Mount a folder on another folder

• Author: Solène
• Date: 22 May 2018
• Tags: openbsd70 openbsd

NILThis 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

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