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.