如何在 Ubuntu 20.04 上设置 NFS 挂载

参考：https://www.digitalocean.com/community/tutorials/how-to-set-up-an-nfs-mount-on-ubuntu-20-04

Introduction

NFS, or Network File System, is a distributed file system protocol that allows you to mount remote directories on your server. This lets you manage storage space in a different location and write to that space from multiple clients. NFS provides a relatively standard and performant way to access remote systems over a network and works well in situations where the shared resources must be accessed regularly.

In this guide, we’ll go over how to install the software needed for NFS functionality on Ubuntu 20.04, configure two NFS mounts on a server and client, and mount and unmount the remote shares.

Prerequisites

We will use two servers in this tutorial, with one sharing part of its filesystem with the other. To follow along, you will need:

• Two Ubuntu 20.04 servers. Each of these should have a non-root user with sudo privileges, a firewall set up with UFW, and private networking, if it’s available to you.
  • For assistance setting up a non-root user with sudo privileges and a firewall, follow our Initial Server Setup with Ubuntu 20.04 guide.
  • If you’re using DigitalOcean Droplets for your server and client, you can read more about setting up a private network in our documentation on How to Create a VPC.

Throughout this tutorial, we refer to the server that shares its directories as the host and the server that mounts these directories as the client. You will need to know the IP address for both. Be sure to use the private network address, if available.

Throughout this tutorial we will refer to these IP addresses by the placeholders host_ip and client_ip. Please substitute as needed.

Step 1 — Downloading and Installing the Components

We’ll begin by installing the necessary components on each server.

On the Host

On the host server, install the nfs-kernel-server package, which will allow you to share your directories. Since this is the first operation that you’re performing with apt in this session, refresh your local package index before the installation:

sudo apt update
sudo apt install nfs-kernel-server

Once these packages are installed, switch to the client server.

On the Client

On the client server, we need to install a package called nfs-common, which provides NFS functionality without including any server components. Again, refresh the local package index prior to installation to ensure that you have up-to-date information:

sudo apt update
sudo apt install nfs-common

Now that both servers have the necessary packages, we can start configuring them.

Step 2 — Creating the Share Directories on the Host

We’re going to share two separate directories, with different configuration settings, in order to illustrate two key ways that NFS mounts can be configured with respect to superuser access.

Superusers can do anything anywhere on their system. However, NFS-mounted directories are not part of the system on which they are mounted, so by default, the NFS server refuses to perform operations that require superuser privileges. This default restriction means that superusers on the client cannot write files as root, reassign ownership, or perform any other superuser tasks on the NFS mount.

Sometimes, however, there are trusted users on the client system who need to perform these actions on the mounted file system but who have no need for superuser access on the host. You can configure the NFS server to allow this, although it introduces an element of risk, as such a user could gain root access to the entire host system.

Example 1: Exporting a General Purpose Mount

In the first example, we’ll create a general-purpose NFS mount that uses default NFS behavior to make it difficult for a user with root privileges on the client machine to interact with the host using those client superuser privileges. You might use something like this to store files which were uploaded using a content management system or to create space for users to easily share project files.

First, make the share directory:

sudo mkdir /var/nfs/general -p

Since we’re creating it with sudo, the directory is owned by the host’s root user:

ls -la /var/nfs/general

Output
drwxr-xr-x 2 root root 4096 May 14 18:36 .

NFS will translate any root operations on the client to the nobody:nogroup credentials as a security measure. Therefore, we need to change the directory ownership to match those credentials.

sudo chown nobody:nogroup /var/nfs/general

You’re now ready to export this directory.

Example 2: Exporting the Home Directory

In our second example, the goal is to make user home directories stored on the host available on client servers, while allowing trusted administrators of those client servers the access they need to conveniently manage users.

To do this, we’ll export the /home directory. Since it already exists, we don’t need to create it. We won’t change the permissions, either. If we did, it could lead to a range of issues for anyone with a home directory on the host machine.

Step 3 — Configuring the NFS Exports on the Host Server

Next, we’ll dive into the NFS configuration file to set up the sharing of these resources.

