Sugestão de sources.list para o Debian 9 (Stretch)

Após entrar no terminal como root

#mousepad /etc/apt/sources.list

Copiar o conteúdo abaixo

#------------------------------------------------------------------------------#
# OFFICIAL DEBIAN REPOS
#------------------------------------------------------------------------------#

###### Debian Main Repos
deb http://ftp.br.debian.org/debian/ stretch main contrib non-free
#deb-src http://ftp.br.debian.org/debian/ stretch main contrib non-free

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

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

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