client.cpp

/*  Copyright (C) 2011-2015  P.D. Buchan ([email protected])

    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>           // close()
#include <string.h>           // strcpy, memset(), and memcpy()

#include <netdb.h>            // struct addrinfo
#include <sys/types.h>        // needed for socket(), uint8_t, uint16_t, uint32_t
#include <sys/socket.h>       // needed for socket()
#include <netinet/in.h>       // IPPROTO_RAW, IPPROTO_IP, IPPROTO_TCP, INET_ADDRSTRLEN
#include <netinet/ip.h>       // struct ip and IP_MAXPACKET (which is 65535)
#include <arpa/inet.h>        // inet_pton() and inet_ntop()
#include <sys/ioctl.h>        // macro ioctl is defined
#include <bits/ioctls.h>      // defines values for argument "request" of ioctl.
#include <net/if.h>           // struct ifreq

#include <errno.h>            // errno, perror()

// Define some constants.
#define IP4_HDRLEN 20         // IPv4 header length

// Function prototypes
uint16_t checksum (uint16_t *, int);
char *allocate_strmem (int);
uint8_t *allocate_ustrmem (int);
int *allocate_intmem (int);

int
main (int argc, char **argv)
{
    int status, sd, *ip_flags;
    const int on = 1;
    char *interface, *target, *src_ip, *dst_ip;
    struct ip iphdr;
    uint8_t *packet;
    struct addrinfo hints, *res;
    struct sockaddr_in *ipv4, sin;
    struct ifreq ifr;
    void *tmp;

    // Allocate memory for various arrays.
    packet = allocate_ustrmem (IP_MAXPACKET);
    interface = allocate_strmem (40);
    target = allocate_strmem (40);
    src_ip = allocate_strmem (INET_ADDRSTRLEN);
    dst_ip = allocate_strmem (INET_ADDRSTRLEN);
    ip_flags = allocate_intmem (4);

    // Interface to send packet through.
    strcpy (interface, "enp2s0");

    // Submit request for a socket descriptor to look up interface.
    if ((sd = socket (AF_INET, SOCK_RAW, IPPROTO_RAW)) < 0) {
        perror ("socket() failed to get socket descriptor for using ioctl() ");
        exit (EXIT_FAILURE);
    }

    // Use ioctl() to look up interface index which we will use to
    // bind socket descriptor sd to specified interface with setsockopt() since
    // none of the other arguments of sendto() specify which interface to use.
    memset (&ifr, 0, sizeof (ifr));
    snprintf (ifr.ifr_name, sizeof (ifr.ifr_name), "%s", interface);
    if (ioctl (sd, SIOCGIFINDEX, &ifr) < 0) {
        perror ("ioctl() failed to find interface ");
        return (EXIT_FAILURE);
    }
    close (sd);
    printf ("Index for interface %s is %i\n", interface, ifr.ifr_ifindex);

    // Source IPv4 address: you need to fill this out
    strcpy (src_ip, "192.168.0.1");

    // Destination URL or IPv4 address: you need to fill this out
    strcpy (target, "192.168.0.168");

    // Fill out hints for getaddrinfo().
    memset (&hints, 0, sizeof (struct addrinfo));
    hints.ai_family = AF_INET;
    hints.ai_socktype = SOCK_RAW;
    hints.ai_flags = hints.ai_flags | AI_CANONNAME;

    // Resolve target using getaddrinfo().
    if ((status = getaddrinfo (target, NULL, &hints, &res)) != 0) {
        fprintf (stderr, "getaddrinfo() failed: %s\n", gai_strerror (status));
        exit (EXIT_FAILURE);
    }
    ipv4 = (struct sockaddr_in *) res->ai_addr;
    tmp = &(ipv4->sin_addr);
    if (inet_ntop (AF_INET, tmp, dst_ip, INET_ADDRSTRLEN) == NULL) {
        status = errno;
        fprintf (stderr, "inet_ntop() failed.\nError message: %s", strerror (status));
        exit (EXIT_FAILURE);
    }
    freeaddrinfo (res);

    // IPv4 header

    // IPv4 header length (4 bits): Number of 32-bit words in header = 5
    iphdr.ip_hl = IP4_HDRLEN / sizeof (uint32_t);

    // Internet Protocol version (4 bits): IPv4
    iphdr.ip_v = 4;

    // Type of service (8 bits)
    iphdr.ip_tos = 0;

    // Total length of datagram (16 bits): IP header
    iphdr.ip_len = htons (IP4_HDRLEN);

    // ID sequence number (16 bits): unused, since single datagram
    iphdr.ip_id = htons (0);

    // Flags, and Fragmentation offset (3, 13 bits): 0 since single datagram

    // Zero (1 bit)
    ip_flags[0] = 0;

    // Do not fragment flag (1 bit)
    ip_flags[1] = 0;

    // More fragments following flag (1 bit)
    ip_flags[2] = 0;

    // Fragmentation offset (13 bits)
    ip_flags[3] = 0;

    iphdr.ip_off = htons ((ip_flags[0] << 15)
                          + (ip_flags[1] << 14)
                          + (ip_flags[2] << 13)
                          +  ip_flags[3]);

    // Time-to-Live (8 bits): default to maximum value
    iphdr.ip_ttl = 255;

    iphdr.ip_p = 200;

    // Source IPv4 address (32 bits)
    if ((status = inet_pton (AF_INET, src_ip, &(iphdr.ip_src))) != 1) {
        fprintf (stderr, "inet_pton() failed.\nError message: %s", strerror (status));
        exit (EXIT_FAILURE);
    }

    // Destination IPv4 address (32 bits)
    if ((status = inet_pton (AF_INET, dst_ip, &(iphdr.ip_dst))) != 1) {
        fprintf (stderr, "inet_pton() failed.\nError message: %s", strerror (status));
        exit (EXIT_FAILURE);
    }

    // IPv4 header checksum (16 bits): set to 0 when calculating checksum
    iphdr.ip_sum = 0;
    iphdr.ip_sum = checksum ((uint16_t *) &iphdr, IP4_HDRLEN);

    // Prepare packet.

