Synchronet v3.18b-Win32 (install) has been released (Sept-2020).

Synchronet v3.19a, now under development, requires libarchive-dev to build successfully.

You can donate to the Synchronet project using PayPal.

Use DOS Doors with Synchronet on Linux

Instructions for using DOSEMU to run DOS doors on Synchronet-Linux.

:!: DOSEMU has been discontinued for more than a decade. However, it is the only viable solution for most distributions to run DOS doors, and is supported in Synchronet.

What is this?

Using DOSEMU, you can run DOS external programs inside of your Linux Synchronet BBS.

Getting Started

Install DOSEMU

Install DOSEMU into your system. For Debian and others, you will need to find a repository for an older version in order to get the DOSEMU package. For other systems, you may be able to compile it or obtain it in other ways.

For Debian, once you install a prior repository, simply:

 apt-get install dosemu dosemu-freedos	  	 

For other distributions,​ you will need to first install and configure dosemu, and then install FreeDOS (or your DOS of choice) and set it up to work with dosemu. Ensure that you can at least get to a DOS prompt through standalone DOSEMU before even trying to use it with Synchronet.

Edit sbbs.ini

Modify ctrl/sbbs.ini to set UseDOSemu = true.

You can also modify the DOSemuPath and DOSemuConfPath values if you need something other than the defaults. However, normally, the Synchronet-provided dosemu.conf will be in your Synchronet ctrl dir and dosemu.bin will be in your system bin directory, so the default values should work fine.

Install DOS Utils

Make a dosutils directory, preferably in your exec dir but you can also put it in your xtrn dir. (Note: if you put in your xtrn dir, you can use it for external editors).

Inside the dosutils directory, you'll need a FOSSIL driver (x00.exe works well, but you can also use, etc.).

If you aren't sure what to use, go with X00. You can get a copy at

Alternatively, you can get a copy of BNU at

You may also need if it is not found in your dosemu drive.

You can install additional utilities here that your doors may need, but the above is all you will normally need.

Modify external.bat

The external.bat file found in your exec dir is the template used to build the batch file which executes each external program. Inside this file is where you will place your call to your FOSSIL driver, within the :NOEMU section, like this:

IF "$RUNTYPE" == "FOSSIL" x00.exe eliminate >NUL

And you will also have share:

@share >NUL

Setup External Programs

Proceed to setup your external program using the normal Installation methods.

Advanced Usage


Historically, the integration used emusetup.bat in your ctrl directory to setup the fossil drivers, etc. This is still supported for existing users, but it is no longer required. Just put what you need in the :NOEMU section of ctrl/external.bat. This batch file configures the DOS environment, and can be used to load additional TSRs, change drive letters around, or make any other last minute changes to the environment before the door is loaded. It is required to at least load a FOSSIL driver in this batch file.

The file looked like:

set PATH=%PATH%;e:\dosutils
REM fossil driver, such as x00, bnu, or dosemu
rem /P1 /L0=11520
rem x00.exe
REM share.exe for multinode file locking
REM for console i/o programs

NOTE: append “eliminate” to x00.exe to skip the 5 second commercial ad when it is invoked (ie; “x00.exe eliminate” instead of just x00.exe). This will prevent a 5 second delay when starting doors and invoking external dos editors.

DOSEmu Path

The default dosemu binary is /usr/bin/dosemu.bin. If you have a different one that you would like all DOS programs that Synchronet launches to use, configure DOSemuPath in sbbs.ini.



NOTE: Be sure to point to dosemu.bin and not the dosemu wrapper script.

DOSEmu Conf

By default, it will check for dosemu.conf in your startup/external program directory, and then look at the path you configured as DOSemuConfPath in sbbs.ini. If neither is found, it will look at /etc/dosemu/dosemu.conf and /etc/dosemu.conf


The file exec/dosemu.ini is used to determine the command line execution of dosemu. You should not normally need to edit this file, but you can if you need to. The global cmd entry is used for FOSSIL, COM, and UART programs. The [stdio] cmd entry is used for intercept I/O programs.

