Support multi-connect on non-dual-mode IPv6

src/libraries/System.Net.Sockets/src/System/Net/Sockets/Socket.Unix.cs

@@ -130,6 +130,21 @@ internal void ReplaceHandleIfNecessaryAfterFailedConnect()
             _handle.LastConnectFailed = false;
         }
 
+        // On platforms that lack IPv6 dual-mode support (e.g. OpenBSD), a socket created for one address
+        // family cannot connect to an address of the other. When the socket has not yet connected, re-create
+        // its handle for the requested family so a pending connect attempt can use it. Returns false if the
+        // family is unchanged or the handle could not be replaced.
+        internal bool TryReplaceHandleForAddressFamily(AddressFamily addressFamily)
+        {
+            if (_addressFamily == addressFamily)
+            {
+                return false;
+            }
+
+            _addressFamily = addressFamily;
+            return ReplaceHandle() == SocketError.Success;
+        }
+
         internal unsafe SocketError ReplaceHandle()
         {
             // Collect values of trackable socket options marked by SafeSocketHandle.TrackSocketOption().

src/libraries/System.Net.Sockets/src/System/Net/Sockets/Socket.Windows.cs

@@ -21,6 +21,10 @@ public partial class Socket
 
 #pragma warning disable CA1822
         internal void ReplaceHandleIfNecessaryAfterFailedConnect() { /* nop on Windows */ }
+
+        // Windows always supports IPv6 dual-mode, so OSSupportsIPv6DualMode short-circuits before this is
+        // ever called; the stub exists only to satisfy the cross-platform call site.
+        internal bool TryReplaceHandleForAddressFamily(AddressFamily _) => false;
         internal bool CanProceedWithMultiConnect => true;
 #pragma warning restore CA1822
 

src/libraries/System.Net.Sockets/src/System/Net/Sockets/SocketAsyncEventArgs.cs

@@ -752,8 +752,12 @@ async Task Core(MultiConnectSocketAsyncEventArgs internalArgs, Task<IPAddress[]>
                         if (_currentSocket != null)
                         {
                             // If this SocketAsyncEventArgs was configured with a socket, then use it.
-                            // If that instance doesn't support this address, move on to the next.
-                            if (!_currentSocket.CanTryAddressFamily(address.AddressFamily))
+                            // If that instance doesn't support this address, move on to the next. The exception
+                            // is platforms without IPv6 dual-mode (e.g. OpenBSD), where a single socket can't
+                            // reach both families: re-create the still-unconnected handle for this address'
+                            // family so the connect can proceed. Dual-mode platforms skip this entirely.
+                            if (!_currentSocket.CanTryAddressFamily(address.AddressFamily) &&
+                                (Socket.OSSupportsIPv6DualMode || !_currentSocket.TryReplaceHandleForAddressFamily(address.AddressFamily)))
                             {
                                 continue;
                             }