我昨天尝试安装 VLC Player，但出现了错误Requires installation of untrusted packages。我花了几个小时寻找这个问题的解决方案，最后终于找到了一个建议，说要更改用于下载更新的服务器。我照做了，但当我在更新管理器中单击“检查”时，出现了 404 错误。我尝试将服务器切换回主服务器（之后又切换了其他几个服务器），但现在仍然出现 404 错误：
我在下载或更新时遇到的另一个错误：
编辑：输出cat /etc/apt/sources.list：
deb-src http://us.archive.ubuntu.com/ubuntu/ precise main restricted #Added by software-properties
# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://us.archive.ubuntu.com/ubuntu/ precise main restricted multiverse
deb-src http://us.archive.ubuntu.com/ubuntu/ precise multiverse universe #Added by software-properties
## Major bug fix updates produced after the final release of the
## distribution.
deb http://us.archive.ubuntu.com/ubuntu/ precise-updates main restricted multiverse
deb-src http://us.archive.ubuntu.com/ubuntu/ precise-updates restricted main multiverse universe #Added by software-properties
## 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.
deb http://us.archive.ubuntu.com/ubuntu/ precise universe
deb http://us.archive.ubuntu.com/ubuntu/ precise-updates universe
## 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://us.archive.ubuntu.com/ubuntu/ precise-backports main restricted universe multiverse
deb-src http://us.archive.ubuntu.com/ubuntu/ precise-backports main restricted universe multiverse #Added by software-properties
deb http://us.archive.ubuntu.com/ubuntu/ precise-security main restricted multiverse
deb-src http://us.archive.ubuntu.com/ubuntu/ precise-security restricted main multiverse universe #Added by software-properties
deb http://us.archive.ubuntu.com/ubuntu/ precise-security 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.
答案1
根据您在我之前的回答的评论部分中粘贴的错误，我还有另一种可能的解决方案：
sudo apt-get clean
cd /var/lib/apt
sudo mv lists lists.old
sudo mkdir -p lists/partial
sudo apt-get clean
sudo apt-get update
这将重建缓存。
礼貌：http://ubuntuforums.org/showthread.php?t=1983220#8
答案2
此链接可能有帮助。它解决了一个非常相似的问题。
基本上，你打开一个终端并输入sudo apt-get update && sudo apt-get upgrade。对我来说就是这样的。
答案3
接受的答案对我的情况（Ubuntu 13.10）没有帮助，但我能够通过命令行而不是通过软件更新程序解决更新问题（以防其他人遇到此问题）。
sudo aptitude upgrade
向我展示了来自不受信任来源（Google 的音乐管理器）的特定软件包，并询问我是否愿意接受并升级。
答案4
我尝试更新系统指示器时遇到了完全相同的错误。
就我而言，这是因为缺少 GPG 密钥。这里有一个答案，它将指导您安装 PPA 管理器，它可以尝试导入所有丢失的密钥。我这样做了，它没有给我任何确认丢失的密钥是否已导入的确认，但我随后运行了更新，它安装得很好。
在这种情况下，我必须说这是一个可怕的（无用的）错误信息。