IIS 8 rewrite websocket get 502 Bad Gateway

I'm trying to rewrite http:// to ws://, I will call http://server:port/ws/ and it should rewrite to ws://server:port/ws/

but I get POST http://server:port/ws/ 502 (Bad Gateway)

I enabled iis websocket protocol from control panel -> add roles and features wizard 

my web.config:

<?xml version="1.0" encoding="UTF-8"?><configuration><system.webServer><rewrite><rules><rule name="Rewrite rule1 for /ws" stopProcessing="true"><match url="(.*)" /><action type="Rewrite" url="ws://server:7070/ws/"/></rule></rules></rewrite></system.webServer></configuration>

Anyone can guide me? Thanks.