Internet connection via proxy established but apt not working [duplicate]

Question

My machine is situated behind a Proxy (not sure what type, accessible via http, no user credentials required) which is automatically configured.

I'm connected to the internet but only firefox seems to be able to resolve URLs. APT does not.

So it seems like the proxy lets firefox resolve names but not other apps? Is it possible to somehow configure apt to circumvent this limitation?

When I enter the IP address of de.archive.ubuntu.com manually in the sources.list, I get an "no route to host" error.

As of now, the proxy server is configured with Ubuntus network settings gui.

Ouptut of apt:

Err:1 http://security.ubuntu.com/ubuntu cosmic-security InRelease
  Something wicked happened resolving 'security.ubuntu.com:http' (-5 - No address associated with hostname)

Err:2 http://de.archive.ubuntu.com/ubuntu cosmic InRelease
  Something wicked happened resolving 'de.archive.ubuntu.com:http' (-5 - No address associated with hostname)

Err:3 http://de.archive.ubuntu.com/ubuntu cosmic-updates InRelease
  Something wicked happened resolving 'de.archive.ubuntu.com:http' (-5 - No address associated with hostname)

Err:4 http://de.archive.ubuntu.com/ubuntu cosmic-backports InRelease
  Something wicked happened resolving 'de.archive.ubuntu.com:http' (-5 - No address associated with hostname)

Reading package lists... Done

Building dependency tree

Reading state information... Done

All packages are up to date.

W: Failed to fetch http://de.archive.ubuntu.com/ubuntu/dists/cosmic/InRelease  Something wicked happened resolving 'de.archive.ubuntu.com:http' (-5 - No address associated with hostname)

W: Failed to fetch http://de.archive.ubuntu.com/ubuntu/dists/cosmic-updates/InRelease  Something wicked happened resolving 'de.archive.ubuntu.com:http' (-5 - No address associated with hostname)

W: Failed to fetch http://de.archive.ubuntu.com/ubuntu/dists/cosmic-backports/InRelease  Something wicked happened resolving 'de.archive.ubuntu.com:http' (-5 - No address associated with hostname)

W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/cosmic-security/InRelease  Something wicked happened resolving 'security.ubuntu.com:http' (-5 - No address associated with hostname)

W: Some index files failed to download. They have been ignored, or old ones used instead.

Answer 1

Firefox (like other browsers) has its own internal proxy settings. CLI applications such as apt-get don't see this, they rely on an http_proxy (yes, lowercase, AFAIK) environment variable:

export http_proxy="http://PROXY_SERVER:PORT"