From bare Ubuntu to fully working DOS doors, step by step.

If you prefer an opinionated, pre-packaged setup, Nigel’s DOSEMU2 tarball gives you a working configuration you can adapt:

curl -O https://www.endofthelinebbs.com/dosemu2.tar.gz
tar -xzf dosemu2.tar.gz
# Read the included README and move files into /sbbs/ and your home directory as directed.

Use this guide alongside that bundle to understand what each piece is doing and tune it for your own BBS.

Before you start, you should already have:

• Ubuntu Linux server (SSH access, sudo available)
• Synchronet installed at /sbbs/ and running
• A dedicated sbbs user (non-root)

• Install DOSEMU2 and grant Synchronet permission to bind ports
• Copy FreeDOS into a place DOSEMU2 can boot from
• Install DOS utilities used by many doors
• Create DOSEMU2 + FreeDOS configs
• Wire everything into sbbs.ini and SCFG

Add the official DOSEMU2 PPA and install DOSEMU2 plus FreeDOS support:

sudo add-apt-repository ppa:dosemu2/ppa
sudo apt update

# DOSEMU2 + FreeDOS Pseudo-DOS Package
sudo apt install dosemu2 fdpp

sudo setcap 'cap_net_bind_service=+ep' \
  /sbbs/repo/src/sbbs3/gcc.linux.x64.exe.release/sbbs

We’ll unpack the FreeDOS 1.4 full USB image into your DOSEMU2 home.

# As the sbbs user
cd ~
wget https://www.ibiblio.org/pub/micro/pc-stuff/freedos/files/distributions/1.4/official/FD14FULL.img

mkdir -p ~/.dosemu/freedos
sudo mount -o loop,offset=32256 FD14FULL.img /tmp/freedos_mount
cp -r /tmp/freedos_mount/* ~/.dosemu/freedos/
sudo umount /tmp/freedos_mount

chmod -R u+w ~/.dosemu/freedos/

DOSEMU2 expects a drive_c directory in ~/.dosemu. Point that at your new FreeDOS tree:

ln -s ~/.dosemu/freedos ~/.dosemu/drive_c

Create a shared utilities directory for your doors:

sudo mkdir -p /sbbs/exec/dosutils

cd /tmp
wget https://www.bbsing.com/bbsfossil/x00150.zip
sudo unzip x00150.zip -d /sbbs/exec/dosutils/

# Many setups simply treat X00.SYS as X00.EXE
sudo cp /sbbs/exec/dosutils/X00.SYS /sbbs/exec/dosutils/X00.EXE

cd ~/.dosemu/freedos
unzip packages/base/share.zip
cp BIN/SHARE.COM /sbbs/exec/dosutils/

cp /usr/share/dosemu/dosemu2-cmds-0.3/dosemu/fossil.com /sbbs/exec/dosutils/
cp /usr/share/dosemu/dosemu2-cmds-0.3/dosemu/exitemu.com ~/.dosemu/freedos/

sudo chown -R sbbs:sbbs /sbbs/exec/dosutils

Edit ~/.dosemu/freedos/fdconfig.sys and replace the existing content with:

!COUNTRY=001,858:\FREEDOS\BIN\COUNTRY.SYS
!LASTDRIVE=Z
!BUFFERS=20
!FILES=40
DOS=HIGH
DOS=UMB
DOSDATA=UMB
DEVICE=\FREEDOS\BIN\HIMEMX.EXE
DEVICE=C:\EMUFS.SYS
REM DEVICE=C:\NANSI.SYS
SHELLHIGH=\FREEDOS\BIN\COMMAND.COM \FREEDOS\BIN /E:2048 /P=\FDAUTO.BAT

Commenting out NANSI.SYS avoids “line too long” problems that can appear with Synchronet’s long REM lines in batch files.

Tell Synchronet to use DOSEMU2

In /sbbs/ctrl/sbbs.ini, add or verify these under the [BBS] section:

[BBS]
UseDOSemu = true
DOSemuPath = /usr/bin/dosemu
DOSemuConfPath = /sbbs/ctrl/dosemu.conf

Create dosemu.conf

Create /sbbs/ctrl/dosemu.conf:

$_xms = (8192)
$_ems = (2048)
$_hdimage = "/home/sbbs/.dosemu/freedos /sbbs/node1 /sbbs/xtrn /sbbs/ctrl /sbbs/data /sbbs/exec"
$_lredir_paths = "/sbbs"
$_bootdrive = "c"
$_term_color = (on)
$_term_char_set = "cp437"
$_fixed_term_size = "80x25"

This maps your FreeDOS volume plus key Synchronet directories into the DOS environment as C:–H: for node 1. Additional nodes are handled via DOSEMU’s -d flags below.

