Linux使用原始套接字从链路层开始封包组包（IP数据包）

UDP校验方式：

1. 在对UDP校验的时候需要在UDP报文之间加上伪头部。
2. IP校验的时候不需要伪头部
   
   注意：伪头部中的源ip、目的IP必须和 IP报文中源ip、目的IP 一致。

#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
using namespace std;

typedef struct{
    u_int32_t saddr;//源IP
    u_int32_t daddr;//目的IP
    u_int8_t flag;//标记（0）
    u_int8_t protocol;//udp协议 17
    u_int16_t len;//长度
}FAKEHDR;
//校验函数，通信两边的校验方法需要保持一致
unsigned short checksum(unsigned short *buf, int len)
{
    int nword = len/2;
    unsigned long sum;

    if(len%2 == 1)
        nword++;
    for(sum = 0; nword > 0; nword--)
    {
        sum += *buf;
        buf++;
    }
    sum = (sum>>16) + (sum&0xffff);
    sum += (sum>>16);
    return ~sum;
}

int main(int argc, char *argv[])
{
    int sockfd = socket(PF_PACKET,SOCK_RAW,htons(ETH_P_ALL));
    if(sockfd < 0)
    {
        perror("socket error");
        return -1;
    }

    struct ifreq ethreq;
    strncpy(ethreq.ifr_ifrn.ifrn_name,"ens33",IFNAMSIZ);
    if(ioctl(sockfd,SIOCGIFINDEX,ðreq) == -1)
    {
        perror("socket error");
        close(sockfd);
        return -2;
    }

    cout<< "请输入要发送的内容：" << endl;
    char data[128] = "";
    fgets(data,sizeof(data),stdin);
    data[strlen(data) - 1 ] = 0;//去掉回车
    int dataLen = strlen(data);
    //udp需要数据长度为偶数个字节，不是偶数补0
    if(dataLen %2 != 0)
        dataLen++;

    struct sockaddr_ll sll;
    bzero(&sll,sizeof(sll));
    sll.sll_ifindex = ethreq.ifr_ifindex;

    unsigned char dstMac[6] = {0x00,0xFF,0xB4,0x31,0xD1,0xE2};
    unsigned char srcMac[6] = {0x00,0x0c,29,0xee,0x18,0x53};
    unsigned char dstIp[4] = {192,168,123,249};
    unsigned char srcIp[4] = {192,168,243,128};
    unsigned char msg[1024] = "";

    //1.ether
    struct ether_header *macHeader = (struct ether_header*)msg;
    memcpy(macHeader->ether_dhost,dstMac,6);
    memcpy(macHeader->ether_shost,srcMac,6);
    macHeader->ether_type  = htons(0x0800);

    //2.IP
    struct iphdr *ipHeader = (struct iphdr*)(msg+14);
    ipHeader->version = 4;//ipv4版本
    ipHeader->ihl = 5;//IP头部长度 单位4B 所以赋值5其实就是5*4=20B
    ipHeader->tos = 0;//服务类型
    ipHeader->tot_len = htons(20+8+dataLen);//总长度=IP首部长度+UDP首部长度+IP数据长度
    ipHeader->id = htons(0);//标识
    ipHeader->frag_off = htons(0);//标志 + 片偏移
    ipHeader->ttl = 128;//64或128都可以 生命周期
    ipHeader->protocol = 17;//协议类型:UDP-17;TCP-6
    memcpy(&ipHeader->daddr,dstIp,4);
    memcpy(&ipHeader->saddr,srcIp,4);
    //ip报文头部校验
    ipHeader->check = checksum((unsigned short *)ipHeader,20);

    //3.UDP
    struct udphdr *udpHeader = (struct udphdr*)(msg+14+20);
    udpHeader->source = htons(8000);//源端口
    udpHeader->dest = htons(9000);//目的端口
    udpHeader->len = htons(8+dataLen);//udp总长度=udp报文头+数据长度
    //UDP校验
    unsigned char fakeHdr[256] = "";
    FAKEHDR* fakeHeader = (FAKEHDR*)fakeHdr;
    memcpy(&fakeHeader->saddr,srcIp,4);
    memcpy(&fakeHeader->daddr,dstIp,4);
    fakeHeader->protocol = 17;
    fakeHeader->flag = 0;
    fakeHeader->len = htons(8+dataLen);//udp的总长度
    //将msg中的udp头部信息以及data数据 拷贝到伪头部后方
    memcpy(fakeHdr+12,udpHeader,8+dataLen);
    //校验udp： 伪头部+udp头部+data部分
    udpHeader->check = checksum((unsigned short *)fakeHdr,12+8+dataLen);

    //4.要发送的数据
    memcpy(msg+14+20+8,data,dataLen);

    sendto(sockfd,msg,14+20+8+dataLen,0,(struct sockaddr*)&sll,sizeof(sll));

    close(sockfd);
    return 0;
}