使用apt-get时出现 “no public key available” 的解决方法

在 Debian 和 Ubuntu 下运行 apt-get update 刷新更新源的时候，经常遇到 “There is no public key available for the following key IDs” 的问题，具体表现为以下错误提示：

W:There is no public key available for the following key IDs: 3B4FE6ACC0B21F32,
W:There is no public key available for the following key IDs: 3B4FE6ACC0B21F32,

解决方法如下，

sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 3B4FE6ACC0B21F32

把 3B4FE6ACC0B21F32 替换为你出现问题的IDs。另外，如果有如下问题，是因为加入了 natty-backports 源，删掉即可：

W:Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/natty-backports/main/source/Sources 404 Not Found [IP: 91.189.91.13 80],
W:Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/natty-backports/restricted/source/Sources 404 Not Found [IP: 91.189.91.13 80],
W:Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/natty-backports/universe/source/Sources 404 Not Found [IP: 91.189.91.13 80],
W:Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/natty-backports/multiverse/source/Sources 404 Not Found [IP: 91.189.91.13 80],

运行如下命令：

sudo sed -i '/natty\-backports/d' /etc/apt/sources.list{,.d/*}