5

Is there a way to tell the SSH daemon, "when a remote device attempts to connect via SSH, verify against authorized_keys, but if there is no key offered, accept a one-time password so the user can use ssh-copy-id to provide it for subsequent attempts"?

context: I would like to be able to set up a linux server so that a user can SSH into the box, but I want to use RSA keypairs for authentication. I also have several devices that I want to be able to connect from, so that means providing multiple key-pairs (not a problem).

Essentially, I want to add entries to authorized_keys using OTP and ssh-copy-id, from a device that, currently would be denied access (because it hasn't presented a public key yet). I currently have to SSH in from a machine that can connect, edit the sshd_config file to allow passwords, then copy over the id with a static password, turn password authentication off and reboot the daemon. I would prefer a way to get a OTP from the system and then just use that and keep working...

  • You might want to write a PAM module for SSH that gets run if key auth fails. PAM is generally how you configure alternative auth mechanisms; that having been said, I don't know if there's already a better way out there, so I'm not giving that as an answer. – Parthian Shot Jul 29 '15 at 18:11
2

Public key authentication is turned on by default and has higher priority then password authentication (handled by PAM or directly).

You can set up in sshd_config option UsePAM yes (by default on Red Hat), which will defer authentication to PAM -- this is configured in /etc/pam.d/sshd (can differ a bit on some systems).

For OTP you can use google_authenticator, or some other open implementation of one-time passwords. There are many how-to's around here. You can try to search for two-factor authentication, but basically it is adding one line like this

 auth            required        pam_google_authenticator.so

in the /etc/pam.d/sshd and do some configuration: Arch has nice instruction for this: https://wiki.archlinux.org/index.php/Google_Authenticator

Using only one-time passwords, without the second factor can be potential risk if the one-time password or token gets lost -- I recommend you not to go this way and implement rather the two factor authentication than only one-time password.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.