Sharing Folders between macOS and FreeBSD under Virtual Box

I use VirtualBox to host a number of guest operating systems on a Mac, mainly for testing. I write a few tests for the emerging Red programming language. Although still in the Alpha phase, Red runs under Windows, macOS, Linux and FreeBSD.

Windows and Linux are well supported by VirtualBox and after installing the VirtualBox Guest Additions you can share folders between the host and guest operating systems. (The additions also provide mouse integration, clipboard sharing and other sharing features). This allows me to keep a single copy of the Red git repository and access it from the different virtual machines.

Virtual Box support of FreeBSD is not quite at the level of the others operating systems. Specifically, the guest additions on FreeBSD don't support folder sharing. In the current alpha, the Red GUI is not implemented for FreeBSD. However, I found that there is a simple method to share folders between the macOS host and the FreeBSD guest using NFS. I was able to get it working quickly and reliably, thanks to lionoceros's clear and thorough post on the FreeBSD forums. 

Note: Installing the VirtualBox additions under FreeBSD is straightforward as there is a package available. Be aware that installing the package requires 1GB of disk space. If you plan to install the additions, these instructions should help.