SSHFS: Remote directory over ssh

Often one wants shared access to files across machines. Traditionally one uses the network file system (nfs). The network file server works as follows: There is an nfs server that exports some directories in its filesystem hiearchy to various nfs clients that mount these directory over the network into their file system hierarchy. As a result, each of the clients shares the directories exported by the nfs server. However nfs is probably the worst protocol when it comes to security and is rightly called network failure system.

This is a tutorial on sshfs or ssh file system. The idea is to provide a nfs like mount which is secured by the very dependable ssh (the sftp subsystem of ssh).

Using sshfs.

1. First mount the remote directory onto a local directory

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$ sshfs ppk@remote: path/to/mount

where path/to/mount is the point where you want the remote file system to be mounted.

2. After step 1, path/to/mount on your local machine is actually the home directory of the remote machine. So you can use it just like a local machine. Expect slow response if your network connection to remote machine is slow though.

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$ cd path/to/mount
$ emacs myfavoritprogram.hs
$ ghc myfavoritprogram.hs

3. After you are done with the work on the remote machine you may unmount the file system

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$ fusermount -u path/to/mount

How it works.

Sshfs is a userspace file system (fuse) that works over ssh, or rather sftp. Fuse is an implementation of filesystem primitives in userspace rather than in kernel space. This essentially means that users can mount and unmount file system without having to be root. Sshfs makes use of the sftp subsystem to do the remote file system operations. Thus all the great features of ssh holds true, i.e. key based authentication, use of ssh-agents. See my tutorial blog on ssh for more details on how to use ssh.

Installing sshfs.

All linux distros have a prebuilt package for sshfs. On Debian/Ubuntu and Arch the relevant package is sshfs. So all you need to do is to install it.

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$ aptitude install sshfs # as root.
$ sudo aptitude install sshfs # if you are on Unbutu
$ pacman -S sshfs # as root on an Arch machine

On Fedora it looks like it is called fuse-sshfs so something like this should work.

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$ yum install fuse-sshfs

Ssh is working but not sshfs.

A common error that people have reported is that ssh works but sshfs fails. If this happens, check whether your sftp subsystem is working. Most probably this too would fail or work incorrectly. One of the main reasons why sshfs/sftp does not work is because your startup scripts in the remote machine prints stuff on the screen. To check this out, try the following command.

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$ ssh ppk@remote /bin/true

If this command produces any output then you are in trouble. You have to fix your startup script in your remote machine — .bash_profile and .bashrc, if you are using bash as your default shell. The startup script should check whether the standard output is a terminal before it outputs something. For this protect your output generating commands inside a test -t 1 block as follows

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$ cat .bash_profile

if [ -t 1 ] # Check if stdout is connected to a terminal
then
    echo "The answer is 42"
fi

See the openssh FAQ for more details.