在Ubuntu和Debian下运行apt-get update刷新更新源的时候却经常遇到“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
额外的，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/*}
via：https://www.vmvps.com/apt-get-error-no-public-key-available-solution.html
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