de:Network File System hu:NFS ja:NFS zh-hans:NFS

From Wikipedia:

1. Network File System (NFS) is a distributed file system protocol originally developed by Sun Microsystems in 1984, allowing a user on a client computer to access files over a network in a manner similar to how local storage is accessed.

The NFS FAQ lists file systems well tested and details limitations regarding FAT32.

Installation

Both client and server only require the installation of the nfs-utils package.

Server configuration

Global configuration options are set in /etc/nfs.conf. Users of simple configurations should not need to edit this file.

The NFS server needs a list of directories to share, in the form of exports (see exports(5) for details) which one must define in /etc/exports or /etc/exports.d/*.exports. By default, the directories are exported with their paths as-is; for example:

/data/music    192.168.1.0/24(rw)

/etc/exports

The above will make the directory /data/music mountable as MyServer:/data/music for both NFSv3 and NFSv4.

Custom export root

Shares may be relative to the so-called NFS root. A good security practice is to define a NFS root in a discrete directory tree which will keep users limited to that mount point. Bind mounts are used to link the share mount point to the actual directory elsewhere on the filesystem.

Consider this following example wherein:

1. The NFS root is /srv/nfs.

2. The export is /srv/nfs/music via a bind mount to the actual target /mnt/music.

# mkdir -p /srv/nfs/music /mnt/music
# mount --bind /mnt/music /srv/nfs/music

To make the bind mount persistent across reboots, add it to fstab:

/mnt/music /srv/nfs/music  none   bind   0   0

/etc/fstab

Add directories to be shared and limit them to a range of addresses via a CIDR or hostname(s) of client machines that will be allowed to mount them in /etc/exports, e.g.:

/srv/nfs        192.168.1.0/24(rw,fsid=root)
/srv/nfs/music  192.168.1.0/24(rw,sync)
/srv/nfs/home   192.168.1.0/24(rw,sync)
/srv/nfs/public 192.168.1.0/24(ro,all_squash,insecure) desktop(rw,sync,all_squash,anonuid=99,anongid=99) # map to user/group - in this case nobody

/etc/exports

When using NFSv4, the option fsid=root or fsid=0 denotes the "root" export; if such an export is present, then all other directories must be below it. The rootdir option in the /etc/nfs.conf file has no effect on this. The default behavior, when there is no fsid=0 export, is to behave the same way as in NFSv3.

In the above example, because /srv/nfs is designated as the root, the export /srv/nfs/music is now mountable as MyServer:/music via NFSv4 – note that the root prefix is omitted.

It should be noted that modifying /etc/exports while the server is running will require a re-export for changes to take effect:

# exportfs -arv

To view the current loaded exports state in more detail, use:

# exportfs -v

For more information about all available options see exports(5).

Starting the server

• To provide both NFSv3 and NFSv4 service, start and enable nfs-server.service.

• To provide NFSv4 service exclusively, start and enable nfsv4-server.service.

Users of protocol version 4 exports will probably want to mask at a minimum both rpcbind.service and rpcbind.socket to prevent superfluous services from running. See 76453. Additionally, consider masking nfs-server.service which is pulled in for some reason as well.

Restricting NFS to interfaces/IPs

By default, starting nfs-server.service will listen for connections on all network interfaces, regardless of /etc/exports. This can be changed by defining which IPs and/or hostnames to listen on.

[nfsd]
host=192.168.1.123
# Alternatively, use the hostname.
# host=myhostname

/etc/nfs.conf

Restart nfs-server.service to apply the changes immediately.

Firewall configuration

To enable access of NFSv4-servers through a firewall, TCP port 2049 must be opened for incoming connections. (NFSv4 uses a static port number; it does not use any auxiliary services such as mountd or portmapper.)

To enable access of NFSv3 servers, you will additionally need to open TCP/UDP port 111 for the portmapper (rpcbind), as well as the MOUNT (rpc.mountd) port. By default, rpc.mountd selects a port dynamically, so if you're behind a firewall you will want to edit /etc/nfs.conf to set a static port instead. Use rpcinfo -p to examine the exact ports in use on the NFSv3 server:

100003    3   tcp   2049  nfs
100003    4   tcp   2049  nfs
100227    3   tcp   2049  nfs_acl
...

$ rpcinfo -p

Client configuration

Users intending to use NFS4 with Kerberos need to start and enable nfs-client.target.

