NFS

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From Wikipedia:

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.
Note:
  • NFS is not encrypted. Tunnel NFS through an encrypted protocol like Kerberos or (secure) VPN when dealing with sensitive data.
  • Unlike Samba, NFS does not have any user authentication by default, client access is restricted by their IP-address/hostname.
  • NFS expects the user and/or user group IDs are the same on both the client and server. Enable NFSv4 idmapping or overrule the UID/GID manually by using anonuid/anongid together with all_squash in /etc/exports.

Installation

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

It is highly recommended to use a time synchronization daemon to keep client/server clocks in sync. Without accurate clocks on all nodes, NFS can introduce unwanted delays.

Configuration

Server

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 exports (see exports(5) for details) which are defined in /etc/exports or /etc/exports.d/*.exports. These shares are 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
Note: ZFS filesystems require special handling of bindmounts, see ZFS#Bind mount.

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

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

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.: 

/etc/exports
/srv/nfs        192.168.1.0/24(rw,sync,crossmnt,fsid=0)
/srv/nfs/music  192.168.1.0/24(rw,sync)
/srv/nfs/home   192.168.1.0/24(rw,sync,nohide)
/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
Note: When using NFSv4, the nfs root directory is specified by the entry denoted by fsid=0, other directories must be below it. The rootdir option in the /etc/nfs.conf file has no effect on this.
Tip:
  • The crossmnt option makes it possible for clients to access all filesystems mounted on a filesystem marked with crossmnt and clients will not be required to mount every child export separately. Note this may not be desirable if a child is shared with a different range of addresses.
  • Instead of crossmnt, one can also use the nohide option on child exports so that they can be automatically mounted when a client mounts the root export. Being different from crossmnt, nohide still respects address ranges of child exports.
  • Use an asterisk (*) to allow access from any interface.

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).

Tip: ip2cidr is a tool to convert an IP ranges to correctly structured CIDR specification.
Note: If the target export is a tmpfs filesystem, the fsid=1 option is required.

Starting the server

Start and enable nfs-server.service.

Warning: A hard dependency of serving NFS (rpc-gssd.service) will wait until the random number generator pool is sufficiently initialized possibly delaying the boot process. This is particularly prevalent on headless servers. It is highly recommended to populate the entropy pool using a utility such as Rng-tools (if TPM is supported) or Haveged in these scenarios.
Note: If exporting ZFS shares, also start/enable zfs-share.service. Without this, ZFS shares will no longer be exported after a reboot. See ZFS#NFS.

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. 

/etc/nfs.conf
[nfsd]
host=192.168.1.123
# Alternatively, use the hostname.
# host=myhostname

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

Firewall configuration

To enable access through a firewall, TCP and UDP ports 111, 2049, and 20048 may need to be opened when using the default configuration; use rpcinfo -p to examine the exact ports in use on the server: 

$ rpcinfo -p | grep nfs
100003    3   tcp   2049  nfs
100003    4   tcp   2049  nfs
100227    3   tcp   2049  nfs_acl

When using NFSv4, make sure TCP port 2049 is open. No other port opening should be required: 

/etc/iptables/iptables.rules
-A INPUT -p tcp -m tcp --dport 2049 -j ACCEPT

When using an older NFS version, make sure other ports are open: 

# iptables -A INPUT -p tcp -m tcp --dport 111 -j ACCEPT
# iptables -A INPUT -p tcp -m tcp --dport 2049 -j ACCEPT
# iptables -A INPUT -p tcp -m tcp --dport 20048 -j ACCEPT
# iptables -A INPUT -p udp -m udp --dport 111 -j ACCEPT
# iptables -A INPUT -p udp -m udp --dport 2049 -j ACCEPT
# iptables -A INPUT -p udp -m udp --dport 20048 -j ACCEPT

To have this configuration load on every system start, edit /etc/iptables/iptables.rules to include the following lines: 

/etc/iptables/iptables.rules
-A INPUT -p tcp -m tcp --dport 111 -j ACCEPT
-A INPUT -p tcp -m tcp --dport 2049 -j ACCEPT
-A INPUT -p tcp -m tcp --dport 20048 -j ACCEPT
-A INPUT -p udp -m udp --dport 111 -j ACCEPT
-A INPUT -p udp -m udp --dport 2049 -j ACCEPT
-A INPUT -p udp -m udp --dport 20048 -j ACCEPT

The previous commands can be saved by executing: 

# iptables-save > /etc/iptables/iptables.rules
Warning: This command will override the current iptables start configuration with the current iptables configuration!

