3 of 4

File Systems, NFS and AutofsLesson 3 of 4

Mounting on demand with autofs

Mount NFS shares only when someone uses them, with indirect, direct and wildcard maps, and unmount them again when idle.

  • autofs
  • /etc/auto.master
  • /etc/auto.master.d
  • systemctl reload autofs
  • automount -m
  • findmnt
  • use_nfs_home_dirs

45 min5 exercises

After this lesson you can

  • Explain how autofs differs from an fstab mount and when to prefer it.
  • Write a master map entry and a map file for indirect, direct and wildcard (home directory) mounts.
  • Enable the service, trigger a mount, verify it, and diagnose a map that does nothing.

The idea

An /etc/fstab NFS line mounts a share at boot and keeps it mounted whether anyone uses it or not. With hundreds of shares or an unreliable server, that is wasteful and fragile. autofs mounts a share the moment someone accesses its directory and unmounts it after it has been idle for a while.

The service is autofs (package autofs; NFS support from nfs-utils). Its configuration has two layers:

  1. The master map says where autofs manages a directory and which map file describes it.
  2. A map file says, for each name under that directory, what to mount.
/mnt/remote   /etc/auto.remote   --timeout=60
share   -rw,sync   node1:/srv/share

Together: accessing /mnt/remote/share mounts node1:/srv/share. autofs creates the share directory itself; it does not exist until accessed, and ls /mnt/remote may even look empty. That is normal, not a fault.

Three kinds of map

KindMaster entryMap lineResult
Indirect/mnt/remote /etc/auto.remoteshare -rw node1:/srv/shareMounts under the parent: /mnt/remote/share
Direct/- /etc/auto.direct/data/shared -rw node1:/srv/shareMounts at an exact path
Wildcard/rhome /etc/auto.rhome* -rw node1:/home/remoteusers/&One line serves every user: /rhome/NAME mounts /home/remoteusers/NAME

In the wildcard line, * matches whatever name was requested and & substitutes that same name. This is the standard way to provide home directories for many users.

Rather than editing /etc/auto.master, put a file ending in .autofs into /etc/auto.master.d/; it is read as part of the master map. That keeps each task in its own small file.

Service behaviour

  • Start and enable: systemctl enable --now autofs.
  • Master map changes (new file, new timeout) need systemctl reload autofs.
  • Map file changes take effect on the next access, no reload.
  • Do not also mount the same path from fstab, and do not put your own files in the autofs directory: the contents are hidden while it is managed.
  • --timeout=SECONDS is how long the mount stays idle before unmounting. The default is 300.

For users logging in with an NFS home directory, SELinux must allow it: setsebool -P use_nfs_home_dirs on. Without it, login succeeds and the home directory access is denied.

Try it

sudo dnf install -y autofs nfs-utils
echo '/mnt/remote /etc/auto.remote --timeout=60' | sudo tee /etc/auto.master.d/remote.autofs
echo 'share -rw,sync node1:/srv/share' | sudo tee /etc/auto.remote
sudo systemctl enable --now autofs
/mnt/remote /etc/auto.remote --timeout=60
share -rw,sync node1:/srv/share
Created symlink '/etc/systemd/system/multi-user.target.wants/autofs.service' → '/usr/lib/systemd/system/autofs.service'.
findmnt -t nfs4
ls /mnt/remote/share
findmnt -t nfs4
hello.txt
TARGET              SOURCE           FSTYPE OPTIONS
/mnt/remote/share   node1:/srv/share nfs4   rw,relatime,vers=4.2,...

The first findmnt printed nothing: no mount existed until the ls. After an idle minute, it disappears again.

sudo automount -m | head -8
autofs dump map information
===========================

global options: none configured

Mount point: /mnt/remote

source(s):

  instance type(s): file
  map: /etc/auto.remote

  share | -rw,sync node1:/srv/share

This shows exactly what autofs parsed. If your entry is missing here, the master file was not read: check its name ends in .autofs and that you reloaded.

Home directories with one line

echo '/rhome /etc/auto.rhome' | sudo tee /etc/auto.master.d/rhome.autofs
echo '* -rw,sync node1:/home/remoteusers/&' | sudo tee /etc/auto.rhome
sudo setsebool -P use_nfs_home_dirs on
sudo systemctl reload autofs
ls /rhome/remoteuser1
Documents  notes.txt

Diagnosing

If the directory stays empty and nothing mounts:

systemctl is-active autofs
sudo automount -m | grep -A3 'Mount point'
sudo journalctl -u autofs -n 10 --no-pager
getent hosts node1
showmount -e node1

For a detailed trace, stop the service and run the daemon in the foreground: sudo systemctl stop autofs; sudo automount -f -v, reproduce in a second terminal, then Ctrl+C and start the service again.

