How to Use autofs for On‑Demand Mounts in Linux

Static mounting via /etc/fstab automatically mounts all listed remote resources at boot, which can overload network bandwidth and server hardware when many shares are defined but seldom used. Manually running mount each time avoids the waste but is inconvenient.

What autofs Does

The autofs daemon watches for access to a path that is not yet mounted; when a user attempts to open a file or change directory, autofs automatically mounts the required filesystem, saving bandwidth and hardware resources.

Install autofs

[root@localhost ~]# yum install autofs
Loaded plugins: langpacks, product-id, subscription-manager
......
Running transaction
Installing : hesiod-3.2.1-3.el7.x86_64 1/2
Installing : 1:autofs-5.0.7-40.el7.x86_64 2/2
Verifying : hesiod-3.2.1-3.el7.x86_64 1/2
Verifying : 1:autofs-5.0.7-40.el7.x86_64 2/2
Installed:
autofs.x86_64 1:5.0.7-40.el7
Dependency Installed:
hesiod.x86_64 0:3.2.1-3.el7
Complete!

Configure the Master Map

To keep the master map concise, use the mount‑point sub‑map format. The mount point is the directory under which autofs will look for further maps.

# 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).
/media /etc/iso.misc
/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 /etc/auto.master.d/*.autofs
+dir:/etc/auto.master.d
# 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

Create a Sub‑map for the CD-ROM

Define a sub‑map (any name ending with .misc) that describes each device under the chosen mount directory.

# /etc/iso.misc
iso   -fstype=iso9660,ro,nosuid,nodev :/dev/cdrom

Start and Enable autofs

[root@localhost ~]# systemctl start autofs
[root@localhost ~]# systemctl enable autofs
ln -s '/usr/lib/systemd/system/autofs.service' '/etc/systemd/system/multi-user.target.wants/autofs.service'

Test the Dynamic Mount

Before accessing the CD, df -h shows no entry for /dev/cdrom. After navigating to /media/iso, autofs mounts the device automatically.

# df -h (before)
Filesystem            Size  Used Avail Use% Mounted on
/dev/mapper/rhel-root  18G  3.0G   15G  17% /
devtmpfs               905M     0  905M   0% /dev
... (other filesystems)
# cd /media
# ls
# cd iso
# ls -l
total 812
dr-xr-xr-x. 4 root root 2048 May 7 2017 addons
... (list of CD contents)
# df -h (after)
Filesystem            Size  Used Avail Use% Mounted on
/dev/cdrom            3.5G  3.5G     0 100% /media/iso
... (other filesystems)

Thus, autofs provides on‑demand mounting, eliminating unnecessary resource consumption while keeping the workflow seamless for users.