基本 apt-get install librdkafka1 在Debian8.x上工作，但在Debian9.x上失败。这看起来像是关于libssl的依赖版本问题。Debian8.x有libssl1.0.0，Debian9.x有libssl1.0.2和libssl1.1，但没有libssl1.0.0，这个版本的bump只会导致librdkafka1安装中断。
这在最新的docker debian 9官方图片上很容易复制：

docker pull debian:9
docker run --rm -it debian:9

然后在虚拟机中

cat /etc/debian_version

# 9.4

apt-get update

# Get basics to setup Confluent repository

apt-get install -y wget gnupg2 software-properties-common libssl1.0.2

wget -qO - http://packages.confluent.io/deb/4.1/archive.key | apt-key add -
add-apt-repository "deb [arch=amd64] http://packages.confluent.io/deb/4.1 stable main"
apt-get update
``` `apt-cache policy libssl1.0.2` 结果：

libssl1.0.2:
Installed: 1.0.2l-2+deb9u3
Candidate: 1.0.2l-2+deb9u3
Version table:

***1.0.2l-2+deb9u3 500

    500 http://security.debian.org/debian-security stretch/updates/main amd64 Packages
    100 /var/lib/dpkg/status
 1.0.2l-2+deb9u2 500
    500 http://deb.debian.org/debian stretch/main amd64 Packages

``` apt-cache policy librdkafka1 结果：

librdkafka1:
  Installed: (none)
  Candidate: 0.11.4~1confluent4.1.0-1
  Version table:
     0.11.4~1confluent4.1.0-1 500
        500 http://packages.confluent.io/deb/4.1 stable/main amd64 Packages
     0.9.3-1 500
        500 http://deb.debian.org/debian stretch/main amd64 Packages

以及 apt-get install librdkafka1 结果：

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:
 librdkafka1 : Depends: libssl1.0.0 (>= 1.0.0) but it is not installable
E: Unable to correct problems, you have held broken packages.
``` `apt-cache search libssl` 产生以下结果。请注意，有一个 `libssl1.0.2` 以及 `libssl1.1` 但是没有 `libssl1.0.0` ```
libssl-ocaml - OCaml bindings for OpenSSL (runtime)
libssl-ocaml-dev - OCaml bindings for OpenSSL
libssl-dev - Secure Sockets Layer toolkit - development files
libssl-doc - Secure Sockets Layer toolkit - development documentation
libssl1.1 - Secure Sockets Layer toolkit - shared libraries
libssl1.0-dev - Secure Sockets Layer toolkit - development files
libssl1.0.2 - Secure Sockets Layer toolkit - shared libraries

更新：这个问题在Confluent4.1、librdkafka0.11.4和Debian9.4中仍然存在。