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.
这个错误信息是什么意思？据我所知，我安装了正确版本的软件包（0.6.0-1~bpo8+1是>= 0.5.0，对吗？）为什么软件包管理器拒绝查看它？
答案1
我怀疑过渡到时存在一些问题certbot以及虚拟包的使用...（直到最近，虚拟包还不支持版本化依赖项。）
由于certbot现在位于 Jessie 向后移植中并替换了letsencrypt，因此我建议安装它：
apt-get install -t jessie-backports certbot