Netty: How to implement a telnet client handler which needs authentication - authentication

Question

This is my first time ask question through this platform. I am sorry. I am not good in English. I will try my best to let you understand my questions.
I am totally beginner in Netty. I would like to implement a program to send commands to a telnet server and receive response message. I modified the sample telnet program to connect and get response from the serve when there is no authentication of serve.
The question is that
When the authentication processes are setup in server. (Require login name and password)
How to implement the client side program?
How can I receive the serve login request and response it?
Should I implement another handler to handle the authentication?
below shows how i send the commands to the server
EventLoopGroup group = new NioEventLoopGroup();
try {
Bootstrap b = new Bootstrap();
b.group(group)
.channel(NioSocketChannel.class)
.handler(new TelnetClientInitializer(sslCtx));
// Start the connection attempt.
ChannelFuture lastWriteFuture = null;
lastWriteFuture = b.connect(HOST, PORT).sync();
Channel ch = lastWriteFuture.channel();
lastWriteFuture = ch.writeAndFlush("ls" + "\r\n", ch.newPromise());
lastWriteFuture = ch.writeAndFlush("status" + "\r\n");
lastWriteFuture = ch.writeAndFlush("ls" + "\r\n");
lastWriteFuture = ch.writeAndFlush("exit" + "\r\n");
// Wait until the connection is closed.
lastWriteFuture.channel().closeFuture().sync();
} finally {
// Shut down the event loop to terminate all threads.
group.shutdownGracefully();
}
but what should i do before send the above commands to login into the serve?
The following picture shows what i want to do in the program
Thank you very much!!!

Answer 1

If we talk about TELNET as a protocol you should know that Telnet client from Netty examples does not support TELNET protocol. His name is just confusing and you can't connect to any standard telnet servers. You can read more about TELNET protocol here - THE TELNET PROTOCOL .
I see 2 ways:
write your implementation for TELNET on Netty
use another implementation for examples Apache Commons Net
Example for the first way - modified netty client, i tested him on Linux servers. He has several dirty hacks like a timer but he works.
Example for the second - Java – Writing An Automated Telnet Client:
import org.apache.commons.net.telnet.*;
import java.io.InputStream;
import java.io.PrintStream;
public class AutomatedTelnetClient {
private TelnetClient telnet = new TelnetClient();
private InputStream in;
private PrintStream out;
private String prompt = "~>";
public AutomatedTelnetClient(String server) {
try {
// Connect to the specified server
telnet.connect(server, 8023);
TerminalTypeOptionHandler ttopt = new TerminalTypeOptionHandler("VT100", false, false, true, false);
EchoOptionHandler echoopt = new EchoOptionHandler(true, false, true, false);
SuppressGAOptionHandler gaopt = new SuppressGAOptionHandler(true, true, true, true);
try {
telnet.addOptionHandler(ttopt);
telnet.addOptionHandler(echoopt);
telnet.addOptionHandler(gaopt);
} catch (InvalidTelnetOptionException e) {
System.err.println("Error registering option handlers: " + e.getMessage());
}
// Get input and output stream references
in = telnet.getInputStream();
out = new PrintStream(telnet.getOutputStream());
} catch (Exception e) {
e.printStackTrace();
}
}
// public void su(String password) {
// try {
// write(“su”);
// readUntil(“Password: “);
// write(password);
// prompt = “#”;
// readUntil(prompt + ” “);
// } catch (Exception e) {
// e.printStackTrace();
// }
// }
public String readUntil(String pattern) {
try {
char lastChar = pattern.charAt(pattern.length() - 1);
StringBuffer sb = new StringBuffer();
boolean found = false;
char ch = (char) in.read();
while (true) {
System.out.print(ch);
sb.append(ch);
if (ch == lastChar) {
if (sb.toString().endsWith(pattern)) {
return sb.toString();
}
}
ch = (char) in.read();
}
} catch (Exception e) {
e.printStackTrace();
}
return null;
}
public void write(String value) {
try {
out.println(value);
out.flush();
System.out.println(value);
} catch (Exception e) {
e.printStackTrace();
}
}
public String sendCommand(String command) {
try {
write(command);
return readUntil(prompt + " ");
} catch (Exception e) {
e.printStackTrace();
}
return null;
}
public void disconnect() {
try {
telnet.disconnect();
} catch (Exception e) {
e.printStackTrace();
}
}
public static void main(String[] args) {
String user = "test";
String password = "test";
AutomatedTelnetClient telnet = new AutomatedTelnetClient("localhost");
// Log the user on
telnet.readUntil("login:");
telnet.write(user);
telnet.readUntil("Password:");
telnet.write(password);
// Advance to a prompt
telnet.readUntil(telnet.prompt + " ");
telnet.sendCommand("ps -ef");
telnet.sendCommand("ls");
telnet.sendCommand("w");
telnet.disconnect();
}
}

Answer 2

Telnet has no real concept of a password packet, a password prompt is just like any normal text output. This means that you can just send the username and password when connection as separate lines, and the telnet server will use them correctly.
ch.writeAndFlush("administrator" + "\r\n");
ch.writeAndFlush("LetMeIn4!!" + "\r\n");
If you require connecting to server that don't always require the password, then you should read the output from the server, check if it contains "username", send the username, then keep reading if it contains "password" and send the password. This is prone to breaking as servers are not required to send those strings, and legit output may also contain those. This is the downside of the telnet protocol.

