My solutions to Protohackers

make image-X, where X is the number of a problem, i.e. 0-11

netcat was designed to work until both sides have closed the connection. You still have one side left open. When the server closes the connection with FIN the connection remains in a half-open state where you can send data and the server can receive it. The connection will remain in this state until you close netcat's stdin (type Ctrl-D), which makes it send a FIN packet to the server.

nc [host] [port]

1. Type text, send with enter
2. Use ctrl + D to send FIN and close the connection