I notice that raddb/mods-available/otp has a link to <http://www.tri-dsystems.com/> That domain is broken - there are REFUSED responses from both their nameservers. If they have gone out of business, perhaps the file should link to this fork instead? https://code.google.com/p/otpd/ That hasn't had a release since Nov 2009, although SVN has commits from Sep 2012. I'm just trying to work out whether this is a viable option going forward (specifically for yubikey authentication, in place of using a PAM module) I notice that the README file says that you will need: b. A plugin for your auth server, that talks to otpd. You can download plugins from the TRI-D Systems web site, or write your own. Your authentication server software vendor might also supply plugins for otpd. But if their site has gone away, that means it's probably not very useful :-( Regards, Brian.