ubuntu软件源更新及其中会遇到的各种问题（以下ID的密钥没有可用的公钥）

本文转自http://blog.csdn.net/humanspider1/article/details/41441679，所有权力归原作者所有，重点看以下ID的密钥没有可用的公钥的解决办法。
ubuntu如果下载软件慢，原因可能是下载源是国外或资源不多，可改变下载源地址，可按如下方法更改
打开/etc/apt/sources.list，添加以下国内软件源

deb http://mirrors.163.com/ubuntu/ trusty
main restricted universe multiverse 

deb http://mirrors.163.com/ubuntu/ trusty-security main restricted universe multiverse 

deb http://mirrors.163.com/ubuntu/ trusty-updates main restricted universe multiverse 

deb http://mirrors.163.com/ubuntu/ trusty-proposed main restricted universe multiverse 

deb http://mirrors.163.com/ubuntu/ trusty-backports main restricted universe multiverse 

deb-src http://mirrors.163.com/ubuntu/ trusty main restricted universe multiverse 

deb-src http://mirrors.163.com/ubuntu/ trusty-security main restricted universe multiverse 

deb-src http://mirrors.163.com/ubuntu/ trusty-updates main restricted universe multiverse 

deb-src http://mirrors.163.com/ubuntu/ trusty-proposed main restricted universe multiverse 

deb-src http://mirrors.163.com/ubuntu/ trusty-backports main restricted universe multiverse 

deb http://mirrors.sohu.com/ubuntu/ trusty main restricted universe multiverse 

deb http://mirrors.sohu.com/ubuntu/ trusty-security main restricted universe multiverse 

deb http://mirrors.sohu.com/ubuntu/ trusty-updates main restricted universe multiverse 

deb http://mirrors.sohu.com/ubuntu/ trusty-proposed main restricted universe multiverse 

deb http://mirrors.sohu.com/ubuntu/ trusty-backports main restricted universe multiverse 

deb-src http://mirrors.sohu.com/ubuntu/ trusty main restricted universe multiverse 

deb-src http://mirrors.sohu.com/ubuntu/ trusty-security main restricted universe multiverse 

deb-src http://mirrors.sohu.com/ubuntu/ trusty-updates main restricted universe multiverse 

deb-src http://mirrors.sohu.com/ubuntu/ trusty-proposed main restricted universe multiverse 

deb-src http://mirrors.sohu.com/ubuntu/ trusty-backports main restricted universe multiverse 

deb http://mirrors.oschina.net/ubuntu/ trusty main restricted universe multiverse 

deb http://mirrors.oschina.net/ubuntu/ trusty-backports main restricted universe multiverse 

deb http://mirrors.oschina.net/ubuntu/ trusty-proposed main restricted universe multiverse 

deb http://mirrors.oschina.net/ubuntu/ trusty-security main restricted universe multiverse 

deb http://mirrors.oschina.net/ubuntu/ trusty-updates main restricted universe multiverse 

deb-src http://mirrors.oschina.net/ubuntu/ trusty main restricted universe multiverse 

deb-src http://mirrors.oschina.net/ubuntu/ trusty-backports main restricted universe multiverse 

deb-src http://mirrors.oschina.net/ubuntu/ trusty-proposed main restricted universe multiverse 

deb-src http://mirrors.oschina.net/ubuntu/ trusty-security main restricted universe multiverse 

deb-src http://mirrors.oschina.net/ubuntu/ trusty-updates main restricted universe multiverse

最后 sudo apt-get update使之生效
如果出现：W: 以下ID的密钥没有可用的公钥
3B4FE6ACC0B21F32
则执行以下命令即可解决
gpg --keyserver subkeys.pgp.net --recv C0B21F32

gpg --export --armor C0B21F32| sudo apt-key add -
其中数据为公钥的后8位
再次apt-get update试试