我在代理后面。每次执行 apt-get update 命令时，屏幕上的输出都是这样
Err http://extras.ubuntu.com trusty InRelease
Err http://extras.ubuntu.com trusty Release.gpg
Unable to connect to extras.ubuntu.com:http:
Err http://security.ubuntu.com trusty-security InRelease
Err http://security.ubuntu.com trusty-security Release.gpg
Unable to connect to security.ubuntu.com:http: [IP: 91.189.91.15 80]
Err http://in.archive.ubuntu.com trusty InRelease
Err http://in.archive.ubuntu.com trusty-updates InRelease
Err http://in.archive.ubuntu.com trusty-backports InRelease
Err http://in.archive.ubuntu.com trusty Release.gpg
Unable to connect to in.archive.ubuntu.com:http: [IP: 91.189.88.149 80]
Err http://in.archive.ubuntu.com trusty-updates Release.gpg
Unable to connect to in.archive.ubuntu.com:http: [IP: 91.189.88.149 80]
Err http://in.archive.ubuntu.com trusty-backports Release.gpg
Unable to connect to in.archive.ubuntu.com:http: [IP: 91.189.88.149 …apt ×1