如果没有事先 ping 通，UDP 数据包不会到达

Question

我有一个代码可以将 UDP 数据包从 linux 设备（服务器）发送到另一个（客户端）。我在执行此操作时遇到问题：

1. 重启服务器。
2. 运行程序发送数据包，客户端没有收到。

如果然后我对客户端 ip 执行 ping 操作，然后再次运行程序，现在数据包已被客户端接收到

这是代码：

  int main (int argc, char **argv){
  int i, status, datalen, frame_length, sd, bytes;
  char *interface, *target, *src_ip, *dst_ip;
  struct ip6_hdr iphdr;
  struct udphdr udphdr;
  uint8_t *data, *src_mac, *dst_mac, *ether_frame;
  struct addrinfo hints, *res;
  struct sockaddr_in6 *ipv6;
  struct sockaddr_ll device;
  struct ifreq ifr;
  void *tmp;

  // Allocate memory for various arrays.
  src_mac = allocate_ustrmem (6);
  dst_mac = allocate_ustrmem (6);
  data = allocate_ustrmem (IP_MAXPACKET);
  ether_frame = allocate_ustrmem (IP_MAXPACKET);
  interface = allocate_strmem (INET6_ADDRSTRLEN);
  target = allocate_strmem (INET6_ADDRSTRLEN);
  src_ip = allocate_strmem (INET6_ADDRSTRLEN);
  dst_ip = allocate_strmem (INET6_ADDRSTRLEN);

  // Interface to send packet through.
  strcpy (interface, "usb0");

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

  // Use ioctl() to look up interface name and get its MAC address.
  memset (&ifr, 0, sizeof (ifr));
  snprintf (ifr.ifr_name, sizeof (ifr.ifr_name), "%s", interface);
  if (ioctl (sd, SIOCGIFHWADDR, &ifr) < 0) {
    perror ("ioctl() failed to get source MAC address ");
    return (EXIT_FAILURE);
  }
  close (sd);

  // Copy source MAC address.
  memcpy (src_mac, ifr.ifr_hwaddr.sa_data, 6 * sizeof (uint8_t));

  // Report source MAC address to stdout.
  printf ("MAC address for interface %s is ", interface);
  for (i=0; i<5; i++) {
    printf ("%02x:", src_mac[i]);
  }
  printf ("%02x\n", src_mac[5]);

  // Find interface index from interface name and store index in
  // struct sockaddr_ll device, which will be used as an argument of sendto().
  memset (&device, 0, sizeof (device));
  if ((device.sll_ifindex = if_nametoindex (interface)) == 0) {
    perror ("if_nametoindex() failed to obtain interface index ");
    exit (EXIT_FAILURE);
  }
  printf ("Index for interface %s is %i\n", interface, device.sll_ifindex);

  // Set destination MAC address
  dst_mac[0] = 0x00;
  dst_mac[1] = 0x11;
  dst_mac[2] = 0x7d;
  dst_mac[3] = 0x30;
  dst_mac[4] = 0x7f;
  dst_mac[5] = 0xd0;

  // Source IPv6 address
  strcpy (src_ip, "fe80::11:7dff:fe30:8013");

  // Destination URL or IPv6 address
  strcpy (target, "fe80:0000:0000:0000:0211:7d00:0030:7fd0");

  // Fill out hints for getaddrinfo().
  memset (&hints, 0, sizeof (hints));
  hints.ai_family = AF_INET6;
  hints.ai_socktype = SOCK_STREAM;
  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);
  }
  ipv6 = (struct sockaddr_in6 *) res->ai_addr;
  tmp = &(ipv6->sin6_addr);
  if (inet_ntop (AF_INET6, tmp, dst_ip, INET6_ADDRSTRLEN) == NULL) {
    status = errno;
    fprintf (stderr, "inet_ntop() failed.\nError message: %s", strerror (status));
    exit (EXIT_FAILURE);
  }
  freeaddrinfo (res);

  // Fill out sockaddr_ll.
  device.sll_family = AF_PACKET;
  memcpy (device.sll_addr, src_mac, 6 * sizeof (uint8_t));
  device.sll_halen = 6;

