Check if a tcp connection can connect back to the same host via it's public IP (hint, it can!)

main.go

package main

import (
	"context"
	"fmt"
	"net"
	"time"
)

func main() {
	l, err := net.Listen("tcp", ":9091")
	if err != nil {
		panic(err)
	}
	go func() {
		c, err := l.Accept()
		if err != nil {
			panic(err)
		}

		msg := make([]byte, 5)
		n, err := c.Read(msg)
		if err != nil {
			panic(err)
		}
		fmt.Printf("read %d bytes from connection %q\n", n, string(msg))

	}()

	ctx, cancel := context.WithTimeout(context.Background(), time.Second)
	defer cancel()
	var d net.Dialer
	c, err := d.DialContext(ctx, "tcp", "your_public_ip")
	if err != nil {
		panic(err)
	}
	n, err := c.Write([]byte("hello"))
	if err != nil {
		panic(err)
	}

	fmt.Printf("wrote %d bytes of hello to connection\n", n)
	time.Sleep(time.Second)

}