Nfs – FreeBSD 10: NFS unable to export folder

Finally I can mount the nfs using another linux machine, and I found that my freebsd rc.conf doesn't have the below line seems to be the root cause of the problem:-

nfs_client_enable="YES"

What does that line do? Can a FreeBSD server be a NFS client without booting up with nfs_client_enable="YES" inside the rc.conf?

I think you have a cart before the horse problem here in that when you export a file system using NFS it locks on to the source direcotory. You are trying to not even have that source directory available at that time and only put something there via a mount later.

This will not work, because once you give NFS a handle on something to share, it will always share that thing, even if it ends up underneath a layer of mounts.

Let's say you setup a directory with a file 'frog' and export it with nfs:

[server] $ mkdir /mnt/test && touch /mnt/test/frog
[server] $ echo '/mnt/test *(ro)' >> /etc/exports
[server] $ exportfs -a

Then you mount it on a client somewhere, you will see the file frog as expected:

[client] $ mkdir /mnt/test
[client] $ mount -t nfs server:/mnt/test /mnt/test
[client] $ ls $/mnt/test
frog

Now let's say you mount something else on top of that folder on the server:

[server] $ mkdir /mnt/test2 && touch /mnt/test2/fish
[server] $ mount -o bind /mnt/test2 /mnt/test
[server] $ ls /mnt/test
fish

Spiffy. But what is nfs serving up?

[client] $ ls /mnt/test
frog

You can't even get to that file frog on the server because it's got a different thing mounted on top of it, but NFS is serving up that under layer!

To make a long story short, if you want to export your file systems via NFS you will need to have them mounted up properly at the time NFS starts up and exports them and they will need to stick around. Exporting file systems that are themselves mounted using autofs will never work quite right. You will need to permanently mount those ISOs in order to export them via NFS.