我們在更新Ubuntu軟件源爲國內源（比如163）之後，update出現下面這樣的錯誤：
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.
這是由於GFW的原因，我們獲取到的文件和官方源上的文件大小不一致，導致md5值也不一樣，所以會出現Hash Sum mismatch。我們可以通過設置代理的方式解決這個問題。
更簡單的方法，我們可以使用aliyun的軟件源，我換了之後就沒再出現過這個問題了。
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
