/*  Copyright (C) 2011-2026  P.D. Buchan (pdbuchan@gmail.com)

    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/>.
*/

// Send an IPv4 UDP packet via raw socket at the link layer (Ethernet frame).
// Need to have destination MAC address.
// Includes some UDP data.

#define _GNU_SOURCE           // Sometimes required for GNU/Linux-specific interfaces. e.g., SO_BINDTODEVICE
#define __FAVOR_BSD           // Use BSD-style networking structures. e.g., struct tcphdr
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>           // close()
#include <string.h>           // memset(), memcpy()
#include <stdint.h>           // uint8_t, uint16_t, uint32_t

#include <netdb.h>            // struct addrinfo
#include <sys/socket.h>       // socket()
#include <netinet/in.h>       // IPPROTO_UDP, INET_ADDRSTRLEN
#include <netinet/ip.h>       // struct ip, IP_MAXPACKET (which is 65535)
#include <netinet/udp.h>      // struct udphdr
#include <arpa/inet.h>        // inet_pton(), inet_ntop()
#include <sys/ioctl.h>        // macro ioctl is defined
#include <net/if.h>           // struct ifreq
#include <linux/if_ether.h>   // ETH_HLEN, ETH_P_IP, ETH_P_ALL
#include <linux/if_packet.h>  // struct sockaddr_ll (see man 7 packet)
#include <time.h>             // time()

#include <errno.h>            // errno

// Define some constants.
#define ETH_HDRLEN ETH_HLEN   // Ethernet header length
#define MAC_LEN 6             // Length of a hardware (MAC) address
#define IP4_HDRLEN 20         // IPv4 header length
#define UDP_HDRLEN  8         // UDP header length, excludes data
#define HOSTNAME_LEN 255      // Maximum FQDN length including terminating null byte

// Function prototypes
uint16_t checksum (uint8_t *, int);
uint16_t udp4_checksum (struct ip, struct udphdr, uint8_t *, int);
char *allocate_strmem (int);
uint8_t *allocate_ustrmem (int);

int
main (void) {

  int i, n, status, udp_datalen, sd, ip_flags[4] = {0}, frame_length;
  ssize_t bytes;
  char *interface, *target, *src_ip, *dst_ip;
  struct ip iphdr;
  struct udphdr udphdr;
  uint8_t src_mac[MAC_LEN] = {0}, *ether_frame;
  struct addrinfo hints, *res;
  struct sockaddr_in dst;
  struct sockaddr_ll device;
  struct ifreq ifr;

  memset (&iphdr, 0, sizeof (iphdr));
  memset (&udphdr, 0, sizeof (udphdr));

  // Allocate memory for various arrays.
  ether_frame = allocate_ustrmem (ETH_HDRLEN + IP_MAXPACKET);
  interface = allocate_strmem (sizeof (ifr.ifr_name));
  target = allocate_strmem (HOSTNAME_LEN);
  src_ip = allocate_strmem (INET_ADDRSTRLEN);
  dst_ip = allocate_strmem (INET_ADDRSTRLEN);

  // Random number seed
  srand ((unsigned) time (NULL));

  // Interface to send packet through.
  snprintf (interface, sizeof (ifr.ifr_name), "enp7s0");

  // Submit request for a socket descriptor to look up interface.
  if ((sd = socket (AF_INET, SOCK_DGRAM, 0)) < 0) {
    status = errno;
    fprintf (stderr, "socket() failed to get socket descriptor for using ioctl().\nError message: %s\n", strerror (status));
    exit (EXIT_FAILURE);
  }

  // Use ioctl() to look up interface name and get its MAC address.
  memset (&ifr, 0, sizeof (ifr));
  n = snprintf (ifr.ifr_name, sizeof (ifr.ifr_name), "%s", interface);
  if ((n < 0) || (n >= (int) sizeof (ifr.ifr_name))) {
    fprintf (stderr, "Invalid interface name: %s\n", interface);
    exit (EXIT_FAILURE);
  }
  if (ioctl (sd, SIOCGIFHWADDR, &ifr) < 0) {
    fprintf (stderr, "ioctl(SIOCGIFHWADDR) failed to get source MAC address.\nError message: %s\n", strerror (errno));
    close (sd);
    exit (EXIT_FAILURE);
  }
  close (sd);