  // UDP data
  datalen = 4;
  data[0] = 'T';
  data[1] = 'E';
  data[2] = 'S';
  data[3] = 'T';

  // IPv6 header

  // IPv6 version (4 bits), Traffic class (8 bits), Flow label (20 bits)
  iphdr.ip6_flow = htonl ((6 << 28) | (0 << 20) | 0);

  // Payload length (16 bits): UDP header + UDP data
  iphdr.ip6_plen = htons (UDP_HDRLEN + datalen);

  // Next header (8 bits): 17 for UDP
  iphdr.ip6_nxt = IPPROTO_UDP;

  // Hop limit (8 bits): default to maximum value
  iphdr.ip6_hops = 255;

  // Source IPv6 address (128 bits)
  if ((status = inet_pton (AF_INET6, src_ip, &(iphdr.ip6_src))) != 1) {
    fprintf (stderr, "inet_pton() failed.\nError message: %s", strerror (status));
    exit (EXIT_FAILURE);
  }

  // Destination IPv6 address (128 bits)
  if ((status = inet_pton (AF_INET6, dst_ip, &(iphdr.ip6_dst))) != 1) {
    fprintf (stderr, "inet_pton() failed.\nError message: %s", strerror (status));
    exit (EXIT_FAILURE);
  }

  // UDP header

  // Source port number (16 bits): pick a number
  udphdr.source = htons (61616);

  // Destination port number (16 bits): pick a number
  udphdr.dest = htons (61616);

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

  // UDP checksum (16 bits)
  udphdr.check = udp6_checksum (iphdr, udphdr, data, 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 = 6 + 6 + 2 + IP6_HDRLEN + UDP_HDRLEN + datalen;

  // Destination and Source MAC addresses
  memcpy (ether_frame, dst_mac, 6 * sizeof (uint8_t));
  memcpy (ether_frame + 6, src_mac, 6 * sizeof (uint8_t));

  // Next is ethernet type code (ETH_P_IPV6 for IPv6).
  // http://www.iana.org/assignments/ethernet-numbers
  ether_frame[12] = ETH_P_IPV6 / 256;
  ether_frame[13] = ETH_P_IPV6 % 256;

  // Next is ethernet frame data (IPv6 header + UDP header + UDP data).

  // IPv6 header
  memcpy (ether_frame + ETH_HDRLEN, &iphdr, IP6_HDRLEN * sizeof (uint8_t));

  // UDP header
  memcpy (ether_frame + ETH_HDRLEN + IP6_HDRLEN, &udphdr, UDP_HDRLEN * sizeof (uint8_t));

  // UDP data
  memcpy (ether_frame + ETH_HDRLEN + IP6_HDRLEN + UDP_HDRLEN, data, datalen * sizeof (uint8_t));

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

  // Send ethernet frame to socket.
  if ((bytes = sendto (sd, ether_frame, frame_length, 0, (struct sockaddr *) &device, sizeof (device))) <= 0) {
    perror ("sendto() failed");
    exit (EXIT_FAILURE);
  }

  // Close socket descriptor.
  close (sd);

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

  return (EXIT_SUCCESS);
}

Answer 1

如果dst mac 不在ARP 表中，您选择发送UDP 数据包的方法似乎会失败。不知何故，这样做会导致内核对于dst mac 不是arp（可能是因为您已手动将其插入数据包中）-但是-数据包传输仍然失败，因为它不存在于@ 987654326@ 表。 ping 导致 ARP 表被填充，然后它开始工作。

只是一个理论。我们将不得不查看内核路径来确认这一点。

你可以尝试几个实验来证实这个理论。

1. 不要ping。运行该程序几次，看看它在后续运行中是否有效。
2. 首先ping，然后尝试该程序（它应该可以工作），然后刷新ARP 表，再次尝试该程序（它应该失败）。使用sudo ip -s -s neigh flush all 刷新ARP 表。

您是否有任何理由不使用SOCK_DGRAM，这是执行UDP 的标准方式？

编辑

正如 cmets 中所指出的，我错过了 OP 使用 IPV6 的事实。将所有提及的ARP 替换为neighbor（和等效项）。