conntrackd software package provides Connection tracking daemon, you can install in your Debian 9 (Stretch) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install conntrackd
conntrackd is installed in your system.
Make ensure the conntrackd package were installed using the commands given below,
$ sudo dpkg-query -l | grep conntrackd *
You will get with conntrackd package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Network |
| Package | conntrackd |
| Version | 1:1.4.4-5 |
| Description | Connection tracking daemon |
| Sub Section | Net |