  // Copy source MAC address.
  memcpy (src_mac, ifr.ifr_hwaddr.sa_data, sizeof (src_mac));

  // Report source MAC address to stdout.
  fprintf (stdout, "MAC address for interface %s is ", interface);
  for (i = 0; i < (int) sizeof (src_mac); i++) {
    fprintf (stdout, "%02x%s", src_mac[i], (i < (int) sizeof (src_mac) - 1) ? ":" : "\n");
  }

  // Destination Ethernet MAC address: You need to fill these out.
  // For off-link destinations, this is normally the next-hop router's MAC address.
  uint8_t dst_mac[MAC_LEN] = {0x02, 0x00, 0x00, 0x00, 0x00, 0x01};

  // Source IPv4 address: You need to fill this out.
  snprintf (src_ip, INET_ADDRSTRLEN, "192.168.0.9");

  // Destination hostname or IPv4 address: You need to fill this out.
  snprintf (target, HOSTNAME_LEN, "www.google.com");

  // Fill out hints for getaddrinfo().
  memset (&hints, 0, sizeof (hints));
  hints.ai_family = AF_INET;
  hints.ai_socktype = 0;  // Address resolution only; any socket type.
  hints.ai_flags = hints.ai_flags | AI_CANONNAME;

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

  // Fill out device's sockaddr_ll struct.
  memset (&device, 0, sizeof (device));
  device.sll_family = AF_PACKET;
  device.sll_protocol = htons (ETH_P_IP);
  if ((device.sll_ifindex = if_nametoindex (interface)) == 0) {
    status = errno;
    fprintf (stderr, "if_nametoindex(\"%s\") failed to obtain interface index.\nError message: %s\n", interface, strerror (status));
    exit (EXIT_FAILURE);
  }
  fprintf (stdout, "Index for interface %s is %d\n", interface, device.sll_ifindex);
  memcpy (device.sll_addr, dst_mac, sizeof (dst_mac));
  device.sll_halen = sizeof (dst_mac);

  // UDP data
  uint8_t udp_data[] = {'T', 'e', 's', 't'};
  udp_datalen = sizeof (udp_data);

  // 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 + UDP header + UDP data
  iphdr.ip_len = htons (IP4_HDRLEN + UDP_HDRLEN + udp_datalen);

  // IPv4 Identification field (16 bits)
  iphdr.ip_id = htons ((uint16_t) (rand () & 0xffff));

  // 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;

  // Transport layer protocol (8 bits): 17 for UDP
  iphdr.ip_p = IPPROTO_UDP;

  // Source IPv4 address (32 bits)
  if ((status = inet_pton (AF_INET, src_ip, &(iphdr.ip_src))) != 1) {
    if (status == 0) {
      fprintf (stderr, "inet_pton() failed for source address.\nError message: Invalid address\n");
    } else if (status < 0) {
      fprintf (stderr, "inet_pton() failed for source address.\nError message: %s\n", strerror (errno));
    }
    exit (EXIT_FAILURE);
  }

  // Destination IPv4 address (32 bits)
  if ((status = inet_pton (AF_INET, dst_ip, &(iphdr.ip_dst))) != 1) {
    if (status == 0) {
      fprintf (stderr, "inet_pton() failed for destination address.\nError message: Invalid address\n");
    } else if (status < 0) {
      fprintf (stderr, "inet_pton() failed for destination address.\nError message: %s\n", strerror (errno));
    }
    exit (EXIT_FAILURE);
  }

  // IPv4 header checksum (16 bits): Set to 0 when calculating checksum.
  iphdr.ip_sum = 0;
  iphdr.ip_sum = checksum ((uint8_t *) &iphdr, IP4_HDRLEN);

  // UDP header

  // Source port number (16 bits): Pick a number.
  udphdr.source = htons (4950);

  // Destination port number (16 bits): Pick a number.
  udphdr.dest = htons (4950);

  // Length of UDP datagram (16 bits): UDP header + UDP data
  udphdr.len = htons (UDP_HDRLEN + udp_datalen);

  // UDP checksum (16 bits): Set to 0 for checksum calculation.
  udphdr.check = 0;
  udphdr.check = udp4_checksum (iphdr, udphdr, udp_data, udp_datalen);

  // Fill out Ethernet frame header.

