burble.dn42 / Retro42 / Modem Emulator

Modem Emulator

No modem ? no problem.

If you don’t have a physical modem you can still get that retro vibe by accessing dn42 using the modem emulator service. This service presents a series of TCP/IP ports that emulate a subset of the Hayes AT command set and which are speed limited to specific baud rates. The modem can dial directly to the fr-par1 shell service for that CLI experience, or get full dn42 access via a PPP session.

The service is intended to be used as a fake serial port in a retro VM or connected to directly using a serial terminal.

Accessing the modem emulator service

  • dialup.burble.dn42:(10000 + baud/100)

The modem emulator is accessed by connecting to dialup.burble.dn42 on ports corresponding to specific baud rates, as detailed in the table below.

10003 300 10012 1200 10024 2400
10048 4800 10096 9600 10144 14400
10192 19200 10288 28800 10336 33600
10560 56000 11150 115000
Low baud rates give a great experience when using a serial terminal and the shell service but aren’t so much fun when using PPP. Conversely, using a serial terminal and higher baud rates is a bit dull and they are better suited to the PPP service.

The emulated modem is able to dial two services depending on the number used.

Dial Number Service
54311 fr-par1 shell server
4242 PPP dialup service

Dialling the fr-par1 shell server

Use a terminal emulator and dial 54311 to access the fr-par1 shell server.

Once the modem connects, use your burble.dn42 username and password to log in.

You can set a burble.dn42 password using the services portal.
Don’t forget to set a terminal type once you’ve logged in, e.g. export TERM=vt102

Dialing the PPP server

  • Dial 4242 to access the PPP server.
  • Use username: dn42, password: dn42 to log in.
  • Once you see the PPP initiation string, connect your PPP client.
  • PPP authentication should not be required.

Example shell session using socat and minicom

Example on how to connect to the shell server using socat and minicom at 2400 baud

$ socat -d -d pty,rawer tcp:dialup.burble.dn42:10024 &
... socat[xxx] N PTY is /dev/pts/2
$ minicom -D /dev/pts/2

Within minicom type ATDT54311 to dial the shell server.

Welcome to minicom 2.8

Port /dev/pts/2, 14:01:58

Press CTRL-A Z for help on special keys

Ubuntu 22.04.1 LTS
shell-fr-par1 login: 

You can now log in using your burble.dn42 username and password.

Example PPP connection using QEMU and Windows XP

Add a virtual serial port to QEMU using -chardev and -device.

e.g. for windows XP connecting at 115k

qemu-system-i386 -enable-kvm -cpu host -m 1024 -vga std -device AC97 -hda winxp.img -chardev socket,id=bdn42,port=11150,host=dialup.burble.dn42,server=off -device pci-serial,chardev=bdn42

Create a PPP dialup connection with username dn42 and password dn42, enable the option to prompt for username and password on connect.

Enable chat prompt

Start the connection and when then chat box opens:

  • Type ATDT4242 to dial the PPP service
  • Log in using username dn42 / password dn42
  • Hit continue when you see the PPP connect string

The following chat script provided by 04dco should provide automatic login for win95 onwards:

; This is a script file that automatically logs you in
; to establish a dn42 dial-up PPP connection.

; Main entry point to script
proc main

   integer nTries = 3

   ; This is the login prompt and timeout values

   string szLogin = "modem-server Login: "
   integer nLoginTimeout = 5

   ; This is the password prompt and timeout values

   string szPW = "Password: "
   integer nPWTimeout = 5

   ; This is the prompt once your password is verified

   string szPrompt = "Last login: "

   ; Attempt to login at most 'nTries' times

   while 0 < nTries do

      ; Wait for the login prompt before entering
      ; the user ID, timeout after x seconds

      waitfor szLogin then DoLogin 
        until nLoginTimeout

      transmit "^M"        ; ping
      nTries = nTries - 1


   goto BailOut

   ; Enter user ID

   transmit $USERID, raw
   transmit "^M"

   ; Wait for the password prompt 

   waitfor szPW until nPWTimeout
   if FALSE == $SUCCESS then
      goto TryAgain

   ; Send the password

   transmit $PASSWORD, raw
   transmit "^M"

   ; Wait for the prompt

   waitfor szPrompt

   goto Done

   ; Something isn't responding.  Halt the script
   ; and let the user handle it manually.

   set screen keyboard on



Source code

The modem emulator is a small golang server and the source code is available on the burble.dn42 git server