xref: /recipes/faketcp/icmpecho.cc

#include "faketcp.h"

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <netinet/ip.h>
#include <linux/if_ether.h>

int main()
{
  char ifname[IFNAMSIZ] = "tun%d";
  int fd = tun_alloc(ifname);

  if (fd < 0)
  {
    fprintf(stderr, "tunnel interface allocation failed\n");
    exit(1);
  }

  printf("allocted tunnel interface %s\n", ifname);
  sleep(1);

  for (;;)
  {
    union
    {
      unsigned char buf[ETH_FRAME_LEN];
      struct iphdr iphdr;
    };

    const int iphdr_size = sizeof iphdr;

    int nread = read(fd, buf, sizeof(buf));
    if (nread < 0)
    {
      perror("read");
      close(fd);
      exit(1);
    }
    printf("read %d bytes from tunnel interface %s.\n", nread, ifname);

    const int iphdr_len = iphdr.ihl*4;
    if (nread >= iphdr_size
        && iphdr.version == 4
        && iphdr_len >= iphdr_size
        && iphdr_len <= nread
        && iphdr.tot_len == htons(nread)
        && in_checksum(buf, iphdr_len) == 0)
    {
      const void* payload = buf + iphdr_len;
      if (iphdr.protocol == IPPROTO_ICMP)
      {
        icmp_input(fd, buf, payload, nread);
      }
    }
    else
    {
      printf("bad packet\n");
      for (int i = 0; i < nread; ++i)
      {
        if (i % 4 == 0) printf("\n");
        printf("%02x ", buf[i]);
      }
      printf("\n");
    }
  }

  return 0;
}