gethostbyaddr返回NULL，即使IP有效

Question

我尝试使用IP连接服务器，但我在使用gethostbyaddr函数时得到了error 11004，我使用的函数如下：

WSADATA wsaData;
DWORD dwError;
struct hostent *remoteHost;
char host_addr[] = "127.0.0.1"; //or any other IP
struct in_addr addr = { 0 };

addr.s_addr = inet_addr(host_addr);
if (addr.s_addr == INADDR_NONE) {
  printf("The IPv4 address entered must be a legal address\n");
  return 1;
} else
  remoteHost = gethostbyaddr((char *) &addr, 4, AF_INET);


if (remoteHost == NULL) {
   dwError = WSAGetLastError();
   if (dwError != 0)
      printf("error: %d", dwError)
}

我得到了这个错误：11004 by WSAGetLastError函数：

WSANO_DATA
11004



Valid name, no data record of requested type.

The requested name is valid and was found in the database, but it does not have the correct
associated data being resolved for. The usual example for this is a host name-to-address 
translation attempt (using gethostbyname or WSAAsyncGetHostByName) which uses the DNS 
(Domain Name Server). An MX record is returned but no A record—indicating the host itself 
exists, but is not directly reachable.

PS:当我使用gethostbyname时，我的代码工作正常。

Answer 1

你忘了初始化winsock了吗？

#include <stdio.h>
#include <winsock2.h>

#pragma comment(lib, "Ws2_32.lib")

int main() {
   WSADATA wsaData;
   // Initialize Winsock
   int wret = WSAStartup(MAKEWORD(2,2), &wsaData);
   if (wret != 0) {
      printf("WSAStartup failed: %d\n", wret);
      return 1;
   }

   DWORD dwError;
   struct hostent *remoteHost;
   char host_addr[] = "127.0.0.1"; //or any other IP
   struct in_addr addr = { 0 };

   addr.s_addr = inet_addr(host_addr);
   if (addr.s_addr == INADDR_NONE) {
      printf("The IPv4 address entered must be a legal address\n");
      return 1;
   } else
      remoteHost = gethostbyaddr((char *) &addr, 4, AF_INET);


   if (remoteHost == NULL) {
      dwError = WSAGetLastError();
      if (dwError != 0)
         printf("error: %d", dwError);
   }
   else {
      printf("Hostname: %s\n", remoteHost->h_name);
   }

   WSACleanup();

   return 0;
}

上面的代码适用于我的127.0.0.1和我局域网上的一些远程主机。但有趣的是，对于我的路由器，它可能没有主机名，我得到了同样的11004 winsock错误。

在这种情况下。

struct hostent* hostbyname = gethostbyname(host_addr);

if (hostbyname == NULL) {
   dwError = WSAGetLastError();
   if (dwError != 0)
      printf("error: %d", dwError);
}
else {
   printf("Hostname: %s\n", hostbyname->h_name);
}

也不返回主机名，并导致相同的错误- 11004。

对你的经历感到迷惑。