[arch-general] Forward all PPTP VPN packets to another proxy?

[Bill Sun]

Hi,

Currently I have a arch linux server running PPTP VPN (for iOS devices
only), and I forward all PPTP VPN packets to local eth0:
    iptables -A POSTROUTING -s <ip_addr_for_pptp> -o eth0 -j MASQUERADE

Now I got a external proxy, and I want all the traffic currently through
the PPTP VPN then through that extenal proxy (it's a http proxy, and
requires authentication). If my description is confusing, please consult
this chart:

iOS --> Server 1 (PPTP VPN) --> Server 2 (http proxy, authentication
required) --> Web

Server {1,2} both have dedicated external IP address. I have root
permission for Server 1, and nothing for Server 2 (except for the http
proxy account).

I'm thinking about running a http server that handles all the
authentication for Server 2 (such as dante?), then forward all packets
to that server (running in Server 1). But I don't know how to write
corresponding iptables rules. So how to write such rules?

Regards.