制作 TCP/IP 数据包

Question

我正在学习使用 C/C++ 进行套接字编程，我认为最好的方法是深入研究它。我可以使用 socket.h send() 将数据发送到套接字，因此我想通过制作网络数据包来更深入。

我尝试了但仍然无法确定我的数据的哪一部分是无效的，因为我得到了 Invalid argument errno 22。这是我的十六进制 IP 标头：

45 00 28 00 
d4 31 00 00 
ff 06 3c 6e 
c0 a8 01 06 
c0 a8 01 01

这是我的 TCP 标头：

00 50 00 50 
00 00 00 00 
00 00 00 00 
50 02 16 d0 
15 1b 00 00 

感谢任何提示。

注意：我正在阅读 beej.us 和 here 以供学习。

编辑：这是我的代码：

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;
};

int main(int argc, char* argv[]) {
    int sockfd = socket (PF_INET, SOCK_RAW, IPPROTO_TCP);
    if (sockfd == -1) {
        perror("Failed to create socket");
        exit(1);
    }

    // Datagram to represent the packet
    char datagram[4096];
    memset(datagram, 0, 4096); // zero out the packet buffer

    //Data part
    char *data = datagram + sizeof(struct ip) + sizeof(struct tcphdr);
    strcpy(data, "");

    // some address resolution
    char source_ip[32];
    strcpy(source_ip, "192.168.1.6");
    struct sockaddr_in sai;
    sai.sin_family = AF_INET;
    sai.sin_port = htons(80);

    sai.sin_addr.s_addr = inet_addr("192.168.1.1");
    cout << "sai.sin_addr.s_addr=" << sai.sin_addr.s_addr << endl;

    //Fill in the IP Header
    struct ip *iph = (struct ip *) datagram;
    iph->ip_hl = 5;
    iph->ip_v = 4;
    iph->ip_tos = 0;
    iph->ip_len = sizeof(struct ip) + sizeof(struct tcphdr) + strlen(data);
    iph->ip_id = htons(54321);
    iph->ip_off = 0;
    iph->ip_ttl = 255;
    iph->ip_p = IPPROTO_TCP;
    iph->ip_sum = 0;
    iph->ip_src.s_addr = inet_addr(source_ip);
    iph->ip_dst.s_addr = sai.sin_addr.s_addr;

    //Ip checksum
    unsigned short checksum = csum((unsigned short *) datagram, iph->ip_len);
    iph->ip_sum = checksum;
    cout << "iph->ip_sum=" << checksum << endl;

    unsigned char *pIph = (unsigned char *) datagram;
    for (int i = 0; i < 20; i++) {
        cout << setfill('0') << setw(2) << hex << (int) pIph[i] << " ";
        if (i + 1 >= 4 && (i + 1) % 4 == 0) {
            cout << endl;
        }
    }

    //TCP Header
    struct tcphdr *tcph = (struct tcphdr *) (datagram + sizeof(struct ip));
    struct pseudo_header psh;
    tcph->th_sport = htons(80);
    tcph->th_dport = htons(80);
    tcph->th_seq = 0;
    tcph->th_ack = 0;
    tcph->th_off = 5;
    tcph->th_flags = TH_SYN;
    tcph->th_win  = htons(5840); /* maximum allowed window size */
    tcph->th_sum = 0;
    tcph->th_urp = 0;

    //Now the TCP checksum
    psh.source_address = inet_addr(source_ip);
    psh.dest_address = sai.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);

    char *pseudogram = malloc(psize);

    memcpy(pseudogram, (char*) &psh, sizeof(struct pseudo_header));
    memcpy(pseudogram + sizeof(struct pseudo_header), tcph, sizeof(struct tcphdr) + strlen(data));

    checksum = csum((unsigned short*) pseudogram, psize);
    tcph->th_sum = checksum;
    cout << "tcph->th_sum=" << checksum << endl;

    unsigned char *pTcph = (unsigned char *) tcph;
    for (int i = 0; i < 20; i++) {
        cout << setfill('0') << setw(2) << hex << (int) pTcph[i] << " ";
        if (i + 1 >= 4 && (i + 1) % 4 == 0) {
            cout << endl;
        }
    }

    //IP_HDRINCL to tell the kernel that headers are included in the packet
    int one = 1;
    const int *val = &one;
    if (setsockopt(sockfd, IPPROTO_IP, IP_HDRINCL, val, sizeof(one)) < 0) {
        perror("Error setting IP_HDRINCL");
        exit(0);
    }

    struct sockaddr *pSa = (struct sockaddr *) &sai;

    // Send the packet
    if (sendto(sockfd, datagram, iph->ip_len, 0, pSa, sizeof(sai)) < 0) { // failed here
        perror("sendto failed");

    } else { //Data send successfully
        printf("Packet Send. Length : %d \n", iph->ip_len);
    }

    return 1;
}

Answer 1

在您的 IPv4 标头中：

45 00[28 00] 
d4 31 00 00 
ff 06 3c 6e
c0 a8 01 06 
c0 a8 01 01

你的数据包长度是 10240 (0x2800) 吗？

你确定不是 40 (0x0028)？

45 00[00 28] 
d4 31 00 00 
ff 06[64 46] // checksum updated
c0 a8 01 06 
c0 a8 01 01

编辑：既然您已经发布了代码……

你应该替换：

iph->ip_len = sizeof(struct ip) + sizeof(struct tcphdr) + strlen(data);

作者：

iph->ip_len = htons(sizeof(struct ip) + sizeof(struct tcphdr) + strlen(data));