cat <<EOF > /etc/apt/sources.list 
deb http://mirrors.ustc.edu.cn/debian/ bullseye main non-free contrib
deb-src http://mirrors.ustc.edu.cn/debian/ bullseye main non-free contrib
deb http://mirrors.ustc.edu.cn/debian-security/ bullseye-security main
deb-src http://mirrors.ustc.edu.cn/debian-security/ bullseye-security main
deb http://mirrors.ustc.edu.cn/debian/ bullseye-updates main non-free contrib
deb-src http://mirrors.ustc.edu.cn/debian/ bullseye-updates main non-free contrib
deb http://mirrors.ustc.edu.cn/debian/ bullseye-backports main non-free contrib
deb-src http://mirrors.ustc.edu.cn/debian/ bullseye-backports main non-free contrib
EOF