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Two-Factor Authentication for SSH with PAM Support (pam_duo).
Duo Unix includes a PAM module or alternatively a stand alone executable that can be used to protect programs such as SSH or Sudo.
Duo can be easily added to any Unix system to protect remote (SSH) or local logins with the addition of a simple pam_duo PAM module. It has been tested on Linux (RedHat, Fedora, CentOS, Debian, Ubuntu, Amazon Linux), BSD (FreeBSD, NetBSD, OpenBSD), Solaris, and AIX.