无法验证 TCP 校验和

Question

I have been using raw sockets to create custom TCP packets in C. To verify, I sent them to the loopback interface, and when I checked the received packets using TCPDUMP, the checksum did not match for the TCP packet. Here are the fields in the TCP header:

        tcp->th_sport = temp_port;      // The TCP structure. The source port, spoofed, we accept through the command line
        tcp->th_dport = atoi(argv[2]);  // The destination port, we accept through command line
        tcp->th_seq = htonl(random_id()%1000);
        tcp->th_ack = htonl(random_id()%1000);
        tcp->th_off = 5;
        tcp->th_flags = TH_SYN;
        tcp->th_win = 10000;
        tcp->th_sum = 0;
        tcp->th_urp = 0;
        
        //pseudo header for TCP checksum calculation
        p_hdr->source = t1->s_addr;
        p_hdr->dest = t2->s_addr;
        p_hdr->reserved = 0;
        p_hdr->protocol = IPPROTO_TCP;  //TCP
        p_hdr->tcp_size = sizeof(struct tcphdr);
        memcpy(buffer2 + sizeof(struct pseudo_tcp_header) , tcp , sizeof(struct tcphdr) );
        tcp->th_sum = htons(csum((unsigned short *) (buffer2 ), sizeof(struct tcphdr) + sizeof(struct pseudo_tcp_header)));

This is the random_id function:

int random_id()
{
    int lower = 1, upper = 65535,number;
    number = (rand() % (upper - lower + 1)) + lower;
    return number;
}

And the checksum is computed by the function,

unsigned short csum(unsigned short *buf, int len)
{
    unsigned long sum;
    for(sum=0; len>0; len--)
        sum += *buf++;
    sum = (sum >> 16) + (sum &0xffff);
    sum += (sum >> 16);
    return (unsigned short)(~sum);
}

Is there a default function to compute the tcpchecksum in C?

Answer 1

I understand that there is no default function for calculating the ip / tcp checksum. It can be found by the following code:

unsigned short csum(unsigned short *buf, int len)
{
    unsigned long sum;
    for(sum=0; len>0; len--)
        sum += *buf++;
    while (sum>>16)
        sum = (sum >> 16) + (sum &0xffff);
    return (unsigned short)(~sum);
}

Where the input, buf if the data with the pseudo header.

This calculation is only required if the data packet only traverses through the localhost . If it passes the network card to an external network, leave the sum field blank. The network card will automatically compute the header value.