ubuntu 今天更新出错，发现无法更新

细节：W:Failed to fetch bzip2:/var/lib/apt/lists/partial/extras.ubuntu.com_ubuntu_dists_precise_main_source_Sources  Hash Sum mismatch, W:Failed to fetch bzip2:/var/lib/apt/lists/partial/extras.ubuntu.com_ubuntu_dists_precise_main_binary-amd64_Packages  Hash Sum mismatch, W:Failed to fetch bzip2:/var/lib/apt/lists/partial/extras.ubuntu.com_ubuntu_dists_precise_main_binary-i386_Packages  Hash Sum mismatch, E:Some index files failed to download. They have been ignored, or old ones used instead.

发现是运营商从中作梗，偷偷修改了数据包，原因不明

解决方法：