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There are two types of special maps: -hosts and -null. By default, the -hosts map
is used with the /net directory and assumes that
the map entry is the hostname of the NFS server. The automountd daemon dynamically creates a map entry from the server's list of
exported filesystems. For example, a reference to /net/Casey/usr initiates an automatic mount of all the exported filesystems from Casey which can be mounted by the client. References to
the directory under /net/Casey refers to the
corresponding directory relative to the root directory of Casey. The -null map, when included
in a map entry, cancels a map for the directory indicated. This is
especially useful in the AutoFS master map, /etc/auto_master, for cancelling entries that would otherwise be inherited from the +auto_master include entry. To be effective, the -null entry must be inserted before the included
map entry. For information on how to include a map in another map,
see “Including a Map in Another Map”. Automounting All Exported Directories from Any Host Using the -hosts Map | |
To automount all exported directories using the -hosts map, follow these steps: The -hosts map is a built-in
AutoFS map. It enables AutoFS to mount exported directories from any
NFS server found in the hosts database, whenever a user or a process
requests access to one of the exported directories from that server. | | | |  | CAUTION: You may inadvertently cause an NFS mount over X.25 or SLIP,
which is unsupported, or through a slow router or gateway, because
the -hosts map allows NFS access
to any reachable remote system. Mounts over slow links can cause excessive
retransmissions and degrade the performance for all users. | | | | |
When a user or a process requests a directory from an NFS server,
AutoFS creates a subdirectory named after the NFS server, under the
local mount-point you configured in the AutoFS master map. (The conventional
mount-point for the -hosts map is /net). AutoFS then mounts the exported directories
from that server. These exported directories are now accessible. The -hosts map is an indirect
map. It uses the hosts database (the /etc/hosts file, the NIS hosts map, LDAP,
or BIND [DNS]) to discover the host on the network. The NSS configuration
determines which name services to search for host information. For
more information on configuring the NSS, see nsswitch.conf(4). For example, if the server, Basil exports /opt and /apps, and a user on your NFS client enters the
following command: the subdirectory, /basil is
created under /net, and /opt is mounted under /basil. Figure 3-8 shows the automounted
file structure after the user enters the command. In the following example, the server thyme exports the directory /exports/proj1, and a user enters the following command: more /net/thyme/exports/proj1/readme |
The subdirectory /thyme is
created under /net, and /exports/proj1 is mounted under /thyme. Figure 3-9 shows the automounted
directory structure after the user enters the second command. Turning Off an AutoFS Map Using the -null Map | |
To turn off a map using the -null map, follow
these steps: Add a line with the following syntax in the AutoFS
master map: If AutoFS is running, enter the following command
on each client that uses the map, to force AutoFS to reread its maps: This enables AutoFS to ignore the map entry that does not apply
to your host.
Using Executable Maps | |
An executable map is a map whose entries are generated dynamically
by a program or a script. AutoFS determines whether a map is executable,
by checking whether the execute bit is set in its
permissions string. If a map is not executable, ensure that its execute bit is not set. An executable map is an indirect
map. When the AutoFS daemon locates a map whose execute bit is set, then instead
of opening the file and searching for an entry, the daemon executes
the file as a program and passes the key to be located within the
map as an argument. Executable AutoFS maps return a map entry on the
standard output. If they cannot supply a map entry for the key, they
do not return anything. You can list executable AutoFS maps in the
master map or include them in the local AutoFS map files. However,
executable maps cannot be used with NIS and LDAP. For example, an executable map implemented as a shell script,
emulates the AutoFS built-in -hosts map for /net. It obtains
a list of exported file systems from an NFS server (its name is the
key argument). It then formats the path names into a multiple-mount
map entry and sorts the list to order the mounts correctly into a
top-down hierarchy, as follows: !/bin/sh
Server=$1
showmount -e $1 | awk ‘NR > 1 {print $1 “\t'$Server’:” $1 ” \\ “}’ | sort |
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