Centos – Where is linux automount’s config

automountcentosmountnfs

I have a centos box, it has some NFS mounts. I'm trying to figure out these NFS filesystems get mounted. And I figured out if I rename /usr/sbin/automount to some else name, after reboot the box, those NFS will not be mount. So I can be sure that automount does those NFS mounts.

But /etc/auto.master show nothing about those NFS's info. I wonder what possibilities there could be about how automount how which NFS to mount?

$ cat /etc/auto.master
#
# Sample auto.master file
# This is an automounter map and it has the following format
# key [ -mount-options-separated-by-comma ] location
# For details of the format look at autofs(5).
#
/misc   /etc/auto.misc
#
# NOTE: mounts done from a hosts map will be mounted with the
#   "nosuid" and "nodev" options unless the "suid" and "dev" 
#   options are explicitly given.
#
/net    -hosts
#
# Include central master map if it can be found using
# nsswitch sources.
#
# Note that if there are entries for /net or /misc (as
# above) in the included master map any keys that are the
# same will not be seen as the first read key seen takes
# precedence.
#
+auto.master

But under /var/run, I find somethings like the following:

prw-------  1 root      root         0 Jan 20 04:36 autofs.fifo-bldmnt
prw-------  1 root      root         0 Jan 20 04:36 autofs.fifo-blr

Best Answer

The file auto.master usually contains this line:

/net    -hosts

An older alternative is (was):

/net    /etc/auto.net

The first line is the so called builtin map referring to the file /etc/hosts and the second example is a so called program map (usually a simple shell script), which may better explain to you how the automounter works.

I guess you have some symbolic links on your system pointing to some path on remote servers under /net (or whatever autofs path prefix is used instead of /net in your copy of the /etc/auto.master).

Whenever a path below the /net anchor is visited the automounter catches this attempt and tries to mount the referenced directory there.

Example:

Assume the automounter is running and in the network there exists a NFS server named HostA which exports a directory named Documents in his /etc/exports file. Then using the command

cd /net/HostA/Documents

can automatically mount this remote directory there without further configuration on the NFS client.

Internals:

Like other services automount uses named pipes for some internal interprocess communication. So the files in /var/run/ mentioned in the question are created by the automount process for this purpose prior to really mounting anything.

In early versions of the autofs package /etc/auto.net was a script which calls the command showmount or kshowmount --no-headers -e NFS-SERVER to obtain a list of exported filesystems from the NFS server.

showmount on the client displays the directories defined in the file /etc/exports on the server side. This file might contain the directory /. But due to security concerns this was never the default on any Linux distribution I've ever seen.

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