Answer 3

I hope this my article is helpful to someone.
Netty | Implement Telnet Automated Authentication
I had to use Telnet to control the sub-equipment while developing the space ground station software. Except for the authentication, Telnet is quite similar to regular TCP server communication. So, I implemented a Handler that automatically handles Telnet authentication to communicate with the Telnet server. When connecting to the Telnet server, the following introductory message, “Username: “, “Passwrod: “ messages are displayed in sequence, and user authentication is requested. Handler automatically handles the authentication process as if a human would input account information. Below is a brief description of the implementation.
c:\> telnet 192.168.0.1 12345
Power On Self Test (POST) Passed.
Integrated Control Unit (ICU) Build xxx (Build:xxxxxx) - Feb 7 2022, 17:57:16 (Network/TCP)
Date and Time: 2022-02-16 20:01:19 (GMT)
MAC Address : [00:xx:xx:xx:C6:8F]
Username: User
Password: 1234
>
Handler
TelnetAuthenticator Handler simply works as follows.
If the message contains the string “Username: “, send the username.
If the message contains the string “Password: “, the password is sent.
If the message contains the string “>” waiting for input, delete the authentication handler from the Pipeline. After authentication, TelnetAuthenticator Handler is unnecessary.
If the account is not registered on the Telnet server or the password does not match, the string “Username: “ or “Password: “ is repeatedly received. The authentication failure error is unrecoverable, notifying the user of a failed authentication process and forcing them to disconnect.
#Slf4j
#RequiredArgsConstructor
public class TelnetAuthenticator extends SimpleChannelInboundHandler<String> {
private final ChannelSpec channelSpec;
private boolean alreadyUserTried = false;
private boolean alreadyPasswordTried = false;
#Override
protected void channelRead0(ChannelHandlerContext ctx, String msg) {
// If the message contains the string “Username: “, send the username.
if (msg.contains(channelSpec.getReqUserTag())) {
if (alreadyUserTried) {
processFail(ctx);
}
ctx.channel().writeAndFlush(channelSpec.getAccount().getUser() + channelSpec.getEndLine());
alreadyUserTried = true;
return;
}
// If the message contains the string “Password: “, the password is sent.
if (msg.contains(channelSpec.getReqPasswordTag())) {
if (alreadyPasswordTried) {
processFail(ctx);
}
ctx.channel().writeAndFlush(channelSpec.getAccount().getPassword() + channelSpec.getEndLine());
alreadyPasswordTried = true;
return;
}
// If the incoming message contains an input waiting message, the Pipeline deletes the current handler.
if (msg.contains(channelSpec.getStandByTag())) {
ctx.pipeline().remove(this.getClass());
}
}
private void processFail(ChannelHandlerContext ctx) {
ctx.fireUserEventTriggered(ErrorMessage.AUTHENTICATE_FAIL);
ctx.close();
}
}
Initialize ChannelPipeline
A ChannelPipeline configuration with a TelnetAuthenticator Handler can be: First, register InboundHandlers as follows.
First, add DelimiterBasedFrameDecoder with “Username: “, “Password: “, “>” strings as delimiters. The stripDelimiter option is set to false because all delimiters must be received to recognize the authentication process.
Add StringDecoder.
Add the implemented TelnetAuthenticator Handler.
Add other necessary business logic.
Simply add StringEncoder to Outbound. You can add other Handlers as needed.
public class PipelineInitializer extends ChannelInitializer<SocketChannel> {
private ChannelSpec channelSpec;
public void init(ChannelSpec channelSpec) {
this.channelSpec = channelSpec;
}
#Override
protected void initChannel(SocketChannel ch) throws Exception {
ch.pipeline()
// Inbound
.addLast(new DelimiterBasedFrameDecoder(1024, false,
channelSpec.getDelimiter().reqUserTag(),
channelSpec.getDelimiter().reqPasswordTag(),
channelSpec.getDelimiter().standByTag()))
.addLast(new StringDecoder())
.addLast(new TelnetAuthenticator(channelSpec))
.addLast(new BusinessLogic())
// Outbound
.addLast(new StringEncoder());
}
}
ChannelSpec
ChannelSpec defines specifications required for communication with Telnet server. Manage server IP, port, account information, separator, etc.
#Getter
public class ChannelSpec {
private final String serverIp = "192.168.0.1";
private final int serverPort = 12345;
private final String endLine = "\r\n";
private final String standByTag = ">";
private final String reqUserTag = "Username: ";
private final String reqPasswordTag = "Password: ";
private final Account account = new Account("User", "1234");
private final Delimiter delimiter = new Delimiter();
public class Delimiter {
public ByteBuf standByTag() {
return toByteBuf(standByTag);
}
public ByteBuf reqUserTag() {
return toByteBuf(reqUserTag);
}
public ByteBuf reqPasswordTag() {
return toByteBuf(reqPasswordTag);
}
private ByteBuf toByteBuf(String input) {
ByteBuf delimiterBuf = Unpooled.buffer();
delimiterBuf.writeCharSequence(input, StandardCharsets.UTF_8);
return delimiterBuf;
}
}
}
#RequiredArgsConstructor
#Getter
public class Account {
private final String user;
private final String password;
}