VB6 winsock control: closing a connection and connecting to another server

Question

I'm working on a legacy VB6 app. The app uses the RAS API to establish a modem connection, then uses a winsock control to connect to an IP address & port.

I'm now adding a "failover" feature to this app where if it can't connect to a server, it tries to connect to the next one in the list.

Let's say I have two servers, server app A and server app B. (During my tests I've swapped these around to verify that both servers are accessible and there are no routing / firewall issues.)

If I stop server app A, then something quite strange happens when the app tries to connect to server app A and then server app B:

mySocket.close
mySocket.Connect serverA, portA

mySocket.state = 6 sckConnecting
socket connect event is called
mySocket.state = 7 sckConnected
mySocket.state = 8 sckClosing

(which seems slightly odd: I'm not sure why it appears to connect for a few moments.)

The socket remains in sckClosing state. After a few hundred milliseconds I move on to try to connect to server B:

mySocket.close

mySocket.state = 0 sckClosed

.

mySocket.Connect serverB, portB

mySocket.state = 6 sckConnecting
socket error event is called with error 10060 (WSAETIMEDOUT)
mySocket.state = 9 sckError

At this point if I start server app A, do a RAShangup and a RASdial, and try to connect to server A, all works OK.

It's as if the sequence

socket.connect ip, port
socket.close
socket.connect newIP, newPort

doesn't work properly unless a RAShangup and RASdial is inserted. Instead it fails with WSAETIMEDOUT.

Is there anything I need to do between a close and connect call?

Note: I've tried making sure that the close call has really closed, but this doesn't help:

Private Sub closeSocket(ByRef w As Winsock)
  w.Close
  Do While (w.State <> sckClosed)
    DoEvents
  Loop
End Sub

Simon Elliott Simon Elliott · Accepted Answer · 2011-06-15T10:35:58

After looking again at the project, it turns out that a previous developer set the LocalPort property of the winsock control to a nonzero value. This was preventing the reuse of the winsock control.

Setting this to zero causes a random local port to be selected, which allows the control to be reused.

Lesson learned: when dealing with VB6 I need to look at the property settings as well as the source code.

VB6 winsock control: closing a connection and connecting to another server

2 Answers