anchor.diff:  
   In this mode of configuration, OpenVPN server will be authenticating users
   by verifying username and user’s password against system password file 
  -“/etc/passwd” and verifying OTP (one time password generated from YubiKey)
  -against Yubico’s OTP validation server.
  +`/etc/passwd` and verifying OTP (one time password generated from YubiKey)
  +against Yubico's OTP validation server.
   
   We assume that OpenVPN server is already installed on the server.
   
  
positive.code: 
  Successful configuration of the Yubico PAM module to support two factor authentication for OpenVPN has the following prerequisites:
  
    * Operating System: Any Unix operating system which supports PAM (Pluggable Authentication Module)
      (http://www.kernel.org/pub/linux/libs/pam/)
    * Complier : GNU GCC complier (http://gcc.gnu.org/)
    * Yubico PAM Module: Yubico PAM Module Version 1.8. (https://developers.yubico.com/yubico-pam/)
    * OpenVPN: OpenVPN Version 2.0.9. (http://openvpn.net/index.php/downloads.html)
    * FreeRADIUS: FreeRADIUS Version: 1.1.7 or later. (http://freeradius.org/download.html)
    * Pam_Radius: pam_radius Version 1.3.17. (ftp://ftp.freeradius.org/pub/radius/pam_radius-1.3.17.tar.gz)
  
  Configuration
  -------------
  
negative.code:   and password authentication:
  
  ------
  plugin <Absolute path of  “openvpn-auth-pam.so” file> <PAM configuration file name for OpenVPN
  client-cert-not-required
  username-as-common-name
  ------
  
  (For e.g.: plugin /usr/lib/openvpn/plugin/lib/openvpn-auth-pam.so openvpn)
  