[Updated 06/22/2006 - fixed typo in code]
Lately, I've been debugging an issue in an application that I wrote that was a bit frustrating, though until today I've pretty much ignored the error. I decided to bite the bullet and dig in and try to figure out the problem.
I have an application that creates some datagram (UDP) endpoints using the Socket class found within the System.Net.Sockets namespace. As it turned out, there were times during my application's execution path in which the sockets would suddenly stop responding, though the port was still open (this could be ascertained via netstat or TCPView). To make the situation more perplexing, I had another application that used the exact same objects and it didn't exhibit the same behaviors - it continued to run as expected.
So I dug in a bit and within about 30 seconds figured out the problem - at least what was causing the problem: the point at which the application always had this problem of stopping to respond it would always send a datagram packet to localhost (127.0.0.1) to determine if it could find a particular listener/host. Well, if I had a listener at localhost there was never a problem, but if no socket was bound to that endpoint my UDP socket would throw a SocketException during the EndReceiveFrom().
I put together some tests and found that the WinSock error being thrown was WSAECONNRESET; in other words the socket was forcibly closed by the remote host. This was disconcerting for a little while, but I found that if I simply called BeginReceiveFrom() again, it would be up and running - but this wasn't necessarily the right fix.
Microsoft made some changes to their WinSock implementation between Windows NT 4.0 and Windows 2000. One of these was that the Recvfrom() function would return WSAECONNRESET instead of continuing to block or time out if the sendto() function resulted in an “ICMP port unreachable”. In other words, if you sent a datagram to a listening port all was cosher and behaved normally, however, if you attempted to send a datagram to an endpoint without a listening application, a pending/blocking call to Recvfrom() would throw. Previously, my SocketException handling code would simply return without attempting to re-listen, assuming that if the error were received the socket was closing down.
It is possible, fortunately, to achieve Windows NT 4.0 behavior rather than having the socket return an error result. This is accomplished by calling the WSAIoctl() function in Ws2_32.dll. Well, if you're writing code using the .NET framework, it's not necessary to dip down into P/Invoke to call the function, but rather you can use the IOControl() method on the Socket class. The key lies in passing in the control code of SIO_UDP_CONNRESET and a value of False (0) to turn off the 'new', Windows 2000 behavior - thus restoring the expected Windows NT 4.0 behavior.
const uint SIO_UDP_CONNRESET = 0x9800000C;
It's interesting, and I feel unfortunate, that the IOControl() function accepts control codes of type Int32. Many of the control codes are UInt32 values. So instead of taking the default value for SIO_UDP_CONNRESET and passing that in (which we can't because it's out of the range for an Int32), we must take the same value and convert it to a signed value.
// 0x9800000C == 2440136844 (uint) == -1744830452 (int) == 0x9800000C
const int SIO_UDP_CONNRESET = -1744830452;
byte[] inValue = new byte[] { 0, 0, 0, 0 }; // == false
byte[] outValue = new byte[] { 0, 0, 0, 0 }; // initialize to 0
_socket.IOControl(SIO_UDP_CONNRESET, inValue, outValue);
Simply make that call during the initialization of the socket and the WSAECONNRESET error will not be raised when a datagram is sent to a closed/invalid end point.
Happy coding!