    // First part is an IPv4 header.
    memcpy (packet, &iphdr, IP4_HDRLEN * sizeof (uint8_t));

    // The kernel is going to prepare layer 2 information (ethernet frame header) for us.
    // For that, we need to specify a destination for the kernel in order for it
    // to decide where to send the raw datagram. We fill in a struct in_addr with
    // the desired destination IP address, and pass this structure to the sendto() function.
    memset (&sin, 0, sizeof (struct sockaddr_in));
    sin.sin_family = AF_INET;
    sin.sin_addr.s_addr = iphdr.ip_dst.s_addr;

    // Submit request for a raw socket descriptor.
    if ((sd = socket (AF_INET, SOCK_RAW, IPPROTO_RAW)) < 0) {
        perror ("socket() failed ");
        exit (EXIT_FAILURE);
    }

    // Set flag so socket expects us to provide IPv4 header.
    if (setsockopt (sd, IPPROTO_IP, IP_HDRINCL, &on, sizeof (on)) < 0) {
        perror ("setsockopt() failed to set IP_HDRINCL ");
        exit (EXIT_FAILURE);
    }

    // Bind socket to interface index.
    if (setsockopt (sd, SOL_SOCKET, SO_BINDTODEVICE, &ifr, sizeof (ifr)) < 0) {
        perror ("setsockopt() failed to bind to interface ");
        exit (EXIT_FAILURE);
    }

    // Send packet.
    if (sendto (sd, packet, IP4_HDRLEN, 0, (struct sockaddr *) &sin, sizeof (struct sockaddr)) < 0)  {
        perror ("sendto() failed ");
        exit (EXIT_FAILURE);
     }

    printf("packet sent\n");

    // Close socket descriptor.
    close (sd);

    // Free allocated memory.
    free (packet);
    free (interface);
    free (target);
    free (src_ip);
    free (dst_ip);
    free (ip_flags);

    return (EXIT_SUCCESS);
}

// Computing the internet checksum (RFC 1071).
uint16_t
checksum (uint16_t *addr, int len)
{
    int count = len;
    register uint32_t sum = 0;
    uint16_t answer = 0;

    // Sum up 2-byte values until none or only one byte left.
    while (count > 1) {
        sum += *(addr++);
        count -= 2;
    }

    // Add left-over byte, if any.
    if (count > 0) {
        sum += *(uint8_t *) addr;
    }

    // Fold 32-bit sum into 16 bits; we lose information by doing this,
    // increasing the chances of a collision.
    // sum = (lower 16 bits) + (upper 16 bits shifted right 16 bits)
    while (sum >> 16) {
        sum = (sum & 0xffff) + (sum >> 16);
    }

    // Checksum is one's compliment of sum.
    answer = ~sum;

    return (answer);
}

// Allocate memory for an array of chars.
char *
allocate_strmem (int len)
{
    char *tmp;

    if (len <= 0) {
        fprintf (stderr, "ERROR: Cannot allocate memory because len = %i in allocate_strmem().\n", len);
        exit (EXIT_FAILURE);
    }

    tmp = (char *) malloc (len * sizeof (char));
    if (tmp != NULL) {
        memset (tmp, 0, len * sizeof (char));
        return (tmp);
    } else {
        fprintf (stderr, "ERROR: Cannot allocate memory for array allocate_strmem().\n");
        exit (EXIT_FAILURE);
    }
}

// Allocate memory for an array of unsigned chars.
uint8_t *
allocate_ustrmem (int len)
{
    uint8_t *tmp;

    if (len <= 0) {
        fprintf (stderr, "ERROR: Cannot allocate memory because len = %i in allocate_ustrmem().\n", len);
        exit (EXIT_FAILURE);
    }

    tmp = (uint8_t *) malloc (len * sizeof (uint8_t));
    if (tmp != NULL) {
        memset (tmp, 0, len * sizeof (uint8_t));
        return (tmp);
    } else {
        fprintf (stderr, "ERROR: Cannot allocate memory for array allocate_ustrmem().\n");
        exit (EXIT_FAILURE);
    }
}

// Allocate memory for an array of ints.
int *
allocate_intmem (int len)
{
    int *tmp;

    if (len <= 0) {
        fprintf (stderr, "ERROR: Cannot allocate memory because len = %i in allocate_intmem().\n", len);
        exit (EXIT_FAILURE);
    }

    tmp = (int *) malloc (len * sizeof (int));
    if (tmp != NULL) {
        memset (tmp, 0, len * sizeof (int));
        return (tmp);
    } else {
        fprintf (stderr, "ERROR: Cannot allocate memory for array allocate_intmem().\n");
        exit (EXIT_FAILURE);
    }
}