网络层（L3）原始套接字，IPv4，UDP发送数据包

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netinet/ip.h>
#include <netinet/udp.h>
#include <arpa/inet.h>
#include <net/if.h>

// 计算校验和
unsigned short
checksum(unsigned short* addr, int len)
{
    int nleft = len;
    int sum = 0;
    unsigned short* w = addr;
    unsigned 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;
}

// 计算UDP校验和
unsigned short
udp_checksum(struct iphdr* ip, struct udphdr* udp, char* data, int data_len)
{
    struct pseudo_header
    {
        u_int32_t source_address;
        u_int32_t dest_address;
        u_int8_t placeholder;
        u_int8_t protocol;
        u_int16_t udp_length;
    } pseudo_header;

    pseudo_header.source_address = ip->saddr;
    pseudo_header.dest_address = ip->daddr;
    pseudo_header.placeholder = 0;
    pseudo_header.protocol = IPPROTO_UDP;
    pseudo_header.udp_length = htons(sizeof(struct udphdr) + data_len);

    int psize = sizeof(struct pseudo_header) + sizeof(struct udphdr) + data_len;
    char* pseudogram = malloc(psize);

    memcpy(pseudogram, &pseudo_header, sizeof(struct pseudo_header));
    memcpy(pseudogram + sizeof(struct pseudo_header), udp, sizeof(struct udphdr));
    memcpy(pseudogram + sizeof(struct pseudo_header) + sizeof(struct udphdr), data, data_len);

    unsigned short csum = checksum((unsigned short*)pseudogram, psize);
    free(pseudogram);
    return csum;
}

int
main(int argc, char* argv[])
{
    if (argc < 2) {
        fprintf(stderr, "Usage: %s <message>\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    // 要发送的消息
    char* message = argv[1];
    int msg_len = strlen(message);

    // 创建原始套接字
    int sockfd = socket(AF_INET, SOCK_RAW, IPPROTO_UDP);
    if (sockfd < 0) {
        perror("socket");
        exit(EXIT_FAILURE);
    }

    // 设置IP_HDRINCL选项，我们自己构建IP头部
    int one = 1;
    if (setsockopt(sockfd, IPPROTO_IP, IP_HDRINCL, &one, sizeof(one)) < 0) {
        perror("setsockopt IP_HDRINCL");
        close(sockfd);
        exit(EXIT_FAILURE);
    }

    // 设置SO_BINDTODEVICE选项，绑定到ens37接口
    if (setsockopt(sockfd, SOL_SOCKET, SO_BINDTODEVICE, "ens33", strlen("ens33")) < 0) {
        perror("setsockopt SO_BINDTODEVICE");
        close(sockfd);
        exit(EXIT_FAILURE);
    }

    // 准备目标地址结构
    struct sockaddr_in dest_addr;
    memset(&dest_addr, 0, sizeof(dest_addr));
    dest_addr.sin_family = AF_INET;
    dest_addr.sin_port = htons(12345); // 目标端口
    if (inet_pton(AF_INET, "192.168.243.1", &dest_addr.sin_addr) <= 0) {
        perror("inet_pton dest_addr");
        close(sockfd);
        exit(EXIT_FAILURE);
    }

    // 准备IP头部
    struct iphdr* ip = (struct iphdr*)malloc(sizeof(struct iphdr));
    memset(ip, 0, sizeof(struct iphdr));
    ip->ihl = 5;                                                                 // IP头部长度（5 * 4 = 20字节）
    ip->version = 4;                                                             // IPv4
    ip->tos = 0;                                                                 // 服务类型
    ip->tot_len = htons(sizeof(struct iphdr) + sizeof(struct udphdr) + msg_len); // 总长度
    ip->id = htonl(54321);                                                       // 标识符
    ip->frag_off = 0;                                                            // 分片偏移
    ip->ttl = 64;                                                                // 生存时间
    ip->protocol = IPPROTO_UDP;                                                  // 上层协议为UDP
    ip->check = 0;                                                               // 先设为0，后面计算校验和

    // 设置源IP地址
    if (inet_pton(AF_INET, "10.88.243.156", &ip->saddr) <= 0) {
        perror("inet_pton src_ip");
        free(ip);
        close(sockfd);
        exit(EXIT_FAILURE);
    }

    // 设置目标IP地址
    if (inet_pton(AF_INET, "192.168.243.1", &ip->daddr) <= 0) {
        perror("inet_pton dest_ip");
        free(ip);
        close(sockfd);
        exit(EXIT_FAILURE);
    }

    // 计算IP校验和
    ip->check = checksum((unsigned short*)ip, sizeof(struct iphdr));

    // 准备UDP头部
    struct udphdr* udp = (struct udphdr*)malloc(sizeof(struct udphdr));
    memset(udp, 0, sizeof(struct udphdr));
    udp->source = htons(54321); // 源端口
    udp->dest = htons(12345);   // 目标端口
    udp->len = htons(sizeof(struct udphdr) + msg_len);
    udp->check = 0; // 先设为0，后面计算校验和

    // 计算UDP校验和
    udp->check = udp_checksum(ip, udp, message, msg_len);
    if (udp->check == 0) {
        udp->check = 0xffff; // RFC 768规定，校验和为0时用0xffff代替
    }

    // 构建完整的数据包
    int packet_size = sizeof(struct iphdr) + sizeof(struct udphdr) + msg_len;
    char* packet = malloc(packet_size);
    memset(packet, 0, packet_size);

    // 复制各部分到数据包
    memcpy(packet, ip, sizeof(struct iphdr));
    memcpy(packet + sizeof(struct iphdr), udp, sizeof(struct udphdr));
    memcpy(packet + sizeof(struct iphdr) + sizeof(struct udphdr), message, msg_len);

    // 发送数据包
    if (sendto(sockfd, packet, packet_size, 0, (struct sockaddr*)&dest_addr, sizeof(dest_addr)) < 0) {
        perror("sendto");
        free(ip);
        free(udp);
        free(packet);
        close(sockfd);
        exit(EXIT_FAILURE);
    }

    printf("Sent IPv4 UDP packet from 192.168.243.2:54321 to 192.168.243.1:12345 via ens37: %s\n", message);

    // 清理资源
    free(ip);
    free(udp);
    free(packet);
    close(sockfd);

    return 0;
}