When connecting to a Bluetooth serial port from Ubuntu:

adi@Poppy:~$ sudo rfcomm -r connect 0
Connected /dev/rfcomm0 to 1C:3E:84:FE:4F:69 on channel 8
Press CTRL-C for hangup

on the client side I receive the following data which is automatically sent from Ubuntu on connect:

41 54 5e 53 51 50 4f 52 54 3f 0d 41 54 0d 41 54 0d 41 54 0d 7e 00 78 f0 7e 7e 00 78 f0 7e

AT^SQPORT?
AT
AT
AT
~

This interferes with what I want to do so I need to prevent Ubuntu from sending any unsolicited data to the client.

Thanks.

I'm assuming your /etc/bluetooth configuration directory files are all defaults? There should be an rfcomm.conf file in there.

Otherwise, I'm unsure what the problem is, if it's not some odd configuration set up in that file.

Of course rfcomm.conf file exists and it has the classic configuration since the connection is successfully created and I can exchange packets.
My problem is that immediately after the connection is done, Ubuntu(15) sends that data through serial channel and confuses the client (which expects a specific protocol).
My guess is that rfcomm is doing some modem negotiation on initialization or there is some service in Ubuntu who does that, but I have no idea how to investigate this.

rfcomm0 {
bind no;
device 1C:3E:84:FE:4F:69;
channel 8;
comment "Serial Port";
}

You're running SPP? I'm assuming that PPP is not involved with this. I'd default assume that anyways, but wondering. Seems more like wvdial or ppp is sending AT commands to establish a call of some type.

Yes, it's SPP. Hmm, I never came across wvdial or ppp so I don't know how they can be involved. I'll have to try to disable them and see what happens.

LE
wvdial and ppp are not installed.

After connecting to a port, minicom can function only with root(and that confused me). For any other user I get:

adi@Poppy:~$ minicom -D /dev/rfcomm1
minicom: cannot open /dev/rfcomm1: Device or resource busy

adi@Poppy:~$ ls -lsa /dev/rfcomm1
0 crw-rw-rw- 1 root dialout 216, 1 aug 3 19:46 /dev/rfcomm1

So I did this:

adi@Poppy:~$ sudo fuser /dev/rfcomm1
[sudo] password for adi:
/dev/rfcomm1: 696

adi@Poppy:~$ ps -p 696
PID TTY TIME CMD
696 ? 00:00:00 ModemManager

Aha, got you!

After killing the process, what do you think? No AT commands on connect.