我正在使用 Ubuntu 11.04。更新管理器曾经发现新版本“oneiric”，当我使用 ssh 登录时仍然显示此屏幕：
Welcome to Ubuntu 11.04 (GNU/Linux 2.6.38-14-generic x86_64)
* Documentation: https://help.ubuntu.com/
0 packages can be updated.
0 updates are security updates.
New release 'oneiric' available.
Run 'do-release-upgrade' to upgrade to it.
Last login: Wed Apr 25 16:22:48 2012 from ***
但是我当时没有升级，也没有更改我的 apt 源。现在我无法升级到“oneiric”。do-relase-upgrade显示：
$ sudo do-release-upgrade
Checking for a new ubuntu release
No new release found
$
并apt-get dist-upgrade显示：
$ sudo apt-get dist-upgrade
Reading package lists... Done
Building dependency tree
Reading state …