Monday, October 31, 2016

Creating and uploading ssh keypair using ansible

As some of you have noticed from previous posts, I do use docker. However, I also use Ansible to install packages on and configure hosts. I am not going to talk about when to use each one. Instead, I will try to make another (hopefully) quick post, this time ansible-related.

Let's say I have an user abd I want to set said user up so I can later on ssh to the target machine as that user using ssh key pair. From the machine I am running ansible from. To do so I need to create a ssh keypair and then put the public key to the user's authorized_keys file. And that might require creating said file and ~/.ssh if not available. Now that is a mouthfull so let's do it in steps.

  1. Decide where to put the key pair: This is rather important since we not only need to have it somewhere we can find when we aare ready to copy it to the target server but also we might want not to recreate the key pair every time we run this task.

    For now I will put it in roles/files/, which is not ideal for many reasons. Just to name one, we might want to put it somewhere encrypted or with minimum access by suspicious individuals. But for this discussion it is a great starting place for the key pair. So, let's see how I would verify if the pair already exists or not:

    - name: Check if user_name will need a ssh keypair
        local_action: stat path="roles/files/{{user_name}}.ssh"
        register: sshkeypath
        ignore_errors: True

    The first line (name) writes something to the screen to show where we are in the list of steps. It is rather useful because when we run ansible we then can see which task we are doing. The local_action uses stat to determine if the file roles/files/{{user_name}}.ssh exists. The return/result code is then written to the variable ssheypath. Finally I do not care about any other messages from stat.

  2. Create key pair: there are probably really clever ways that are purely ansible native to do it, but I am not that bright so I will use openssh instead, specifically ssh-keygen. Ansible has a way to call shell scripts, the shell module. Now, I want to run tt>ssh-keygen on the machine I am running ansible from, the local machine in ansible parlance. And for that reason we have the local_action command. Here is one way to do the deed:

    - name: Create user_name ssh keypair if non-existent
        local_action: shell ssh-keygen -f roles/files/"{{user_name}}".ssh -t rsa -b 4096 -N "" -C "{{user_name}}"
        when: not sshkeypath.stat.exists

    As you guessed, we are using ssh-keygen to create the key pair roles/files/user_name.ssh and roles/files/ The other ssh-keygen are just because I would like my keys to be a bit more secure than those created by the average bear. So far so good: we are here using local_action and shell like in the previous step.

    The interesting part if the when statement. What it is saying is to only create the key pair if it does not already exist. And that is why we did check if roles/files/user_name.ssh existed in the previous step. Of course that meant we assumed that if roles/files/user_name.ssh existed, roles/files/ must also exist. There are ways to check for both but I will leave that to you.

  3. Copy public key to remote server: Now we know we have a ssh key pair, we do need to copy it. You can do it using the file module, which will require a few extra tasks (create the ~/.ssh directory with the proper permissions comes to mind). Or, you can use the appropriately named authorized_key module. I am lazy so I will pick the last option.

    - name: Copy the public key to the user's authorized_keys
        # It also creates the dir as needed
          user: "{{user_name}}"
          key: '{{ lookup("file", "roles/files/" ~ user_name ~ "") }}'
          state: present

    What this does is create ~/.ssh and ~/.ssh/authorized_keys and then copy the key to ~/.ssh/authorized_keys. The most important statement here is the key one. Since it expects us to pass the key as a string, we need to read the contents of the public key. And that is why we called upon the lookup command; think of it here as using cat. And, why we have all the crazy escaping in "roles/files/" ~ user_name ~ "").

And that is pretty much it. If you want to be fancy you could use a few local_actions to copy the private key to your (local) ~/.ssh and then create/edit ~/.ssh/config to make your life easier. I did not do it here because this user will be used as is by ansible; I need no access to this user as me outside ansible.

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