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Enable passwordless sudo for ansible

In order to fully control a remote machine we need to be able to execute command on the remote machines as user root. There are a number of ways to accomplish this. Each one with a slightly different security implication.

Some of our options to execute commands as root

We can log in to the remote server as user root providing password on each login.

We can log in to the remote server as user root using ssh keys.

We can login as an unprivileged user and then use sudo after providing the password of the user.

We can login as an unprivileged user and then use sudo without providing a password.

We'll try he last two now.

Our inventory.cfg looks like this: 

[all]
192.168.56.11
192.168.56.12

[all:vars]
ansible_python_interpreter=/usr/bin/python3

We are running on our manager machine as user foo and we are accessing the remote machine as user foo.

Let's check if we can use Ansible at all. 

$ ansible -i inventory.cfg all -m ping


192.168.56.11 | SUCCESS => {
    "changed": false,
    "ping": "pong"
}
192.168.56.12 | SUCCESS => {
    "changed": false,
    "ping": "pong"
}

grep ^root: /etc/shadow

This is not very sophisticated or useful command. We just want to display the information about user root in the /etch/shadow file using grep. The point is, that only user root has the rights to do this. 

$ ansible -i inventory.cfg all -a "grep ^root: /etc/shadow"

It fails as expected: 

192.168.56.12 | FAILED | rc=2 >>
grep: /etc/shadow: Permission deniednon-zero return code

192.168.56.11 | FAILED | rc=2 >>
grep: /etc/shadow: Permission deniednon-zero return code

Only user root can read the /etc/shadow file.

Become user root

Adding the -b or --become flag tells Ansible to become another user on the remote server. The "other" can be configured, but defaults to root which is rather convenient. 

$ ansible -i inventory.cfg all -a "grep ^root: /etc/shadow"  -b

It fails now in a different way. It tries to use sudo but fails because sudo needs a password. 

192.168.56.12 | FAILED! => {
    "changed": false,
    "module_stderr": "Shared connection to 192.168.56.12 closed.\r\n",
    "module_stdout": "sudo: a password is required\r\n",
    "msg": "MODULE FAILURE",
    "rc": 1
}
192.168.56.11 | FAILED! => {
    "changed": false,
    "module_stderr": "Shared connection to 192.168.56.11 closed.\r\n",
    "module_stdout": "sudo: a password is required\r\n",
    "msg": "MODULE FAILURE",
    "rc": 1
}

Prompt for password

We can use the -K or --ask-become-pass flag to tell Ansible to ask for the sudo password. 

$ ansible -i inventory.cfg all -a "grep ^root: /etc/shadow"  -b -K

It asks for the SUDO password and then uses that on both machines. I guess if the passwords were different on the two machines then it will notice this and ask for the other password as well. I have never tried that. 

SUDO password:
192.168.56.11 | SUCCESS | rc=0 >>
root:!:17596:0:99999:7:::

192.168.56.12 | SUCCESS | rc=0 >>
root:!:17596:0:99999:7:::

Allow passwordless sudo

Telling ansible ask for the password has the security advantage that only people who know what is the password can execute code but it can be a bit inconvenient on the long run.

Instead we can configure the the remote user we use to be able to execute all, or certain commands using sudo even without supplying a password. In this case we need to protect the user account of the manager machine that has its public ssh-key installed on the remote server. Anyone who can access this machine would be able to control the remote servers.

Who can run sudo command, what are theses command and whether password is required is controlled in the /etc/suduers file. It can be edited manually using the visudo command or we can ask Ansible to edit it. You can also edit the file with any editor, but if you save an incorrectly formatted version, you can easily lock yourself out from user root. Hence it is strongly recommended that you use the visudo command that will validate the syntax of the file before you save it.

Manually editing the sudoers

ssh to the remote server. 

