Virtualbox / Kubuntu / proxy

Virtualbox:

Network / Karta 1: Intel PRO/1000 MT Desktop (NAT)

Kubuntu:

At the end of the file /etc/environment add lines:

http_proxy=http://IP:port/
https_proxy=https://IP:port/
ftp_proxy=http://IP:port/
tcp_proxy=http://IP:port/

Content of file http-proxy.conf:

[Service]
Environment="http_proxy=http://IP:port/"

Content of file https-proxy.conf:

[Service]
Environment="https_proxy=http://IP:port/"

From file /lib/systemd/system/snapd.service you have to comment line starts with: EnvironmentFile

NOTE
Look at the places marked as bold. In the second file with the proxy configuration for HTTPS we provide the HTTP protocol.

After all the only thing to do is to restart out system.