NOTE: CentOS Enterprise Linux 5 is built from the Red Hat Enterprise Linux source code. Other than logo and name changes CentOS Enterprise Linux 5 is compatible with the equivalent Red Hat version. This document applies equally to both Red Hat and CentOS Enterprise Linux 5.
One drawback to using /etc/fstab is that, regardless of how infrequently a user accesses the NFS mounted file system, the system must dedicate resources to keep the mounted file system in place. This is not a problem with one or two mounts, but when the system is maintaining mounts to many systems at one time, overall system performance can be affected. An alternative to /etc/fstab is to use the kernel-based automount utility. An automounter consists of two components. One is a kernel module that implements a file system, while the other is a user-space daemon that performs all of the other functions. The automount utility can mount and unmount NFS file systems automatically (on demand mounting) therefore saving system resources. The automount utility can be used to mount other file systems including AFS, SMBFS, CIFS and local file systems.
autofs uses /etc/auto.master (master map) as its default primary configuration file. This can be changed to use another supported network source and name using the autofs configuration (in /etc/sysconfig/autofs) in conjunction with the Name Service Switch mechanism. An instance of the version 4 daemon was run for each mount point configured in the master map and so it could be run manually from the command line for any given mount point. This is not possible with version 5 because it uses a single daemon to manage all configured mount points, so all automounts must be configured in the master map. This is in line with the usual requirements of other industry standard automounters. Mount point, hostname, exported directory, and options can all be specified in a set of files (or other supported network sources) rather than configuring them manually for each host. Please ensure that you have the autofs package installed if you wish to use this service.
18.3.1. What's new in autofs version 5?
Direct map support
Autofs direct maps provide a mechanism to automatically mount file systems at arbitrary points in the file system hierarchy. A direct map is denoted by a mount point of "/-" in the master map. Entries in a direct map contain an absolute path name as a key (instead of the relative path names used in indirect maps).
Lazy mount and unmount support
Multimount map entries describe a hierarchy of mount points under a single key. A good example of this is the "-hosts" map, commonly used for automounting all exports from a host under "/net/<host>" as a multi-mount map entry. When using the "-hosts" map, an 'ls' of "/net/<host>" will mount autofs trigger mounts for each export from <host> and mount and expire them as they are accessed. This can greatly reduce the number of active mounts needed when accessing a server with a large number of exports.
Enhanced LDAP support
The Lightweight Directory Access Protocol, or LDAP, support in autofs version 5 has been enhanced in several ways with respect to autofs version 4. The autofs configuration file (/etc/sysconfig/autofs) provides a mechanism to specify the autofs schema that a site implements, thus precluding the need to determine this via trial and error in the application itself. In addition, authenticated binds to the LDAP server are now supported, using most mechanisms supported by the common LDAP server implementations. A new configuration file has been added for this support: /etc/autofs_ldap_auth.conf. The default configuration file is self-documenting, and uses an XML format.
Proper use of the Name Service Switch (nsswitch) configuration.
The Name Service Switch configuration file exists to provide a means of determining from where specific configuration data comes. The reason for this configuration is to allow administrators the flexibility of using the back-end database of choice, while maintaining a uniform software interface to access the data. While the version 4 automounter is becoming increasingly better at handling the name service switch configuration, it is still not complete. Autofs version 5, on the other hand, is a complete implementation. See the manual page for nsswitch.conf for more information on the supported syntax of this file. Please note that not all nss databases are valid map sources and the parser will reject ones that are invalid. Valid sources are files, yp, nis, nisplus, ldap and hesiod.
Multiple master map entries per autofs mount point
One thing that is frequently used but not yet mentioned is the handling of multiple master map entries for the direct mount point "/-". The map keys for each entry are merged and behave as one map.
An example is seen in the connectathon test maps for the direct mounts below:
The primary configuration file for the automounter is /etc/auto.master, also referred to as the master map which may be changed as described in the introduction section above. 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>
where:
mount-point is the autofs mount point e.g /home.
map-name is 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. The syntax for a map entry is described below.
options if supplied, will apply to all entries in the given map provided they don't themselves have options specified. This behavior is different from autofs version 4 where the options where cumulative. This has been changed to meet our primary goal of mixed environment compatibility.
The following is a sample /etc/auto.master file:
$ cat /etc/auto.master
/home /etc/auto.misc
The general format of maps is similar to the master map, however the "options" appear between the mount point and the location instead of at the end of the entry as in the master map:
<mount-point> [<options>] <location>
where:
<mount-point> is the autofs mount point. This can be a single directory name for an indirect mount or the full path of the mount point for direct mounts. Each direct and indirect map entry key (<mount-point> above) may be followed by a space separated list of offset directories (sub directory names each beginning with a "/") making them what is known as a mutli-mount entry.