On the host machine, open the /etc/exports file in your text editor with root privileges:

sudo nano /etc/exports

The file has comments showing the general structure of each configuration line. The syntax is as follows:

                                                 /etc/exports
directory_to_share    client(share_option1,...,share_optionN)

We’ll need to create a line for each of the directories that we plan to share. Be sure to change the client_ip placeholder shown here to your actual IP address:

                                                 /etc/exports
/var/nfs/general    client_ip(rw,sync,no_subtree_check)
/home               client_ip(rw,sync,no_root_squash,no_subtree_check)

Here, we’re using the same configuration options for both directories with the exception of no_root_squash. Let’s take a look at what each of these options mean:

• rw: This option gives the client computer both read and write access to the volume.
• sync: This option forces NFS to write changes to disk before replying. This results in a more stable and consistent environment since the reply reflects the actual state of the remote volume. However, it also reduces the speed of file operations.
• no_subtree_check: This option prevents subtree checking, which is a process where the host must check whether the file is actually still available in the exported tree for every request. This can cause many problems when a file is renamed while the client has it opened. In almost all cases, it is better to disable subtree checking.
• no_root_squash: By default, NFS translates requests from a root user remotely into a non-privileged user on the server. This was intended as security feature to prevent a root account on the client from using the file system of the host as root. no_root_squash disables this behavior for certain shares.

When you are finished making your changes, save and close the file. Then, to make the shares available to the clients that you configured, restart the NFS server with the following command:

sudo systemctl restart nfs-kernel-server

Before you can actually use the new shares, however, you’ll need to be sure that traffic to the shares is permitted by firewall rules.

Step 4 — Adjusting the Firewall on the Host

First, let’s check the firewall status to see if it’s enabled and, if so, to see what’s currently permitted:

sudo ufw status

Output
Status: active

To                         Action      From
--                         ------      ----
OpenSSH                    ALLOW       Anywhere
OpenSSH (v6)               ALLOW       Anywhere (v6)

On our system, only SSH traffic is being allowed through, so we’ll need to add a rule for NFS traffic.

With many applications, you can use sudo ufw app list and enable them by name, but nfs is not one of those. However, because ufw also checks /etc/services for the port and protocol of a service, we can still add NFS by name. Best practice recommends that you enable the most restrictive rule that will still allow the traffic you want to permit, so rather than enabling traffic from just anywhere, we’ll be specific.

Use the following command to open port 2049 on the host, being sure to substitute your client IP address:

sudo ufw allow from client_ip to any port nfs

You can verify the change by typing:

sudo ufw status

You should see traffic allowed from port 2049 in the output:

Output
Status: active

To                         Action      From
--                         ------      ----
OpenSSH                    ALLOW       Anywhere                 
2049                       ALLOW       203.0.113.24        
OpenSSH (v6)               ALLOW       Anywhere (v6)

This confirms that UFW will only allow NFS traffic on port 2049 from our client machine.

Step 5 — Creating Mount Points and Mounting Directories on the Client

Now that the host server is configured and serving its shares, we’ll prepare our client.

In order to make the remote shares available on the client, we need to mount the directories on the host that we want to share to empty directories on the client.

Note: If there are files and directories in your mount point, they will become hidden as soon as you mount the NFS share. To avoid the loss of important files, be sure that if you mount in a directory that already exists that the directory is empty.

We’ll create two directories for our mounts:

sudo mkdir -p /nfs/general
sudo mkdir -p /nfs/home

Now that we have a location to put the remote shares and we’ve opened the firewall, we can mount the shares using the IP address of our host server:

sudo mount host_ip:/var/nfs/general /nfs/general
sudo mount host_ip:/home /nfs/home

These commands will mount the shares from the host computer onto the client machine. You can double-check that they mounted successfully in several ways. You can check this with a mount or findmnt command, but df -h provides a more readable output:

df -h

Output
Filesystem                       Size  Used Avail Use% Mounted on
udev                             474M     0  474M   0% /dev
tmpfs                             99M  936K   98M   1% /run
/dev/vda1                         25G  1.8G   23G   8% /
tmpfs                            491M     0  491M   0% /dev/shm
tmpfs                            5.0M     0  5.0M   0% /run/lock
tmpfs                            491M     0  491M   0% /sys/fs/cgroup
/dev/vda15                       105M  3.9M  101M   4% /boot/efi
tmpfs                             99M     0   99M   0% /run/user/1000
10.132.212.247:/var/nfs/general   25G  1.8G   23G   8% /nfs/general
10.132.212.247:/home              25G  1.8G   23G   8% /nfs/home

Both of the shares we mounted appear at the bottom. Because they were mounted from the same file system, they show the same disk usage. To see how much space is actually being used under each mount point, use the disk usage command du and the path of the mount. The -s flag provides a summary of usage rather than displaying the usage for every file. The -h prints human-readable output.

