For reasons unknown one of my favourite and most essential tools arping is not available in OPNsense's package repository. On OPNsense, the preferred method of incorporating mainline FreeBSD packages (according to the official documentation) is to use opnsense-code to install the ports system which, bringing back bittersweet memories of my Gentoo days (search these articles for portage) builds software and its dependencies directly on the local machine, from source code - wholesome like a fresh squeezed glass of lemonade.
opnsense-code ports cd /usr/ports/net/arping make install