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Most servers will be initially deployed with a direct SSH access for root with a SSH key or only a password so you can configure the server. Because of security issues, I want to configure a new user with sudo privileges and disable the direct access for root.

I want to create an Ansible Playbook which can do this task for me, however I do not know if it is possible to set ansible_user, ansible_become and so on depending on a condition like "Can you ssh as root?". Is it even possible to let Ansible detect this while running the playbook?

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Yes, it's possible, but most solutions are going to look pretty complicated at first glance. You might be better off simply defining a "bootstrap" playbook that you only ever execute once. That's what I have to install python and setup initial user.

However, to answer the question directly, here's a solution that should work:

# Experiment to "fix" `ansible_user` depending upon host availability
- hosts: all
  gather_facts: false  # Otherwise initial connection will fail

  vars:
    - bootstrap_user: root

  tasks:
    - debug:
        msg: |
          ansible_user: {{ ansible_user | d('unset') }};
          remote_user: {{ remote_user | d('unset') }}

    - action: ping
      ignore_unreachable: true  # requires Ansible >= 2.7
      ignore_errors: yes
      register: pingtest

    # Works (mostly) for Ansible >= 2.2.
    # Might think this alone would work, but only if NOT *ALL* hosts failed up to
    # this point, which makes running the playbook on only a single host pointless.
    # Therefore, also set `ignore_unrechable` in `ping` above.
    # - https://github.com/ansible/ansible/issues/26362
    # - https://github.com/ansible/ansible/issues/19673
    # - https://github.com/ansible/ansible/issues/18075
    - meta: clear_host_errors

    - name: set ansible_user if no ping failed
      set_fact:
        ansible_user: "{{ bootstrap_user }}"
      when: pingtest.failed | d(pingtest.unreachable) | d(false)

    - debug:
        msg: "ansible_user: {{ ansible_user | d('unset') }}"

    # Connect as ansible_user from here on
    - name: Show remote user
      shell: "echo $USER"
      changed_when: false

In general, the following two refs will help you with understanding variables (and precedence, which is important here) as well as error recovery:

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