Multicast UDP Tools

A pair of simple command-line utilities for sending and receiving binary SDR data over UDP multicast.

Author

Edson Pereira, PY2SDR

Overview

This package contains two programs:

• msend - Multicast sender that reads binary data from stdin and transmits it via UDP multicast
• mrecv - Multicast receiver that listens for UDP multicast packets and writes them to stdout

These tools are useful for streaming binary data (such as audio samples, sensor data, or SDR I/Q data) across a network to multiple receivers simultaneously. Receivers can be added and removed transparently, enabling use cases like live spectrum monitoring, instrumentation, and multiple demodulators.

Building

Compile both programs using gcc:

gcc -Wall -O2 -o msend msend.c
gcc -Wall -O2 -o mrecv mrecv.c

Installation

sudo cp msend mrecv /usr/local/bin

Usage

Multicast Sender (msend)

msend -a <multicast_group> -p <port> -i <interface> [-t <ttl>]

Required arguments:

• -a <multicast_group> - Multicast IP address (e.g., 239.0.0.11)
• -p <port> - UDP port number (e.g., 15004)
• -i <interface> - Network interface name (e.g., dummy0, eth0)

Optional arguments:

• -t <ttl> - Time-to-live (default: 1, range: 1-255)

Examples:

# Send binary file to multicast group
msend -a 239.0.0.11 -p 15004 -i dummy0 < audio_samples.bin

# Send with TTL=2 (reaches devices 2 hops away)
msend -a 239.0.0.11 -p 15004 -i dummy0 -t 2 < data.bin

# Pipe data from another program
another_program | msend -a 239.0.0.11 -p 15004 -i dummy0

Multicast Receiver (mrecv)

mrecv -a <multicast_group> -p <port> -i <interface>

Required arguments:

• -a <multicast_group> - Multicast IP address (must match sender)
• -p <port> - UDP port number (must match sender)
• -i <interface> - Network interface name (e.g., dummy0, eth0)

Examples:

# Receive and save to file
mrecv -a 239.0.0.11 -p 15004 -i dummy0 > received_data.bin

# Receive and pipe to another program
mrecv -a 239.0.0.11 -p 15004 -i dummy0 | another_program

Technical Details

Data Format

• Both programs handle binary data (IQ, audio, etc)
• The sender reads data in chunks of 512 int16_t values (1024 bytes)
• Data is transmitted exactly as received with no encoding or transformation

Multicast Groups

• Uses IPv4 multicast addresses in the range 224.0.0.0 to 239.255.255.255
• The example address 239.0.0.11 is in the organization-local scope (239.0.0.0/8)

Example Use Cases

RTL-SDR

# Transmitter side - send I/Q samples
rtl_sdr -f 145M -s 2048000 -g 40 - | msend -a 239.0.0.11 -p 15004 -i dummy0

# Receiver side - receive and process
mrecv -a 239.0.0.11 -p 15004 -i dummy0 | csdr convert_u8_f | ...

It is advisable to add a bandpass filter before the RTL-SDR in order to prevent issues with strong out of band signals.

QSD-SDR

# Sender
arecord -f S16_LE -r 96000 -c 2 | msend -a 239.0.0.11 -p 15004 -i dummy0

# Receiver
mrecv -a 239.0.0.11 -p 15004 -i dummy0 | csdr convert_u16_f | ... | aplay -f S16_LE -r 48000 -c 2

The arecord sample rate should match the sample rate of your sound card

Data Distribution

# One sender
data_generator | msend -a 239.0.0.11 -p 15004 -i dummy0

# Multiple receivers (can run simultaneously)
mrecv -a 239.0.0.11 -p 15004 -i dummy0 | csdr ... | ...  # Receiver 1
mrecv -a 239.0.0.11 -p 15004 -i dummy0 | csdr ... | ...  # Receiver 2
mrecv -a 239.0.0.11 -p 15004 -i dummy0 | analyzer        # Receiver 3
mrecv -a 239.0.0.11 -p 15004 -i dummy0 > output.bin      # Receiver 4

Local Dummy Network Setup

To prevent multicast data from being transmitted over WiFi or other physical networks, you can set up a private dummy network interface on your local machine. By specifying the dummy interface with the -i flag, all multicast traffic stays isolated to your system.

On Linux, create a dummy network interface:

ip link add dummy0 type dummy
ip addr add 192.168.10.1/24 dev dummy0
ip link set dummy0 multicast on
ip link set dummy0 up

Since both msend and mrecv explicitly specify the interface via the -i flag, no additional routing configuration is needed. This is particularly useful for:

• Testing multicast applications locally
• Preventing multicast traffic on WiFi networks (WiFi has issues with IP Multicast)
• Running sender and receiver on the same machine
• Development and debugging without network interference

Verifying the Setup

Verify no multicast leaks to your physical interfaces using tcpdump:

# Monitor WiFi interface (replace wlan0 by your WiFi interface) - should see NO multicast traffic
sudo tcpdump -i wlan0 multicast

# Monitor dummy interface - should see your multicast packets
sudo tcpdump -i dummy0 multicast

Cleanup

To remove the dummy interface when done:

ip link delete dummy0

Note: These commands require root privileges. The dummy interface configuration will not persist across reboots unless added to your network configuration files. On my system, I have placed the command in /etc/rc.local and enabled rc.local in systemd.

Troubleshooting

No data received

1. Check that sender and receiver use the same multicast group, port, and interface
2. Verify firewall settings allow UDP traffic on the specified port
3. Ensure multicast routing is enabled on your network
4. Try increasing TTL if devices are on different subnets

Limitations

• UDP provides no guarantee of delivery or ordering (I never had a problem with this)
• No built-in error correction or retransmission (I never had a problem with this)
• Receiver must be running before sender starts (normally not an issue)
• No encryption or authentication (not an issue on local networks)