Suppose you have a linux network setup with automounter maps that come from the network (via
LDAP etc.) and you want to block some of them acting on a particular system. In our case we have an automount map that acts on
/opt and mounts various software packages from network shares. The problem with this is that you can’t then install your own stuff locally to
/opt, which is what a lot of Debian/Ubuntu packages expect to be able to do.
It turns out there is a option in the automounter for this sort of situation. There is a built-in map called
-null that blocks any further automounts to a particular mountpoint. In our case we want to block
auto.opt, so we add a line to
auto.master (somewhere before the bottom
Then restart the
autofs service (if stuff was mounted on
/opt then unmount it). Or reboot the system. You should find that you can put stuff in the local
To check the map is blocked you can also run
(also handy for checking what is actually meant to be mapped where).
Another way of doing this that leaves the system
auto.master untouched is to create a file
/etc/auto.master.d/opt.autofs (the first part of the name can be anything you want). Put the same contents in the file, e.g.
Note that using this mechanism normally requires two files – one in
/etc/auto.master.d/ and a map file that it refers to. In this case
-null is a built-in map.
Unfortunately this option is not well documented. Places where it is referred to are:
There are also other built-in maps, e.g.
-fedfs. Of these only the
-hosts map is documented in the
auto.master(5) man page.
-null is confirmed to work in CentOS 7, CentOS 8, Ubuntu 20.04, Debian 10.