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Creating a TCP client using the net package in Go allows developers to connect to servers seamlessly, mirroring the functionality of the telnet command. By initializing a Go project and employing the net.Dial method, users can establish a connection to a specified address, handling potential errors effectively. This lesson explores how to send input from standard input to a server using io.Copy and addresses common challenges, such as blocking calls, by leveraging goroutines for concurrent execution. The final outcome is a simple yet operational telnet client that communicates with an echo server, demonstrating the practical application of Go’s networking capabilities.
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As well as creating servers, the net package can also be used to create clients. Let's go ahead and create a TCP client that will act pretty much the same as our telnet function already is. To do so, I'm going to go ahead and create a new project called telnet and initialize it using the go mod init command as we've seen before.
package main
func main() {
Then inside of this function, let's go ahead and create a new TCP client using the net/dial method. Again, the dial method also has a number of other specializations depending on which protocol that you're using, such as dial IP, dial TCP, dial UDP, and dial UNIX. For the meantime, let's go ahead and use the simple dial function, which will cover most of our use case. Again, we need to pass in the network, which in this case is going to be TCP, and the address we want to connect to. In this case, let's set it to be 0.27.0.0.1:8080.
Now let's go ahead and capture the return values of which we have a connection and an error. And if an error exists, we'll go ahead and log.Fatal line, letting the user know that something went wrong.
Failed to dial connect to TCP server.
And let's go ahead and log out the error message so we can take action on it. Next, let's go ahead and close this connection whenever we exit the function, and our code should now connect.
Next, in order to create our telnet server, we're going to want to copy the contents of the standard input into our connection. Again, we can do this using the io.Copy function, passing in our connection as the writer, and os.Stdin as the reader.
Now if I go ahead and... Now if I go ahead and change back to our code, let's exit this. And restart our server again using the go run . command. Now if I head on over to the telnet function, and use go run, if I pass in messages, you can see they now appear on our server.
However, one thing you'll notice is that we're not writing any of the messages that we've received from the server to stdout. Fortunately, we can do this pretty easy, by using the io.Copy command, passing in stdout as our writer, and the connection as our io.Reader.
Unfortunately, however, if we go to run this, you'll see we still no longer get any of the data being printed out. This is because the first io.Copy on line 17 is blocking before we can get to the io.Copy on line 18.
Fortunately, this is pretty easy to solve by just running the first io.Copy inside of a go routine. Now if I go ahead and run this code again, you can see it works the exact same way as our telnet function did before. With that, we've managed to make a really simple telnet client in order to use with our echo server, which is working pretty much like telnet would.