SSHFS File Systems

SSHFS is a FUSE filesystem that allows easy access to directories on a remote machine using the SSH File Transfer Protocol (SFTP). It means that if you have SSH access to a machine, no additional setup is needed to mount a directory.

Interactive mounting

In NixOS, SSHFS is packaged as sshfs. Once installed, mounting a directory interactively is simple as running:

$ sshfs my-user@example.com:/my-dir /mnt/my-dir

Like any other FUSE file system, the directory is unmounted using:

$ fusermount -u /mnt/my-dir

Non-interactive mounting

Mounting non-interactively requires some precautions because sshfs will run at boot and under a different user (root). For obvious reason, you can't input a password, so public key authentication using an unencrypted key is needed. To create a new key without a passphrase you can do:

$ ssh-keygen -t ed25519 -P '' -f example-key
Generating public/private ed25519 key pair.
Your identification has been saved in test-key
Your public key has been saved in test-key.pub
The key fingerprint is:
SHA256:yjxl3UbTn31fLWeyLYTAKYJPRmzknjQZoyG8gSNEoIE my-user@workstation

To keep the key safe, change the ownership to root:root and make sure the permissions are 600: OpenSSH normally refuses to use the key if it's not well-protected.

The file system can be configured in NixOS via the usual fileSystems option. Here's a typical setup:

{
  fileSystems."/mnt/my-dir" = {
    device = "my-user@example.com:/my-dir/";
    fsType = "sshfs";
    options =
      [ # Filesystem options
        "allow_other"          # for non-root access
        "_netdev"              # this is a network fs
        "x-systemd.automount"  # mount on demand

        # SSH options
        "reconnect"              # handle connection drops
        "ServerAliveInterval=15" # keep connections alive
        "IdentityFile=/var/secrets/example-key"
      ];
  };
}

More options from ssh_config(5) can be given as well, for example you can change the default SSH port or specify a jump proxy:

{
  options =
    [ "ProxyJump=bastion@example.com"
      "Port=22"
    ];
}

It's also possible to change the ssh command used by SSHFS to connect to the server. For example:

{
  options =
    [ (builtins.replaceStrings [" "] ["\\040"]
        "ssh_command=${pkgs.openssh}/bin/ssh -v -L 8080:localhost:80")
    ];

}

::: {.note} The escaping of spaces is needed because every option is written to the /etc/fstab file, which is a space-separated table. :::

Troubleshooting

If you're having a hard time figuring out why mounting is failing, you can add the option "debug". This enables a verbose log in SSHFS that you can access via:

$ journalctl -u $(systemd-escape -p /mnt/my-dir/).mount
Jun 22 11:41:18 workstation mount[87790]: SSHFS version 3.7.1
Jun 22 11:41:18 workstation mount[87793]: executing <ssh> <-x> <-a> <-oClearAllForwardings=yes> <-oServerAliveInterval=15> <-oIdentityFile=/var/secrets/wrong-key> <-2> <my-user@example.com> <-s> <sftp>
Jun 22 11:41:19 workstation mount[87793]: my-user@example.com: Permission denied (publickey).
Jun 22 11:41:19 workstation mount[87790]: read: Connection reset by peer
Jun 22 11:41:19 workstation systemd[1]: mnt-my\x2ddir.mount: Mount process exited, code=exited, status=1/FAILURE
Jun 22 11:41:19 workstation systemd[1]: mnt-my\x2ddir.mount: Failed with result 'exit-code'.
Jun 22 11:41:19 workstation systemd[1]: Failed to mount /mnt/my-dir.
Jun 22 11:41:19 workstation systemd[1]: mnt-my\x2ddir.mount: Consumed 54ms CPU time, received 2.3K IP traffic, sent 2.7K IP traffic.

::: {.note} If the mount point contains special characters it needs to be escaped using systemd-escape. This is due to the way systemd converts paths into unit names. :::

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SSHFS File Systems

Interactive mounting

Non-interactive mounting

Troubleshooting

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