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Installation and configuration of NFS in RHEL7

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Introduction

To configure NFS Server, we have to install nfs-utils package. Usually, this package is automatically installed during installation of  RHEL or CentOS 7. However, you can install it anytime using yum command.

There are three ways to configure an NFS server under RHEL:

This page discusses the classic Unix way: manually editing /etc/exports and using the /usr/sbin/exportfs command to export NFS file systems.

On RHEL 7, NFS 4 is the default version of NFS. If when making an NFS mount the NFS server offers a previous version of NFS, the client falls automatically back to that version. From a client, you can also force a specific NFS version to be used for the mount, by using the nfsvers= mount option. This can prove useful if you are connecting to a server or a device that offers NFS 3 only.

Installation

First you need to install two packages, unless they are already installed
yum -y install nfs-utils rpcbind	
Then you need to activate then via systemctl
systemctl enable nfs-server
systemctl enable rpcbind
Only for RHEL 7.0. Starting from 7.1 this is not needed as tthe daemon below are the dependencies of nfs-server
systemctl enable nfs-lock  # From RHEL 7.1 on, it is not necessary to enable nfs-lock.service manually. This service will be started automatically, if needed.
systemctl enable nfs-idmap # Same as above 

NOTE:

According to to Enable service `nfs-lock` results in error- No such file or directory - Red Hat Customer Portal  since RHEL7.1 "the nfs-utils get an update to version 1.3.0-0.8.el7. By this, the nfs-lock.service is defined as dependency of rpc-bind.target. So manual enabling service is impossible now. The service is started automatically, if needed."

Start all the services

#  systemctl start rpcbind
#  systemctl start nfs-server
Only for RHEL 7.0. Starting from 7.1 this is done automatically as dependencies of nfs-server
#  systemctl start nfs-lock
#  systemctl start nfs-idmap
After that you need to check the status of nfs
systemctl status nfs-server
systemctl status rpcbind

You can also check the netstat output for listening TCP and UDP ports. At this point nmap from any client should show NFS running. 

NFS Server Configuration

Unlike Solaris and other classic Unixes, RHEL does not use share command. Instead it uses the file /etc/exports which controls which file systems are exported to remote hosts and specifies mode of this export. 

The syntax is pretty typical: blank lines are ignored, comments can be made by starting a line with the hash mark (#), and long lines can be wrapped with a backslash (\).

 

Each line for an exported file system should has the following structure:

<exportred_directory> <host1>(<options>) <hostN>(<options>)...

As you see, each exported file system should be on its own individual line, and any lists of authorized hosts placed after an exported file system must be separated by space characters. Options for each of the hosts must be placed in parentheses directly after the host identifier, without any spaces separating the host and the first parenthesis.

In this structure, replace <export> with the directory being exported, replace <host1> with the host or network to which the export is being shared, and replace (<options> with the options for that host or network. Additional hosts can be specified in a space separated list.  For example:

/sge server.example.com(options)
/home/joeuser 10.194.137.1.0/24(rw)
/Apps 10.194.186.254(rw,no_root_squash) 10.194.186.224(rw,no_root_squash)

The following methods can be used to specify host names:

In its simplest form, /etc/exports need only specify the exported directory and the hosts permitted to access it, as in the following example:

/exported/directory bob.example.com

In the example, bob.example.com can mount /exported/directory/. Because no options are specified in this example, the following default NFS options take effect:

Once you configure NFS server and have an /etc/exports file setup, you can use systemctl to to tell the NFS server processes to reread the config and export all file systems specified in the /etc/exports file. Or you can use exportfs command:

[root@centos-7 ~]# exportfs -a

NOTE

  1. The command exportfs -v lists the currently exported NFS shares on the server. This command will also show the default permissions applied to the NFS share.
  2. Use the showmount -e  command to find out which shares are available.
  3. Use netstat -an | grep your.nfs.server.ip:port to verify the availability of the mount.

By default, access control lists (ACLs) are supported by NFS under RHEL. To disable this feature, specify the no_acl option when exporting the file system. For more about this feature, refer to the chapter titled Network File System (NFS) in the RHEL System Administration Guide.

Each default for every exported file system must be explicitly overridden. For example, if the rw option is not specified, then the exported file system is shared as read-only. The following is a sample line from /etc/exports which overrides two default options:

/Apps 192.168.0.3(rw,sync)

In this example 192.168.0.3 can mount /Apps read/write and all transfers to disk are committed to the disk before the write request by the client is completed.

Additionally, other options are available where no default value is specified. These include the ability to disable sub-tree checking, allow access from insecure ports, and allow insecure file locks (necessary for certain early NFS client implementations). Refer to the exports man page for details on these lesser used options.

NOTES:
The format of the /etc/exports file is very precise, particularly in regards to use of the space character. Remember to always separate exported file systems from hosts and hosts from one another with a space character. However, there should be no other space characters in the file except on comment lines.

For example, the following two lines do not mean the same thing:

/home node25.example.com(rw)
/home node25.example.com (rw)

The first line allows only users from node25.example.com read/write access to the /home directory. The second line allows users from node25.example.com to mount the directory read-only (the default), but the rest of the world can mount it read/write.

For detailed instructions on configuring an NFS server by editing /etc/exports, refer to the chapter titled Network File System (NFS) in the RHEL System Administration Guide.

Restart both rcpbind and nfs to read the configuration file

To restart those two daemons use the command:

systemctl restart nfs 

If the firewall is enabled, ports for NFS will need to be opened.

You can also use

netstat -an | grep your.nfs.server.ip:port 

to verify the availability of the mount.

From the client you can use nmap to see if NFS is available on the server.

The exportfs command

Every file system being exported to remote users via NFS, as well as the access level for those file systems, are listed in the /etc/exports file. When the nfs service starts, the /usr/sbin/exportfs command launches and reads this file, and passes to rpc.mountd and rpc.nfsd the file systems available to remote users.

When issued manually, the /usr/sbin/exportfs command allows the root user to selectively export or unexport directories without restarting the NFS service. When passed the proper options, the /usr/sbin/exportfs command writes the exported file systems to /var/lib/nfs/xtab. Since rpc.mountd refers to the xtab file when deciding access privileges to a file system, changes to the list of exported file systems take effect immediately.

The following is a list of commonly used options available for /usr/sbin/exportfs:

If no options are passed to the /usr/sbin/exportfs command, it displays a list of currently exported file systems.

For more information about the /usr/sbin/exportfs command, refer to the exportfs man page.


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Last modified: December 17, 2020