dpkg 抱怨缺少已安装的软件包

Question

尝试letsencrypt在 Debian Jessie上安装时，我遇到了以下我不太明白的情况。我已经安装了python-acme：

dpkg -l | grep acme
ii  python-acme  0.6.0-1~bpo8+1  all  ACME protocol library for Python 2

现在我正在尝试安装python-letsencrypt：

apt-get install python-letsencrypt
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 python-letsencrypt : Depends: python-acme (>= 0.5.0) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

此错误消息的含义是什么？据我所知，我安装了正确版本的包（0.6.0-1~bpo8+1是>= 0.5.0，对吗？）为什么包管理器拒绝看到它？

Answer 1

我怀疑过渡到certbot和使用虚拟包存在一些问题......（直到最近，虚拟包不支持版本化依赖项。）

由于certbot现在在 Jessie backports 中，并且替换了letsencrypt，我建议改为安装它：

apt-get install -t jessie-backports certbot