https://abcdxyzk.github.io/blog/2015/04/14/kernel-net-sock-raw/

IP报头格式

IP header的结构由RFC 791定义，如图(标尺每个数字代表一个bit位)：
    0                   1                   2                   3 
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Version|  IHL  |Type of Service|          Total Length         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Identification        |Flags|      Fragment Offset    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Time to Live |    Protocol   |         Header Checksum       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Source Address                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Destination Address                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Options                    |    Padding    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

然后TCP header的结构是由RFC 793定义的：
    0                   1                   2                   3 
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Source Port          |       Destination Port        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Sequence Number                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Acknowledgment Number                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Data |           |U|A|P|R|S|F|                               |
   | Offset| Reserved  |R|C|S|S|Y|I|            Window             |
   |       |           |G|K|H|T|N|N|                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Checksum            |         Urgent Pointer        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Options                    |    Padding    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

摘自：

1. https://www.binarytides.com/raw-sockets-c-code-linux/

2. https://www.binarytides.com/raw-socket-programming-in-python-linux/

3. https://wordpress.youran.me/python-raw-socket-programming/

/*
    Raw TCP packets
    Silver Moon (m00n.silv3r@gmail.com)
*/
#include<stdio.h>   //for printf
#include<string.h> //memset
#include<sys/socket.h>  //for socket ofcourse
#include<stdlib.h> //for exit(0);
#include<errno.h> //For errno - the error number
#include<netinet/tcp.h> //Provides declarations for tcp header
#include<netinet/ip.h>  //Provides declarations for ip header

/* 
    96 bit (12 bytes) pseudo header needed for tcp header checksum calculation 
*/
struct pseudo_header
{
    u_int32_t source_address;
    u_int32_t dest_address;
    u_int8_t placeholder;
    u_int8_t protocol;
    u_int16_t tcp_length;
};

/*
    Generic checksum calculation function
*/
unsigned short csum(unsigned short *ptr,int nbytes) 
{
    register long sum;
    unsigned short oddbyte;
    register short answer;

    sum=0;
    while(nbytes>1) {
        sum+=*ptr++;
        nbytes-=2;
    }
    if(nbytes==1) {
        oddbyte=0;
        *((u_char*)&amp;oddbyte)=*(u_char*)ptr;
        sum+=oddbyte;
    }

    sum = (sum>>16)+(sum &amp; 0xffff);
    sum = sum + (sum>>16);
    answer=(short)~sum;
    
    return(answer);
}

int main (void)
{
    //Create a raw socket
    int s = socket (PF_INET, SOCK_RAW, IPPROTO_TCP);
    
    if(s == -1)
    {
        //socket creation failed, may be because of non-root privileges
        perror("Failed to create socket");
        exit(1);
    }
    
    //Datagram to represent the packet
    char datagram[4096] , source_ip[32] , *data , *pseudogram;
    
    //zero out the packet buffer
    memset (datagram, 0, 4096);
    
    //IP header
    struct iphdr *iph = (struct iphdr *) datagram;
    
    //TCP header
    struct tcphdr *tcph = (struct tcphdr *) (datagram + sizeof (struct ip));
    struct sockaddr_in sin;
    struct pseudo_header psh;
    
    //Data part
    data = datagram + sizeof(struct iphdr) + sizeof(struct tcphdr);
    strcpy(data , "ABCDEFGHIJKLMNOPQRSTUVWXYZ");
    
    //some address resolution
    strcpy(source_ip , "192.168.1.2");
    sin.sin_family = AF_INET;
    sin.sin_port = htons(80);
    sin.sin_addr.s_addr = inet_addr ("1.2.3.4");
    
    //Fill in the IP Header
    iph->ihl = 5;
    iph->version = 4;
    iph->tos = 0;
    iph->tot_len = sizeof (struct iphdr) + sizeof (struct tcphdr) + strlen(data);
    iph->id = htonl (54321);    //Id of this packet
    iph->frag_off = 0;
    iph->ttl = 255;
    iph->protocol = IPPROTO_TCP;
    iph->check = 0;     //Set to 0 before calculating checksum
    iph->saddr = inet_addr ( source_ip );   //Spoof the source ip address
    iph->daddr = sin.sin_addr.s_addr;
    
    //Ip checksum
    iph->check = csum ((unsigned short *) datagram, iph->tot_len);
    
    //TCP Header
    tcph->source = htons (1234);
    tcph->dest = htons (80);
    tcph->seq = 0;
    tcph->ack_seq = 0;
    tcph->doff = 5; //tcp header size
    tcph->fin=0;
    tcph->syn=1;
    tcph->rst=0;
    tcph->psh=0;
    tcph->ack=0;
    tcph->urg=0;
    tcph->window = htons (5840);    /* maximum allowed window size */
    tcph->check = 0;    //leave checksum 0 now, filled later by pseudo header
    tcph->urg_ptr = 0;
    
    //Now the TCP checksum
    psh.source_address = inet_addr( source_ip );
    psh.dest_address = sin.sin_addr.s_addr;
    psh.placeholder = 0;
    psh.protocol = IPPROTO_TCP;
    psh.tcp_length = htons(sizeof(struct tcphdr) + strlen(data) );
    
    int psize = sizeof(struct pseudo_header) + sizeof(struct tcphdr) + strlen(data);
    pseudogram = malloc(psize);
    
    memcpy(pseudogram , (char*) &amp;psh , sizeof (struct pseudo_header));
    memcpy(pseudogram + sizeof(struct pseudo_header) , tcph , sizeof(struct tcphdr) + strlen(data));
    
    tcph->check = csum( (unsigned short*) pseudogram , psize);
    
    //IP_HDRINCL to tell the kernel that headers are included in the packet
    int one = 1;
    const int *val = &amp;one;
    
    if (setsockopt (s, IPPROTO_IP, IP_HDRINCL, val, sizeof (one)) < 0)
    {
        perror("Error setting IP_HDRINCL");
        exit(0);
    }
    
    //loop if you want to flood :)
    while (1)
    {
        //Send the packet
        if (sendto (s, datagram, iph->tot_len , 0, (struct sockaddr *) &amp;sin, sizeof (sin)) < 0)
        {
            perror("sendto failed");
        }
        //Data send successfully
        else
        {
            printf ("Packet Send. Length : %d \n" , iph->tot_len);
        }
    }
    
    return 0;
}

//Complete