<options> if supplied, are the mount options for the map entries that do not specify their own options.
<location> is the file system location such as a local file system path (preceded with the Sun map format escape character ":" for map names beginning with "/"), an NFS file system or other valid file system location.
The first column in a map file indicates the autofs mount point (sales and payroll from the server called personnel). The second column indicates the options for the autofs mount while the third column indicates the source of the mount. Following the above configuration, the autofs mount points will be /home/payroll and /home/sales. The -fstype= option is often omitted and is generally not needed for correct operation.
The automounter will create the directories if they do not exist. If the directories exist before the automounter was started, the automounter will not remove them when it exits. You can start or restart the automount daemon by issuing the following command:
$/sbin/service autofs start
or
$/sbin/service autofs restart
Using the above configuration, if a process requires access to an autofs unmounted directory such as /home/payroll/2006/July.sxc, the automount daemon automatically mounts the directory. If a timeout is specified, the directory will automatically be unmounted if the directory is not accessed for the timeout period.
You can view the status of the automount daemon by issuing the following command in your terminal:
$/sbin/service/autofs status
18.3.3. autofs Common Tasks
18.3.3.1. Overriding or augmenting site configuration files
It can be useful to override site defaults for a specific mount point on a
client system. For example, assuming that the automounter maps are stored in
NIS and the /etc/nsswitch.conf file has the following directive:
automount: files nis
and the NIS auto.master map file contains the following:
/home auto.home
Also assume the NIS auto.home map contains the following:
beth fileserver.example.com:/export/home/beth
joe fileserver.example.com:/export/home/joe
* fileserver.example.com:/export/home/&
and the file map /etc/auto.home does not exist.
For the above example, lets assume that the client system needs to mount home
directories from a different server. In this case, the client will need to use the following /etc/auto.master map:
/home /etc/auto.home2
+auto.master
And the /etc/auto.home2 map contains the entry:
* labserver.example.com:/export/home/&
Because only the first occurrence of a mount point is processed, /home will contain the contents of /etc/auto.home2 instead of the NIS auto.home map.
Alternatively, if you just want to augment the site-wide
auto.home
map with a few entries, create a /etc/auto.home file map, and in it put your new entries and at the end, include the NIS auto.home map. Then the /etc/auto.home file map might look similar to:
mydir someserver:/export/mydir
+auto.home
Given the NIS auto.home map listed above, an ls of /home would now give:
$ ls /home
beth joe mydir
This last example works as expected because autofs knows not to include the contents of a file map of the same name as the one it is reading and so moves on to the next map source in the nsswitch configuration.
18.3.3.2. Using LDAP to Store Automounter Maps
LDAP client libraries must be installed on all systems which are to retrieve automounter maps from LDAP. On RHEL 5, the openldap package should be installed automatically as a dependency of the automounter. To configure LDAP access, modify /etc/openldap/ldap.conf. Ensure that BASE and URI are set appropriately for your site. Please also ensure that the schema is set in the configuration.
The most recently established schema for storing automount maps in LDAP is
described by rfc2307bis. To use this schema it is necessary to set it in the autofs configuration (/etc/sysconfig/autofs) by removing the comment characters from the schema definition. For example:
Ensure that these are the only schema entries not commented in the configuration. Please also note that the automountKey replaces the cn attribute in the rfc2307bis schema. An LDIF of a sample configuration is described below:
Any number of maps can be combined into a single map in this manner. This
feature is no longer present in v5. This is because Version 5 supports
included maps which can be used to attain the same results. Consider the following multi-map example: /home file /etc/auto.home -- nis auto.home
This can be replaced by the following configuration for v5:
/etc/nsswitch.conf must list:
automount: files nis
/etc/auto.master should contain:
/home auto.home
/etc/auto.home should contain:
<entries for the home directory>
+auto.home
In this way, the entries from /etc/auto.home and the nis auto.home map are combined.
Multiple master maps
In autofs version 4, it is possible to merge the contents of master maps
from each source, such as files, nis, hesiod, and LDAP. The version 4
automounter looks for a master map for each of the sources listed in
/etc/nsswitch.conf. The map is read if it exists and its contents are merged into one large auto.master map.
In version 5, this is no longer the behaviour. Only the first master map found from the list of sources in nsswitch.conf is consulted. If it is desirable to merge the contents of multiple master maps, included maps can be used. Consider the following example:
The above configuration will merge the contents of the file-based
auto.master and the NIS-based auto.master. However, because included map entries are only allowed in file maps, there is no way to include both an
NIS auto.master and an LDAP auto.master.
This limitation can be overcome by creating a master maps that have a
different name in the source. In the example above if we had an LDAP
master map named auto.master.ldap we could also add "+auto.master.ldap" to the file based master map and provided that "ldap" is listed as a source in our nsswitch configuration it would also be included.
