一、 概述
安装完Ubuntu系统，系统默认选择了Ubuntu的repositoy源，有3个，security、archive、extra。但是这3个源无法用于apt-get update。会有报错，所以需要更改源。
二、 报错信息：
W: GPG error: http://extras.ubuntu.com trusty Release: The following signatures couldn‘t be verified because the public key is not available: NO_PUBKEY 16126D3A3E5C1192
三、分析
应该是所指向的源需要有正确的pubkey，所以简单的说就是这个源不可用。
四、 解决方案
更换源
4.1 备份源的配置文件
sudo cp /etc/apt/sources.list /etc/apt/sources.list_backup
4.2 编辑源的配置文件
vim /etc/apt/sources.list
把所有内容都删除，只保留最上面的注释就行了(因为怕有格式的问题而不可用，所以就想保留格式。)
#
#deb cdrom `````````
#deb cdrom
4.3 把如下的内容复制进文件
deb http://archive.ubuntu.com/ubuntu/ trusty main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ trusty-security main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ trusty-updates main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ trusty-proposed main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ trusty-backports main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ trusty main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ trusty-security main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ trusty-updates main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ trusty-proposed main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ trusty-backports main restricted universe multiverse
4.4 保存并推出即可。
4.5 更新一下
sudo apt-get update.
4.6 大功告成
五、 总结
就是源不可用，还可以把源设置成自己的光盘也行。
六、 如下博客帮了我大忙，转发一下。
Ubuntu 14.04 LTS更新源
-
首先备份源列表:
sudo cp /etc/apt/sources.list /etc/apt/sources.list_backup
- 而后用gedit或其他编辑器打开:
sudo gedit /etc/apt/sources.list
- 从下面列表中选择合适的源，替换掉文件中所有的内容，保存编辑好的文件:
注意：一定要选对版本
- 然后，刷新列表:
sudo apt-get update
Ubuntu 官方更新服务器（欧洲，此为官方源，国内较慢，但无同步延迟问题，电信、移动/铁通、联通等公网用户可以使用)：
deb http://archive.ubuntu.com/ubuntu/ trusty main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ trusty-security main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ trusty-updates main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ trusty-proposed main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ trusty-backports main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ trusty main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ trusty-security main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ trusty-updates main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ trusty-proposed main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ trusty-backports main restricted universe multiverse
Ubuntu官方提供的其他软件（第三方闭源软件等）：
deb http://archive.canonical.com/ubuntu/ trusty partner
deb http://extras.ubuntu.com/ubuntu/ trusty main
骨头兄亲自搭建并维护的 Ubuntu 源（该源位于浙江杭州百兆共享宽带的电信机房)，包含 Deepin 等镜像：
deb http://ubuntu.srt.cn/ubuntu/ trusty main restricted universe multiverse
deb http://ubuntu.srt.cn/ubuntu/ trusty-security main restricted universe multiverse
deb http://ubuntu.srt.cn/ubuntu/ trusty-updates main restricted universe multiverse
deb http://ubuntu.srt.cn/ubuntu/ trusty-proposed main restricted universe multiverse
deb http://ubuntu.srt.cn/ubuntu/ trusty-backports main restricted universe multiverse
deb-src http://ubuntu.srt.cn/ubuntu/ trusty main restricted universe multiverse
deb-src http://ubuntu.srt.cn/ubuntu/ trusty-security main restricted universe multiverse
deb-src http://ubuntu.srt.cn/ubuntu/ trusty-updates main restricted universe multiverse
deb-src http://ubuntu.srt.cn/ubuntu/ trusty-proposed main restricted universe multiverse
deb-src http://ubuntu.srt.cn/ubuntu/ trusty-backports main restricted universe multiverse
网易163更新服务器（广东广州电信/联通千兆双线接入），包含其他开源镜像：
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
搜狐更新服务器（山东联通千兆接入，官方中国大陆地区镜像跳转至此） ，包含其他开源镜像：
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
LupaWorld 更新服务器（浙江杭州电信/联通双线服务器），包含其他开源镜像：
deb http://mirror.lupaworld.com/ubuntu trusty main restricted universe multiverse
deb http://mirror.lupaworld.com/ubuntu trusty-security main restricted universe multiverse
deb http://mirror.lupaworld.com/ubuntu trusty-updates main restricted universe multiverse
deb http://mirror.lupaworld.com/ubuntu trusty-backports main restricted universe multiverse
deb http://mirror.lupaworld.com/ubuntu trusty-proposed main restricted universe multiverse
deb-src http://mirror.lupaworld.com/ubuntu trusty main restricted universe multiverse
deb-src http://mirror.lupaworld.com/ubuntu trusty-security main restricted universe multiverse
deb-src http://mirror.lupaworld.com/ubuntu trusty-updates main restricted universe multiverse
deb-src http://mirror.lupaworld.com/ubuntu trusty-backports main restricted universe multiverse
deb-src http://mirror.lupaworld.com/ubuntu trusty-proposed main restricted universe multiverse
常州贝特康姆软件技术有限公司 更新服务器（江苏常州电信服务器），包含其他开源镜像。
( CN99域名 系 “常州市亚科技术有限公司” 所注册，但此公司与常州贝特康姆的法人代表和营业地址相同。所以本镜像实际即原 CN99。但现在 CN99镜像的域名已改为跳转至网易镜像）:
deb http://centos.bitcomm.cn/ubuntu trusty main restricted universe multiverse
deb http://centos.bitcomm.cn/ubuntu trusty-security main restricted universe multiverse
deb http://centos.bitcomm.cn/ubuntu trusty-updates main restricted universe multiverse
deb http://centos.bitcomm.cn/ubuntu trusty-backports main restricted universe multiverse
deb http://centos.bitcomm.cn/ubuntu trusty-proposed main restricted universe multiverse
deb-src http://centos.bitcomm.cn/ubuntu trusty main restricted universe multiverse
deb-src http://centos.bitcomm.cn/ubuntu trusty-security main restricted universe multiverse
deb-src http://centos.bitcomm.cn/ubuntu trusty-updates main restricted universe multiverse
deb-src http://centos.bitcomm.cn/ubuntu trusty-backports main restricted universe multiverse
deb-src http://centos.bitcomm.cn/ubuntu trusty-proposed main restricted universe multiverse