Sending binary data over Bluetooth/RFCOMM/SPP converts 0x0A to 0x0D 0x0A

Question

I have a Bluetooth RFCOMM service running on Linux via the commands:

sdptool add --channel 1 SP
rfcomm watch hci0 1 "$COMMAND" {}
# ^ here

The $COMMAND will write binary data to the file passed as a parameter. I have tested that it behaves correctly by doing:

FIFO=$(tempfile)
mkfifo "$FIFO"
"$COMMAND" "$FIFO" &
cat "$FIFO" | hexdump -C # <- output is correct

However, when connecting to the service from a different device via SPP/RFCOMM discovery (UUID 00001101-0000-1000-8000-00805F9B34FB), I see that every instance of 0x0A (LF) is replaced by 0x0D 0x0A (CR LF) in the stream. The issue isn't on the receiving end, because I have tried connecting to a hardware serial device that also sends binary data, and there the conversion doesn't happen. It must be the command in the first snippet (above the # ^ here line) that performs the replacement.

Why does the rfcomm tool do this replacement and how can I disable it?

Answer 1

It looks like you are being bitten by the line discipline of the TTY (remember that rfcomm does not create a fifo, but a tty).

You may try changing the TTY to raw mode, that is, without any magic. The easiest way is with stty --file <tty> raw. I don't know if rfcomm will admit several commands in its command line but you can do easily with a script:

command_raw

#!/bin/bash
stty --file "$1" raw
"$COMMAND" "$1"

And then run:

sdptool add --channel 1 SP
rfcomm watch hci0 1 ./command_raw {}

If you have the source of the command to be run, you can also change it easily in C:

 #include <termios.h>
 #include <unistd.h>

 //WARNING: error checking left as an exercise to the reader!
 void make_raw(int fd)
 {
     struct termios ios;

     //Not a TTY: nothing to do
     if (!isatty(fd))
         return; 

     tcgetattr(fd, &ios);
     cfmakeraw(&ios);
     tcsetattr(fd, TCSANOW, &ios);
 }