If using NFSv3 and the above listed static ports for rpc.statd and lockd the following ports may also need to be added to the configuration: 

/etc/iptables/iptables.rules
-A INPUT -p tcp -m tcp --dport 32765 -j ACCEPT
-A INPUT -p tcp -m tcp --dport 32803 -j ACCEPT
-A INPUT -p udp -m udp --dport 32765 -j ACCEPT
-A INPUT -p udp -m udp --dport 32803 -j ACCEPT

To apply changes, Restart iptables.service.

Enabling NFSv4 idmapping

Tango-view-fullscreen.pngThis article or section needs expansion.Tango-view-fullscreen.png

Reason: Missing lookup information, static binding examples, etc. (Discuss in Talk:NFS)
Note:
  • NFSv4 idmapping does not work with the default sec=sys mount option. [1]
  • NFSv4 idmapping needs to be enabled on both the client and server.
  • Another option is to make sure the user and group IDs (UID and GID) match on both the client and server.
  • Enabling/starting nfs-idmapd.service should not be needed as it has been replaced with a new id mapper:
# dmesg | grep id_resolver
[ 3238.356001] NFS: Registering the id_resolver key type
[ 3238.356009] Key type id_resolver registered

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. See nfsidmap(8) for details.

Even though idmapd may be running, it may not be fully enabled. If /sys/module/nfs/parameters/nfs4_disable_idmapping or /sys/module/nfsd/parameters/nfs4_disable_idmapping returns Y on a client/server, enable it by:

Note: The kernel modules nfs4 and nfsd need to be loaded (respectively) for the following paths to be available.

On the client: 

# echo N > /sys/module/nfs/parameters/nfs4_disable_idmapping

On the server: 

# echo N > /sys/module/nfsd/parameters/nfs4_disable_idmapping

Set as module option to make this change permanent, i.e.: 

/etc/modprobe.d/nfsd.conf
options nfs nfs4_disable_idmapping=0
options nfsd nfs4_disable_idmapping=0

To fully use idmapping, make sure the domain is configured in /etc/idmapd.conf on both the server and the client: 

/etc/idmapd.conf
# The following should be set to the local NFSv4 domain name
# The default is the host's DNS domain name.
Domain = domain.tld

See [2] for details.

Client

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 server:/ /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
Note: Server name needs to be a valid hostname (not just IP address). Otherwise mounting of remote share will hang.

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. 

/etc/fstab
servername:/music   /mountpoint/on/client   nfs   defaults,timeo=900,retrans=5,_netdev	0 0
Note: Consult nfs(5) and mount(8) for more mount options.

Some additional mount options to consider:

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).
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 a NFS request after retrans retransmissions have been sent, causing the NFS client to return an error to the calling application.
Warning: A so-called soft timeout can cause silent data corruption in certain cases. As such, use the soft option only when client responsiveness is more important than data integrity. Using NFS over TCP or increasing the value of the retrans option may mitigate some of the risks of using the soft option.
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.
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).
_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.
Note: Setting the sixth field (fs_passno) to a nonzero value may lead to unexpected behaviour, e.g. hangs when the systemd automount waits for a check which will never happen.

Mount using /etc/fstab with systemd

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

/etc/fstab
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

To make systemd aware of the changes to fstab, reload systemd and restart remote-fs.target [3].

Tango-inaccurate.pngThe factual accuracy of this article or section is disputed.Tango-inaccurate.png

Reason: Not everyone uses NetworkManager. Refer to Systemd#Running services after the network is up instead? (Discuss in Talk:NFS)
Tip:
  • The noauto mount option will not mount the NFS share until it is accessed: use auto for it to be available immediately.
    If experiencing any issues with the mount failing due to the network not being up/available, enable NetworkManager-wait-online.service. It will ensure that network.target has all the links available prior to being active.
  • The users mount option would allow user mounts, but be aware it implies further options as noexec for example.
  • The x-systemd.idle-timeout=1min option will unmount the NFS share automatically after 1 minute of non-use. Good for laptops which might suddenly disconnect from the network.
  • If shutdown/reboot holds too long because of NFS, enable NetworkManager-wait-online.service to ensure that NetworkManager is not exited before the NFS volumes are unmounted.
  • Do not add the x-systemd.requires=network-online.target mount option as this can lead to ordering cycles within systemd [4]. systemd adds the network-online.target dependency to the unit for _netdev mount automatically.
  • Using the nocto option may improve performance for read-only mounts, but should be used only if the data on the server changes only occasionally.

As systemd unit

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

Note: Make sure the filename corresponds to the mountpoint you want to use. E.g. the unit name mnt-home.mount can only be used if you are going to mount the share under /mnt/home. Otherwise the following error might occur: systemd[1]: mnt-home.mount: Where= setting does not match unit name. Refusing..

What= path to share

Where= path to mount the share

Options= share mounting options

Note:
  • Network mount units automatically acquire After dependencies on remote-fs-pre.target, network.target and network-online.target, and gain a Before dependency on remote-fs.target unless nofail mount option is set. Towards the latter a Wants unit is added as well.
  • Append noauto to Options preventing automatically mount during boot (unless it is pulled in by some other unit).
  • If you want to use a hostname for the server you want to share (instead of an IP address), add nss-lookup.target to After. This might avoid mount errors at boot time that do not arise when testing the unit.
/etc/systemd/system/mnt-home.mount
[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
Tip: In case of an unreachable system, append ForceUnmount=true to [Mount], allowing the export to be (force-)unmounted.

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: 

/etc/systemd/system/mnt-home.automount
[Unit]
Description=Automount home

[Automount]
Where=/mnt/home

[Install]
WantedBy=multi-user.target

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

Tip: Append TimeoutIdleSec to enable auto unmount. See systemd.automount(5) for details.

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

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: 

/etc/nfs.conf
[nfsd]
threads=16

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
Note: This is mainly useful for 32-bit servers when dealing with the large numbers of nfsd threads. Lowering the max_block_size may decrease NFS performance on modern hardware.

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

/etc/tmpfiles.d/nfsd-block-size.conf
w /proc/fs/nfsd/max_block_size - - - - 32768

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. 

/etc/exports
/srv/nfs        192.168.1.0/24(rw,async,crossmnt,fsid=0)
/srv/nfs/music  192.168.1.0/24(rw,async)
Warning: Using async comes with a risk of possible data loss or corruption if the server crashes or restarts uncleanly.

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 [5].

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

/etc/fstab
lithium:/mnt/data           /mnt/data	        nfs noauto 0 0
lithium:/var/cache/pacman   /var/cache/pacman	nfs noauto 0 0
Note:
  • Use hostnames in fstab for this to work, not IP addresses.
  • In order to mount NFS shares with non-root users the users option has to be added.
  • The noauto mount option tells systemd to not automatically mount the shares at boot, otherwise this may cause the boot process to stall.

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: 

/usr/local/bin/auto_share
#!/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 $MOUNT_POINT
    fi
  fi
done
Note: Test using a TCP probe instead of ICMP ping (default is tcp port 2049 in NFS4) then replace the line: 
# Check if the server is reachable
ping -c 1 "${SERVER}" &>/dev/null

with: 

# Check if the server is reachable
timeout 1 bash -c ": < /dev/tcp/${SERVER}/2049"
in the auto_share script above.

Make sure the script is executable.

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

Cron

# crontab -e
* * * * * /usr/local/bin/auto_share

systemd/Timers 

/etc/systemd/system/auto_share.timer
[Unit]
Description=Automount NFS shares every minute

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

[Install]
WantedBy=timers.target
/etc/systemd/system/auto_share.service
[Unit]
Description=Automount NFS shares
After=syslog.target network.target

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

[Install]
WantedBy=multi-user.target

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: 

/etc/NetworkManager/dispatcher.d/30-nfs.sh
#!/bin/bash

# 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: NetworkManager connection 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
Note: This script ignores mounts with the noauto option, remove this mount option or use auto to allow the dispatcher to manage these mounts.

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

Troubleshooting

There is a dedicated article NFS/Troubleshooting.

See also