Always use IPv6 for the server if a "dual stack" is available. This means a server on vista (as well as most linux systems) will support IPv6 by default. IPv4 requests will be mapped by the OS.

src/net/common/socket_startup_cmn.cpp

@@ -21,20 +21,20 @@
 #include <net/socket_helper.h>
 
 
-bool
-socket_has_sctp()
-{
-#ifdef IPPROTO_SCTP
-	SOCKET test = socket(AF_INET, SOCK_STREAM, IPPROTO_SCTP);
-
-	if (test == INVALID_SOCKET)
-		return false;
-	CLOSESOCKET(test);
-	return true;
-#else
-	return false;
-#endif
-}
+bool
+socket_has_sctp()
+{
+#ifdef IPPROTO_SCTP
+	SOCKET test = socket(AF_INET, SOCK_STREAM, IPPROTO_SCTP);
+
+	if (test == INVALID_SOCKET)
+		return false;
+	CLOSESOCKET(test);
+	return true;
+#else
+	return false;
+#endif
+}
 
 bool
 socket_has_ipv6()
@@ -47,3 +47,19 @@ socket_has_ipv6()
 	return true;
 }
 
+bool
+socket_has_dual_stack()
+{
+	bool retVal = false;
+	SOCKET test = socket(AF_INET6, SOCK_STREAM, 0);
+
+	if (test != INVALID_SOCKET)
+	{
+		int ipv6only = 0;
+		if (setsockopt(test, IPPROTO_IPV6, IPV6_V6ONLY, (char *)&ipv6only, sizeof(ipv6only)) == 0)
+			retVal = true;
+		CLOSESOCKET(test);
+	}
+	return retVal;
+}
+