Title: Mount a folder on another folder

	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.