Manual mounting

For NFSv3 use this command to show the server's exported file systems:

$ showmount -e servername

For NFSv4 mount the root NFS directory and look around for available mounts:

# mount servername:/ /mountpoint/on/client

Then mount omitting the server's NFS export root:

# mount -t nfs -o vers=4 servername:/music /mountpoint/on/client

If mount fails try including the server's export root (required for Debian/RHEL/SLES, some distributions need -t nfs4 instead of -t nfs):

# mount -t nfs -o vers=4 servername:/srv/nfs/music /mountpoint/on/client

Mount using /etc/fstab

Using fstab is useful for a server which is always on, and the NFS shares are available whenever the client boots up. Edit /etc/fstab file, and add an appropriate line reflecting the setup. Again, the server's NFS export root is omitted.

servername:/music   /mountpoint/on/client   nfs   defaults,timeo=900,retrans=5,_netdev	0 0

/etc/fstab

Some additional mount options to consider:

1. rsize and wsize: The rsize value is the number of bytes used when reading from the server. The wsize value is the number of bytes used when writing to the server. By default, if these options are not specified, the client and server negotiate the largest values they can both support (see nfs(5) for details). After changing these values, it is recommended to test the performance (see #Performance tuning).

2. soft or hard: Determines the recovery behaviour of the NFS client after an NFS request times out. If neither option is specified (or if the hard option is specified), NFS requests are retried indefinitely. If the soft option is specified, then the NFS client fails an NFS request after retrans retransmissions have been sent, causing the NFS client to return an error to the calling application.

1. timeo: The timeo value is the amount of time, in tenths of a second, to wait before resending a transmission after an RPC timeout. The default value for NFS over TCP is 600 (60 seconds). After the first timeout, the timeout value is doubled for each retry for a maximum of 60 seconds or until a major timeout occurs. If connecting to a slow server or over a busy network, better stability can be achieved by increasing this timeout value.

2. retrans: The number of times the NFS client retries a request before it attempts further recovery action. If the retrans option is not specified, the NFS client tries each request three times. The NFS client generates a "server not responding" message after retrans retries, then attempts further recovery (depending on whether the hard mount option is in effect).

3. _netdev: The _netdev option tells the system to wait until the network is up before trying to mount the share - systemd assumes this for NFS.

Mount using /etc/fstab with systemd

Another method is using the x-systemd.automount option which mounts the filesystem upon access:

servername:/home   /mountpoint/on/client  nfs  _netdev,noauto,x-systemd.automount,x-systemd.mount-timeout=10,timeo=14,x-systemd.idle-timeout=1min 0 0

/etc/fstab

To make systemd aware of the changes to fstab, reload systemd and restart remote-fs.target https://bbs.archlinux.org/viewtopic.php?pid=1515377#p1515377.

As systemd unit

Create a new .mount file inside /etc/systemd/system, e.g. mnt-home.mount. See systemd.mount(5) for details.

What= path to share

Where= path to mount the share

Options= share mounting options

[Unit]
Description=Mount home at boot

[Mount]
What=172.16.24.192:/home
Where=/mnt/home
Options=vers=4
Type=nfs
TimeoutSec=30

[Install]
WantedBy=multi-user.target

/etc/systemd/system/mnt-home.mount

To use mnt-home.mount, start the unit and enable it to run on system boot.

automount

To automatically mount a share, one may use the following automount unit:

[Unit]
Description=Automount home

[Automount]
Where=/mnt/home

[Install]
WantedBy=multi-user.target

/etc/systemd/system/mnt-home.automount

Disable/stop the mnt-home.mount unit, and enable/start mnt-home.automount to automount the share when the mount path is being accessed.

Mount using autofs

Using autofs is useful when multiple machines want to connect via NFS; they could both be clients as well as servers. The reason this method is preferable over the earlier one is that if the server is switched off, the client will not throw errors about being unable to find NFS shares. See autofs#NFS network mounts for details.

Tips and tricks

NFSv4 idmapping

[ 3238.356001] NFS: Registering the id_resolver key type
[ 3238.356009] Key type id_resolver registered

# dmesg  grep id_resolver

The NFSv4 protocol represents the local system's UID and GID values on the wire as strings of the form user@domain. The process of translating from UID to string and string to UID is referred to as ID mapping.

