原始套接字-自定义IP首部和TCP首部

 /* =====================================================================================
 *
 *       Filename:    raw.c
 *    Description:    使用原始套接字发送TCP协议，并外带自己的数据。
 *
 * ====================================================================================*/
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <sys/socket.h>
 #include <netinet/in.h>
 #include <netinet/ip.h>
 #include <netinet/tcp.h>
 #include <arpa/inet.h>
 #define DATA "hello"
 #define PACKET_SIZE sizeof(struct iphdr) + sizeof(struct tcphdr) + sizeof(DATA)

 /*---------------------------------------------------------
  Function Name : check_sum()
    Descrypthon : 校验和计算，摘自UNP源码
 ------------------------------------------------------------*/
 unsigned short check_sum(unsigned short *addr, int len)
 {
     int nleft = len;
     int sum = ;
     unsigned short *w = addr;
     short answer = ;
     while (nleft > )
     {
         sum += *w++;
         nleft -=;
     }
     if (nleft == )
     {
         *(unsigned char *)(&answer) = *(unsigned char *)w;
         sum += answer;
     }
     sum = (sum >> ) + (sum & 0xffff);
     sum += (sum >> );
     answer = ~sum;
     return answer;
 }

 /*---------------------------------------------------------
  Function Name : init_socket()
    Descrypthon : 初始化socket，使用原始套接字
    parameter   : P1 一个待初始化的原始套接字，P2 待初始化的目标地址结构，P3 目标地址，P4 目标端口
    return      : 返回一个原始套接字，在函数体内将目标地址结构进行初始
 ------------------------------------------------------------*/
 int init_socket(int sockfd, struct sockaddr_in *target,const char *dst_addr, const char *dst_port)
 {
     const int flag = ;
     //目标协议簇
     target->sin_family = AF_INET;
     //目标端口
     target->sin_port = htons(atoi(dst_port));

     //将dst_addr中的ASCII-IP地址更新到target->sin_addr结构中
     if (inet_aton(dst_addr, &target->sin_addr) == )
     {
         perror("inet_aton fail\n");
         exit(-);
     }
     //初始化原始套接字
     if((sockfd = socket(AF_INET, SOCK_RAW, IPPROTO_TCP)) < )
     {
         perror("error");
         exit(-);
     }
     //设置套接字×××
     if (setsockopt(sockfd,IPPROTO_IP, IP_HDRINCL, &flag, sizeof(flag)) < )
     {
         perror("setsockopt fail \n");
         exit(-);
     }
     return sockfd;
 }

 /*---------------------------------------------------------------
  Function Name : buile_iphdr()
    Descrypthon : 构建IP头部数据， 源地址使用伪随机地址
 -----------------------------------------------------------------*/
 void buile_iphdr(struct sockaddr_in *target, char *buffer)
 {
     struct iphdr *ip = (struct iphdr *)(buffer);
     ip->version = ;//版本
     ip->ihl = ;//首部长度 5*4 = 20
     ip->tos = ;//8位服务类型
     ip->tot_len = htons(PACKET_SIZE);//16位总长度
     ip->id = ;//16位标识符
     ip->frag_off = ;//3位标志
     ip->ttl = ;//生存时间
     ip->protocol = IPPROTO_TCP;//协议
     ip->check = check_sum((unsigned short *)ip, sizeof(struct iphdr) + sizeof(DATA));//16位首部校验和
     ip->saddr = random();//源ip地址
     ip->daddr = target->sin_addr.s_addr;//目标ip地址
 }

 /*---------------------------------------------------------------
  Function Name : buile_tcphdr()
    Descrypthon : 构建TCP头部信息，并加入一些自己的数据，然后进行
                  校验计算。
 -----------------------------------------------------------------*/
 void buile_tcphdr(struct sockaddr_in *target, const char *src_port, char *buffer)
 {
     struct tcphdr *tcp = (struct tcphdr *)(buffer);
     tcp->source = htons(atoi(src_port));//16位源端口号
     tcp->dest = target->sin_port;//16位目的端口号
     tcp->seq = random();//32位序号
     tcp->doff = ;//
     tcp->syn = ;//同步序号
     buffer += sizeof(struct tcphdr);
     tcp->check = check_sum((unsigned short *)tcp, sizeof(struct tcphdr) + sizeof(DATA));//16位检验和
     memcpy(buffer, DATA, sizeof(DATA));//将DATA中的数据拷贝sizeof(DATA)字节到buffer所指的地址中
 }
 int main(int argc, const char *argv[])
 {
     char *buffer;
     char *buffer_head = NULL;
     int sockfd = ;
     struct sockaddr_in *target;
     if (argc != )
     {
         printf("usage: destination addresss, destination port, source port \n");
         exit(-);
     }
     const char *dst_addr = argv[];
     const char *dst_port = argv[];
     const char *src_port = argv[];

     target = calloc(sizeof(struct sockaddr_in),);
     //calloc在动态分配完内存后，自动初始化该内存空间为零，而malloc不初始化，里边数据是随机的垃圾数据。
     buffer = calloc(PACKET_SIZE, );
     buffer_head = buffer;

     //初始化套接字
     sockfd = init_socket(sockfd, target, dst_addr, dst_port);
     //创建IP首部
     buile_iphdr(target, buffer);
     buffer += sizeof(struct iphdr);//指针下移
     //创建TCP首部
     buile_tcphdr(target, src_port, buffer);
     //发送
     sendto(sockfd, buffer_head, PACKET_SIZE, ,(struct sockaddr *)target, sizeof(struct sockaddr_in));

     //下两行是对calloc申请的释放
     free(buffer_head);
     free(target);
     return ;
 }