$ ssh foo@192.168.56.11

On the remote server run: 

$ sudo visudo

It will ask for your password and then open the default editor which happens to be nano these days. The default version of file looks like this: 

#
# This file MUST be edited with the 'visudo' command as root.
#
# Please consider adding local content in /etc/sudoers.d/ instead of
# directly modifying this file.
#
# See the man page for details on how to write a sudoers file.
#
Defaults	env_reset
Defaults	mail_badpass
Defaults	secure_path="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/snap/bin"

# Host alias specification

# User alias specification

# Cmnd alias specification

# User privilege specification
root	ALL=(ALL:ALL) ALL

# Members of the admin group may gain root privileges
%admin ALL=(ALL) ALL

# Allow members of group sudo to execute any command
%sudo	ALL=(ALL:ALL) ALL

# See sudoers(5) for more information on "#include" directives:

#includedir /etc/sudoers.d

We need to edit the line 

%sudo   ALL=(ALL:ALL) ALL

and look like this: 

%sudo  ALL=(ALL:ALL) NOPASSWD: ALL

We can save the file and exit.

In order to verify that it works properly log out from the server (e.g. type exit or press Ctrl-d. Then ssh to the server again and run 

sudo grep root /etc/shadow

We needed to logout and login again for the verification because normally even if sudo requires a password it retains the access rights for a few minutes or until you log out. So we wanted to make sure we can use sudo because of the change in the sudoers file and not because of this grace period.

We can now log out again and on the management machine run the ansible command again with -b but without -K 

$ ansible -i inventory.cfg all -a "grep ^root: /etc/shadow"  -b

The result looks promising: 

192.168.56.12 | FAILED! => {
    "changed": false,
    "module_stderr": "Shared connection to 192.168.56.12 closed.\r\n",
    "module_stdout": "sudo: a password is required\r\n",
    "msg": "MODULE FAILURE",
    "rc": 1
}
192.168.56.11 | SUCCESS | rc=0 >>
root:!:17596:0:99999:7:::

One machine where we changed the sudoers file worked as expected, the other one without the passwordless access failed as expected.

(We could use the -K and then it would ask for a password again, but instead of that we'd like to allow passwordless sudo commands on the other machine as well.

Ansible playbook to set passwordless sudo

This is the set_sudoer.yml file. 

---
- hosts: all
  tasks:
    - lineinfile:
        path: /etc/sudoers
        state: present
        regexp: '^%sudo'
        line: '%sudo ALL=(ALL) NOPASSWD: ALL'
        validate: 'visudo -cf %s'

It says: work on the file in the given path. Replace the line matched by regexp with the string in line. Before saving the file run the validate command to verify that format is correct.

It will match the following line: 

%sudo   ALL=(ALL:ALL) ALL

and replace it with this line: 

%sudo  ALL=(ALL:ALL) NOPASSWD: ALL

We can run this with the following command: 

$ ansible-playbook -i inventory.cfg --limit 192.168.56.12 set_sudoer.yml -b -K

  • We only run it on the selected host.
  • We provide the -b as we need to become root for this operation.
  • We also supply -K because sudo still requires password.

The output looks like this: 

SUDO password:

PLAY [all] ****************************************************************************************************************************

TASK [Gathering Facts] ****************************************************************************************************************
ok: [192.168.56.12]

TASK [lineinfile] *********************************************************************************************************************
changed: [192.168.56.12]

PLAY RECAP ****************************************************************************************************************************
192.168.56.12              : ok=2    changed=1    unreachable=0    failed=0

Once this is done, we can run the previous command without providing a password and it will run on both servers: 

$ ansible -i inventory.cfg all -a "grep ^root: /etc/shadow"  -b


192.168.56.12 | SUCCESS | rc=0 >>
root:!:17596:0:99999:7:::

192.168.56.11 | SUCCESS | rc=0 >>
root:!:17596:0:99999:7:::

Conclusion

Having passwordless access is great for automation, but you need to be aware of the security implications we discussed above.

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