Requirements for which I will offer solutions, as bullet points:
- Passwordless root console login
- Passwordless root remote login from pre-authorised users
- Passwordless remote login for specified accounts from pre-authorised users
- Passwordless remote login for any account from pre-authorised users
The following examples are based on Debian, since that's what I've got here for testing. However, I see no reason why the principles cannot be applied to any distribution (or indeed any PAM-based *ix derivative).
Passwordless root console login
I think the way I would approach this would be to leverage PAM and the /etc/securetty configuration file.
As a pre-requisite, a "sufficiently secure" root password must be set. This is not required for console login but exists to make brute force cracking attempts unrealistic. The account is otherwise a perfectly normal root account.
In /etc/pam.d/login I have the following standard set of lines for authentication (those beginning with keyword auth):
auth optional pam_faildelay.so delay=3000000
auth [success=ok new_authtok_reqd=ok ignore=ignore user_unknown=bad default=die] pam_securetty.so
auth requisite pam_nologin.so
@include common-auth
auth optional pam_group.so
The referenced common-auth include file contains the following relevant lines:
auth [success=1 default=ignore] pam_unix.so nullok_secure
auth requisite pam_deny.so
auth required pam_permit.so
auth optional pam_cap.so
The common-auth file instructs PAM to skip one rule (the deny) if a "UNIX login" succeeds. Typically this means a match in /etc/shadow.
The auth ... pam_securetty.so line is configured to prevent root logins except on tty devices specified in /etc/securetty. (This file already includes all the console devices.)
By modifying this auth line slightly it is possible to define a rule that permits a root login without a password from a tty device specified in /etc/securetty. The success=ok parameter needs to be amended so that the ok is replaced by the number of auth lines to be skipped in the event of a successful match. In the situation shown here, that number is 3, which jumps down to the auth ... pam_permit.so line:
auth [success=3 new_authtok_reqd=ok ignore=ignore user_unknown=bad default=die] pam_securetty.so
Passwordless root remote login from pre-authorised users
This is a straightforward inclusion of ssh keys for those authorised users being added to the root authorized_keys file.
Passwordless remote login for specified accounts from pre-authorised users
This is also a straightforward inclusion of ssh keys for authorised users being added to the appropriate and corresponding user's .ssh/authorized_keys file. (The typical remote user chris wants passwordless login to local user chris scenario.)
Note that accounts can remain in the default locked state after creation (i.e. with just ! in the password field for /etc/shadow) but permit SSH key based login. This requires root to place the key in the new user's .ssh/authorized_keys file. What is not so obvious is that this approach is only available when UsePAM Yes is set in /etc/ssh/sshd_config. PAM differentiates ! as "account locked for password but other access methods may be permitted" and !... "account locked. Period." (If UsePAM No is set, then OpenSSH considers any presence of ! starting the password field to represent a locked account.)
Passwordless remote login for any account from pre-authorised users
It wasn't entirely clear to me whether you wanted this facility or not. Namely, certain authorised users would be able to ssh login without a password to any any every local account.
I cannot test this scenario but I believe this can be achieved with OpenSSH 5.9 or newer, which permits multiple authorized_keys files to be defined in /etc/ssh/sshd_config. Edit the configuration file to include a second file called /etc/ssh/authorized_keys. Add your selected authorised users' public keys to this file, ensuring permissions are such that it is owned by root and has write access only by root (0644).
Best Answer
The answer to the password question is:
/etc/ssh/sshd_configfile to ensure that passwords are enabled.Then restart the ssh service (HT - @tonioc). This will work for sysvinit systems:
And this should work for systemd systems:
And then either:
Or (better):
Or (even better):
~/.ssh/authorized_keysfile if they don't know how to copy it themselves.However, for the least number of changes but rather poor security, you can simply add another key to:
on the server.
You can have as many keys as you want in the authorized_keys file. It's one key per line with options prepended.
There are many options that can be added to the
authorized_keysfile.See here
And/or:
Of course, as others have pointed out, it's not a good idea to have more than one user per account unless it's run by a team. Temporary privileged access or accounts are probably not a good idea.