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-keys  

apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 648ACFD622F3D138 0E98404D386FA1D9

4.再次执行更新软件源：apt update -y