DNS query with winsock was failed

Eugene Pustovoyt

Hi, everybody I understand winsock has gethostbyname for this. But I need to debug for embedded application and first I try the code on Windows. Here is a my code:

sockaddr\_in dns\_addr;
int dns\_size = sizeof(dns\_addr);
ZeroMemory(&dns\_addr, dns\_size);
dns\_addr.sin\_family = AF\_INET;
dns\_addr.sin\_port = htons(53);
dns\_addr.sin\_addr.s\_addr = inet\_addr("8.8.8.8");

SOCKET out = socket(AF\_INET, SOCK\_DGRAM, 0);

char msg\[\] = {
	0x12, 0x34,			// ID 
	0x01, 0x00,			// QR ... RCODE
	0x00, 0x01,			// QDCOUNT
	0x00, 0x00,			// ANCOUNT
	0x00, 0x00,			// NSCOUNT
	0x00, 0x00,			// ARCOUNT
	0x03, 
	0x77, 0x77, 0x77,		// www
	0x03, 
	0x74, 0x75, 0x74,		// tut
	0x02, 
	0x62, 0x79,			// by
	0x00, 
	0x00, 0x01,			// TYPE\_A
	0x00, 0x01			// CLASS\_IN
};
int msg\_size = sizeof(msg);
int iSendOk = sendto(out, msg, msg\_size, 0, (sockaddr\*)&dns\_addr, dns\_size);

if (iSendOk != SOCKET\_ERROR)
{
	char bufstr\[100\];
	sockaddr\_in dest\_addr;
	int dest\_addr\_size = sizeof(dest\_addr);
	int iReceived = recvfrom(out, bufstr, 100, 0,(sockaddr\*)&dest\_addr, &dest\_addr\_size);
}

closesocket(out);

And this code freezes when recvfrom function is execution. The function is executed with an infinite loop and never returns. What do I do wrong?

Eugene Pustovoyt Soft and Hard Developer CPPMessageBox v1.0 CPPToolTip v2.1 CPPDumpCtrl v1.2 CPPHtmlStatic v1.2

Lost User

sockaddr_in dns_addr;
int dns_size = sizeof(dns_addr);

What's the "size" of the "uninitialized" dns_addr? What other values can iSendOk take if it isn't SOCKET_ERROR?

It was only in wine that he laid down no limit for himself, but he did not allow himself to be confused by it. ― Confucian Analects: Rules of Confucius about his food

Eugene Pustovoyt

You're right. Getting size of structure better after initialization. But in my cause this structure has constant length and her size will be same after initialization. Second, I check other returning values but I cut this code for this example. When I'm debugging this code sendto function return 28. This value is equal to the length of sent data.

Eugene Pustovoyt Soft and Hard Developer CPPMessageBox v1.0 CPPToolTip v2.1 CPPDumpCtrl v1.2 CPPHtmlStatic v1.2

Lost User

It is most likely that the remote device is not responding to the datagram. If there is no data to be read from the socket then recvfrom will block forever, or until the connection is broken. You should use a non-blocking method to check whether any data has been received and use some form of timed loop to allow the program to abort the activity after a fixed time period. See recvfrom function (winsock.h) - Win32 apps | Microsoft Docs[^] for full details.

Eugene Pustovoyt

I tried to specify the reading size to 1 byte instead 100 from example. But I got same result.

Eugene Pustovoyt Soft and Hard Developer CPPMessageBox v1.0 CPPToolTip v2.1 CPPDumpCtrl v1.2 CPPHtmlStatic v1.2

Lost User

Of course you did. The socket is still waiting for a response from the remote address.

Eugene Pustovoyt

Does it mean that the DNS server does not respond if there is not a single byte in response? But why?

Eugene Pustovoyt Soft and Hard Developer CPPMessageBox v1.0 CPPToolTip v2.1 CPPDumpCtrl v1.2 CPPHtmlStatic v1.2

Lost User

That server is owned by Google, so you should check with them how to get information from it. I am not sure that direct messages are accepted. See Get Started - Public DNS - Google Developers[^]

Lost User

Hi, The code you posted is implemented as a blocking socket. It will literally block the entire thread until it receives a response from the other side. To avoid this you can set the SO_RCVTIMEO[^] socket option. Best Wishes, -David Delaune