How do I receive data from NTP server?

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

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).