Problem: I created a raw socket with the UDP protocol, sent a packet consisting of a UDP header and data, everything is fine in the PP, the packet is correct, I also created the same socket in another program, and it accepted the data that I sent, but along with the IP header. How so? Why isn’t this written anywhere?
That is, we send DATA+UDP, and receive from the same socket DATA+UDP+IP.

The second program that receives data only works if you add 20 to the pointer, that is, the size of the IP header.

#include <stddef.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <linux/udp.h>
#include <arpa/inet.h>
#include <sys/socket.h>
#include <netinet/ether.h>
#include <sys/types.h>
#include <sys/socket.h>
#define __USE_XOPEN2K
#include <netdb.h>

#define AF_INETv4 0x02
#define AF_INETv6 0x0A

int main(int argc, char **argv)
{
    int m_socket = socket(AF_INETv4, SOCK_RAW, IPPROTO_UDP);

    struct sockaddr_in bnd;
    bnd.sin_family = AF_INET;
    bnd.sin_port = htons(55555);
    bnd.sin_addr.s_addr = inet_addr("127.0.0.1");

    bind(m_socket, &bnd, sizeof(bnd));
    perror("bind: ");

    struct iovec iov;

    char buffer[1500 - 14 - 20] = {0};
    iov.iov_base = &buffer;
    iov.iov_len  = sizeof(buffer);
    struct udphdr *udp = iov.iov_base;
    udp->source = htons(55555);
    udp->dest   = htons(22222);
    udp->len    = htons(8);
    udp->check = 0;

    struct  msghdr  msg = {0};

    msg.msg_flags = 0;
    msg.msg_iov = &iov;
    msg.msg_iovlen = 1;

    struct sockaddr_in in;
    in.sin_family = AF_INETv4;
    in.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
    in.sin_port = htons(22222);

    msg.msg_name = &in;
    msg.msg_namelen = sizeof(struct sockaddr_in);

    for(int i = 0; i < 10; i++)
    {
        sendmsg(m_socket, &msg, 0);
        perror("");
    }

    return 0;
}

#include <stddef.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <linux/udp.h>
#include <arpa/inet.h>
#include <sys/socket.h>
#include <netinet/ether.h>
#include <sys/types.h>
#include <sys/socket.h>
#define __USE_XOPEN2K
#include <netdb.h>

#define AF_INETv4 0x02
#define AF_INETv6 0x0A

int main(int argc, char **argv)
{
    int m_socket = socket(AF_INETv4, SOCK_RAW, IPPROTO_UDP);

    struct sockaddr_in bnd;
    bnd.sin_family = AF_INET;
    bnd.sin_port = htons(22222);
    bnd.sin_addr.s_addr = inet_addr("127.0.0.1");

    bind(m_socket, &bnd, sizeof(bnd));
    perror("bind: ");

    struct iovec iov;

    char buffer[1500 - 14 - 20] = {0};
    iov.iov_base = &buffer;
    iov.iov_len  = sizeof(buffer);

    struct  msghdr  msg = {0};

    msg.msg_flags = 0;
    msg.msg_iov = &iov;
    msg.msg_iovlen = 1;

    struct sockaddr_in in;
    in.sin_family = AF_INETv4;
    in.sin_addr.s_addr = 0;
    in.sin_port = 0;

    msg.msg_name = &in;
    msg.msg_namelen = sizeof(struct sockaddr_in);

    for(int i = 0; i < 10; i++)
    {
        recvmsg(m_socket, &msg, MSG_ZEROCOPY);
        perror("");

        struct udphdr *udp = (uint8_t*)iov.iov_base + 20;
        uint16_t src = htons(udp->source );
        uint16_t dst = htons(udp->dest);
        uint16_t len = htons(udp->len);
        uint16_t chk = htons(udp->check);

        fprintf(stderr, "src: %hu dst: %hu len: %hu chk: %hun", src, dst, len, chk);
    }

    return 0;
}