Ubuntu使用apt update更新时出现NO_PUBKEY 6A030B21BA07F4FB

1.Ubuntu在更新时出现如下错误：

W: GPG error: http://mirrors.ustc.edu.cn/kubernetes/apt kubernetes-xenial InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6A030B21BA07F4FB

2.出现此错误是缺少公钥，添加即可

apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 缺少的公钥（上述截图中红框的公钥）
例：
apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 6A030B21BA07F4FB