I'm running squid-deb-proxy server in my local network. It works well for all package downloads while running all APT commands. It is configured for all possible known URLs of the mirrors including old-releases. The Ubuntu 19.10 client has installed client side - the squid-deb-proxy-client package.

$ dpkg -l squid-deb-proxy-client | tail -n1

ii  squid-deb-proxy-client 0.8.14+nmu2  all          automatic proxy discovery for APT based on Avahi



$ sudo apt update 

Hit:1 http://archive.ubuntu.com/ubuntu eoan InRelease

Hit:2 http://archive.ubuntu.com/ubuntu eoan-updates InRelease

Hit:3 http://archive.ubuntu.com/ubuntu eoan-backports InRelease

Hit:4 http://security.ubuntu.com/ubuntu eoan-security InRelease

Reading package lists... Done

Building dependency tree       

Reading state information... Done

All packages are up to date.

But when I try to start upgrade process to get 20.04 LTS - using sudo do-release-upgrade in the terminal I got the following error message:

$ sudo do-release-upgrade 

Checking for a new Ubuntu release

Your Ubuntu release is not supported anymore.

For upgrade information, please visit:

http://www.ubuntu.com/releaseendoflife



Get:1 Upgrade tool signature [1 554 B]                                                                                                              

Get:2 Upgrade tool [1 342 kB]                                                                                                                       

Fetched 1 343 kB in 0s (0 B/s)                                                                                                                      

authenticate 'focal.tar.gz' against 'focal.tar.gz.gpg' 

Authentication failed

Authenticating the upgrade failed. There may be a problem with the network or with the server. 

I do not see anything special in the logs on squid-deb-proxy server side:

==> /var/log/squid-deb-proxy/access.log <==

1595069772.862    212 192.168.3.47 TCP_MISS/200 3349 CONNECT changelogs.ubuntu.com:443 - HIER_DIRECT/91.189.95.15 -

1595069774.726      2 192.168.3.47 TCP_MEM_HIT/200 1950 GET http://archive.ubuntu.com/ubuntu/dists/focal-updates/main/dist-upgrader-all/current/focal.tar.gz.gpg - HIER_NONE/- application/x-gzip

1595069775.410    681 192.168.3.47 TCP_HIT/200 1342082 GET http://archive.ubuntu.com/ubuntu/dists/focal-updates/main/dist-upgrader-all/current/focal.tar.gz - HIER_NONE/- application/x-gzip

How should I fix the problem with Authentication failed to run the upgrade through proxy server normally? I do not want to switch proxy server (or exclude its configuration from the client) off to fix the problem.

Notes:

1. For possible close-voters: the same happening when I try to upgrade 18.04 LTS to 20.04 LTS with sudo do-release-upgrade -d.


2. I have reported bug 1888058 to launchpad.


3. Removing squid-deb-proxy-client deb-package and specifying proxy by Acquire::http::Proxy "http://192.168.12.34:8000"; in /etc/apt/apt.conf does not change anything.