Domain

Display the system's effective NFSv4 domain name on stdout.

domain.tld

# nfsidmap -d

Edit to match up the Domain on the server and/or client:

[General]
Domain = guestdomain.tld

/etc/idmapd.conf

static mapping

7 .id_resolver keys found:
 uid:nobody
 user:1
 uid:bin@domain.tld
 uid:foo@domain.tld
 gid:foo@domain.tld
 uid:remote_user@domain.tld
 uid:root@domain.tld

# nfsidmap -l

These steps are only needed if the server and client have different user/group names. Changes are only done in the clients config file.

[Translation]
# The default is nsswitch and other methods exist.
method = static,nsswitch

[Static]
foo@domain.tld = local_foo
remote_user@domain.tld = user

/etc/idmapd.conf

fallback mapping

Only in the client configuration. Local user/group name to be used when a mapping cannot be completed:

[Mapping]
Nobody-User = nobody
Nobody-Group = nobody

/etc/idmapd.conf

Performance tuning

When using NFS on a network with a significant number of clients one may increase the default NFS threads from 8 to 16 or even a higher, depending on the server/network requirements:

[nfsd]
threads=16

/etc/nfs.conf

It may be necessary to tune the rsize and wsize mount options to meet the requirements of the network configuration.

In recent linux kernels (>2.6.18) the size of I/O operations allowed by the NFS server (default max block size) varies depending on RAM size, with a maximum of 1M (1048576 bytes), the max block size of the server will be used even if nfs clients requires bigger rsize and wsize. See https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/5/html/5.8_technical_notes/known_issues-kernel It is possible to change the default max block size allowed by the server by writing to the /proc/fs/nfsd/max_block_size before starting nfsd. For example, the following command restores the previous default iosize of 32k:

# echo 32768 > /proc/fs/nfsd/max_block_size

To make the change permanent, create a systemd-tmpfile:

w /proc/fs/nfsd/max_block_size - - - - 32768

/etc/tmpfiles.d/nfsd-block-size.conf

To mount with the increased rsize and wsize mount options:

# mount -t nfs -o rsize=32768,wsize=32768,vers=4 servername:/srv/nfs/music /mountpoint/on/client

Furthermore, despite the violation of NFS protocol, setting async instead of sync or sync,no_wdelay may potentially achieve a significant performance gain especially on spinning disks. Configure exports with this option and then execute exportfs -arv to apply.

/srv/nfs        192.168.1.0/24(rw,async,crossmnt,fsid=0)
/srv/nfs/music  192.168.1.0/24(rw,async)

/etc/exports

Automatic mount handling

This trick is useful for NFS-shares on a wireless network and/or on a network that may be unreliable. If the NFS host becomes unreachable, the NFS share will be unmounted to hopefully prevent system hangs when using the hard mount option https://bbs.archlinux.org/viewtopic.php?pid=1260240#p1260240.

Make sure that the NFS mount points are correctly indicated in fstab:

lithium:/mnt/data           /mnt/data	        nfs noauto 0 0
lithium:/var/cache/pacman   /var/cache/pacman	nfs noauto 0 0

/etc/fstab

Create the auto_share script that will be used by cron or systemd/Timers to use ICMP ping to check if the NFS host is reachable:

#!/bin/bash

function net_umount {
  umount -l -f $1 &>/dev/null
}

function net_mount {
  mountpoint -q $1 || mount $1
}

NET_MOUNTS=$(sed -e '/^.*#/d' -e '/^.*:/!d' -e 's/\t/ /g' /etc/fstab | tr -s " ")$'\n'b

printf %s "$NET_MOUNTS" | while IFS= read -r line
do
  SERVER=$(echo $line | cut -f1 -d":")
  MOUNT_POINT=$(echo $line | cut -f2 -d" ")

  # Check if server already tested
  if [[ "${server_ok[@]}" =~ "${SERVER}" ]]; then
    # The server is up, make sure the share are mounted
    net_mount $MOUNT_POINT
  elif [[ "${server_notok[@]}" =~ "${SERVER}" ]]; then
    # The server could not be reached, unmount the share
    net_umount $MOUNT_POINT
  else
    # Check if the server is reachable
    ping -c 1 "${SERVER}" &>/dev/null

    if [ $? -ne 0 ]; then
      server_notok[${#server_notok[@]}]=$SERVER
      # The server could not be reached, unmount the share
      net_umount $MOUNT_POINT
    else
      server_ok[${#server_ok[@]}]=$SERVER
      # The server is up, make sure the share are mounted
      net_mount $MOUNT_POINT
    fi
  fi
done

/usr/local/bin/auto_share

Make sure the script is executable.

