由于debian9 停止维护，目前可用的软件源如下：
deb http://archive.debian.org/debian/ stretch main contrib non-free
deb-src http://archive.debian.org/debian/ stretch main contrib non-free
deb http://archive.debian.org/debian-security/ stretch/updates main contrib non-free
deb-src http://archive.debian.org/debian-security/ stretch/updates main contrib non-free
deb http://archive.debian.org/debian/ stretch-backports main contrib non-free
1.设置软件源
cat >/etc/apt/sources.list<
2.更新软件源：apt update -y
3.提示gpg证书错误处理
报错信息：
W: GPG error: http://archive.debian.org/debian stretch-backports InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 648ACFD622F3D138 NO_PUBKEY 0E98404D386FA1D9
安装需要的依赖和软件：
apt-get install software-properties-common dirmngr -y
添加gpg证书验证：
apt-key adv --keyserver keyserver.ubuntu.com --recv-keysapt-key adv --keyserver keyserver.ubuntu.com --recv-keys 648ACFD622F3D138 0E98404D386FA1D9
4.再次执行更新软件源：apt update -y