Create dosemu.ini (templates)

Create /sbbs/exec/dosemu.ini:

inicmd=/usr/bin/env HOME=/sbbs/ctrl $DOSEMUBIN \
  -quiet \
  -d $NODEDIR -d $XTRNDIR -d $CTRLDIR -d $DATADIR -d $EXECDIR \
  -I"video { none }" \
  -I"serial { virtual com 1 }" \
  -f$DOSEMUCONF \
  -E$EXTBAT \
  -o$NODEDIRdosemu_boot.log $EXTLOG

[stdio]
cmd=/usr/bin/env HOME=/sbbs/ctrl $DOSEMUBIN \
  -quiet \
  -d $NODEDIR -d $XTRNDIR -d $CTRLDIR -d $DATADIR -d $EXECDIR \
  -I"video { none }" \
  -I'keystroke "\n"' \
  -I"serial { virtual com 1 }" \
  -f$DOSEMUCONF \
  -E$EXTBAT \
  -o$NODEDIRdosemu_boot.log $EXTLOG

The [stdio] section is used for text-mode doors where Synchronet handles terminal I/O.

Create external.bat

Create /sbbs/exec/external.bat with:

@echo off
E:
IF "%STARTDIR%"=="" D:
IF NOT "%STARTDIR%"=="" CD %STARTDIR%

IF EXIST EMUSETUP.BAT GOTO EMULOCAL
IF EXIST F:\EMUSETUP.BAT GOTO EMUGLOBAL
IF EXIST E:\DOSUTILS\NUL GOTO NOEMU
IF EXIST H:\DOSUTILS\NUL GOTO NOEMU

ECHO ERROR: No emusetup.bat or DOSUTILS missing
GOTO EXEC

:EMULOCAL
CALL EMUSETUP.BAT
GOTO EXEC

:EMUGLOBAL
CALL F:\EMUSETUP.BAT
GOTO EXEC

:NOEMU
@set PATH=%PATH%;E:\dosutils;H:\dosutils
IF "$RUNTYPE" == "FOSSIL" x00.exe eliminate >NUL
@share >NUL

:EXEC
$CMDLINE
IF NOT "%1" == "TEST" exitemu

Synchronet expands environment variables like $STARTDIR, $CMDLINE and $RUNTYPE before DOSEMU2 sees this file.

Run SCFG as the sbbs user:

/sbbs/exec/scfg

Navigate to External Programs → Online Programs and add a new entry. These examples assume your doors live under /sbbs/xtrn.

Name:                      Wordle
Internal Code:             WORDLE
Start-up Directory:        /sbbs/xtrn/wordle
Command Line:              wordle.exe /D%f
Multiple Concurrent Users: No
Native Executable:         No
I/O Method:                FOSSIL or UART
Use Shell:                 No
Modify User Data:          Yes
BBS Drop File Type:        DOOR.SYS (52 lines)
Place Drop File In:        Node Directory

Name:                      Yacht
Internal Code:             YACHT
Start-up Directory:        /sbbs/xtrn/yact
Command Line:              door.bat
Multiple Concurrent Users: No
Native Executable:         No
I/O Method:                FOSSIL or UART
Use Shell:                 No
Modify User Data:          No
BBS Drop File Type:        DOOR.SYS (52 lines)
Place Drop File In:        Start-Up Directory

Speed up DOSEMU boot

You can shave a bit off door startup by trimming ~/.dosemu/freedos/fdauto.bat. Comment out anything you don’t need:

REM call CDROM.BAT
REM FDAPM APMDOS
REM DOSLFN

You already disabled CD-ROM support in fdconfig.sys, so this mainly avoids unneeded device probing on each door launch.

When a door won’t start

# Synchronet logs
tail -f /sbbs/data/logs/*.log

# DOSEMU2 boot log for this node
cat /sbbs/node1/dosemu_boot.log

# Fix log permissions if needed
sudo chown -R sbbs:sbbs /sbbs/data/logs/
sudo chmod -R 755 /sbbs/data/logs/

# Verify DOS utils ownership
sudo chown -R sbbs:sbbs /sbbs/exec/dosutils

Display & terminal issues

• Keep $_fixed_term_size = "80x25" in dosemu.conf.
• Do not use DOSEMU’s -dumb flag — it can prevent external.bat from running.
• Ensure your telnet / rlogin clients are using a CP437-friendly font for line-drawing characters.