
unsigned char reserved_1:4; //保留6位中的4位首部长度
unsigned char reseverd_2:2; //保留6位中的2位
unsigned char flag:6;    //6位标志 
#endif
unsigned short wnd_size;   //16位窗口大小
unsigned short chk_sum;   //16位TCP检验和
unsigned short urgt_p;    //16为紧急指针
}tcp_hdr;  www.2cto.com  
 
//UDP头部，总长度8字节
typedef struct _udp_hdr
{
unsigned short src_port; //远端口号
unsigned short dst_port; //目的端口号
unsigned short uhl;   //udp头部长度
unsigned short chk_sum; //16位udp检验和
}udp_hdr;
 
//ICMP头部，总长度4字节
typedef struct _icmp_hdr
{
unsigned char icmp_type;   //类型
unsigned char code;    //代码
unsigned short chk_sum;   //16位检验和
}icmp_hdr;