Notes on the automounter

Contents

Wanting to make it easier (and faster) to access archival data stored in /var/local on penguin, I set up an automounter on sparrow.

The automount master map: /etc/auto.master

The master map lists autofs-controlled mount points on the system, and their corresponding configuration files or network sources known as automount maps. The format of the master map is as follows:

mount-point map-name options

These options are:

mount-point

The autofs mount point; for example, /home.

map-name

The name of a map source which contains a list of mount points, and the file system location from which those mount points should be mounted. Map source types are prefixed with type:, where type is one of file: (the default, so it doesn’t need to be specified,) program: (or exec:), yp:, nisplus:, hesiod, ldap:/ldaps:, multi: or dir:. Consult the the auto.master(5) man page for a full description of these types.

options

If supplied, these applies to all entries in the given map provided they do not themselves have options specified. This behavior is different from autofs version 4 where options were cumulative. This has been changed to implement mixed environment compatibility.

An alternative to updating /etc/auto.master for every new mount point is to create a file ending with .autofs in /etc/auto.master.d. The file can contain one or more master entries.

mount-point map files

If map-name refers to a file, the file contains information for mounting devices such as partitions or NFS exports over one or more mount points under the mount-point named in the master map. For example, if the master map contains:

/home    /etc/auto.home

and the /etc/auto.home map contains:

joe     -fstype=nfs,intr sales://home/joe
kyle    -fstype=nfs,intr accounting://home/kyle

then when joe logs in, the automounter will create /home/joe and attempt to mount the /home/joe directory exported from server sales.

mount-point program maps

The program map type can do all sorts of interesting things, because it can create a directory on the fly and populate with files. (This may not be correct.)

The program receives a key (the name of the directory the user requested with the cd command and a set of variables in the environment:

AUTOFS_UID=
AUTOFS_USER=
AUTOFS_GID=
AUTOFS_GROUP=
AUTOFS_HOME=
AUTOFS_SHOST=

The program starts in the directory specified by the managed mount point. It’s expected to return an entry; that is, a line similar to ones in a map file:

-fstype=bind :/path/to/directory

Note that the backing store for the automounted directory is not deleted when the mount point expires or autofs shuts down, so they will accumulate unless you take steps to remove them manually.

My setup for accessing penguin:/var/local

In /etc/auto.master.d/mnt.autofs:

/mnt    /etc/auto.mnt

In /etc/auto.mnt:

# Automounter map for /mnt
penguin  -fstype=nfs,intr     penguin://var/local
cd       -fstype=iso9660,ro   :/dev/cdrom