Стандартный /etc/apt/sources.list для Debian 9 (stretch):
deb http://deb.debian.org/debian/ stretch main contrib non-free
deb-src http://deb.debian.org/debian/ stretch main contrib non-free
deb http://deb.debian.org/debian/ stretch-updates main contrib non-free
deb-src http://deb.debian.org/debian/ stretch-updates main contrib non-free
deb http://security.debian.org/debian-security/ stretch/updates main contrib non-free
deb-src http://security.debian.org/debian-security/ stretch/updates main contrib non-free