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Java – Writing An Automated Telnet Client

Telnet is a common protocol that we normally used to connect to a remote server, especially in Linux or Unix environment. To programmatically automate a telnet session so that we can execute or retrieve information from a remote server can be easily done using Apache Commons Net.

Below is the code that can be used to do a ps -ef on the remote Linux/Unix server, and retrieve the output.

import org.apache.commons.net.telnet.TelnetClient;

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, String user, String password) {
try {
// Connect to the specified server
telnet.connect(server, 23);

// Get input and output stream references
in = telnet.getInputStream();
out = new PrintStream(telnet.getOutputStream());

// Log the user on
readUntil(“login: “);
write(user);
readUntil(“Password: “);
write(password);

// Advance to a prompt
readUntil(prompt + ” “);
}
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) {
try {
AutomatedTelnetClient telnet = new AutomatedTelnetClient(“127.0.0.1″,
“username”,
“password”);
telnet.sendCommand(“ps -ef “);
telnet.disconnect();
}
catch (Exception e) {
e.printStackTrace();
}
}
}

You need to modify the prompt variable so that it matches what you configured in the server.

I used it together with Twitter for remote server monitoring so that I can connect and execute commands on the servers behinds firewall from the DMZ.

Note: This is potential security threat if not used properly.


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RSS Feed for This Post7 Comment(s)

  1. subpop | Feb 13, 2008 | Reply

    Cool! fantastic! thanks

  2. James | Dec 17, 2008 | Reply

    Okay I need code that will compile. I got the source code for org.apache.commons.net.telnet.TelnetClient, saved it as a file and it does not compile. Not sure what I’m doing wrong.

  3. James Huang | Mar 30, 2009 | Reply

    It is a great job. One command is working properly. I tried to add several cammands in a loop, it only stay in the first command. I am not sure if this is the prompt. On SunOS, the prompt should be PS1 setting in the profile something like PS1=”`/usr/bin/uname -n`($LOGNAME)$PWD > “, so the prompt should be /usr/bin/uname -n`($LOGNAME)$PWD >. Could you please help?
    Thanks!

  4. Raja M | Jul 6, 2009 | Reply

    It is a great job. One command is working properly. I tried to add several commands in a loop, it only stay in the first command.can you help me pls

  5. Abdullah | Aug 20, 2009 | Reply

    Thank you it is great work.
    Guys I run commands in loop and it is working fine for me

  6. Julio | Feb 12, 2010 | Reply

    Hi Abdullah, can you tell me how can run commands in loop ????

    I tried to do that, but I don’t know how.

    thanks in advanced.

    Julio

  7. Abdullah | Apr 6, 2010 | Reply

    Hi Hulio,
    Just read the input stream and search the response after successful login. Write the command you want execute. like below code lines:

    if(s.endsWith(“>”)){
    write(“ls 000\n”);
    }

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