mod_authn_otp is an Apache web server module for two-factor authentication using one-time passwords (OTP) generated via the HOTP/OATH algorithm defined in RFC 4226. This creates a simple way to protect a web site with one-time passwords, using any RFC 4226-compliant token device, including software tokens that run on cell phones such as Google Authenticator. mod_authn_otp also supports the obsolete Mobile-OTP algorithm.
mod_authn_otp supports both event and time based one-time passwords. It also supports "lingering" which allows the repeated re-use of a previously used one-time password up to a configurable maximum linger time. This allows one-time passwords to be used directly in HTTP authentication without forcing the user to enter a new one-time password for every page load. No additional infrastructure other than the mod_authn_otp module is required to add one-time password support to any Apache web server.
mod_authn_otp supports both basic and digest authentication, and will auto-synchronize with the user's token within a configurable maximum offset (auto-synchronization is not supported with digest authentication).
mod_authn_otp is especially useful for setting up protected web sites that require more security than simple username/password authentication yet also don't require users to install special VPN software.
Also included are otptool, a one-time password command line utility, and genotpurl. otptool can be used on a simple call-out basis to integrate two-factor authentication into any existing authentication solution. genotpurl generates oathtoken:// URLs suitable for Google Authenticator token distribution.
See the wiki for a detailed description including supported Apache configuration directives.