Problem statement

I wanted to mount a directory from a local desktop in my laptop regardless of the OS it might be running at the moment. Also, not just mount it. Automount it either whenever I boot or on demand.

At the moment in my particular case, both laptop and desktop are running Arch Linux.

First let’s do a bit of a primer:

SMB Primer

SMB is a network protocol (that also acts as a network/distributed file system) used for sharing file and print services between Windows clients. The original version was also known as CIFS, and since then both names are used interchangeably.

Samba is a FOSS implementation of the SMB protocol that runs on *nix systems.

The other competitor to SMB is NFS, but the former is more Windows oriented as it was originally created for the Microsoft ecosystem, while the latter was created towards a more *nix friendly environment.

Using SMB is the traditionally way of sharing directories between Linux/Windows.

TL;DR: SMB for Windows/Linux. NFS for Linux/Linux.

What to install

You need the install cifs-utils which provides user-space tools for mounting SMB/CIFS file systems (or shares as they are also called).

Basically, cifs-utils add the possibility of selecting cifs as filesystem to mount.

Credentials

We create a file containing the credentials used for that. Unfortunately there’s no other way that I’m aware of, of storing the credentials other than plaintext.

The file should contain two entries as such:

username=USERNAME
password=PASSWORD

You should store it somewhere in /etc/samba/ and set its permissions to 600 so that only root has read/write access.

Mount file

The mount file contains the information on how to mount the filesystem.

One important note is that the filename should encode the mount point, otherwise systemd will fail at mounting it.

The filename should be of the pattern: directory1-directory2-directory3.mount

Where the actual path where it is mounted is:

/directory1/directory2/directory3

That is, the internal forward slashes of the path should be the minuses (“-“) in the filename.

Otherwise you will get an error like this

$ systemctl start mntdir.mount
$ systemctl status mntdir.mount
...
...
mntdir.mount's Where= setting doesn't match unit name. Refusing

This file should contain something like this:

[Unit]
Description=DESCRIPTION
Requires=systemd-networkd.service
After=network-online.target
Wants=network-online.target

[Mount]
What=PATH_IN_SMB_SERVER
Where=PATH_TO_MOUNT_POINT
Options=vers=2.1,credentials=CREDENTIALS_PATH,iocharset=utf8,rw,x-systemd.automount
Type=cifs
TimeoutSec=30

[Install]
WantedBy=multi-user.target

Replacing the upper case values with the relevant parameters in your case.

Automount file

The automount file contains the information as to where and how to automount. This automount works on-demand, i.e: it mounts the filesystem when you navigate to that directory.

Just like the mount file: the name of the file should encode the mount location. i.e: it should have the same name as the .mount file but with the file extension: .automount

Its contents should be something like this:

[Unit]
Description=DESCRIPTION
Requires=network-online.target

[Automount]
Where=PATH_TO_MOUNT_POINT
TimeoutIdleSec=0

[Install]
WantedBy=multi-user.target

Replacing the upper case values with the relevant parameters in your case, but keeping in mind that the PATH_TO_MOUNT_POINT should be the same as the one in the .mount file.

This file should be enabled so that it runs at boot.

systemctl enable mnt-dir.automount

Asumming the mount point is /mnt/dir (and therefore the filename is mnt-dir.automout).

Tips

In my particulaar case no matter what Requires, After and/or Wants service you select, enabling the .mount file so that it runs automatically at boot, the system will fail to mount it.

Even more, I found that most guides and tutorials used the automount as the actual enabled unit, which mounts on demand and not on boot.