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Connecting to Raspberry Pi's using 56k Modems

Dial Up Connections on Linux

In this tutorial we will detail how to connect two linux hosts via 56k modems. To do this we will use the following components:

Wiring up

We need to configure our gear.

  • Plug one USB Modem into each Pi
  • Plug one end of the RJ-11 cable into each modem
  • Plug the other end into the RJ-11 splitter
  • Plug the RJ-11 splitter into the SPA-2102

Hardware Configuration

The SPA-2102 serves as a "dial-tone generator" for the modems; that is to say the modem RJ-11 lines need voltage and the SPA-2102 supplies that voltage. You cannot plug the modems directly into each other. The modems lack the ability to generate the current necessary for establishing commmunications between themselves.

Software configuration

The modems will show up on the Pis as serial interfaces. Run dmesg to see more information:

[    1.238294] usb 1-1.4: new full-speed USB device number 3 using xhci_hcd                               
[    1.390434] usb 1-1.4: New USB device found, idVendor=0572, idProduct=1340, bcdDevice= 1.00            
[    1.390478] usb 1-1.4: New USB device strings: Mfr=1, Product=2, SerialNumber=3                        
[    1.390510] usb 1-1.4: Product: USB Modem                                                              
[    1.390534] usb 1-1.4: Manufacturer: Conexant                                                          
[    1.390557] usb 1-1.4: SerialNumber: 12345678 

On my Pis the modem appeared at /dev/ttyACM0, but your mileage may vary. We will need to use PPP as the IP layer between the two modems. We will need to create a "connect" script to pass to pppd so it knows how to handle the modem serial interface. You can see generic instructions on how to do so here: http://tldp.org/HOWTO/PPP-HOWTO/x1188.html.

The connect script I used for the client (connection initiator):

#!/bin/sh
#
# This is part 2 of the ppp-on script. It will perform the connection
# protocol for the desired connection.
#
/usr/sbin/chat -v                                                 \
        TIMEOUT         3                               \
        ABORT           '\nBUSY\r'                      \
        ABORT           '\nNO ANSWER\r'                 \
        ABORT           '\nRINGING\r\n\r\nRINGING\r'    \
        ''              \rAT                            \
        'OK-+++\c-OK'   ATH0                            \
        TIMEOUT         30                              \
        OK              ATD \
        CONNECT         ''                              \

The connect script I used for the server (connection receiver):

#!/bin/sh
#
# This is part 2 of the ppp-on script. It will perform the connection
# protocol for the desired connection.
#
/usr/sbin/chat -v                                                 \
        TIMEOUT         3                               \
        ABORT           '\nBUSY\r'                      \
        ABORT           '\nNO ANSWER\r'                 \
        ABORT           '\nRINGING\r\n\r\nRINGING\r'    \
        ''              \rAT                            \
        'OK-+++\c-OK'   ATH0                            \
        TIMEOUT         30                              \
        OK              ATA                  \
        CONNECT         ''                              \

The difference between these two scripts is the second to last line: OK ATA on the receiver versus OK ATD on the intiator.

Copy those scripts onto each Pi as chat.sh. Make the chat.sh executable using chmod +x chat.sh and then you will be able to run the following commands as root:

Intiator

pppd /dev/ttyACM0 9600 noauth local lock defaultroute debug nodetach 172.16.1.1:172.16.1.2 ms-dns 8.8.8.8 connect ./chat.sh

Receiver:

pppd noauth local lock defaultroute debug nodetach /dev/ttyACM0 connect ./chat.sh

The path prefix to the chat.sh binary seems important, so you will need the ./ in front of chat.sh when you use it in the pppd command

You should see the ppp connection establish successfully, though it might take up to a minute for the IP link to come up.

Looking under the hood at the modem connection

It is possible to send "modem commands" directy to the modem. A list of semi-standardized commands is available here: https://en.wikipedia.org/wiki/Hayes_command_set

If you desire to write commands directly to the modem, I recommend using the cu utility. Install with:

apt install cu

You can then establish AT communications sessions with:

Client:

cu -l /dev/ttyACM0
Connected.
ATD
CONNECT 9600

Server:

cu -l /dev/ttyACM0
Connected.
ATA
CONNECT 9600

Where ATD is the "AT" command run on the client and ATA is the "AT" command run on the server.

Once you have an "AT" connection established, any input from one terminal will be displayed in the opposite terminal. This is the basis for IP communication over 56k modem.

To exit cu, from the cu prompt run:

~.

Why would anyone want to do this

Fax machines are still commonly used, these techniques could be used to fuzz fax modem connections.

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