Question

Have some problems in receiving packets.
I can receive and read incoming packets, but I think i do not get a handshake with any host.
I only want to send a packet to a remote computer with an open port on receiving an answer to see the TTL(time to live) and the window size.
Does anyone have an idea where the errors are? (I don’t have very deep knowledge in C programming)

CODE:

#include <sys/types.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <errno.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <net/ethernet.h>
#include <netinet/in.h>
#include <netinet/ip.h>
#include <netinet/tcp.h>

struct pseudohdr {
    u_int32_t src_addr;
    u_int32_t dst_addr;
    u_int8_t padding;
    u_int8_t proto;
    u_int16_t length;
};

struct data_4_checksum {
    struct pseudohdr pshd;
    struct tcphdr tcphdr;
    char payload[1024];
};
unsigned short comp_chksum(unsigned short *addr, int len) {
    long sum = 0;

    while (len > 1) {
        sum += *(addr++);
        len -= 2;
    }

    if (len > 0)
        sum += *addr;

    while (sum >> 16)
        sum = ((sum & 0xffff) + (sum >> 16));

    sum = ~sum;

    return ((u_short) sum);

}

int main(int argc, char *argv[]) {

    int sock, bytes, on = 1;
    char buffer[1024];
    struct iphdr *ip;
    struct tcphdr *tcp;
    struct sockaddr_in to;
    struct pseudohdr pseudoheader;
    struct data_4_checksum tcp_chk_construct;

    if (argc != 2) {
        fprintf(stderr, "Usage: %s ", argv[0]);
        fprintf(stderr, "<dest-addr>\n");
        return 1;
    }

    sock = socket(AF_INET, SOCK_RAW, IPPROTO_TCP);
    if (sock == -1) {
        perror("socket() failed");
        return 1;
    }else{
        printf("socket() ok\n");
    }

    if (setsockopt(sock, IPPROTO_IP, IP_HDRINCL, &on, sizeof(on)) == -1) {
        perror("setsockopt() failed");
        return 2;
    }else{
        printf("setsockopt() ok\n");
    }

    ip = (struct iphdr*) buffer;
    tcp = (struct tcphdr*) (buffer + sizeof(struct tcphdr));

    int iphdrlen = sizeof(struct iphdr);
    int tcphdrlen = sizeof(struct tcphdr);
    int datalen = 0;
    printf("Typecasting ok\n");

    ip->frag_off = 0;
    ip->version = 4;
    ip->ihl = 5;
    ip->tot_len = htons(iphdrlen + tcphdrlen);
    ip->id = 0;
    ip->ttl = 40;
    ip->protocol = IPPROTO_TCP;
    ip->saddr = inet_addr("192.168.165.135");
    ip->daddr = inet_addr(argv[1]);
    ip->check = 0;

    tcp->source     = htons(12345);
    tcp->dest       = htons(80);
    tcp->seq        = random();
    tcp->doff       = 5;
    tcp->ack        = 0;
    tcp->psh        = 0;
    tcp->rst        = 0;
    tcp->urg        = 0;
    tcp->syn        = 1;
    tcp->fin        = 0;
    tcp->window     = htons(65535);

    pseudoheader.src_addr = ip->saddr;
    pseudoheader.dst_addr = ip->daddr;
    pseudoheader.padding = 0;
    pseudoheader.proto = ip->protocol;
    pseudoheader.length = htons(tcphdrlen + datalen);

    tcp_chk_construct.pshd = pseudoheader;
    tcp_chk_construct.tcphdr = *tcp;

    int checksum = comp_chksum((unsigned short*) &tcp_chk_construct,
            sizeof(struct pseudohdr) + tcphdrlen + datalen);

    tcp->check = checksum;

    printf("TCP Checksum: %i\n", checksum);
    printf("Destination : %i\n", ntohs(tcp->dest));
    printf("Source: %i\n", ntohs(tcp->source));

    to.sin_addr.s_addr = ip->daddr;
    to.sin_family = AF_INET;
    to.sin_port = tcp->dest;

    bytes = sendto(sock, buffer, ntohs(ip->tot_len), 0, (struct sockaddr*) &to,
            sizeof(to));

    if (bytes == -1) {
        perror("sendto() failed");
        return 1;
    }

    recv(sock, buffer, sizeof(buffer), 0);
    printf("TTL= %d\n", ip->ttl);
    printf("Window= %d\n", tcp->window);
    printf("ACK= %d\n", tcp->ack);
    printf("%s:%d\t --> \t%s:%d \tSeq: %d \tAck: %d\n",
                    inet_ntoa(*(struct in_addr*) &ip->saddr), ntohs(tcp->source),
                    inet_ntoa(*(struct in_addr *) &ip->daddr), ntohs(tcp->dest),
                    ntohl(tcp->seq), ntohl(tcp->ack_seq));

    return 0;
}

Answer 1

1. You’re receiving and storing packets in buffer, but you’re printing data from ip and tcp without parsing that buffer. You should parse the packet from buffer after receiving it, and before printing.
2. Your code assumes all packets are TCP, which is not the case. RAW sockets only support Layer 3 protocols (IP, ICMP, etc). In other words, using IPPROTO_TCP is misleading when creating a RAW socket. Stick to IPPROTO_IP, and add the necessary conditions to your code for each protocol you care about (TCP, UDP, etc). This happens to be working because the Linux Kernel validates the protocol number, and fallbacks to IPPROTO_IP. However, this might not work in other systems.
3. Review if your network communication is using the correct byte-order. The network-byte-order is Big-Endian, while the host-byte-order depends on your architecture, so you may need to convert multi-byte fields back and forth.
4. Your tcp->seq might have an invalid value, because TCP only accepts values up to 65535, while random() returns values from 0 to RAND_MAX (0x7fffffff). Try tcp->seq = htonl(random() % 65535);
5. Your offset calculation for the TCP header is incorrect. It should be sizeof(struct iphdr) rather than sizeof(struct tcphdr).