I set up an HAProxy for socks 5 proxy server on my edgerouters, listening on 0.0.0.0:31080, forwarding to proxy server tunneled at 10.99.99.2:31080. You can find it: UBNT VPN + Socks5 代理.
I chose this because of failure on setting destination NAT.
The BAD thing is, HAProxy DOES NOT SUPPORT UDP. If I set foxyproxy resolving names from remote, connections are lost.
I tried many many times, and find out why I got failed setting DNAT.