How automount works

automount operates during two stages:

• startup time

• mount times

automount starts when the command automount(NADM) is executed on a host that will operate as an NFS client. Execution typically takes place at boot time from the command /etc/nfs, but automount can also be executed from the command line. Mandatory for automount operation are one or more files, called ``maps''. These maps contain entries that define what remote filesystems are to be mounted on this client, the mount point for each filesystem, the server that will be the source of each filesystem, and what NFS mount options will characterize each mount. When automount starts, it forks a daemon to serve the mount points specified in the maps. It does this by establishing the daemon as the NFS server for the specified mount points; in effect, it mounts the daemon at these mount points.

When one of these mount points is accessed or crossed, the daemon fields the NFS protocol request as would the real NFS server daemon, nfsd(NADM). The daemon reads the file /etc/mnttab to check whether the remote filesystem is already mounted. If not, the daemon calls a mount routine to mount the filesystem, as specified in the maps. The daemon also records the mount in the file /etc/mnttab. When a predetermined amount of time has elapsed without the filesystem's being accessed, the daemon automatically unmounts the filesystem and removes the record of the mount from the file /etc/mnttab.

Actual and virtual mount points

automount actually mounts all filesystems under the directory /tmp_mnt. It then uses the UNIX operating system's symbolic link support to associate the user-visible mount point with the one in /tmp_mnt. This symbolic link is conceptually similar to a standard UNIX system link(ADM), but it can be established between pathnames residing on different filesystems. The result is that the filesystems, actually mounted under /tmp_mnt, appear to the user to be mounted in a more logical location. See the figures in ``Direct and indirect mounting'' for examples.

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