Proxmox needs NFS to map user and group to root and wheel, respectively. FreeNAS requires reverse lookups. So...
- On FreeNAS, set up zpool. Create datasets (e.g. backups, snapshots, images)
- Then create NFS shares, Mapall User: root, Mapall Group: wheel
- SSH into FreeNAS, edit /etc/hosts to add host declaration for forward/reverse lookups