Month: August 2016

Microsoft has finally landed its anniversary update for Windows 10. Among all the random useless features, is an actual Ubuntu Linux subsystem within Windows, with the ridiculously silly name “Bash on Ubuntu on Windows”. Goodbye Cygwin?

One of the first things I wanted to try was to setup the SSH server, so I can remote-login from my real box. Getting one up was actually fairly easy, if you can deal with a few problems with weird, weird red-herring error messages.

The problem

To recap, installing the OpenSSH server is as easy as popping open a bash shell (I assume you figured out how to get the Linux subsystem installed already and popped open Ubuntu )

sudo apt-get install openssh-server

If you try to now start the server with

sudo service ssh start

It’ll respond with

initctl: Unable to connect to Upstart: Failed to connect to socket /com/ubuntu/upstart: Connection refused

but subsequently

Starting OpenBSD Secure Shell server sshd [OK]

When you now try to connect in from a remote host, there are two outcomes:

1. There is no SSH server on port 22.
2. There is an SSH server on port 22, but it responds only to your Windows password and not your Ubuntu Linux user password. If login is successful, it drops you into a DOS prompt instead of a bash prompt. Trying to run bash within the prompt generates a response:
   "Error: 0x80070005".

Diagnosis

There are multiple underlying problems here.

1. First, it’s important to note that there exists a separate Windows SSH server (separate from the OpenSSH server from the Ubuntu subsystem installation) now on port 22. If you telnet (yes, telnet) into port 22, and the host greeting is SSH-2.0-MS_1.100, you’ve run into the Microsoft SSH implementation (hence the MS part in the greeting).

   I’m not sure what this server is supposed to do (and, in fact, starting SSH servers on standard ports without explicitly telling the user seems like a potential security problem to me, in addition to being kind of jerk-ish). I do know it is launched when you reboot with Developer Mode on. You know, Developer Mode, the mode you have to turn on to run Bash on Ubuntu on Windows. This server is occupying port 22, so you cannot launch another SSH server to listen on this port. This is the problem that is causing the silent failure of sshd to start up, not a broken Upstart or any of that nonsense the error message being displayed is referring to.

2. Apparently the Ubuntu compatibility layer on Windows does not implement chroot or one of the related system calls needed for OpenSSH privilege separation at the time of this post.
3. If you launched bash from DOS cmd.exe, instead of the Bash on Ubuntu on Windows shortcut, /mnt/c/Users/[Windows Username] (the directory you start with in DOS) is not your Linux home directory. .ssh config files left in this directory will not configure your OpenSSH server.

Symptoms & Solutions

1. To resolve the problem of conflicting SSH servers, set your OpenSSH server on a different port : one other than 22. This can be done by editing /etc/ssh/sshd_config and changing the Port configuration. Be sure to open this new port in your Windows Firewall for inbound connections, as this firewall configuration will not be automatically done for you. I set mine to 60022.
2. To enable login, also change UsePrivilegeSeparation in the same config file to No. Failure to do so will cause the server to respond with Connection reset by nnn.nnn.nnn.nnn
3. To enable public key auth and set other configurations, the Linux subsystem home directory is, as in normal Ubuntu, /home/<yourname>.

Having done this, now you can start or restart in the bash shell

sudo service ssh restart

And connect from remote host with your Ubuntu user credentials and/or public key:
$ ssh -p 60022 10.0.1.208
...
Welcome to Ubuntu 14.04.5 LTS (GNU/Linux 3.4.0+ x86_64)

(And if you telnet, the host message should be something like
SSH-2.0-OpenSSH_6.6.1p1 Ubuntu-2ubuntu2.7
)