user -> internet -> juniper -> vip 1723 -> 192.168.3.9:1723

set vip multi-port
set alg pptp enable

set service "LinuxPPTP" protocol tcp src-port 0-65535 dst-port 1723-1723
set service "LinuxPPTP" + udp src-port 0-65535 dst-port 1723-1723

set interface untrust vip 61.144.230.44 + 1723 "LinuxPPTP" 192.168.3.9

set policy id 82 from "Trust" to "Untrust"  "Any" "Any" "PPTP" permit
set policy id 82 application "PPTP"
set policy id 82