How do I receive data from NTP server? - c++

Question

I have no idea why send data is 48 bytes 010,0,0..., someone can explain? the problem is buffer for data received, I don't know how big he should be, and even if I receive data, how to make normal time from it?
Here's the code:
#include <iostream>
#define _WINSOCK_DEPRECATED_NO_WARNINGS
#define WIN32_MEAN_AND_LEAN
#include <winsock2.h>
#include <windows.h>
#include <time.h>
using namespace std;
class HRException
{
public:
HRException() :
m_pMessage("") {}
virtual ~HRException() {}
HRException(const char *pMessage) :
m_pMessage(pMessage) {}
const char * what() { return m_pMessage; }
private:
const char *m_pMessage;
};
const int REQ_WINSOCK_VER = 2; // Minimum winsock version required
const char DEF_SERVER_NAME[] = "0.pl.pool.ntp.org";
const int SERVER_PORT = 123;
const int TEMP_BUFFER_SIZE = 128;
const char msg[48] = { 010,0,0,0,0,0,0,0,0 };
// IP number typedef for IPv4
typedef unsigned long IPNumber;
IPNumber FindHostIP(const char *pServerName)
{
HOSTENT *pHostent;
// Get hostent structure for hostname:
if (!(pHostent = gethostbyname(pServerName)))
throw HRException("could not resolve hostname.");
// Extract primary IP address from hostent structure:
if (pHostent->h_addr_list && pHostent->h_addr_list[0])
return *reinterpret_cast<IPNumber*>(pHostent->h_addr_list[0]);
return 0;
}
void FillSockAddr(sockaddr_in *pSockAddr, const char *pServerName, int portNumber)
{
// Set family, port and find IP
pSockAddr->sin_family = AF_INET;
pSockAddr->sin_port = htons(portNumber);
pSockAddr->sin_addr.S_un.S_addr = FindHostIP(pServerName);
}
bool RequestHeaders(const char *pServername)
{
SOCKET hSocket = INVALID_SOCKET;
char tempBuffer[TEMP_BUFFER_SIZE];
sockaddr_in sockAddr = { 0 };
bool bSuccess = true;
try
{
// Lookup hostname and fill sockaddr_in structure:
cout << "Looking up hostname " << pServername << "... ";
FillSockAddr(&sockAddr, pServername, SERVER_PORT);
cout << "found.\n";
// Create socket
cout << "Creating socket... ";
if ((hSocket = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) == INVALID_SOCKET)
throw HRException("could not create socket.");
cout << "created.\n";
// Connect to server
cout << "Attempting to connect to " << inet_ntoa(sockAddr.sin_addr)
<< ":" << SERVER_PORT << "... ";
if (connect(hSocket, reinterpret_cast<sockaddr*>(&sockAddr), sizeof(sockAddr)) != 0)
throw HRException("could not connect.");
cout << "connected.\n";
cout << "Sending request... ";
// send request part 1
if (send(hSocket, msg, sizeof(msg) , 0) == SOCKET_ERROR)
throw HRException("failed to send data.");
cout << "request sent.\n";
cout << "Dumping received data...\n\n";
// Loop to print all data
recv(hSocket, tempBuffer, sizeof(tempBuffer), 0); // <-- the problem
///
//part where we take time out of tempBuffer
///
}
catch (HRException e)
{
cerr << "\nError: " << e.what() << endl;
bSuccess = false;
}
if (hSocket != INVALID_SOCKET)
{
closesocket(hSocket);
}
return bSuccess;
}
int main(int argc, char* argv[])
{
int iRet = 1;
WSADATA wsaData;
cout << "Initializing winsock... ";
if (WSAStartup(MAKEWORD(REQ_WINSOCK_VER, 0), &wsaData) == 0)
{
// Check if major version is at least REQ_WINSOCK_VER
if (LOBYTE(wsaData.wVersion) >= REQ_WINSOCK_VER)
{
cout << "initialized.\n";
// Set default hostname:
const char *pHostname = DEF_SERVER_NAME;
// Set custom hostname if given on the commandline:
if (argc > 1)
pHostname = argv[1];
iRet = !RequestHeaders(pHostname);
}
else
{
cerr << "required version not supported!";
}
cout << "Cleaning up winsock... ";
// Cleanup winsock
if (WSACleanup() != 0)
{
cerr << "cleanup failed!\n";
iRet = 1;
}
cout << "done.\n";
}
else
{
cerr << "startup failed!\n";
}
int x;
cin >> x;
return iRet;
}
Most part of code is from madwizard.org

Answer 1

Ok it works, main part of code:
const char msg[48] = { 010,0,0,0,0,0,0,0,0 };
if (send(hSocket, msg, sizeof(msg) , 0) == SOCKET_ERROR)
throw HRException("failed to send data.");
cout << "request sent.\n";
cout << "Dumping received data...\n\n";
char tempBuffer[1024];
int bytes = recv(hSocket, tempBuffer, sizeof(tempBuffer), 0);
cout << "bytes received: " << bytes << endl;
time_t tmit;
tmit = ntohl(((time_t*)tempBuffer)[4]);
tmit -= 2208988800U;
cout << ctime(&tmit);
No idea why data that we send is
msg[48] = { 010,0,0,0,0,0,0,0,0 };
and why received data contains many numbers? for example if change code to
tmit = ntohl(((time_t*)tempBuffer)[6]);
I will get date 2008y, why?
Guys why so many minuses?, still waiting for an explanation :D
Here's whole code http://pastebin.com/Sv3ERGfV , dont forget to link ws2_32.lib

Answer 2

Similar to my issue when trying to query the time from a self-hostet Windows-NTP-Server with the C++ library NTPClient which uses boost for the network tasks, msg[48] = { 010,0,0,0,0,0,0,0,0 }; configures the ntp.flags.mode. After comparing the network traffic of w32tm /stripchart /computer:10.159.96.65 using Wireshark, flag 27 or 11 seem to be the choices for my usecase:
Comparison of NTP network packages
tmit = ntohl(((time_t*)tempBuffer)[6]); extracts the data from the received package. It looks like
4 yields the reference time (last sync with timeserver I assume),
8 the time when server received request and
10 the transmit time (which should be almost equal).