Hello,
can you add bind-device $device directive, for upstream traffic.
You can use SO_BINDTODEVICE under linux. It is very useful when
proxy port is from one network interface and upstream traffic have to pass
through other network interface.
At the moment we are using Bind $ipaddress, but us cumbersome because some times
our outgoing address is changing and we have to modify config file and restart tiny proxy.

Cheers,