Sending/Receiving TCP/UDP Messages in Ubuntu

The server listens for messages, and the client sends messages.

The server must be started before the client, otherwise the client will not connect.

References

Good easy guide for simple TCP chat server. How to send and receive messages with NC in Linux?

I didn't have to make any firewall changes to get this to work, so don't mess with ufw. ufw can mess up existing connections like ssh. Ubuntu | Security - Firewall

How To Use Netcat to Establish and Test TCP and UDP Connections

Netcat (nc) Command with Examples

How to check if port is in use on Linux or Unix

IANA port registration list: Service Name and Transport Protocol Port Number Registry

Wikipedia port registration list: List of TCP and UDP port numbers

Bash Commands

TCP

# Server (IP = 1.2.3.4):
# Listen for TCP messages on port 5169 (machine IP = 1.2.3.4)
netcat -l -p 5169
netcat -lt -p 5169  # Equivalent
nc -l -p 5169  # Equivalent
nc -lt -p 5169  # Equivalent

# Client (any IP on network):
# Send TCP messages to port 5169 of machine 1.2.3.4 (from any IP on same network)
netcat 1.2.3.4 5169
netcat -t 1.2.3.4 5169  # Equivalent
nc 1.2.3.4 5169  # Equivalent
nc -t 1.2.3.4 5169  # Equivalent

UDP

# Server (IP = 1.2.3.4):
# Listen for UDP messages on port 5170 (machine IP = 1.2.3.4)
netcat -lu -p 5170
nc -lu -p 5170  # Equivalent

# Client (any IP on network):
# Send UDP messages to port 5170 of machine 1.2.3.4 (from any IP on same network)
netcat -u 1.2.3.4 5169
nc -u 1.2.3.4 5169  # Equivalent