NFS Client Configuration in Linux

The mount command mounts NFS shares on the client side. Its format is as follows:

# mount -t nfs -o options host:/remote/export /local/directory

This command uses the following variables:

options

A comma-delimited list of mount options;

The hostname, IP address, or fully qualified domain name of the server exporting the file system you wish to mount

/remote/export

The file system or directory being exported from the server, that is, the directory you wish to mount

/local/directory

The client location where /remote/export is mounted

The NFS protocol version used in Red Hat Enterprise Linux 6 is identified by the mount options nfsvers or vers. By default, mount will use NFSv4 with mount -t nfs. If the server does not support NFSv4, the client will automatically step down to a version supported by the server. If the nfsvers/vers option is used to pass a particular version not supported by the server, the mount will fail. The file system type nfs4 is also available for legacy reasons; this is equivalent to running mount -t nfs -o nfsvers=4 host:/remote/export /local/directory.

Refer to man mount for more details.

If an NFS share was mounted manually, the share will not be automatically mounted upon reboot. Red Hat Enterprise Linux offers two methods for mounting remote file systems automatically at boot time: the /etc/fstab file and the autofs service.

Mounting NFS File Systems using /etc/fstab

An alternate way to mount an NFS share from another machine is to add a line to the /etc/fstab file. The line must state the hostname of the NFS server, the directory on the server being exported, and the directory on the local machine where the NFS share is to be mounted. You must be root to modify the /etc/fstab file.

Example Syntax example

The general syntax for the line in /etc/fstab is as follows:

server:/usr/local/pub    /pub   nfs    defaults 0 0

The mount point /pub must exist on the client machine before this command can be executed. After adding this line to /etc/fstab on the client system, use the command mount /pub, and the mount point /pub is mounted from the server.

The /etc/fstab file is referenced by the netfs service at boot time, so lines referencing NFS shares have the same effect as manually typing the mount command during the boot process.

A valid /etc/fstab entry to mount an NFS export should contain the following information:

server:/remote/export /local/directory nfs options 0 0

The variables server, /remote/export, /local/directory, and options are the same ones used when manually mounting an NFS share

 

autofs

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:

• a kernel module that implements a file system, and
• 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. It can be used to mount other file systems including AFS, SMBFS, CIFS, and local file systems.

Important

The nfs-utils package is now a part of both the 'NFS file server' and the 'Network File System Client' groups. As such, it is no longer installed by default with the Base group. Ensure that nfs-utils is installed on the system first before attempting to automount an NFS share.

autofs is also part of the 'Network File System Client' group.

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 (NSS) mechanism. An instance of the autofs 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 autofs version 5, because it uses a single daemon to manage all configured mount points; as such, 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.

Improvements in autofs Version 5 over Version 4

autofs version 5 features the following enhancements over version 4:

Direct map support

Direct maps in autofs 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

Multi-mount 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. These will then 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 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 NSS configuration, it is still not complete. Autofs version 5, on the other hand, is a complete implementation.

Refer to man nsswitch.conf for more information on the supported syntax of this file. 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.

Example Multiple master map entries per autofs mount point

An example is seen in the connectathon test maps for the direct mounts below:

/- /tmp/auto_dcthon

/- /tmp/auto_test3_direct

/- /tmp/auto_test4_direct

 

autofs Configuration

The primary configuration file for the automounter is /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

The variables used in this format are:

mount-point

The autofs mount point, /home, for example.

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. The syntax for a map entry is described below.

options

If supplied, these 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 options were cumulative. This has been changed to implement mixed environment compatibility.

Example /etc/auto.master file

The following is a sample line from /etc/auto.master file (displayed with 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

The variables used in this format are:

mount-point

This refers to 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 multi-mount entry.

options

Whenever supplied, these are the mount options for the map entries that do not specify their own options.

location

This refers to 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 following is a sample of contents from a map file (for example, /etc/auto.misc):

payroll -fstype=nfs personnel:/dev/hda3

sales -fstype=ext3 :/dev/hda4

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 either of the following two commands:

• service autofs start (if the automount daemon has stopped)
• 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:

#  service autofs status

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, consider the following conditions:

• Automounter maps are stored in NIS and the /etc/nsswitch.conf file has the following directive:

automount:    files nis

• The auto.master file contains the following

+auto.master

• The NIS auto.master map file contains the following:

/home auto.home

• 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/&

• The file map /etc/auto.home does not exist.

Given these conditions, let's assume that the client system needs to override the NIS map auto.home and 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.home

+auto.master

The /etc/auto.home map contains the entry:

*    labserver.example.com:/export/home/&

Because the automounter only processes the first occurrence of a mount point, /home will contain the contents of /etc/auto.home instead of the NIS auto.home map.

Alternatively, to augment the site-wide auto.home map with just a few entries, create an /etc/auto.home file map, and in it put the new entries. At the end, include the NIS auto.home map. Then the /etc/auto.home file map will look similar to:

mydir someserver:/export/mydir

+auto.home

Given the NIS auto.home map listed above, ls /home would now output:

beth joe mydir

This last example works as expected because autofs does not include the contents of a file map of the same name as the one it is reading. As such, autofs moves on to the next map source in the nsswitch configuration.

Using LDAP to Store Automounter Maps

LDAP client libraries must be installed on all systems configured to retrieve automounter maps from LDAP. On Red Hat Enterprise Linux, 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, URI, and schema are set appropriately for your site.