  // Ethernet frame length = Ethernet header (MAC + MAC + Ethernet type) + Ethernet data (IP header + UDP header + UDP data)
  frame_length = ETH_HDRLEN + IP4_HDRLEN + UDP_HDRLEN + udp_datalen;

  // Destination and Source MAC addresses
  memcpy (ether_frame, dst_mac, sizeof (dst_mac));
  memcpy (ether_frame + sizeof (dst_mac), src_mac, sizeof (src_mac));

  // EtherType (16 bits): ETH_P_IP
  // http://www.iana.org/assignments/ethernet-numbers
  ether_frame[12] = ETH_P_IP / 256;
  ether_frame[13] = ETH_P_IP % 256;

  // Next is Ethernet frame data (IPv4 header + UDP header + UDP data).

  // IPv4 header
  memcpy (ether_frame + ETH_HDRLEN, &iphdr, IP4_HDRLEN);

  // UDP header
  memcpy (ether_frame + ETH_HDRLEN + IP4_HDRLEN, &udphdr, UDP_HDRLEN);

  // UDP data
  memcpy (ether_frame + ETH_HDRLEN + IP4_HDRLEN + UDP_HDRLEN, udp_data, udp_datalen);

  // Submit request for a raw socket descriptor.
  if ((sd = socket (PF_PACKET, SOCK_RAW, htons (ETH_P_ALL))) < 0) {
    status = errno;
    fprintf (stderr, "socket() failed to get socket descriptor.\nError message: %s\n", strerror (status));
    exit (EXIT_FAILURE);
  }

  // Send Ethernet frame to socket.
  bytes = sendto (sd, ether_frame, frame_length, 0, (struct sockaddr *) &device, sizeof (device));
  if (bytes == -1) {
    status = errno;
    fprintf (stderr, "sendto() failed.\nError message: %s\n", strerror (status));
    exit (EXIT_FAILURE);
  }
  // Check for short send.
  if (bytes != frame_length) {
    fprintf (stderr, "sendto() sent %zd bytes but expected to send %d bytes.\n", bytes, frame_length);
    exit (EXIT_FAILURE);
  }

  // Close socket descriptor.
  close (sd);

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

  return (EXIT_SUCCESS);
}

// Computing the internet checksum (RFC 1071).
// Note that the internet checksum is not guaranteed to preclude collisions.
uint16_t
checksum (uint8_t *addr, int len) {

  int count = len;
  uint32_t sum = 0;
  uint16_t answer = 0;

  // Sum up 2-byte values until none or only one byte left.
  while (count > 1) {
    sum += ((uint16_t) addr[0] << 8) + addr[1];
    addr += 2;
    count -= 2;
  }

  // Add left-over byte, if any. For an odd-length buffer, the
  // remaining byte is the high-order byte of the final 16-bit word.
  if (count > 0) {
    sum += ((uint16_t) addr[0] << 8);
  }

  // Fold the accumulated sum into 16 bits by repeatedly adding
  // carries back into the low 16 bits (one's-complement arithmetic).
  // sum = (lower 16 bits) + (upper 16 bits shifted right 16 bits)
  while (sum >> 16) {
    sum = (sum & 0xffff) + (sum >> 16);
  }

