Title: Mount a folder on another folder
       Author: Solène
       Date: 22 May 2018
       Tags: openbsd70 openbsd
       Description: 
       
       This article will explain quickly how to bind a folder to access it
       from another path. It can be useful to give access to a specific
       folder from a chroot without moving or duplicating the data into the
       chroot.
       
       Real world example: "I want to be able to access my 100GB folder
       /home/my_data/ from my httpd web server chrooted in /var/www/".
       
       The trick on OpenBSD is to use NFS on localhost. It's pretty simple.
       
           # rcctl enable portmap nfsd mountd
           # echo "/home/my_data -network=127.0.0.1 -mask=255.255.255.255" >
       /etc/exports
           # rcctl start portmap nfsd mountd
       
       The order is really important. You can check that the folder is
       available through NFS with the following command:
       
           $ showmount -e
           Exports list on localhost:
           /home/my_data               127.0.0.1
       
       If you don't have any line after "Exports list on localhost:", you
       should kill mountd with `pkill -9 mountd` and start mountd again. I
       experienced it twice when starting all the daemons from the same
       commands but I'm not able to reproduce it. By the way, **mountd** only
       supports reload.
       
       If you modify */etc/exports*, you only need to reload **mountd** using
       `rcctl reload mountd`.
       
       Once you have check that everything was alright, you can mount the
       exported folder on another folder with the command:
       
           # mount localhost:/home/my_data /var/www/htdocs/my_data
       
       You can add `-ro` parameter in the */etc/exports* file on the export
       line if you want it to be read-only where you mount it.
       
       Note: On FreeBSD/DragonflyBSD, you can use `mount_nullfs /from /to`,
       there is no need to setup a local NFS server. And on Linux you can use
       `mount --bind /from /to` and some others ways that I won't cover here.