自己构造IP报文

构造IP报文需要了解IP报文的格式和字段含义，以下是一个简单的示例：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>

#define IP_VERSION 4
#define IP_HEADER_LENGTH 20
#define IP_TTL 64
#define IP_PROTOCOL_TCP 6

struct ip_header {
    uint8_t version_and_header_length;
    uint8_t type_of_service;
    uint16_t total_length;
    uint16_t identification;
    uint16_t flags_and_fragment_offset;
    uint8_t time_to_live;
    uint8_t protocol;
    uint16_t header_checksum;
    uint32_t source_address;
    uint32_t destination_address;
};

int main() {
    char packet[IP_HEADER_LENGTH + 20]; // IP头部长度为20字节，加上TCP报文长度，总长度为40字节
    struct ip_header *ip = (struct ip_header *) packet;
    memset(packet, 0, sizeof(packet)); // 初始化为0

    // 设置IP头部字段
    ip->version_and_header_length = (IP_VERSION << 4) | (IP_HEADER_LENGTH / 4);
    ip->type_of_service = 0;
    ip->total_length = htons(IP_HEADER_LENGTH + 20); // IP头部长度+TCP报文长度
    ip->identification = htons(1234); // 标识符
    ip->flags_and_fragment_offset = htons(0x4000); // 不分片
    ip->time_to_live = IP_TTL;
    ip->protocol = IP_PROTOCOL_TCP;
    ip->header_checksum = 0;
    ip->source_address = inet_addr("192.168.0.1"); // 源IP地址
    ip->destination_address = inet_addr("192.168.0.2"); // 目的IP地址

    // 计算IP头部校验和
    uint32_t sum = 0;
    uint16_t *p = (uint16_t *)packet;
    for (int i = 0; i < IP_HEADER_LENGTH / 2; i++) {
        sum += *p++;
    }
    sum = (sum >> 16) + (sum & 0xffff);
    sum += (sum >> 16);
    ip->header_checksum = htons(~sum);

    // 发送IP报文
    int sockfd = socket(AF_INET, SOCK_RAW, IPPROTO_RAW);
    struct sockaddr_in destination;
    destination.sin_family = AF_INET;
    destination.sin_addr.s_addr = ip->destination_address;
    sendto(sockfd, packet, IP_HEADER_LENGTH + 20, 0, (struct sockaddr *)&destination, sizeof(destination));
    close(sockfd);

    return 0;
}

该示例构造了一个基本的IP报文，设置了源IP地址、目的IP地址、标识符、TTL等字段，然后计算了IP头部的校验和，并将报文发送给目的IP地址。需要注意的是，该示例需要在root权限下执行，因为发送IP报文需要特权。