A door is a computer program, on a bulletin board system, that runs outside of the main bulletin board program. Sometimes called external programs, doors are the most common way to add games, utilities, and other extensions to BBSes. From the 1990s on, most BBS software had the capability to "drop to" doors. Several standards were developed for passing connection and user information to doors; this was usually done with dropfiles.
Most doors were responsible for operating the serial port or other communications device directly until returning control to the BBS. Later development of FOSSIL drivers have allowed both BBSes and their doors to communicate without being responsible for direct operation of the communications hardware.
A major use of doors is for door games: computer games played on the BBS. While many of the most popular and memorable BBS doors have been games, numerous doors had non-entertainment applications such as user polls or the time bank, permitting users to time-shift their rationed BBS use. Frequently they act as a front-end to themed databases on subject such as astrology, numerology and fortune-telling, recipes, weather prediction, personal ads (sometimes with additional match-making functionality), classified ads and "for sale" listings (sometimes permitting auctions), BBS lists, and parting comments from the most recent BBS callers.
A front door operates in the opposite manner from most doors: instead of being launched by the BBS, the front door handles call answering and allows non-BBS calls to be dealt with. This allowed BBSes to accept messages and files, on networks such as FidoNet, without the calling system having to know the operating details of the recipient's BBS.
Another type of door compiles and transfers BBS messages for the use of offline readers. Many doors, including Markmail and Qmail, provided QWK functionality for convenient use with offline readers.
Prodoor provided a different message editor for use with the PCBoard messagebase.
Sysops who were away from the physical BBS sometimes have the capability to use general operating system commands through a special door. One such sysop door was DOORWAY by Marshall Dudley, which could run many DOS programs by modem, including file managers, DOS prompt, and the specialized editing programs that administered the BBS. DOORWAY could also be utilized to safely make certain text mode DOS games playable by BBS users, using various command line switches, without allowing disk writes or dropping out to a command shell.