我无法更新当前版本的 Ubuntu。以下是 sudo apt-get update 的输出：
Err http://archive.canonical.com precise Release.gpg
Could not connect to archive.canonical.com:80 (91.189.92.191). - connect (111: Connection refused) [IP: 91.189.92.191 80]
Err http://archive.ubuntu.com precise Release.gpg
Could not connect to archive.ubuntu.com:80 (91.189.91.15). - connect (111: Connection refused) [IP: 91.189.91.15 80]
Err http://archive.ubuntu.com precise-updates Release.gpg
Unable to connect to archive.ubuntu.com:http: [IP: 91.189.91.15 80]
Err http://archive.ubuntu.com precise-backports Release.gpg
Unable to connect to archive.ubuntu.com:http: [IP: 91.189.91.15 80]
Err http://archive.ubuntu.com precise-security Release.gpg
Unable to connect to archive.ubuntu.com:http: [IP: 91.189.91.15 80]
Err http://archive.ubuntu.com precise-proposed Release.gpg
Unable to connect to archive.ubuntu.com:http: [IP: 91.189.91.15 80]
Reading package lists... Done
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/precise/Release.gpg Could not connect to archive.ubuntu.com:80 (91.189.91.15). - connect (111: Connection refused) [IP: 91.189.91.15 80]
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/precise-updates/Release.gpg Unable to connect to archive.ubuntu.com:http: [IP: 91.189.91.15 80]
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/precise-backports/Release.gpg Unable to connect to archive.ubuntu.com:http: [IP: 91.189.91.15 80]
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/precise-security/Release.gpg Unable to connect to archive.ubuntu.com:http: [IP: 91.189.91.15 80]
W: Failed to fetch http://archive.canonical.com/ubuntu/dists/precise/Release.gpg Could not connect to archive.canonical.com:80 (91.189.92.191). - connect (111: Connection refused) [IP: 91.189.92.191 80]
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/precise-proposed/Release.gpg Unable to connect to archive.ubuntu.com:http: [IP: 91.189.91.15 80]
W: Some index files failed to download. They have been ignored, or old ones used instead.
以下是 /etc/apt/sources.list 的内容：
# deb cdrom:[Ubuntu 12.04.3 LTS _Precise Pangolin_ - Release amd64 (20130820.1)]/ dists/precise/main/binary-i386/
# deb cdrom:[Ubuntu 12.04.3 LTS _Precise Pangolin_ - Release amd64 (20130820.1)]/ dists/precise/restricted/binary-i386/
# deb cdrom:[Ubuntu 12.04.3 LTS _Precise Pangolin_ - Release amd64 (20130820.1)]/ precise main restricted
# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://archive.ubuntu.com/ubuntu precise main restricted universe
## Major bug fix updates produced after the final release of the
## distribution.
deb http://archive.ubuntu.com/ubuntu precise-updates main restricted universe
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team, and may not be under a free licence. Please satisfy yourself as to
## your rights to use the software. Also, please note that software in
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
deb http://archive.ubuntu.com/ubuntu precise-backports main restricted universe
deb http://archive.ubuntu.com/ubuntu precise-security main restricted universe
## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
deb http://archive.canonical.com/ubuntu precise partner
# deb-src http://archive.canonical.com/ubuntu precise partner
## This software is not part of Ubuntu, but is offered by third-party
## developers who want to ship their latest software.
# deb http://extras.ubuntu.com/ubuntu precise main
deb http://archive.ubuntu.com/ubuntu precise-proposed restricted main universe
# deb-src http://extras.ubuntu.com/ubuntu precise main
我的 /etc/apt/sources.list.d/ 目录是空的。有什么办法可以不通过 CD 安装来更新 Ubuntu 吗？
答案1
创建文件：/etc/apt/apt.conf
输入以下行：
acquire::http::proxy "http://username:password@proxy:8080";