IKEv1 XAUTH with Google-Authenticator One Time Passwords (OTP)

Libreswan's IKE daemon pluto can use pam for XAUTH authentication (xauthby=pam). One Time Passwords (OTP) can be supported via pam directives. The following example is for using Google Authenticator. It requires that username's are actual unix system users on the VPN gateway, as their google authenticator files are stored in their home directory. Change the /etc/pam.d/pluto file to include the Google Authenticator directives:

#%PAM-1.0

# /etc/pam.d/pluto with google authenticator

auth required pam_google_authenticator.so forward_pass

auth include system-auth use_first_pass
account required pam_nologin.so
account include system-auth
password include system-auth
session optional pam_keyinit.so force revoke
session include system-auth
session required pam_loginuid.so

As root, create the user and generate their QRCODE / link:

[root@vpn #] useradd paul
[root@vpn #] su - paul
[paul2@vpn ~]$ google-authenticator 
Do you want authentication tokens to be time-based (y/n) y
https://www.google.com/chart?chs=200x200&chld=M|0&cht=qr&chl=otpauth://totp/paul@vpn.nohats.ca%3Fsecret%3DEPS4O5H4YO665V2L

Your new secret key is: EPS4O5H4YO665V2L
Your verification code is 265293
Your emergency scratch codes are:
  23358457
  86045401
  59342655
  93987954
  72038389
Do you want me to update your "/home/paul/.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) y

By default, tokens are good for 30 seconds and 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 1:30min to about 4min. Do you want to do so (y/n) y

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) y

The qrcode or the URL can be given to the user, for instance for their iphone google authenticator application.

Using the OTP code

XAUTH only supports a username and password. To specify the OTP, you concatenate it after the password. There is no separator character.