A Raspberry Pi is located behind a hardware firewall which is configured to block all outgoing communication to any IP except my server (all ports are allowed) and to block all incoming communication except the response requested by the Pi (obviously from my server). I can access the Pi from my server by tunneling within a SSH connection initiated by the Pi to my server. The firewall cannot be reconfigured.
RaspPi ---F/W---> MyServer <---Internet---> Repositories
How can I update the Pi as it cannot communicate directly to the repository URLs?
sudo apt-get -y update