cat /etc/apt/apt.conf
APT::Default-Release "testing";
root@toshiba:/home/user# cat /etc/apt/preferences
Package: *
Pin: release a=testing
Pin-Priority: 900

Package: firefox
Pin: release a=unstable
Pin-Priority: 910

Package: *
Pin: release o=Debian,n=unstable
Pin-Priority: 1
root@toshiba:/home/user# apt-cache policy samba
samba:
Установлен: (отсутствует)
Кандидат: 2:4.3.8+dfsg-1
Таблица версий:
2:4.4.2+dfsg-2 500
500 http://http.debian.net/debian unstable/main amd64 Packages
2:4.3.8+dfsg-1 990
990 http://mirror.yandex.ru/debian testing/main amd64 Packages