使用socket原始套接字发送TCP协议

socket使用AF_INET协议族，并创建SOCK_RAW原始套结字，自己填写IP头和TCP头，就当熟悉下头部结构吧。

* =====================================================================================
 *
 *       Filename:  raw.c
 *
 *    Description:  使用原始套接字发送TCP协议，并外带自己的数据。
 *
 *        Version:  1.0
 *        Created:  2012年09月11日 10时39分00秒
 *       Revision:  
 *       Compiler:  GCC 
 *
 *         Author:  yadon.z
 *
 * =====================================================================================
 */
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#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
------------------------------------------------------------*/
int init_socket(int sockfd, struct sockaddr_in *target,\
                const char *dst_addr, const char *dst_port)
{
    const int flag = 1;
    target->sin_family = AF_INET;
    target->sin_port = htons(atoi(dst_port));
    if (inet_aton(dst_addr, &target->sin_addr) == 0) {
        perror("inet_aton fail\n");
        exit(-1);
    }
    if((sockfd = socket(AF_INET, SOCK_RAW, IPPROTO_TCP)) < 0) {
        perror("error");
        exit(-1);
    }
    if (setsockopt(sockfd,IPPROTO_IP, IP_HDRINCL, &flag, sizeof(flag)) < 0) {
        perror("setsockopt fail \n");
        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;
    ip->ihl = 5;
    ip->tos = 0;
    ip->tot_len = htons(PACKET_SIZE);
    ip->id = 0;
    ip->frag_off = 0;
    ip->ttl = 255;
    ip->protocol = IPPROTO_TCP;
    ip->check = 0;
    ip->saddr = random();
    ip->daddr = target->sin_addr.s_addr;
    
    ip->check = check_sum((unsigned short *)ip, sizeof(struct iphdr) + sizeof(DATA));
}
/*---------------------------------------------------------------
 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));
    tcp->dest = target->sin_port;
    tcp->seq = random();
    tcp->doff = 5;
    tcp->syn = 1;
    buffer += sizeof(struct tcphdr);
    memcpy(buffer, DATA, sizeof(DATA));
    tcp->check = check_sum((unsigned short *)tcp, sizeof(struct tcphdr) + sizeof(DATA)); 
}
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 \n");
        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);
    buffer = calloc(PACKET_SIZE, 1);
    buffer_head = buffer;
    sockfd = init_socket(sockfd, target, dst_addr, dst_port);
    buile_iphdr(target, buffer);
    buffer += sizeof(struct iphdr);
    buile_tcphdr(target, src_port, buffer);
    sendto(sockfd, buffer_head, PACKET_SIZE, 0,\
                              (struct sockaddr *)target, sizeof(struct sockaddr_in));
    free(buffer_head);
    free(target);

    return 0;
}