See the file comments for which variable names you can use in the file to be substituted at run-time. These are used instead of environment variables due to the limited amount of memory available inside the batch file during execution. For example, the text $CTRLDIR will be expanded to point to your actual ctrl directory.

You can also place a copy of this file in the external program's startup directory to override it for that external program.

Drive Letters

Assuming you are using the default remapped drive letters and haven't changed them around in emusetup.bat or external.bat, they are:

  • D: - The Synchronet node directory, where the dropfile should probably be put. In door configurations, specify this drive (D:\) as the path to the dropfile.
  • E: - Under this drive, the parent of the door's startup directory will be mounted. Example: E:\LORD
  • F: - The Synchronet ctrl directory
  • G: - The Synchronet data directory
  • H: - The Synchronet exec directory


If a door will not run or has problems:

  • Try to capture any output in the BBS. In Syncterm, you can do this with Alt-C
  • Cat /sbbs/node(#)/external.bat and look at the bottom REM line. Try running that command and see what happens
  • Manually enter dosemu. Map your drive (lredir E: linux\fs\sbbs\xtrn). Switch to drive E and run “external.bat TEST”. The “TEST” simply keeps it from running “exitemu”.

Check your syslog, your SBBS error log, as well as the dosemu_boot.log in your node dir.

Make sure the door's directory conforms to the DOS 8.3 file format! /usr/local/games/tw2002 would be okay, but /usr/local/games/TradeWars2002 would not. The symptom of a non-conforming directory is a “file not found” when Synchronet tries to change to the DOS door's directory inside DOSEMU.

If the door doesn't start, it's probably a configuration problem of some kind that has left DOSEMU sitting at a DOS prompt. If you have what you think is a hung session, the first thing to try is blindly typing 'exitemu' to see if the session can be exited.

Make SURE that if the door is started with a .bat batch file, that you explicitly name it as such in the “Program Name” field. For instance, LORD's “start.bat” command would be entered as “start.bat %#”, not just “start %#”. Failure to do this will cause the session to be hung inside DOSEMU when the user exits the door. (You can still exit by blindly typing exitemu when this happens.)

Make sure your terminal is 80 chars wide, especially for configuration programs.

Setting up a non-door DOS program as a door

For the most part, setting up a non-door DOS program is done following the same procedure as a door. The key difference is that you set Redirect STDIO to Yes for a program that is not designed specifically to run as a BBS door.

Timed Events

You can also set up timed events (e.g. door daily maintenance programs) in this same manner. Just make sure Native Program is set to “No” and it will work fine.

Startup Directory: /path/to/the/door (or ../xtrn/mydoor) 
Program name: program.exe %params 
Multiuser: Yes/No depending on the program 
Redirect STDIO: Yes 
Native Program: No 
Dropfile: pick something if the program wants/needs it. 
Use Lowercase: Yes 
Dropfile Dir: Place in Node Directory 

Per-Program Configuration Files

You can override the default system files used by placing a customized version in the external program's startup directory. Files that can be overridden in this manner are:

external.bat used instead of exec/external.bat

dosemu.conf used instead of configured dosemu.conf

dosemu.ini used instead of exec/dosemu.ini

Alternate DOS

The most up-to-date freedos distribution can be found here – this is known to fix problems with Tradewars 2002. You might consider trying DR-DOS if you have problems with FreeDOS. However, be certain that you use the autoexec.bat from the FreeDOS distribution, as the Synchronet integration depends on it (specifically, it depends on the DOSDRIVE_D and unix -e behavior from it). Also remember to copy the DOSEMU support files (usually located in /usr/lib/dosemu/commands) into c:\dosemu in the DR-DOS hdimage.


See Also

In Other Languages
Translations of this page: