Nfs is insecure by default. The host can filter on client IP but IP is easy to spoof. Therefore (normal) nfs is insecure. You can use nfs with kerberos authentication and authorization.

A good overview of the kerberos algorithm is: Taming Kerberos - Computerphile

A reasonable guide to setting up nfs with kerberos: Network File System (NFS) - Ubuntu

Most guides, include the Ubuntu one above assume you are a sys admin. You and I are not. So lets go through some of things not mentioned.

Things not mentioned

You need domain names

Good news though, they need not be global. No need to buy anything. I assume nfs and kerberos need a name to refer to a server. You might get away with using the IP adresses.

A simple way to do this is to add each machines hostname and IP to the hosts file in /etc/hosts. I have a file server named asgard, it has local IP 192.168.1.15. Therefore I have this in the hosts file of my workstation:

192.168.1.15 	asgard

You must run kadmin as sudo

At least when export keys, if you do not you will get the error Permission denied while adding key to keytab or Key table file '/etc/krb5.keytab' not found while adding key to keytab. That last one I could not find explained anywhere, lost quite some time on that. These errors occur because without sudo the keytab (usually at /etc/krb5.keytab) can not be read or created.

Given the datasets:

/srv/
/srv/music
/srv/pictures

You can not share only /srv. The share will mount but users will get permission denied on any file operation inside music & pictures.

Full Procedure on the Server

This assumes you are building a basic file server such as a NAS:

You have one server you want to run NFS on
You do not have kerberos installed
You have no other needs for kerberos
All clients are on the same local network as the server

Set up kerberos

This is based on How to install a kerberos server - Ubuntu and my own findings. I recommend you read that in its enterity if you get stuck.

Names

You need to decide on some names before we continue. Specifically you need a:

Realm
Host name for the server

In this guide I will be using the names I picked for my network (If you have not guessed it yet this guide is mostly written for future me). My Realm is named: Yggdrasil and my file server is called Asgard.

Every client on my network gets 192.168.1.15 asgard appended to their hosts file.

Setting up the Kerberos servers

I am using debian 12 “bookworm”, on debian based systems such as Ubuntu this will all work. On other unix names you will have to find equivalent packages.

We start by installing three packages providing:

kadmind: a server handles account creation and deletion and other adminstrative commands.
The user programs to interact with kadmind
The Key Distribution Center (KDC). The server that handle creating tickets. servers. The kadmind

During the installation you should be promted to enter:

your realm name, enter it in all caps. I entered: YGGDRASIL.
Kerberos servers for your realm: This is where the KDC will run, in our case this server. I enter the host name for the server, in my case: asgard.
Administrative server for your Kerberos realm: This is where we are running kadmind, in our case this server. In my case again: argard.

$SERVER: sudo apt install krb5-kdc krb5-admin-server

If you did not get prompted then there are configuration files from a previous installation present. You need to remove those first. Do so with:

$SERVER: sudo apt purge -y krb5-kdc krb5-admin-server krb5-config krb5-locales krb5-user

Then try the install again. If that still does not prompt you will have to search for left over kerberos config files. relevant stack exchange issue

Creating the KDC database

Before we can use the KDC we need to create a database for it. You will be prompted for a database password, store it somewhere securely. I store mine in my personal password manager under the name kerberos_database (now you know where to look hackers!).

$SERVER: sudo kdb5_util create -r YGGDRASIL -s

If you forget to create the database you will get the error: kadmin.local: Cannot open DB2 database '/var/lib/krb5kdc/principal': No such file or directory while initializing kadmin.local interface.

Creating kerberos “users”

In kerberos users are called principles. There are two ways to add principles:

as the root user on the machine where kadmind is running using kadmin.local.
by logging in as a kerberos admin using the kadmin program.

Creating the kerberos admin

On each machine we want to be able to access the NFS server we need to create a kerberos user. That user can only be created on the client machine. To do so we need to log in to the adminstrative server (using kadmin) from the client machine. For that we need a kerberos admin.

We create an admin using kadmin.local (which only works on ther same machine as the admin server and when run by sudo).

The name for this principle can be anything, I have chosen my name followed by the word admin. You will be prompted to enter a password. If you lose that you will have to make this user again.

$SERVER: sudo kadmin.local -q "addprinc david/admin"

We need to give this user the permissions that make it an admin. For that we edit the the kerberos Access Control List (ACL) in /etc/krb5kdc/kadm5.acl.

Adapt the line below and add it to your ACL file.

*/admin@YGGDRASIL     *

I have used a star so any principle whoes name ends in /admin gets all permissions on the Kerberos servers.

Now restart the kerberos admin server using:

$SERVER: sudo systemctl restart krb5-admin-server

Creating the NFS principle

The name for the principle used by the NFS server must follow the format / or NFS will not with kerberos. The must occur in every clients hosts file. My server has the host name asgard.

$SERVER: sudo kadmin -p david/admin -q "addprinc -randkey nfs/asgard"
$SERVER: sudo kadmin -p david/admin -q "ktadd nfs/asgard"

Note: We could do this using sudo kadmin.local however by logging in as admin with -p we can catch any issues with the kerberos setup now. If you get the error: kadmin: Communication failure with server while initializing kadmin interface there is probably something wrong with one of the kerberos servers. Run sudo systemctl list-units --failed to figure out which service failed and then sudo systemctl status <service_name> to see what went wrong.

Set up NFSv4

If you already have an NFS server you might need to remove it first. During install the package will start the required kerberos related NFS services. This does not happen if NFS is already installed.

Install nfs with:

$SERVER: sudo apt install nfs-kernel-server

Now we just need to populate the /etc/exports file, I want to share the /srv directory thus my exports file looks like this:

/srv *(rw,sync,no_subtree_check,sec=krb5)

Note: krb5 makes nfs use kerberos only for authentication, a man in the middle can intercept or inject nfs traffic. In my use case (a home network) the router and switches are trusted therefore authentication is all I need. Alternatively use kerb5p for encrypted & integrity verified NFS traffic.

Then refresh the export using:

$SERVER: sudo exportfs -rav

Configure id mapping

File ownership & operations will be send with @ instead of user and group id’s. Kerberos takes care of authenticating that actually belongs to the user.

The NFS server and client need to now how @ maps to local unix users and groups. The server translates this using idmapd based on the content of /etc/idmapd.conf. Most guides describe how to couple idmap to a large organizations user database. Since we are building a simple NAS we will use a fully menual (static) configuration. For each kerberos principle we describe in idmapd.conf which local user it corrosponds to.

This is a basic idmapd.conf for the realm YGGDRASIL. Actions performed and files owned by Kerberos principle david should be translated to actions performed and files owned by local UNIX user david.

[General]
Verbosity = 0
# set your own domain here, if it differs from FQDN minus hostname
Domain = yggdrasil

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

[Translation]
Method = static
GSS-methods = static

[Static]
david/admin@YGGDRASIL = root
david@YGGDRASIL = david

Procedure for each Client

Add the server to hosts

To set up Kerberos and NFS on the client the client needs to know the IP for the server. The server is referred to by its hostname. We make sure the client knows it by entering it in the clients hosts file.

In my case I append 192.168.1.15 asgard to the hosts file at /etc/hosts.

Install kerberos programs

Install the kerberos user programs to interact with kadmind. You should be prompted for the same kerberos information as when you set up the server. That is:

The default kerberos realm
Kerberos servers for your realm.
Administrative servers for your Kerberos realm.

For the Kerberos servers & adminstrative servers enter the host name you just added to the hosts file.

$CLIENT: sudo apt install krb5-user

If you did not get prompted you probably have exisitng configuration. You can try and remove that using sudo apt purge krb5-user krb5-config.

Add Kerberos user for NFS client

We need two or more kerberos principle for each client.

One for the NFS client software, it must be named: nfs/<host> where host is the hostname for the client machine. Check what it is using hostname.
One for each user on the machine that needs to access the share. It is easiest if it has the same name as the local user who is going to use nfs. Its okay to share one between multiple clients machines.

We will use the kerberos admin server from the client. The advantage to that is that we can use ktadd on the client and the key will then appear in the clients local keytab. We use the kerberos admin server remotly by passing -p <admin user> to kadmin instead of using kadmin.local with sudo. We still pass the command to execute to kadmin using -q.

$CLIENT: sudo kadmin -p david/admin -q "addprinc -randkey nfs/<host>" 
$CLIENT: sudo kadmin -p david/admin -q "ktadd nfs/<host>" 
$CLIENT: sudo kadmin -p david/admin -q "addprinc <local username>" 
$CLIENT: sudo kadmin -p david/admin -q "ktadd <local username>"

Install the NFS client & Mount

We need to do this after installing and setting up Kerberos on the client or the extra kerberos services used for NFS will not be enabled and started.

$CLIENT: sudo apt install nfs-common

Configure id mapping

Just like on the server we have to define a mapping between local users and Kerberos users. The NFS client uses nfsidmap instead of idmapd however they both read she same config file. Copy the one you made for the server to /etc/idmapd.conf.

Testing the mount

Now you need to log in as Kerberos user using kinit.

I have ran into an issue where I get invalid password when trying kinit. Running sudo kadmin -p david/admin -q change_password <local username> to set the password again seems to resolve that.

