Wednesday, February 03, 2016

ssh using key authentication does not work

Here is another short one which was a gotcha to me: in my thread about user with network homedir in boot2docker, when I first tried to connect to it using ssh key pair authentication, it did not work:

raub@desktop:~$ ssh docker's password:


As you can see it tried to use password authentication, which was something I did not want it to do. I mean, I took the time to add an entry in my .ssh/config file to connect to that box. Why was it ignoring it? I tried cranking up the verbose in the ssh client,

ssh -vvv docker
and still did not see any clues... at least nothing that made sense to me. So, I decided to crank up verbose in the server (boot2docker) side:
docker@boot2docker:~$ sudo /usr/local/sbin/sshd -ddd -p 10022
debug2: load_server_config: filename /usr/local/etc/ssh/sshd_config
debug2: load_server_config: done config len = 216
debug2: parse_server_config: config /usr/local/etc/ssh/sshd_config len 216
debug3: /usr/local/etc/ssh/sshd_config:50 setting AuthorizedKeysFile .ssh/autho$
debug3: /usr/local/etc/ssh/sshd_config:115 setting Subsystem sftp       /usr/lo$
debug3: /usr/local/etc/ssh/sshd_config:122 setting UseDNS no

And, yes, it will be spitting out all the verbose messages on that terminal. Those of you who had enough caffeine have noticed I am starting a sshd service on port 10022 so I can keep the standard sshd session running on 22. Then I just connect

ssh -p 10022 docker
to the new service and see what garbage comes back. To make a long story short (I said this was going to be a short article), the line we are interested on is
User ducker not allowed because account is locked

OK, why does it say ducker is locked? Well, the ducker entry in /etc/shadow looks like this


Note the exclamation mark. I originally created the ducker user by doing

adduser -D -u 1003 ducker
but did not create a password to the user; I thought I would not need it at that moment because I just wanted to try the key authentication. Now, take a look at other entries in /etc/shadow. The root one looks something like

As ducker, it has no password (very different than having a blank password), but instead of ! it has an *. If you change the ducker entry to

it will still have no password but now you can login using ssh key authentication. And, setting a password for the same account will also work.

So, what is going on? It turns out the ! means the account is disabled/locked, which is why the ssh key pair authentication failed. Fun, huh?

Bottom Line

  • We can create accounts (say, to run backup scripts) without password that can be reached using ssh key pair authentication. All we need to do is to remember to use * instead of !.
  • Running sshd in debug mode can sometimes answer questions neither the log files nor ssh -vvv can.

See Also

Unable to ssh into locked user in Stackoverflow.