  // Checksum is one's complement of sum. Return it in network byte order
  // so it can be copied directly into the packet header.
  answer = ~sum;

  return (htons (answer));
}

// Build IPv4 UDP pseudo-header and call checksum function.
uint16_t
udp4_checksum (struct ip iphdr, struct udphdr udphdr, uint8_t *udp_data, int udp_datalen) {

  int udp_datagram_len, chksumlen = 0;
  uint8_t *buf, *ptr;
  uint16_t answer = 0;

  if (udp_datalen < 0) {
    fprintf (stderr, "ERROR: udp_datalen must not be negative in udp4_checksum().\n");
    exit (EXIT_FAILURE);
  }
  if ((udp_datalen > 0) && (udp_data == NULL)) {
    fprintf (stderr, "ERROR: udp_data is NULL but udp_datalen > 0 in udp4_checksum().\n");
    exit (EXIT_FAILURE);
  }

  udp_datagram_len = UDP_HDRLEN + udp_datalen;

  // Allocate memory for buffer.
  buf = allocate_ustrmem (12 + udp_datagram_len + 1);  // Add 1 for possible padding.
  ptr = &buf[0];  // ptr points to beginning of buffer buf

  // Copy source IP address into buf (32 bits)
  memcpy (ptr, &iphdr.ip_src.s_addr, sizeof (iphdr.ip_src.s_addr));
  ptr += sizeof (iphdr.ip_src.s_addr);
  chksumlen += sizeof (iphdr.ip_src.s_addr);

  // Copy destination IP address into buf (32 bits)
  memcpy (ptr, &iphdr.ip_dst.s_addr, sizeof (iphdr.ip_dst.s_addr));
  ptr += sizeof (iphdr.ip_dst.s_addr);
  chksumlen += sizeof (iphdr.ip_dst.s_addr);

  // Copy zero field to buf (8 bits)
  *ptr = 0; ptr++;
  chksumlen++;

  // Copy transport layer protocol to buf (8 bits)
  memcpy (ptr, &iphdr.ip_p, sizeof (iphdr.ip_p));
  ptr += sizeof (iphdr.ip_p);
  chksumlen += sizeof (iphdr.ip_p);

  // Copy UDP length to buf (16 bits)
  memcpy (ptr, &udphdr.uh_ulen, sizeof (udphdr.uh_ulen));
  ptr += sizeof (udphdr.uh_ulen);
  chksumlen += sizeof (udphdr.uh_ulen);

  // Copy UDP source port to buf (16 bits)
  memcpy (ptr, &udphdr.uh_sport, sizeof (udphdr.uh_sport));
  ptr += sizeof (udphdr.uh_sport);
  chksumlen += sizeof (udphdr.uh_sport);

  // Copy UDP destination port to buf (16 bits)
  memcpy (ptr, &udphdr.uh_dport, sizeof (udphdr.uh_dport));
  ptr += sizeof (udphdr.uh_dport);
  chksumlen += sizeof (udphdr.uh_dport);

  // Copy UDP length again to buf (16 bits)
  memcpy (ptr, &udphdr.uh_ulen, sizeof (udphdr.uh_ulen));
  ptr += sizeof (udphdr.uh_ulen);
  chksumlen += sizeof (udphdr.uh_ulen);

  // Copy UDP checksum to buf (16 bits)
  // Zero, since we don't know it yet
  *ptr = 0; ptr++;
  *ptr = 0; ptr++;
  chksumlen += 2;

  // Copy UDP data to buf, if any.
  if (udp_datalen > 0) {
    memcpy (ptr, udp_data, udp_datalen);
    ptr += udp_datalen;
    chksumlen += udp_datalen;
  }

  // Pad to the next 16-bit boundary. The padding byte is used only for
  // checksum calculation and is not part of the UDP datagram length.
  if ((udp_datagram_len % 2) != 0) {
    *ptr = 0;
    chksumlen++;
  }

  answer = checksum ((uint8_t *) buf, chksumlen);

  // Free allocated memory.
  free (buf);

  return (answer);
}

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

  void *tmp;

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

  tmp = calloc (len, sizeof (char));
  if (tmp != NULL) {
    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) {

  void *tmp;

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

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