Configure
Keys
apt-key add "path/to/key/file"
Configuration
etc/apt/apt.conf
APT::Default-Release buster;
APT::Get::Show-Versions true;
Dpkg::Progress-Fancy true;
Acquire::Check-Valid-Until false;
etc/apt/preferences
Package: *
Pin: release n=stretch-backports
Pin-Priority: 400
Package: *
Pin: release n=buster
Pin-Priority: 200
Package: *
Pin: release n=sid
Pin-Priority: 100
etc/apt/sources.list
Todo
deb.debian.org ↔ ftp.cc.debian.org
deb-src https://deb.debian.org/debian sid main contrib non-free
deb [arch=amd64,i386] https://deb.debian.org/debian sid main contrib non-free
deb [arch=amd64,i386] https://deb.debian.org/debian bullseye main contrib non-free
deb [arch=amd64,i386] https://deb.debian.org/debian bullseye-updates main contrib non-free
deb [arch=amd64,i386] https://deb.debian.org/debian-security bullseye-security main contrib non-free
deb [arch=amd64,i386] https://deb.debian.org/debian buster main contrib non-free
deb [arch=amd64,i386] https://deb.debian.org/debian buster-backports main contrib non-free
deb [arch=amd64,i386] https://deb.debian.org/debian buster-updates main contrib non-free
deb [arch=amd64,i386] https://deb.debian.org/debian-security buster/updates main contrib non-free
deb [arch=amd64] https://deb.debian.org/debian stretch main contrib non-free
deb [arch=amd64] https://deb.debian.org/debian stretch-backports main contrib non-free
deb [arch=amd64] https://deb.debian.org/debian stretch-updates main contrib non-free
deb [arch=amd64] https://deb.debian.org/debian-security stretch/updates main contrib non-free
deb [arch=amd64] https://deb.debian.org/debian jessie main contrib non-free
deb [arch=amd64] https://deb.debian.org/debian-security jessie/updates main contrib non-free
Warning
apt's file protocol handling fails with locations containing spaces
deb file:/media/deb.debian.org/debian stretch main contrib non-free