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:

Example Setting autofs configuration

DEFAULT_MAP_OBJECT_CLASS="automountMap"

DEFAULT_ENTRY_OBJECT_CLASS="automount"

DEFAULT_MAP_ATTRIBUTE="automountMapName"

DEFAULT_ENTRY_ATTRIBUTE="automountKey"

DEFAULT_VALUE_ATTRIBUTE="automountInformation"

Ensure that these are the only schema entries not commented in the configuration. The automountKey replaces the cn attribute in the rfc2307bis schema. An LDIF of a sample configuration is described below:

Example  LDF configuration

# extended LDIF

#

# LDAPv3

# base <> with scope subtree

# filter: (&(objectclass=automountMap)(automountMapName=auto.master))

# requesting: ALL

#

# auto.master, example.com

dn: automountMapName=auto.master,dc=example,dc=com

objectClass: top

objectClass: automountMap

automountMapName: auto.master

# extended LDIF

#

# LDAPv3

# base <automountMapName=auto.master,dc=example,dc=com> with scope subtree

# filter: (objectclass=automount)

# requesting: ALL

#

# /home, auto.master, example.com

dn: automountMapName=auto.master,dc=example,dc=com

objectClass: automount

cn: /home

automountKey: /home

automountInformation: auto.home

# extended LDIF

#

# LDAPv3

# base <> with scope subtree

# filter: (&(objectclass=automountMap)(automountMapName=auto.home))

# requesting: ALL

#

# auto.home, example.com

dn: automountMapName=auto.home,dc=example,dc=com

objectClass: automountMap

automountMapName: auto.home

# extended LDIF

#

# LDAPv3

# base <automountMapName=auto.home,dc=example,dc=com> with scope subtree

# filter: (objectclass=automount)

# requesting: ALL

#

# foo, auto.home, example.com

dn: automountKey=foo,automountMapName=auto.home,dc=example,dc=com

objectClass: automount

automountKey: foo

automountInformation: filer.example.com:/export/foo

# /, auto.home, example.com

dn: automountKey=/,automountMapName=auto.home,dc=example,dc=com

objectClass: automount

automountKey: /

automountInformation: filer.example.com:/export/&

Common NFS Mount Options

Beyond mounting a file system with NFS on a remote host, it is also possible to specify other options at mount time to make the mounted share easier to use. These options can be used with manual mount commands, /etc/fstab settings, and autofs.

The following are options commonly used for NFS mounts:

intr

Allows NFS requests to be interrupted if the server goes down or cannot be reached.

lookupcache=mode

Specifies how the kernel should manage its cache of directory entries for a given mount point. Valid arguments for mode are all, none, or pos/positive.

nfsvers=version

Specifies which version of the NFS protocol to use, where version is 2, 3, or 4. This is useful for hosts that run multiple NFS servers. If no version is specified, NFS uses the highest version supported by the kernel and mount command.

The option vers is identical to nfsvers, and is included in this release for compatibility reasons.

noacl

Turns off all ACL processing. This may be needed when interfacing with older versions of Red Hat Enterprise Linux, Red Hat Linux, or Solaris, since the most recent ACL technology is not compatible with older systems.

nolock

Disables file locking. This setting is occasionally required when connecting to older NFS servers.

noexec

Prevents execution of binaries on mounted file systems. This is useful if the system is mounting a non-Linux file system containing incompatible binaries.

nosuid

Disables set-user-identifier or set-group-identifier bits. This prevents remote users from gaining higher privileges by running a setuid program.

port=num

port=num — Specifies the numeric value of the NFS server port. If num is 0 (the default), then mount queries the remote host's rpcbind service for the port number to use. If the remote host's NFS daemon is not registered with its rpcbind service, the standard NFS port number of TCP 2049 is used instead.

rsize=num and wsize=num

These settings speed up NFS communication for reads (rsize) and writes (wsize) by setting a larger data block size (num, in bytes), to be transferred at one time. Be careful when changing these values; some older Linux kernels and network cards do not work well with larger block sizes. For NFSv2 or NFSv3, the default values for both parameters is set to 8192. For NFSv4, the default values for both parameters is set to 32768.

sec=mode

Specifies the type of security to utilize when authenticating an NFS connection. Its default setting is sec=sys, which uses local UNIX UIDs and GIDs by using AUTH_SYS to authenticate NFS operations.

sec=krb5 uses Kerberos V5 instead of local UNIX UIDs and GIDs to authenticate users.

sec=krb5i uses Kerberos V5 for user authentication and performs integrity checking of NFS operations using secure checksums to prevent data tampering.

sec=krb5p uses Kerberos V5 for user authentication, integrity checking, and encrypts NFS traffic to prevent traffic sniffing. This is the most secure setting, but it also involves the most performance overhead.

tcp

Instructs the NFS mount to use the TCP protocol.

udp

Instructs the NFS mount to use the UDP protocol.

For a complete list of options and more detailed information on each one, refer to man mount and man nfs.

Starting and Stopping NFS

To run an NFS server, the rpcbind service must be running. To verify that rpcbind is active, use the following command:

    # service rpcbind status

If the rpcbind service is running, then the nfs service can be started. To start an NFS server, use the following command:

    # service nfs start

nfslock must also be started for both the NFS client and server to function properly. To start NFS locking, use the following command:

    # service nfslock start

If NFS is set to start at boot, ensure that nfslock also starts by running chkconfig --list nfslock. If nfslock is not set to on, this implies that you will need to manually run the service nfslock start each time the computer starts. To set nfslock to automatically start on boot, use chkconfig nfslock on.

nfslock is only needed for NFSv2 and NFSv3.

To stop the server, use:

    # service nfs stop

The restart option is a shorthand way of stopping and then starting NFS. This is the most efficient way to make configuration changes take effect after editing the configuration file for NFS. To restart the server type:

    # service nfs restart

The condrestart (conditional restart) option only starts nfs if it is currently running. This option is useful for scripts, because it does not start the daemon if it is not running. To conditionally restart the server type:

    # service nfs condrestart

To reload the NFS server configuration file without restarting the service type:

    # service nfs reload