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Mount a folder on another folder

Written by Solène, on 22 May 2018.
Tags: #openbsd64

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=127.0.0.0 -mask=255.255.255.0" > /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               loopback

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.