idn software package provides Command line and Emacs interface to GNU Libidn, 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 idn
idn is installed in your system.
Make ensure the idn package were installed using the commands given below,
$ sudo dpkg-query -l | grep idn *
You will get with idn package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Miscellaneous |
| Package | idn |
| Version | 1.33-1 |
| Description | Command line and Emacs interface to GNU Libidn |
| Sub Section | Miscellaneous |