dpkg --get-selections

Source: How-To Geek - Show the List of Installed Packages on Ubuntu or Debian

Get the archive (a=) name of the repository to search from:

apt-cache policy

Example output:

root@storage1:~# apt-cache policy
Package files:
 100 /var/lib/dpkg/status
     release a=now
 100 https://debian.snt.utwente.nl/debian bullseye-backports/main amd64 Packages
     release o=Debian Backports,a=bullseye-backports,n=bullseye-backports,l=Debian Backports,c=main,b=amd64
     origin debian.snt.utwente.nl
 500 https://debian.snt.utwente.nl/debian bullseye-updates/main amd64 Packages
     release v=11-updates,o=Debian,a=oldstable-updates,n=bullseye-updates,l=Debian,c=main,b=amd64
     origin debian.snt.utwente.nl
 500 https://debian.snt.utwente.nl/debian-security bullseye-security/non-free amd64 Packages
     release v=11,o=Debian,a=oldstable-security,n=bullseye-security,l=Debian-Security,c=non-free,b=amd64
     origin debian.snt.utwente.nl
 500 https://debian.snt.utwente.nl/debian-security bullseye-security/contrib amd64 Packages
     release v=11,o=Debian,a=oldstable-security,n=bullseye-security,l=Debian-Security,c=contrib,b=amd64
     origin debian.snt.utwente.nl
 500 https://debian.snt.utwente.nl/debian-security bullseye-security/main amd64 Packages
     release v=11,o=Debian,a=oldstable-security,n=bullseye-security,l=Debian-Security,c=main,b=amd64
     origin debian.snt.utwente.nl
 500 https://debian.snt.utwente.nl/debian bullseye/non-free amd64 Packages
     release v=11.11,o=Debian,a=oldstable,n=bullseye,l=Debian,c=non-free,b=amd64
     origin debian.snt.utwente.nl
 500 https://debian.snt.utwente.nl/debian bullseye/contrib amd64 Packages
     release v=11.11,o=Debian,a=oldstable,n=bullseye,l=Debian,c=contrib,b=amd64
     origin debian.snt.utwente.nl
 500 https://debian.snt.utwente.nl/debian bullseye/main amd64 Packages
     release v=11.11,o=Debian,a=oldstable,n=bullseye,l=Debian,c=main,b=amd64
     origin debian.snt.utwente.nl
Pinned packages:

Search the installed packages for packages installed from the repository, in this example from backports/bullseye-backports:

root@storage1:~# aptitude search '?narrow(~i, ~Abackports)'
root@storage1:~# aptitude search '?narrow(~i, ~Abullseye-backports)'

Source: serverfault - How can I list all installed packages from lenny-backports?

Tested on stretch.

To change the apt-get sources.list to only use the ftp.nluug.nl mirror:

#deb http://ftp.nl.debian.org/debian stretch main contrib
deb https://ftp.nluug.nl/debian stretch main contrib

# security updates
#deb http://security.debian.org stretch/updates main contrib
deb https://ftp.nluug.nl/debian stretch-updates main contrib

Cleans out the apt cache in /var/cache/apt/archives/.

apt-get clean

Source:Ask Ubuntu - Is it possible to clean the APT cache from the command line?

For deleting packages that were leftover from previous releases. In this example from debian 7 and 8 on a debian 9 system.

Test removal:

apt-get remove --dry-run `dpkg -l | egrep "deb8|deb7" | awk  '{print $2 " "}' | tr -d "\n"`

Actually remove:

apt-get remove `dpkg -l | egrep "deb8|deb7" | awk  '{print $2 " "}' | tr -d "\n"`

For when apt for some reason thinks it can start using IPv6 in an environment with only IPv4 connectivity.

echo "Acquire::ForceIPv4 \"True\";" > /etc/apt/apt.conf.d/ForceIPv4