Eternal port forwarding and related

[iPherian]

Features

• Eternal port forwarding (requests a new port on expiration of old)
  • can run a custom command when a forwarded port is established (see PIA_ON_PORT_FORWARD)
• Writes forwarded port to file for use by other services (/opt/piavpn-manual/pia_port)

Added Config

ENV Var	Function
PIA_ON_PORT_FORWARD=command	A command to be run when a forwarded port is established. The command will be called with one additional argument of the port number.