SSH tunneling via JSch
My aim is to connect to a server (host) which is behind a firewall. I am able to access this server by connecting to another server (tunnel) in the network and then SSH to this server. However I am not able to implement the same scenario via JSch.
I am not able to have the below code work, which I have written for this purpose. Please let me know if I am doing anything silly here.
public class JschExecutor {
public static void main(String[] args){
JschExecutor t=new JschExecutor();
try{
t.go();
} catch(Exception ex){
ex.printStackTrace();
}
}
public void go() throws Exception{
StringBuilder outputBuffer = new StringBuilder();
String host="xxx.xxx.xxx.xxx"; // The host to be connected finally
String user="user";
String password="passwrd";
int port=22;
String tunnelRemoteHost="xx.xx.xx.xx"; // The host from where the tunnel is created
JSch jsch=new JSch();
Session session=jsch.getSession(user, host, port);
session.setPassword(password);
localUserInfo lui=new localUserInfo();
session.setUserInfo(lui);
session.setConfig("StrictHostKeyChecking", "no");
ProxySOCKS5 proxyTunnel = new ProxySOCKS5(tunnelRemoteHost, 22);
proxyTunnel.setUserPasswd(user, password);
session.setProxy(proxyTunnel);
session.connect(30000);
Channel channel=session.openChannel("exec");
((ChannelExec)channel).setCommand("hostname");
channel.setInputStream(null);
((ChannelExec)channel).setErrStream(System.err);
InputStream in=channel.getInputStream();
BufferedReader ebr = new BufferedReader(new InputStreamReader(in));
channel.connect();
while (true) {
byte[] tmpArray=new byte[1024];
while(in.available()>0){
int i=in.read(tmpArray, 0, 1024);
if(i<0)break;
outputBuffer.append(new String(tmpArray, 0, i)).append("\n");
}
if(channel.isClosed()){
System.out.println("exit-status: "+channel.getExitStatus());
break;
}
}
ebr.close();
channel.disconnect();
session.disconnect();
System.out.println(outputBuffer.toString());
}
class localUserInfo implements UserInfo{
String passwd;
public String getPassword(){ return passwd; }
public boolean promptYesNo(String str){return true;}
public String getPassphrase(){ return null; }
public boolean promptPassphrase(String message){return true; }
public boolean promptPassword(String message){return true;}
public void showMessage(String message){}
}
}
The above code gives the below exception on the session.connect(30000); line.
com.jcraft.jsch.JSchException: ProxySOCKS5: com.jcraft.jsch.JSchException: fail in SOCKS5 proxy
at com.jcraft.jsch.ProxySOCKS5.connect(ProxySOCKS5.java:317)
at com.jcraft.jsch.Session.connect(Session.java:231)
at com.ukris.main.JschExecutor.go(JschExecutor.java:50)
at com.ukris.main.JschExecutor.main(JschExecutor.java:19)
Caused by: com.jcraft.jsch.JSchException: fail in SOCKS5 proxy
at com.jcraft.jsch.ProxySOCKS5.connect(ProxySOCKS5.java:200)
... 3 more
Answer
a SOCKS proxy setting on jsch allows you to connect to a running proxy server on the remote side. An sshd on the remote side would not be considered a SOCKS proxy. What you will have to do is establish a local port forward to the ssh port on the machine you're tunneling to, then establish a secondary ssh connection to this system using the api.
I've taken your example and slightly rewritten it to accomplish this:
import com.jcraft.jsch.*;
import java.io.*;
public class JschExecutor2 {
public static void main(String[] args){
JschExecutor2 t=new JschExecutor2();
try{
t.go();
} catch(Exception ex){
ex.printStackTrace();
}
}
public void go() throws Exception{
StringBuilder outputBuffer = new StringBuilder();
String host="firstsystem"; // First level target
String user="username";
String password="firstlevelpassword";
String tunnelRemoteHost="secondlevelhost"; // The host of the second target
String secondPassword="targetsystempassword";
int port=22;
JSch jsch=new JSch();
Session session=jsch.getSession(user, host, port);
session.setPassword(password);
localUserInfo lui=new localUserInfo();
session.setUserInfo(lui);
session.setConfig("StrictHostKeyChecking", "no");
// create port from 2233 on local system to port 22 on tunnelRemoteHost
session.setPortForwardingL(2233, tunnelRemoteHost, 22);
session.connect();
session.openChannel("direct-tcpip");
// create a session connected to port 2233 on the local host.
Session secondSession = jsch.getSession(user, "localhost", 2233);
secondSession.setPassword(secondPassword);
secondSession.setUserInfo(lui);
secondSession.setConfig("StrictHostKeyChecking", "no");
secondSession.connect(); // now we're connected to the secondary system
Channel channel=secondSession.openChannel("exec");
((ChannelExec)channel).setCommand("hostname");
channel.setInputStream(null);
InputStream stdout=channel.getInputStream();
channel.connect();
while (true) {
byte[] tmpArray=new byte[1024];
while(stdout.available() > 0){
int i=stdout.read(tmpArray, 0, 1024);
if(i<0)break;
outputBuffer.append(new String(tmpArray, 0, i));
}
if(channel.isClosed()){
System.out.println("exit-status: "+channel.getExitStatus());
break;
}
}
stdout.close();
channel.disconnect();
secondSession.disconnect();
session.disconnect();
System.out.print(outputBuffer.toString());
}
class localUserInfo implements UserInfo{
String passwd;
public String getPassword(){ return passwd; }
public boolean promptYesNo(String str){return true;}
public String getPassphrase(){ return null; }
public boolean promptPassphrase(String message){return true; }
public boolean promptPassword(String message){return true;}
public void showMessage(String message){}
}
}
What this code does is create a local port forwarding to the ssh port on the target system, then connects through it. The running of the hostname command illustrates that it is, indeed, running on the forwarded-to system.