ubuntu sudo apt-get update 失败 解决方法

sudo apt-get update

报了一堆错误：

Err http://cn.archive.ubuntu.com trusty InRelease
Err http://cn.archive.ubuntu.com trusty-updates InRelease
Err http://cn.archive.ubuntu.com trusty-backports InRelease
Err http://cn.archive.ubuntu.com trusty Release.gpg      
W: Failed to fetch http://cn.archive.ubuntu.com/ubuntu/dists/trusty/InRelease  

错误显示了源有问题，好像是被墙了。

查找解决办法，修改/etc/apt/source.list文件。将源改成163的或者阿里的：

deb http://mirrors.163.com/ubuntu/ vivid main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ vivid-security main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ vivid-updates main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ vivid-proposed main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ vivid-backports main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ vivid main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ vivid-security main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ vivid-updates main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ vivid-proposed main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ vivid-backports main restricted universe multiverse

但是source.list文件是只读的，修改不了。

要使用sudo gedit /etc/apt/source.list修改，但是gedit 指令没有装，使用sudo apt-get install gedit又安装不了。

解决办法：（修改DNS）

修改dns:

sudo vi /etc/resolvconf/resolv.conf.d/base

在里面插入：
nameserver 8.8.8.8
nameserver 8.8.4.4

修改好保存!wq，

然后执行

sudo resolvconf -u

再看/etc/resolv.conf，最下面就多了2行：

cat /etc/resolv.conf

　显示

# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
# DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN
nameserver 8.8.8.8
nameserver 8.8.4.4

此时执行

sudo apt-get update

可以运行，然后就可以使用sudo apt-get install **指令了。