For example:

du -sh /nfs/home

Output
36K	/nfs/home

This shows us that the contents of the entire home directory is using only 36K of the available space.

Step 6 — Testing NFS Access

Next, let’s test access to the shares by writing something to each of them.

Example 1: The General Purpose Share

First, write a test file to the /var/nfs/general share:

sudo touch /nfs/general/general.test

Then, check its ownership:

ls -l /nfs/general/general.test

Output
-rw-r--r-- 1 nobody nogroup 0 Aug  1 13:31 /nfs/general/general.test

Because we mounted this volume without changing NFS’s default behavior and created the file as the client machine’s root user via the sudo command, ownership of the file defaults to nobody:nogroup. client superusers won’t be able to perform typical administrative actions, like changing the owner of a file or creating a new directory for a group of users, on this NFS-mounted share.

Example 2: The Home Directory Share

To compare the permissions of the General Purpose share with the Home Directory share, create a file in /nfs/home the same way:

sudo touch /nfs/home/home.test

Then look at the ownership of the file:

ls -l /nfs/home/home.test

Copy

Output
-rw-r--r-- 1 root root 0 Aug  1 13:32 /nfs/home/home.test

We created home.test as root using the sudo command, exactly the same way we created the general.test file. However, in this case it is owned by root because we overrode the default behavior when we specified the no_root_squash option on this mount. This allows our root users on the client machine to act as root and makes the administration of user accounts much more convenient. At the same time, it means we don’t have to give these users root access on the host.

Step 7 — Mounting the Remote NFS Directories at Boot

We can mount the remote NFS shares automatically at boot by adding them to /etc/fstab file on the client.

Open this file with root privileges in your text editor:

sudo nano /etc/fstab

At the bottom of the file, add a line for each of our shares. They will look like this:

                                                     /etc/fstab
. . .
host_ip:/var/nfs/general    /nfs/general   nfs auto,nofail,noatime,nolock,intr,tcp,actimeo=1800 0 0
host_ip:/home               /nfs/home      nfs auto,nofail,noatime,nolock,intr,tcp,actimeo=1800 0 0

Note: You can find more information about the options we are specifying here in the NFS man page. You can access this by running the following command:

man nfs

The client will automatically mount the remote partitions at boot, although it may take a few moments to establish the connection and for the shares to be available.

Step 8 — Unmounting an NFS Remote Share

If you no longer want the remote directory to be mounted on your system, you can unmount it by moving out of the share’s directory structure and unmounting, like this:

cd ~
sudo umount /nfs/home
sudo umount /nfs/general

Take note that the command is named umount not unmount as you may expect.

This will remove the remote shares, leaving only your local storage accessible:

df -h

Output
Filesystem                       Size  Used Avail Use% Mounted on
udev                             474M     0  474M   0% /dev
tmpfs                             99M  936K   98M   1% /run
/dev/vda1                         25G  1.8G   23G   8% /
tmpfs                            491M     0  491M   0% /dev/shm
tmpfs                            5.0M     0  5.0M   0% /run/lock
tmpfs                            491M     0  491M   0% /sys/fs/cgroup
/dev/vda15                       105M  3.9M  101M   4% /boot/efi
tmpfs                             99M     0   99M   0% /run/user/1000

If you also want to prevent them from being remounted on the next reboot, edit /etc/fstab and either delete the line or comment it out by placing a # character at the beginning of the line. You can also prevent auto-mounting by removing the auto option, which will allow you to still mount it manually.

Conclusion

In this tutorial, we created an NFS host and illustrated some key NFS behaviours by creating two different NFS mounts, which we shared with a NFS client.

If you’re looking to implement NFS in production, it’s important to note that the protocol itself is not encrypted. In cases where you’re sharing over a private network, this may not be a problem. In other cases, a VPN or some other type of encrypted tunnel will be necessary to protect your data.