Then simply mount the shared directory via:

$CLIENT: sudo mount asgard:/srv /mnt

If this fails you need to restart the client machine. I have no idea why but it works :)

Auto mount on login/boot

Currently we need to mount with sudo and get a kerberos ticket. To get a kerberos ticket we have to type in a password It gets worse kerberos tickets expire (after around a day). Lets automate this by:

storing a keytab for the users principle in its homedir and use a keytab for authentication instead of the password.
set up a systemd serice to request the ticket and renew it periodically after the user logs in.
add the mount to fstab so its always mounted on boot (though without a valid kerberos ticket you can not use it)

Passwordless kerberos tickets

Figure out the KVNO:

$CLIENT: kinit <username>
$CLIENT: kvno <username>
$CLIENT: kdestroy -p <username>

Generate a keytab for the user

$CLIENT: ktutil
ktutil: addent -password -p <NAME> -k <KVNO> -e aes128-cts-hmac-sha1-96 -f
ktutil: wkt /home/<username>/.local/keytab
ktutil: q

This should not prompt for a password:

$CLIENT: kinit -kt ~/.local/keytab -p <principle name>

Automate ticket request and renewal

For this we need the programs: k5start and krenew lets install them:

$CLIENT: sudo apt install kstart

Now we want to run k5start once the user logs followed by krenew. We use systemd to set that up.

This will open up an editor where you can write the service:

$CLIENT: systemctl edit --user --force --full kerberos_mount.service

Enter the following:

[Unit]
Description=Initializes, caches and renews Kerberos ticket
After=default.target

[Service]
Type=exec
RemainAfterExit=yes
ExecStart=/usr/bin/k5start \
	# run in daemon mode, recheck ticket every  30 minutes \
	-K 30 \
	# authenticate using keytab rather then asking for a password \
	-f %h/.local/keytab \
	# store this file as ticket cache \
	-k /tmp/krb5cc_%U \
	# principle to get tickets for \
	%u

[Install]
WantedBy=default.target

Enable and start the service:

$CLIENT: systemctl enable --now --user kerberos_mount.service

Mount on boot in fstab

Add something like this to fstab (use: sudoedit /etc/fstab):

asgard:/srv/music  /home/david/Music          nfs4  nofail,rw,sec=krb5

This will mount the share music in /srv/music on the server named asgard in the host file to the folder /home/david/Music. Do not forget the nofail option or the system will not boot if the file server is down.

Verify fstab entry by:

unmounting the share using sudo umount Music
using sudo mount -a witout rebooting to make sure its correct:

$CLIENT: sudo mount -a

Appendix

Some specific setups I need.

Mpd systemd & kerberos

I run MPD (music play deamon) as a seperate user (named mpd in group mpd). The systemd service listed above will not work since the mpd user is never logged in. Rather the MPD process is started as root and drops privilleges.

Instead of starting the ticket renewel when the mpd user logs in (which it never will) we just start it on boot.

The systemd service for ticket renewel is almost the same. The changes are:

Make it run as user mpd in the group mpd
Hard code the principle as mpd
Hard code the ticket cache location as /tmp/krb5cc_mpd
Use the keytab /mpd_keytab (the mpd user has no home and so no .local)

This results in the systemd service file:

[Unit]
Description=Initializes, caches and renews Kerberos ticket
After=default.target

[Service]
Type=exec
User=mpd
Group=mpd
RemainAfterExit=yes
ExecStart=/usr/bin/k5start \
	# run in daemon mode, recheck ticket every  30 minutes \
	-K 30 \
	# authenticate using keytab rather then asking for a password \
	-f /mpd_keytab \
	# store this file as ticket cache \
	-k /tmp/krb5cc_mpd \
	# principle to get tickets for \
	mpd

[Install]
WantedBy=default.target

I’ve named this service kerberos_mount_for_mpd.

Lets make the MPD service require the systemd service above. That way MPD will report an error if the kerberos_mount_for_mpd service fails. We do this by editing the service using:

$CLIENT: sudo systemctl edit --full mpd

The we simple:

Append kerberos_mount_for_mpd.service to the After= line
Add a Requires=kerberos_mount_for_mpd.service line below the After one

For the rest follow the normal client procedure.

Things not mentioned

You need domain names

You must run kadmin as sudo

You must share ZFS datasets seperatly

Full Procedure on the Server

Set up kerberos

Names

Setting up the Kerberos servers

Creating the KDC database

Creating kerberos “users”

Creating the kerberos admin

Creating the NFS principle

Set up NFSv4

Configure id mapping

Procedure for each Client

Add the server to hosts

Install kerberos programs

Add Kerberos user for NFS client

Install the NFS client & Mount

Configure id mapping

Testing the mount

Auto mount on login/boot

Passwordless kerberos tickets

Automate ticket request and renewal

Mount on boot in fstab

Appendix

Mpd systemd & kerberos