Next check configure the script to run every X, in the examples below this is every minute.

Cron

* * * * * /usr/local/bin/auto_share

# crontab -e

systemd/Timers

[Unit]
Description=Automount NFS shares every minute

[Timer]
OnCalendar=*-*-* *:*:00

[Install]
WantedBy=timers.target

/etc/systemd/system/auto_share.timer

[Unit]
Description=Automount NFS shares
After=network.target

[Service]
Type=oneshot
ExecStart=/usr/local/bin/auto_share

[Install]
WantedBy=multi-user.target

/etc/systemd/system/auto_share.service

Finally, enable and start auto_share.timer.

Using a NetworkManager dispatcher

NetworkManager can also be configured to run a script on network status change.

The easiest method for mount shares on network status change is to symlink the auto_share script:

# ln -s /usr/local/bin/auto_share /etc/NetworkManager/dispatcher.d/30-nfs.sh

However, in that particular case unmounting will happen only after the network connection has already been disabled, which is unclean and may result in effects like freezing of KDE Plasma applets.

The following script safely unmounts the NFS shares before the relevant network connection is disabled by listening for the down, pre-down and vpn-pre-down events, make sure the script is executable:

#!/bin/sh

# Find the connection UUID with "nmcli con show" in terminal.
# All NetworkManager connection types are supported: wireless, VPN, wired...
WANTED_CON_UUID="CHANGE-ME-NOW-9c7eff15-010a-4b1c-a786-9b4efa218ba9"

if [ "$CONNECTION_UUID" = "$WANTED_CON_UUID" ]; then
    
    # Script parameter $1: network interface name, not used
    # Script parameter $2: dispatched event
    
    case "$2" in
        "up")
            mount -a -t nfs4,nfs 
            ;;
        "down"|"pre-down"|"vpn-pre-down")
            umount -l -a -t nfs4,nfs -f >/dev/null
            ;;
    esac
fi

/etc/NetworkManager/dispatcher.d/30-nfs.sh

Create a symlink inside /etc/NetworkManager/dispatcher.d/pre-down to catch the pre-down events:

# ln -s /etc/NetworkManager/dispatcher.d/30-nfs.sh /etc/NetworkManager/dispatcher.d/pre-down.d/30-nfs.sh

TLS encryption

NFS traffic can be encrypted using TLS as of Linux 6.5 using the xprtsec=tls mount option. To begin, install the ktls-utils package on the client and server, and follow the below configuration steps for each.

Server

Create a private key and obtain a certificate containing your server's DNS name (see Transport Layer Security#Obtaining a certificate for more detail). These files do not need to be added to the system's trust store.

Edit /etc/tlshd.conf to use these files, using your own values for x509.certificate and x509.private_key:

[authenticate.server]
x509.certificate= /etc/nfsd-certificate.pem
x509.private_key= /etc/nfsd-private-key.pem

/etc/tlshd.conf

Now start and enable tlshd.service.

Client

Add the server's TLS certificate generated in the previous step to the system's trust store (see Transport Layer Security#Add a certificate to a trust store for more detail).

Start and enable tlshd.service.

Now you should be able to mount the server using the server's DNS name:

# mount -o xprtsec=tls servername.domain:/ /mountpoint/on/client

Checking journalctl on the client should show that the TLS handshake was successful:

Sep 28 11:14:46 client tlshd[227]: Built from ktls-utils 0.10 on Sep 26 2023 14:24:03
Sep 28 11:15:37 client tlshd[571]: Handshake with servername.domain (192.168.122.100) was successful

$ journalctl -b -u tlshd.service

Troubleshooting

There is a dedicated article NFS/Troubleshooting.

See also

• See also Avahi, a Zeroconf implementation which allows automatic discovery of NFS shares.

• HOWTO: Diskless network boot NFS root

• Microsoft Services for Unix NFS Client info

• NFS on Snow Leopard

• http://chschneider.eu/linux/server/nfs.shtml

• How to do Linux NFS Performance Tuning and Optimization

• Linux: Tune NFS Performance

• Configuring an NFSv4-only Server