# official debian repository:
  deb     http://ftp.debian.org/debian/ stretch main contrib non-free
  deb-src http://ftp.debian.org/debian/ stretch main contrib non-free

# security updates:
  deb     http://security.debian.org/ stretch/updates main contrib
  deb-src http://security.debian.org/ stretch/updates main contrib

# backports:
  deb     http://ftp.debian.org/debian/ stretch-backports main contrib non-free
  deb-src http://ftp.debian.org/debian/ stretch-backports main contrib non-free