HOWTO: Sane management of multiple RTL dongles in Linux

[luma]

I was looking to utilize a couple RTL dongles to monitor two ISM band frequencies commonly used in LoRa without buying an SDR with wide enough bandwidth to cover both ranges. I pretty quickly ran into issues with how SpyServer and rtl_tcp enumerate devices, which appears to be based mostly upon the order in which each device had been plugged in. With some work, I think I've come upon a flexible and secure solution to handle an arbitrary number of dongles on one system while maintaining deterministic control of each device. This means I can label an individual dongle, connect it to the desired antenna, and then connect to that dongle on the assigned TCP port every time, without regard to the order in which things have been plugged in.

All of these steps have been performed on a fresh installation of Raspbian Stretch.

Prepare the system

Code: Select all

# get the latest packages and apply any updates
sudo apt-get update && sudo apt-get upgrade
# install required packages
sudo apt install rtl-sdr librtlsdr-dev -y
# get the latest udev rules for common dongles
sudo wget -O /etc/udev/rules.d/20-rtlsdr.rules https://raw.githubusercontent.com/osmocom/rtl-sdr/master/rtl-sdr.rules
# reboot to apply udev rules
sudo reboot

Create unprivileged service user
Here we create a system user "rtltcp" with no login privileges or shell to help protect against shell escapes if opening this service to the internet

Code: Select all

sudo adduser --system rtltcp
sudo addgroup rtltcp
sudo usermod -aG dialout,rtltcp -a rtltcp

Define device serial numbers to match TCP port
We'll define a serial number for each device to match the TCP port. I've applied a physical label to each dongle for easy identification, so I can plug in the desired antenna to the correct device and then connect to the TCP port written on the label.

Code: Select all

# First, run rtl_eeprom to display installed devices
rtl_eeprom
# Then assign the desired SN to each device.  We want the SN to line up w/ the TCP port, so pick a valid value.
# Here I'm setting up two dongles to present on ports 1230 and 1231.  Change these lines to match your requirements!
rtl_eeprom -d0 -s 1230
rtl_eeprom -d1 -s 1231

Create script to identify dongles by SN
rtl_tcp identifies devices by device number, which is non-deterministic. The script below will accept a serial number as an argument, and return the device ID assigned to the provided serial number.

Code: Select all

sudo mkdir /opt/rtltcp
sudo tee /opt/rtltcp/rtlsn2dev.sh << 'EOF'
#!/bin/bash
# usage: ./rtlsn2dev.sh [serial number]
# returns: device ID
if [ $1 ]
then
  rtl_num_devices=$(rtl_eeprom 2>&1 >/dev/null | grep "Found [0-9][0-9]*" | sed -E 's/.*([0-9]+).*/\1/')
  if [ $rtl_num_devices ]
  then
    for i in $(seq 1 $rtl_num_devices);
    do
      rtl_device=$((i-1))
      rtl_serial=$(rtl_eeprom -d$rtl_device 2>&1 >/dev/null | grep "Serial number\:" | sed -E 's/Serial number:[[:blank:]]+//')
      if [ "$1" == "$rtl_serial" ]
      then
        echo $rtl_device
      fi
    done
  fi
fi
EOF
sudo chmod +x /opt/rtltcp/rtlsn2dev.sh
sudo chown -R rtltcp:rtltcp /opt/rtltcp

Create systemd service unit file
Now we'll make a systemd service template which we'll then use for each instance we plan to run.

Code: Select all

sudo tee /etc/systemd/system/[email protected] << 'EOF'
[Unit]
Description=rtl_tcp radio streaming service
After=network.target

[Install]
WantedBy=multi-user.target

[Service]
Type=simple
User=%p
Restart=always
WorkingDirectory=/opt/rtltcp
ExecStart=/bin/sh -c "/usr/bin/rtl_tcp -d $(/opt/rtltcp/rtlsn2dev.sh %i) -a 0.0.0.0 -p %i"
EOF

# Now reload systemd, create a couple instances of this service, and start them
sudo systemctl --system daemon-reload
sudo systemctl enable [email protected]
sudo systemctl enable [email protected]
sudo systemctl start [email protected]
sudo systemctl start [email protected]

And that's it! Now you have a couple services which will run at startup and can be controlled/monitored via conventional methods for linux system administration, and which will "stick" to the assigned dongle.

[MaxFun]

Hi,

Being a newbie to RTL-SDR, I have lots of questions about how to implement two USB SDR dongle on one Ubuntu computer.

I have been able to receive data from a dongle by running rtl_tcp on one terminal window and rtlamr on another terminal window. On another computer, I have been able to receive data from a another dongle by running rtl_tcp on one terminal window and rtl_433 on another terminal window.

When I put both dongles on the same computer, the first instance of rtl_tcp on one terminal window and rtlamr on another terminal window works, but when I try to start the two instance of rtl_tcp I get an error because the two instance of rtl_tcp tries to use the same device the first instance of rtl_tcp is using.

Not sure how to tell each instance of rtl_tcp which device to use because they do not have an address or pot number or each instance of rtlamr and rtl_433 which rtl_tcp to use.

Not sure why rtl_tcp works with a USB device not on the network.

Any help on how would be greatly appreciated. Also any tutorial on what you mean by “can be controlled/monitored via conventional methods for linux system administration”

Thanks

[rtlsdrblog]

You can access the help screen by typing rtl_tcp -h. The flag for choosing another device is '-d'. So -d 0, on one instance, and -d 1 on another.

[MaxFun]

Thanks, using the -h on rtl_433 was also very helpful.