chkrootkit software package provides rootkit detector, 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 chkrootkit
chkrootkit is installed in your system.
Make ensure the chkrootkit package were installed using the commands given below,
$ sudo dpkg-query -l | grep chkrootkit *
You will get with chkrootkit package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Miscellaneous |
| Package | chkrootkit |
| Version | 0.50-3.2 |
| Description | rootkit detector |
| Sub Section | Miscellaneous |