efax is an integrated fax program for Unix-like computer systems, produced by Casas Communications Engineering since 1993. The software allows users to send and receive faxes using a computer, fax modem, and telephone line. It is command-line based, but there are several graphical user interfaces available. efax is open-source and free software, licensed under the GPL. It is included in several major Linux distributions, including Debian, Red Hat, Mandriva, and others. There is also a BSD version, and it forms part of the Apple Mac OS X Darwin system.