deb-src http://ftp.de.debian.org./debian stretch main contrib non-free deb-src http://ftp.de.debian.org./debian stretch-updates main contrib non-free deb-src http://ftp.de.debian.org./debian stretch-proposed-updates main contrib non-free deb-src http://security.debian.org./debian-security stretch/updates main contrib non-free deb-src http://ftp.de.debian.org./debian stretch-backports main contrib non-free