IPv4 to IPv6 address mapping is disabled on OpenBSD, so do not
set DualMode in mono either.

Index: mcs/class/System/System.Net.Sockets/Socket.cs
--- mcs/class/System/System.Net.Sockets/Socket.cs.orig
+++ mcs/class/System/System.Net.Sockets/Socket.cs
@@ -150,7 +150,7 @@ namespace System.Net.Sockets
 						this.NoDelay = false;
 				// The socket was created successfully; enable IPV6_V6ONLY by default for normal AF_INET6 sockets.
 				// This fails on raw sockets so we just let them be in default state.
-				} else if (addressFamily == AddressFamily.InterNetworkV6 && socketType != SocketType.Raw) {
+				} else if (addressFamily == AddressFamily.InterNetworkV6 && socketType != SocketType.Raw && !Platform.IsOpenBSD) {
 					this.DualMode = true;
 				}
 
