升级到 Ubuntu 14.04.3 LTS 后，MOTD 总是说 0 个新包可以更新

我不是服务器专家，所以我在使用 Ubuntu 时遇到了一些升级问题。一点历史：我的 Digital Ocean droplet 运行的是 Ubuntu 12.04 LTS，然后几个月前我升级到更新的 HWE 以继续接收 12.04 的安全更新。最近我将 Ubuntu 升级到 14.04 LTS。一切似乎都工作正常，除了我没有收到任何通知有要更新的软件包。登录到我的服务器后，每日消息继续显示：

0 packages can be updated.
0 updates are security updates.

现在 3 周内不可能是正确的......？

运行sudo apt-get update没有错误并sudo apt-get upgrade说0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.

我有一种感觉，/etc/apt/sources.list在升级到新的 HWE 内核和新的 LTS 之后，我的系统被破坏了。以下是当前列出的来源。有人看到这个列表有什么问题吗？任何意见，将不胜感激。

$ cat /etc/apt/sources.list

#

# deb cdrom:[Ubuntu-Server 12.04.3 LTS _Precise Pangolin_ - Release i386 (20130820.2)]/ precise main restricted

# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
## DigitalOcean mirrors for blazing speed!
deb http://mirrors.digitalocean.com/ubuntu/ trusty main restricted
deb-src http://mirrors.digitalocean.com/ubuntu/ trusty main restricted
# deb http://us.archive.ubuntu.com/ubuntu/ precise main restricted
# deb-src http://us.archive.ubuntu.com/ubuntu/ precise main restricted

## Major bug fix updates produced after the final release of the
## distribution.
## DigitalOcean mirrors for blazing speed!
deb http://mirrors.digitalocean.com/ubuntu/ trusty-updates main restricted
deb-src http://mirrors.digitalocean.com/ubuntu/ trusty-updates main restricted
# deb http://us.archive.ubuntu.com/ubuntu/ precise-updates main restricted
# deb-src http://us.archive.ubuntu.com/ubuntu/ precise-updates main restricted

## 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.
## DigitalOcean mirrors for blazing speed!
deb http://mirrors.digitalocean.com/ubuntu/ trusty universe
deb-src http://mirrors.digitalocean.com/ubuntu/ trusty universe
deb http://mirrors.digitalocean.com/ubuntu/ trusty-updates universe
deb-src http://mirrors.digitalocean.com/ubuntu/ trusty-updates universe
# deb http://us.archive.ubuntu.com/ubuntu/ precise universe
# deb-src http://us.archive.ubuntu.com/ubuntu/ precise universe
# deb http://us.archive.ubuntu.com/ubuntu/ precise-updates universe
# deb-src 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.
## DigitalOcean mirrors for blazing speed!
deb http://mirrors.digitalocean.com/ubuntu/ trusty multiverse
deb-src http://mirrors.digitalocean.com/ubuntu/ trusty multiverse
deb http://mirrors.digitalocean.com/ubuntu/ trusty-updates multiverse
deb-src http://mirrors.digitalocean.com/ubuntu/ trusty-updates multiverse
# deb http://us.archive.ubuntu.com/ubuntu/ precise multiverse
# deb-src http://us.archive.ubuntu.com/ubuntu/ precise multiverse
# deb http://us.archive.ubuntu.com/ubuntu/ precise-updates multiverse
# deb-src http://us.archive.ubuntu.com/ubuntu/ precise-updates multiverse

## 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.
## DigitalOcean mirrors for blazing speed!
deb http://mirrors.digitalocean.com/ubuntu/ trusty-backports main restricted universe multiverse
deb-src http://mirrors.digitalocean.com/ubuntu/ trusty-backports main restricted universe multiverse
# 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

## DigitalOcean mirrors for blazing speed!
deb http://mirrors.digitalocean.com/ubuntu/ trusty-security main restricted
deb-src http://mirrors.digitalocean.com/ubuntu/ trusty-security main restricted
deb http://mirrors.digitalocean.com/ubuntu/ trusty-security universe
deb-src http://mirrors.digitalocean.com/ubuntu/ trusty-security universe
deb http://mirrors.digitalocean.com/ubuntu/ trusty-security multiverse
deb-src http://mirrors.digitalocean.com/ubuntu/ trusty-security multiverse
# deb http://security.ubuntu.com/ubuntu precise-security main restricted
# deb-src http://security.ubuntu.com/ubuntu precise-security main restricted
# deb http://security.ubuntu.com/ubuntu precise-security universe
# deb-src http://security.ubuntu.com/ubuntu precise-security universe
# deb http://security.ubuntu.com/ubuntu precise-security multiverse
# deb-src http://security.ubuntu.com/ubuntu precise-security multiverse

## 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

## Uncomment the following two lines to add software from Ubuntu's
## 'extras' repository.
## 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-src http://extras.ubuntu.com/ubuntu precise main