我们在更新Ubuntu软件源为国内源（比如163）之后，update出现下面这样的错误：
W: Failed to fetch bzip2:/var/lib/apt/lists/partial/mirrors.163.com_ubuntu_dists_trusty-security_main_source_Sources Hash Sum mismatch
E: Some index files failed to download. They have been ignored, or old ones used instead.
这是由于GFW的原因，我们获取到的文件和官方源上的文件大小不一致，导致md5值也不一样，所以会出现Hash Sum mismatch。我们可以通过设置代理的方式解决这个问题。
更简单的方法，我们可以使用aliyun的软件源，我换了之后就没再出现过这个问题了。
deb http://mirrors.aliyun.com/ubuntu/ trusty main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ trusty-security main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ trusty-updates main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ trusty-proposed main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ trusty-backports main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ trusty main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ trusty-security main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ trusty-updates main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ trusty-proposed main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ trusty-backports main restricted universe multiverse
