I m developing an application that detects Source-based games running on the LAN. Following up on the specifications provided by Valve, I have it narrowed down to exactly what I want: making a UDP connection on port 27015, sending an A2S_INFO query (0xFFFFFFFF followed by "TSource Engine Query") and parsing the binary reply.

This is the code I m working with:

Dim sIP As String = "192.168.0.154"
Dim nPort As Integer = 27015

Dim connectionMessage As String = "ÿÿÿÿTSource Engine Query" & Chr(0)

Dim endPoint As New IPEndPoint(IPAddress.Parse(sIP), nPort)

Dim client As Socket = New Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp)
client.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReceiveTimeout, 6000)
client.SendTo(Encoding.ASCII.GetBytes(connectionMessage), endPoint)
Console.WriteLine("Message sent to " & sIP & ":" & nPort & vbCrLf & connectionMessage)
Dim sBuffer(1400) As Byte
Try
    client.ReceiveFrom(sBuffer, endPoint)
    MsgBox(System.Text.Encoding.Unicode.GetString(sBuffer))
Catch ex As SocketException
    MsgBox("Failed to receive response on " & sIP & ":" & nPort & ":" & vbCrLf & ex.Message)
End Try

I keep getting SocketError.TimedOut exceptions, informing me:

Failed to receive response on 192.168.0.154:27015: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond

I m fairly certain this is very close to the answer, because the following very simple PHP version works like a charm:

$ip = "192.168.0.154";
$port = 27015;

$fp = fsockopen("udp://".$ip,$port, $errno, $errstr);
socket_set_timeout($fp, 6);

$prefix = "xffxffxffxff";
$command = "TSource Engine Query";
$msg = "$prefix$command";
fputs($fp, $msg, strlen($msg));

$response = "";
do {
    $response .= fgets($fp, 16);
    $status = socket_get_status($fp);
} while ($status[ unread_bytes ]);
fclose ($fp);

echo $response;

This provides me with a reply in accordance with the specifications.

I m close, but something s amiss with my VB.NET code. What am I doing wrong?

The solution

The encoding type is not valid for transmitting binary data. I replaced the following:

Encoding.ASCII.GetBytes(connectionMessage)

with:

System.Text.Encoding.[Default].GetBytes(connectionMessage)

And it was instantly solved.

A more robust solution, suggested by "pub", might be to specify the code page (although I haven t tested this):

Encoding.GetEncoding("iso-8859-1").GetBytes(connectionMessage)