Autofs and automountd daemon

The AutoFS file system provides a mechanism for automatically mounting NFS file systems on demand and for automatically unmounting these file systems after a predetermined period of inactivity. The mount points are specified using local or distributed automount maps. Typical usage on Solaris includes mounting remote directories (like /export/home/username ) and serving /net mount point.

Whenever a user on a client computer running the automountd daemon tries to access a remote file or directory, the daemon mounts the remote file system to which that file or directory belongs. This remote file system remains mounted for as long as it is needed. If the remote file system is not accessed for a defined period of time, the automountd daemon automatically unmounts the file system.

The AutoFS service mounts and unmounts file systems as required without any user intervention. The user does not need to use the mount and umount commands and does not need to know the superuser password.

The AutoFS file system enables you to do the following:

• Mount file systems on demand
• Unmount file systems automatically
• Centralize the administration of AutoFS mounts through the use of a name service, which can dramatically reduce administration overhead time
• Create multiple mount resources for read/write or read-only file systems

The automount facility contains three components:

• The AutoFS file system defined by automount maps.
• The automountd daemon
• The automount command

An AutoFS file system’s mount points are defined in the automount maps on the client system. After the AutoFS mount points are set up, activity under the mount points can trigger file systems to be mounted under the mount points. If the automount maps are configured, the AutoFS kernel module monitors mount requests made on the client. If a mount request is made for an AutoFS resource not currently mounted, the AutoFS service calls the automountd daemon, which mounts the requested resource.

The /etc/rc2.d/S74autofs script starts the automountd daemon at boot time. The automountd daemon mounts file systems on demand and unmounts idle mount points.

Note – The automountd daemon is completely independent from the automount command. Because of this separation, you can add, delete, or change map information without having to stop and start the automountd daemon process.

The automount command, called at system startup time, reads the master map to create the initial set of AutoFS mounts.

These AutoFS mounts are not automatically mounted at startup time, they are the points under which file systems are mounted on demand

You do not have to stop and restart the automountd daemon after making changes to existing entries in a direct map, because the daemon is stateless:

• You can modify existing entries in the direct map at any time. 
• The new information is used when the automountd daemon next accesses the map entry to perform a mount.
• Any modifications to indirect maps are automatically used by the automountd daemon.

Key points:

• AutoFS is a client side service that provides automatic mounting using the NFS protocol.  The user does not need to use the mount and umount commands and does not need to know the superuser password.
• The AutoFS file system is initialized by the /etc/rc2.d/S74autofs automount script, which runs automatically when a system is booted. This script runs the automount command, which reads the AutoFS configuration files and also starts the automount daemon automountd. The automountd daemon runs continuously, mounting and unmounting remote directories on an as-needed basis.
• The file system resources for automatic mounting are defined in automount maps. These maps describe information similar to the information specified in the /etc/vfstab file for remote file resources.
• The automount facility contains three components:
  • The AutoFS file system that is partially kernel space partially user space and is defined by automount maps.
  • The automountd daemon
  • The automount command

Automount maps. There are four main types of the AutoFS maps:

• Master map – Lists the other maps used for establishing the AutoFS file system. The automount command reads this map at boot time. Typically includes at least three entry (/net, /home, and /-):

  /etc/auto_master

  /net -hosts [options]
  /home auto_home [options]
  /- auto_direct [options]

• Direct map typically is stored in /etc/auto-direct and lists the mount points as absolute path names.

  /etc/auto_direct

  /opt/oss_apps sandbox:/export/opt/oss_apps

• Indirect map (/etc/auto_home)  Lists the mount points as relative path names. This map uses a relative path to establish the mount point on the client.

  /etc/auto_home

  sandbox softpano:/export/home/sandbox
  checkpoint softpano:/export/home/checkpoint
   

• Special – Provides access to NFS servers by using their host names.

The automount maps can be obtained from ASCII data files, NIS maps, NIS+ tables, or from an LDAP database. Together, these maps describe information similar to the information specified in the /etc/vfstab file for remote file resources. The source for automount maps is determined by the automount entry in the /etc/nsswitch.conf file. For example, the entry:

automount: files

tells the automount command that it should look in the /etc directory for its configuration information. Using nis instead of files tells automount to check the NIS maps for its configuration information.

You do not have to stop and restart the automountd daemon after making changes to existing entries in a direct map, because the daemon is stateless:

• You can modify existing entries in the direct map at any time. 
• The new information is used when the automountd daemon next accesses the map entry to perform a mount.
• Any modifications to indirect maps are automatically used by the automountd daemon.

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Last modified: March 12, 2019