Difference between revisions of "Two Factors Authentication With OpenSSH"

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(Adjust PAM instructions for Alpine 3.14+)
 
Line 14: Line 14:
 
{{Note|This configuration does NOT allow password authentication globally}}
 
{{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:
+
Then, configure PAM by adding the following lines, enabling google-authenticator as the package handling authentication:
  
 
* For Alpine 3.14 or newer:<br>
 
* For Alpine 3.14 or newer:<br>
Line 29: Line 29:
  
 
== Time-based One Time Password authentication (TOTP RFC 6238) ==
 
== Time-based One Time Password authentication (TOTP RFC 6238) ==
As user root:
+
As the root user:
 
{{cmd|# google-authenticator}}
 
{{cmd|# google-authenticator}}
 
{{Note|Please take note of <secret>}}
 
{{Note|Please take note of <secret>}}
Line 48: Line 48:
 
Do you want to disallow multiple uses of the same authentication
 
Do you want to disallow multiple uses of the same authentication
 
token? This restricts you to one login about every 30s, but it increases
 
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
+
your chances to detect or prevent man-in-the-middle attacks (y/n) n
  
 
By default, tokens are good for 30 seconds. In order to compensate for
 
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
+
possible time-skew between the client and server, we allow an extra
 
token before and after the current time. If you experience problems with
 
token before and after the current time. If you experience problems with
 
poor time synchronization, you can increase the window from its default
 
poor time synchronization, you can increase the window from its default
size of +-1min (window size of 3) to about +-4min (window size of
+
size of ±1min (window size of 3) to about ±4min (window size of
 
17 acceptable tokens).
 
17 acceptable tokens).
 
Do you want to do so? (y/n) n
 
Do you want to do so? (y/n) n
  
If the computer that you are logging into isn't hardened against brute-force
+
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.
 
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.
 
By default, this limits attackers to no more than 3 login attempts every 30s.
Line 64: Line 64:
 
</pre>
 
</pre>
  
{{Tip|You might want to answer differently at questions 2, 3 and 4 based on your paranoia's level and firewall settings :)}}
+
{{Tip|You might want to answer questions 2, 3 and 4 differently based on your paranoia 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.
 
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 ==
 
== Authentication token ==
Download '''Google Authenticator''' app from your ''App Store''. Startup '''Google Authenticator''' app and enter manually your <secret> key.
+
Download the '''Google Authenticator''' app from the ''App Store''. Startup '''Google Authenticator''' and manually enter your <secret> key.
  
 
== Login ==
 
== Login ==
 
{{cmd|$ ssh -v root@yourbox}}
 
{{cmd|$ ssh -v root@yourbox}}
You should see the last lines saying:
+
The last lines should say:
 
<pre>
 
<pre>
 
Authenticated with partial success.
 
Authenticated with partial success.

Latest revision as of 20:23, 24 July 2021

Configure OpenSSH to use PAM

OpenSSH allows us to configure PAM to handle authentication duties, working nicely with the google-authenticator package.

# apk add google-authenticator openssh-server-pam

First, configure the SSH daemon to use PAM authentication:

# cat /etc/ssh/sshd_config

AuthenticationMethods publickey,keyboard-interactive
ChallengeResponseAuthentication yes
PermitRootLogin yes
UsePAM yes
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:

# cat /etc/pam.d/sshd.pam #create the file if needed

  • For Alpine 3.13 or older:

# cat /etc/pam.d/sshd #create the file if needed

account		include				base-account

auth		required			pam_env.so
auth		required			pam_nologin.so	successok
auth		include				google-authenticator

Time-based One Time Password authentication (TOTP RFC 6238)

As the root user:

# google-authenticator

Note: Please take note of <secret>
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 detect or 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 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
Tip: You might want to answer questions 2, 3 and 4 differently based on your paranoia level and firewall settings :)

Re-run google-authenticator for each user that needs to login via SSH. Don't forget to include .google_authenticator files in your LBU if you're running from RAM.

Authentication token

Download the Google Authenticator app from the App Store. Startup Google Authenticator and manually enter your <secret> key.

Login

$ ssh -v root@yourbox

The last lines should say:

Authenticated with partial success.
debug1: Authentications that can continue: keyboard-interactive
debug1: Next authentication method: keyboard-interactive
Verification code: 

Authenticated with partial success means that pubkey authentication was successful and now the verifier is asking for the verification code generated from the Google Authenticator app.