golang check tcp port open

Question

I need to check if remote address has specific TCP ports open or not. I have chosen to use golang for this purpose. Here is my attempt so far:

func raw_connect(host string, ports []string) {
  for _, port := range ports {
     timeout := time.Second
     conn, err := net.DialTimeout("tcp", host + ":" + port, timeout)
     if err != nil {
        _, err_msg := err.Error()[0], err.Error()[5:]
        fmt.Println(err_msg)
     } else {
        msg, _, err := bufio.NewReader(conn).ReadLine()
        if err != nil {
           if err == io.EOF {
              fmt.Print(host + " " + port + " - Open!\n")
           }
        } else {
           fmt.Print(host + " " + port + " - " + string(msg))
        }
        conn.Close()
     }
   }
 }

This is working just fine for TCP port when application (such as SSH) returns first a string, I read it and print it with no time.

However, when the application above TCP is waiting for command from the client first (such as HTTP), there is a timeout (if err == io.EOF clause).

This timeout is quite long. I would need to know immediately if the port is open or not.

Is there a more suitable technique for this purpose ?

Many thanks!

Answer 1

To check the port, you can check if the connection was successful. For example:

func raw_connect(host string, ports []string) {
    for _, port := range ports {
        timeout := time.Second
        conn, err := net.DialTimeout("tcp", net.JoinHostPort(host, port), timeout)
        if err != nil {
            fmt.Println("Connecting error:", err)
        }
        if conn != nil {
            defer conn.Close()
            fmt.Println("Opened", net.JoinHostPort(host, port))
        }
    }
}