Going further

Map options accept anything mount does, such as -fstype=nfs4,rw,sec=sys. +auto.master includes NIS or LDAP maps, which is how autofs is usually deployed in larger sites. automount --dumpmaps is another way to inspect the configuration. The ghost option makes keys visible before they are mounted, and /etc/autofs.conf holds global defaults such as timeout. Local file systems and CD-ROMs can be handled the same way (-fstype=iso9660,ro :/dev/sr0).

Practice

0 of 5 done

Do these on your own system. They get harder. Check each result, then tick it off.

  1. Warm-up

    An indirect map for one share

    Make node1:/srv/share (from the previous lesson) appear at /mnt/remote/share on the client, mounted automatically on first access. Use a master drop-in file /etc/auto.master.d/remote.autofs, a map file /etc/auto.remote, read-write, and an idle timeout of 60 seconds. Enable autofs at boot.

    Check your result

    ls /mnt/remote shows nothing at first. After ls /mnt/remote/share the file hello.txt is listed and findmnt /mnt/remote/share shows nfs4. After 60+ idle seconds the mount is gone.

    Hint

    Master: /mnt/remote /etc/auto.remote --timeout=60. Map: share -rw,sync node1:/srv/share. Install autofs, systemctl enable --now autofs.

    Solution
    sudo dnf install -y autofs nfs-utils
echo '/mnt/remote /etc/auto.remote --timeout=60' | sudo tee /etc/auto.master.d/remote.autofs
echo 'share -rw,sync node1:/srv/share' | sudo tee /etc/auto.remote
sudo systemctl enable --now autofs
ls /mnt/remote
ls /mnt/remote/share
findmnt /mnt/remote/share
  2. Practice

    Home directories for remote users

    The server exports /home/remoteusers and each user remoteuser1, remoteuser2, ... has a directory there. On the client, make /rhome/remoteuser1 mount node1:/home/remoteusers/remoteuser1 automatically, and the same for any other user, with one map line. Allow SELinux to use NFS home directories.

    Check your result

    ls /rhome/remoteuser1 mounts and lists the files. ls /rhome/remoteuser2 also works without any extra line. getsebool use_nfs_home_dirs prints on.

    Hint

    Wildcard map: * -rw,sync node1:/home/remoteusers/&. The * matches the directory name asked for, & repeats it. setsebool -P use_nfs_home_dirs on.

    Solution
    echo '/rhome /etc/auto.rhome' | sudo tee /etc/auto.master.d/rhome.autofs
echo '* -rw,sync node1:/home/remoteusers/&' | sudo tee /etc/auto.rhome
sudo setsebool -P use_nfs_home_dirs on
sudo systemctl reload autofs
ls /rhome/remoteuser1
getsebool use_nfs_home_dirs
  3. Practice

    A direct map at an exact path

    Mount node1:/srv/share at exactly /data/shared (not under a parent directory) with a direct map. Verify that /data itself is not otherwise managed by autofs.

    Check your result

    ls /data/shared triggers the mount; findmnt /data/shared shows nfs4. The master entry uses /- as the mount point.

    Hint

    Master: /- /etc/auto.direct. Map: /data/shared -rw,sync node1:/srv/share. The map key is the full path.

    Solution
    echo '/- /etc/auto.direct' | sudo tee /etc/auto.master.d/direct.autofs
echo '/data/shared -rw,sync node1:/srv/share' | sudo tee /etc/auto.direct
sudo systemctl reload autofs
ls /data/shared
findmnt /data/shared
  4. Practice

    When is a reload needed?

    Change the map file /etc/auto.remote by adding a second key docs pointing to another export, without restarting anything, and check it works. Then change the master map timeout and show which change needs a reload.

    Check your result

    A new key in a map file works on the next access. A change to the master map (or a new master file) only takes effect after systemctl reload autofs.

    Hint

    Map files are re-read on access; master maps are read at start and reload.

    Solution
    echo 'docs -rw,sync node1:/srv/docs' | sudo tee -a /etc/auto.remote
ls /mnt/remote/docs
sudo sed -i 's/--timeout=60/--timeout=120/' /etc/auto.master.d/remote.autofs
sudo systemctl reload autofs
automount -m | head
  5. Challenge

    The directory is empty and nothing mounts

    Given a map whose server name is misspelled, diagnose and repair. List the checks in order: service running, master map loaded, map syntax, name resolution, export reachable, journal messages. Then repair.

    Check your result

    systemctl status autofs is active; automount -m shows the mount point and map; journalctl -u autofs says mount(nfs): ... failed or lookup(file); after fixing the name, ls /mnt/remote/share works.

    Hint

    Run the automounter in the foreground for detail: sudo systemctl stop autofs; sudo automount -f -v.

    Solution
    systemctl is-active autofs
sudo automount -m
sudo journalctl -u autofs -n 20 --no-pager
getent hosts node1
showmount -e node1
sudo sed -i 's/nod1/node1/' /etc/auto.remote
ls /mnt/remote/share