Two Factor Authentication With OpenSSH: Difference between revisions

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(Adjust PAM instructions for Alpine 3.14+)
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== Configure OpenSSH to use PAM==
#REDIRECT[[HOWTO OpenSSH 2FA with password and Google Authenticator]]
OpenSSH allows us to configure PAM to handle authentication duties, working nicely with the {{pkg|google-authenticator}} package.
 
{{cmd|# apk add google-authenticator openssh-server-pam}}
 
First, configure the SSH daemon to use PAM authentication:
{{cmd|# cat /etc/ssh/sshd_config}}
<pre>
AuthenticationMethods publickey,keyboard-interactive
ChallengeResponseAuthentication yes
PermitRootLogin yes
UsePAM yes
</pre>
{{Note|This configuration does NOT allow password authentication globally}}
 
Then, configure pam by adding the following lines, enabling google-authenticator as the package handling authentication:
 
* For Alpine 3.14 or newer:<br>
    {{cmd|# cat /etc/pam.d/sshd.pam #create the file if needed}}
* For Alpine 3.13 or older:
    {{cmd|# cat /etc/pam.d/sshd #create the file if needed}}
<pre>
account include base-account
 
auth required pam_env.so
auth required pam_nologin.so successok
auth include google-authenticator
</pre>
 
== Time-based One Time Password authentication (TOTP RFC 6238) ==
As user root:
{{cmd|# google-authenticator}}
{{Note|Please take note of <secret>}}
<pre>
Do you want authentication tokens to be time-based (y/n) y
https://www.google.com/<pruned>
Your new secret key is: <secret>
Your verification code is <pruned>
Your emergency scratch codes are:
  <pruned>
  <pruned>
  <pruned>
  <pruned>
  <pruned>
 
Do you want me to update your "/root/.google_authenticator" file? (y/n) y
 
Do you want to disallow multiple uses of the same authentication
token? This restricts you to one login about every 30s, but it increases
your chances to notice or even prevent man-in-the-middle attacks (y/n) n
 
By default, tokens are good for 30 seconds. In order to compensate for
possible time-skew between the client and the server, we allow an extra
token before and after the current time. If you experience problems with
poor time synchronization, you can increase the window from its default
size of +-1min (window size of 3) to about +-4min (window size of
17 acceptable tokens).
Do you want to do so? (y/n) n
 
If the computer that you are logging into isn't hardened against brute-force
login attempts, you can enable rate-limiting for the authentication module.
By default, this limits attackers to no more than 3 login attempts every 30s.
Do you want to enable rate-limiting (y/n) n
</pre>
 
{{Tip|You might want to answer differently at questions 2, 3 and 4 based on your paranoia's level and firewall settings :)}}
 
Re-run <code>google-authenticator</code> for each user that needs to login via SSH. Don't forget to include <code>.google_authenticator</code> files in your [[Alpine_local_backup|LBU]] if you're running from RAM.
 
== Authentication token ==
Download '''Google Authenticator''' app from your ''App Store''. Startup '''Google Authenticator''' app and enter manually your <secret> key.
 
== Login ==
{{cmd|$ ssh -v root@yourbox}}
You should see the last lines saying:
<pre>
Authenticated with partial success.
debug1: Authentications that can continue: keyboard-interactive
debug1: Next authentication method: keyboard-interactive
Verification code:
</pre>
<code>Authenticated with partial success</code> means that pubkey authentication was successful and now the verifier is asking for the verification code generated from the '''Google Authenticator''' app.
 
[[Category:Authentication]]

Latest revision as of 15:48, 29 July 2023