Hirsute
Check after cacher is down in the host

+;enp8s0;IPv4;apt-cacher-ng\032proxy\032on\032Keschdeichel;_apt_proxy._tcp;local
+;enp7s0;IPv4;apt-cacher-ng\032proxy\032on\032Keschdeichel;_apt_proxy._tcp;local
+;enp1s0;IPv4;apt-cacher-ng\032proxy\032on\032Keschdeichel;_apt_proxy._tcp;local
=;enp8s0;IPv4;apt-cacher-ng\032proxy\032on\032Keschdeichel;_apt_proxy._tcp;local;Keschdeichel.local;192.168.122.1;3142;
=;enp7s0;IPv4;apt-cacher-ng\032proxy\032on\032Keschdeichel;_apt_proxy._tcp;local;Keschdeichel.local;192.168.122.1;3142;
=;enp1s0;IPv4;apt-cacher-ng\032proxy\032on\032Keschdeichel;_apt_proxy._tcp;local;Keschdeichel.local;192.168.122.1;314

ubuntu@h-deb-proxy:~$ curl  192.168.122.1:3142
curl: (7) Failed to connect to 192.168.122.1 port 3142: Connection refused

ubuntu@h-deb-proxy:~$ /usr/share/squid-deb-proxy-client/apt-avahi-discover 2>/dev/null
error: uncaptured python exception, closing channel <AptAvahiClient> ('192.168.122.1', 3142): 9223372036854775807 (<class 'ConnectionRefusedError'>:[Errno 111] Connection refused [/usr/lib/python3.9/asyncore.py|read|83] [/usr/lib/python3.9/asyncore.py|handle_read_event|417] [/usr/lib/python3.9/asyncore.py|handle_connect_event|425])
error: uncaptured python exception, closing channel <AptAvahiClient> ('192.168.122.1', 3142): 9223372036854775807 (<class 'ConnectionRefusedError'>:[Errno 111] Connection refused [/usr/lib/python3.9/asyncore.py|read|83] [/usr/lib/python3.9/asyncore.py|handle_read_event|417] [/usr/lib/python3.9/asyncore.py|handle_connect_event|425])
error: uncaptured python exception, closing channel <AptAvahiClient> ('192.168.122.1', 3142): 9223372036854775807 (<class 'ConnectionRefusedError'>:[Errno 111] Connection refused [/usr/lib/python3.9/asyncore.py|read|83] [/usr/lib/python3.9/asyncore.py|handle_read_event|417] [/usr/lib/python3.9/asyncore.py|handle_connect_event|425])

Upgrade

ubuntu@h-deb-proxy:~$ sudo apt install squid-deb-proxy-client
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
The following package was automatically installed and is no longer required:
  net-tools
Use 'sudo apt autoremove' to remove it.
The following packages will be upgraded:
  squid-deb-proxy-client
1 upgraded, 0 newly installed, 0 to remove and 14 not upgraded.
Need to get 4876 B of archives.
After this operation, 0 B of additional disk space will be used.
Get:1 http://archive.ubuntu.com/ubuntu hirsute-proposed/universe amd64 squid-deb-proxy-client all 0.8.15ubuntu0.21.04.1 [4876 B]
Fetched 4876 B in 1s (4131 B/s)            
(Reading database ... 99104 files and directories currently installed.)
Preparing to unpack .../squid-deb-proxy-client_0.8.15ubuntu0.21.04.1_all.deb ...
Unpacking squid-deb-proxy-client (0.8.15ubuntu0.21.04.1) over (0.8.15build1) ...
Setting up squid-deb-proxy-client (0.8.15ubuntu0.21.04.1) ...
Scanning processes...                                                                                                                                                                          
Scanning candidates...                                                                                                                                                                         
Scanning linux images...                                                                                                                                                                       

Restarting services...
Service restarts being deferred:
 /etc/needrestart/restart.d/dbus.service
 systemctl restart systemd-logind.service
 systemctl restart unattended-upgrades.service

No containers need to be restarted.

User sessions running outdated binaries:
 ubuntu @ session #1: sshd[759]
 ubuntu @ user manager service: systemd[762]


Test with new version

ubuntu@h-deb-proxy:~$ /usr/share/squid-deb-proxy-client/apt-avahi-discover 2>/dev/null

No more errors bleed into the output, mark verified