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 = 0;
    unsigned short *w = addr;
    short answer = 0;
    while (nleft > 1)
    {
        sum += *w++;
        nleft -=2;
    }
    if (nleft == 1)
    {
        *(unsigned char *)(&answer) = *(unsigned char *)w;
        sum += answer;
    }
    sum = (sum >> 16) + (sum & 0xffff);
    sum += (sum >> 16);
    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 = 1;
    //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) == 0)
    {
        perror("inet_aton fail
");
        exit(-1);
    }
    //初始化原始套接字
    if((sockfd = socket(AF_INET, SOCK_RAW, IPPROTO_TCP)) < 0)
    {
        perror("error");
        exit(-1);
    }
    //设置套接字×××
    if (setsockopt(sockfd,IPPROTO_IP, IP_HDRINCL, &flag, sizeof(flag)) < 0)
    {
        perror("setsockopt fail 
");
        exit(-1);
    }
    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 = 4;//版本
    ip->ihl = 5;//首部长度 5*4 = 20
    ip->tos = 0;//8位服务类型
    ip->tot_len = htons(PACKET_SIZE);//16位总长度
    ip->id = 0;//16位标识符
    ip->frag_off = 0;//3位标志
    ip->ttl = 255;//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 = 5;//
    tcp->syn = 1;//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 = 0;
    struct sockaddr_in *target;
    if (argc != 4)
    {
        printf("usage: destination addresss, destination port, source port 
");
        exit(-1);
    }
    const char *dst_addr = argv[1];
    const char *dst_port = argv[2];
    const char *src_port = argv[3];

    target = calloc(sizeof(struct sockaddr_in),1);
    //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, 1);
    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, 0,(struct sockaddr *)target, sizeof(struct sockaddr_in));

    //下两行是对calloc申请的释放
    free(buffer_head);
    free(target);
    return 0;
}