#include <string.h>
#include <unistd.h>
#include <netdb.h>
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <sys/uio.h>
#include <arpa/inet.h>

#include <netinet/udp.h>


int main(int argc, char **argv)
{
  int ret;

  if (argc < 2) {
    fprintf(stderr, "Usage: csumtest <bytes-in-bad-packet>\n");
    exit(1);
  }

  struct sockaddr_in addr;
  socklen_t addrLen = sizeof(addr);  
  addr.sin_family = AF_INET;
  addr.sin_addr.s_addr = inet_addr("127.0.0.1");
  addr.sin_port = htons(0);

  int listen = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
  if (listen < 0)
  {
    fprintf(stderr, "failed to create listening socket (err=%d: %s)\n", errno, strerror(errno));
    exit(1);
  }

  ret = bind(listen, (struct sockaddr *) &addr, addrLen);
  if (ret != 0)
  {
    fprintf(stderr, "failed to bind listening socket socket (err=%d: %s)\n", errno, strerror(errno));
    exit(1);
  }

  ret = getsockname(listen, (struct sockaddr *) &addr, &addrLen);
  if (ret != 0)
  {
    fprintf(stderr, "getsockname failed for listening socket socket (err=%d: %s)\n", errno, strerror(errno));
    exit(1);
  }
  else
  {
    printf("listening on port %d\n", ntohs(addr.sin_port));
  }


  
  // Send a packet with deliberately bad checksum
  char rawBuf[4096];
  memset(rawBuf, 0, 4096);
  struct udphdr *udph = (struct udphdr *) rawBuf;

  // Send as UDP data the string "BAD DATA", repeated as necessary
  // to fill the number of bytes requested on the command-line
  char *data = rawBuf + sizeof(struct udphdr);
  int badBytes = atoi(argv[1]);
  int i;
  for (i = 0; i < badBytes; i += 8)
  {
    strncpy(data + i, "BAD DATA", 8);
  }

  // UDP headers contain a bogus checksum, but are otherwise reasonable
  udph->check = htons(0xbadd);
  udph->source = htons(18);
  udph->dest = addr.sin_port;
  udph->len = htons(sizeof(struct udphdr) + badBytes);

  int raw = socket(AF_INET, SOCK_RAW, IPPROTO_UDP);
  ret = sendto(raw, rawBuf,
	       sizeof(struct udphdr) + badBytes, 0,
	       (struct sockaddr *) &addr, addrLen);
  if (ret < 0)
  {
    fprintf(stderr, "raw send failed (err=%d: %s)\n", errno, strerror(errno));
    exit(1);
  }
  
  
  // Send a second, legitimate UDP packet
  int cooked = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
  char *second = "Good data";
  ret = sendto(cooked, second, strlen(second), 0,
	       (struct sockaddr *) &addr, addrLen);
  if (ret < 0)
  {
    fprintf(stderr, "second cooked send failed (err=%d: %s)\n", errno, strerror(errno));
    exit(1);
  }

  // Read what's queued up for us.
  while (1)
  {
    struct msghdr msg;
    struct iovec  iov;
    char readBuf[4096];
    ssize_t bytes;
    
    memset(readBuf, 0, 4096);
    
    iov.iov_base = readBuf;
    iov.iov_len = sizeof(readBuf);

    msg.msg_name = NULL;
    msg.msg_namelen = 0;
    msg.msg_iov = &iov;
    msg.msg_iovlen = 1;
    msg.msg_control = NULL;
    msg.msg_controllen = 0;
    msg.msg_flags = 0;
    
    bytes = recvmsg(listen, &msg, 0);
    if (bytes < 0)
    {
      fprintf(stderr, "getsockname failed for listening socket socket (err=%d: %s)\n", errno, strerror(errno));
      exit(1);
    }

    printf("recvmsg returned %ld bytes: %s\n", bytes, readBuf);

    if (bytes == 9)
      break;
  }
}