虚拟机安装 ubuntu 后，更新源无效，以及无法联网安装软件的问题

虚拟机安装 ubuntu 后，更新源无效，以及无法联网安装软件；
错误提示：

Err http://security.ubuntu.com/ubuntu/ trusty-security/universe putty-tools amd64 0.63-4ubuntu0.1

Could not resolve 'security.ubuntu.com'

E: Failed to fetch http://security.ubuntu.com/ubuntu/pool/universe/p/putty/putty_0.63-4ubuntu0.1_amd64.deb Could not resolve 'security.ubuntu.com'

E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?

apt-get install putty-tools
Reading package lists… Done
Building dependency tree
Reading state information… Done
Suggested packages:
putty-doc
The following NEW packages will be installed:
putty-tools
0 upgraded, 1 newly installed, 0 to remove and 11 not upgraded.
Need to get 298 kB of archives.
After this operation, 1,522 kB of additional disk space will be used.
WARNING: The following packages cannot be authenticated!
putty-tools
Install these packages without verification? [y/N] Y
Err http://us.archive.ubuntu.com/ubuntu/ trusty-updates/universe putty-tools amd64 0.63-4ubuntu0.1
Could not resolve ‘us.archive.ubuntu.com’
Err http://security.ubuntu.com/ubuntu/ trusty-security/universe putty-tools amd64 0.63-4ubuntu0.1
Could not resolve ‘security.ubuntu.com’
E: Failed to fetch http://security.ubuntu.com/ubuntu/pool/universe/p/putty/putty-tools_0.63-4ubuntu0.1_amd64.deb Could not resolve ‘security.ubuntu.com’
E: Unable to fetch some archives, maybe run apt-get update or try with –fix-missing?

修改DNS，编辑如下文件：

# edit /etc/resolv.conf
nameserver 8.8.8.8
nameserver 8.8.4.4