The SCUs are registered in the ACC-LAN so that they don't have access to the internet. Hence, apt can't be used in the default way. This can be bypassed by using a proxy server which has access to the internet.

To configure a Debian system that does not have direct internet access but is connected to an Ubuntu system that has internet access, you can set up a proxy server on the Ubuntu system. Then, configure the Debian system to use this proxy server. Here is a step-by-step guide:

Squid is a popular proxy server. Install it with:

sudo apt update
sudo apt install squid

The default Squid configuration should suffice for basic proxy needs. Open the Squid configuration file:

sudo nano /etc/squid/squid.conf

Ensure that the following section is present and configured to allow connections from your network. Add or edit this section:

acl localnet src 192.168.1.0/24    # Example network, replace this with your actual network
http_access allow localnet

Note: Replace 192.168.1.0/24 with your network's subnet in CIDR notation. /24 means that the 24st bit of the subnet mask are 1s. So it is equal to 255.255.255.0.

After editing the configuration file, you need to restart Squid for the changes to take effect:

sudo systemctl restart squid

Create or edit the file /etc/apt/apt.conf.d/95proxies:

sudo vi /etc/apt/apt.conf.d/95proxies

Add the following lines (replace ubuntu-system-ip and port with the IP address of the Ubuntu system and the Squid port, usually 3128):

Acquire {
    HTTP::proxy "http://ubuntu-system-ip:3128";
    HTTPS::proxy "http://ubuntu-system-ip:3128";
}

Save the file and close the editor.

You can also set environment variables for the proxy by editing the file /etc/environment:

sudo nano /etc/environment

Add the following lines:

http_proxy="http://ubuntu-system-ip:3128"
https_proxy="http://ubuntu-system-ip:3128"

Save the file and reload the environment variables:

source /etc/environment

Update the APT cache on the Debian system to ensure the new proxy settings are effective:

sudo apt update

Test installing a package to ensure the Debian system can download packages from the internet via the proxy server:

sudo apt install <package-name>

If everything is configured correctly, the Debian system should now be able to download and install packages from the internet via the proxy server on the Ubuntu system.