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Using JSch to SFTP when one must also switch user

I am using JSch in a Java client to connect to a remote server and get some files using SFTP. The following code has been working fine for me: -
JSch ssh = new JSch();
JSch.setConfig(FileTransferConstants.STRICT_HOST_KEY_CHECKING, FileTransferConstants.NO);
Session session = ssh.getSession(userName, host, port);
session.setPassword(password);
session.connect();
Channel channel = session.openChannel(FileTransferConstants.SFTP);
channel.connect();
ChannelSftp sftp = (ChannelSftp) channel;
sftp.cd(remoteDirectoryPath);
sftp.lcd(localDirectoryPath);
sftp.get(remoteDirectoryPath + remoteFileName, remoteFileName);
The problem is that there has now been a change of site policy. I am no longer permitted to log on directly as this user (userName above). I must first log on as my personal user and then su into the user that has access to the files I want to SFTP.
I don't think there is anyway I can refactor the above code to achieve this and so I have started looking at using a shell or exec channel instead. I have had little success so far and cannot seem to find any examples on the web, so I would be very grateful for any advice or pointers in the right direction. Many thanks.

I do not think you can do this directly with JSch. But with some modification of its code, it's probably doable.
Note that my answer assumes that the server is *nix-based (what is backed by your reference to su) and uses OpenSSH SFTP server.
You have to open SSH "exec" channel, to execute something like:
sudo /bin/sftp-server
But on top of that channel, you need to build the ChannelSftp instance, not ChannelExec.
So you will need to implement Session.openChannel-like method, that will open exec channel, but create ChannelSftp for it.
For some background, see how it's possible to do sudo with WinSCP SFTP client.
Note that while the FAQ claims, that you won't be able to use password for the sudo, that's true for WinSCP. But as you have a full control of the session with JSch, you may be able to feed the password to sudo.
For that you might override the ChannelSftp.start() to write the password to the channel input, before starting the actual SFTP session.
You still need the requiretty option be off, as the SFTP cannot work with TTY.
For general considerations when automating operations using a different/root account, see:
Allowing automatic command execution as root on Linux using SSH

Java program to copy files from Linux remote server to windows local machine [duplicate]

I am trying to connect to a SFTP remote server using JSCH library version 0.1.49. Every time I run the program I receive the following error :
Initializing...
Connection to SFTP server is successfully
com.jcraft.jsch.JSchException: Unable to connect to SFTP server.com.jcraft.jsch.JSchException: failed to send channel request
at shell.MainClass.JschConnect(MainClass.java:95)
at shell.MainClass.main(MainClass.java:30)
line 30 is : sftpChannel.connect() from the code below :
System.out.println("Initializing...");
JSch jsch = new JSch();
Session session = null;
try {
session = jsch.getSession(ProjectConstants.rmUsername,ProjectConstants.rmHost, 22);
session.setPassword(ProjectConstants.rmPassword);
java.util.Properties config = new java.util.Properties();
config.put("StrictHostKeyChecking", "no");
session.setConfig(config);
session.connect();
if (session.isConnected() == true) {
System.out.println("Connection to SFTP server is successfully");
}
ChannelSftp sftpChannel = (ChannelSftp) session.openChannel("sftp");
try {
sftpChannel.connect();
} catch (Exception e) {
throw new JSchException("Unable to connect to SFTP server. "
+ e.toString());
}
the credentials I am using are correct ( it connects through FileZilla using the same data ), and I also disabled the proxy for that server ( either way I get the same error with or without proxy )
If anyone could help me I would greatly appreciate it as I am stuck with this error for about a week now ...
Thank you.

Check if SFTP server is started and running.
I had encountered the same issue - I was not able to open SFTP channel to my server, but I could connect with WinSCP. It took me some time to notice that WinSCP would fallback to SCP hence confusing me. Starting the server solved this issue.

Check Subsystem sftp /usr/lib/openssh/sftp-server in /etc/ssh/sshd_config

In /etc/ssh/sshd_config I changed:
Subsystem sftp /usr/lib/openssh/sftp-server
to:
Subsystem sftp internal-sftp
It helps.

Problems with web sockets secure connection using jetty webserver

I have been having problems with getting my websocket secure (wss://) connection to work.
I recently had to switch servers and got a new SSL certificate, on my previous server wss:// connection use to work fine. On new server I generated a new keystore using instruction provided by the CA, I gave the keystore the same name and password as it had on previous server as well I placed the keystore in the same directory which the code refers to. I did all that so I don't have to change the code.
I am using the exact same .JAR file from previous server, now every time I run my application i get following error message in the console
WebSocket connection to 'wss://example.com:8080/' failed: Error in
connection establishment: net::ERR_CONNECTION_REFUSED
Websockets work fine when I turn off SSL and run it on the same port, I also made sure keystore was created properly and it was. I can't figure out what may the problem be, I would really appreciate help and guidance.

This sounds like a network issue rather than a jetty/SSL issue. Is port 8080 open? Try to run the command "telnet hostname 8080". If you get a connection refused error then the port is not open.

JSch causes packet size exceeds maximum allowed error

I am attempting to use the JSch class (Java Secure Channel; jsch-0.1.50.jar) to connect to an SFTP server and send a file from within a ColdFusion (9.0.2) application (which runs atop Java 1.7.0_15). The basic code in question is:
jsch = classLoader.create("com.jcraft.jsch.JSch").init(); // ColdFusion-specific to load the jar
jschSession = jsch.getSession("myusername", "ftp.example.com", 22);
jschSession.setConfig("StrictHostKeyChecking", "no");
jschSession.setTimeout(60000);
jschSession.setPassword("mypassword");
jschSession.connect();
Upon connection to a Serv-U SFTP server it is giving me the following error on the Serv-U side immediately after the connection opens:
SSH Protocol Error: packet size exceeds maximum allowed.
Serv-U then closes the session, at which point JSch throws the exception:
Session.connect: java.io.IOException: End of IO Stream Read
I am new to the JSch class, and it's possible I'm missing something obvious, but I am at a loss as to where the error may lie. Connecting to the same SFTP server from the same origin with WinSCP gives no errors. Any tips on what the code is doing wrong or where to turn next for troubleshooting?

SSH Protocol Error: packet size exceeds maximum allowed
This means that the local client received some data from the remote server which wasn't properly formatted as an SFTP protocol message. The usual reason is that the server sent some kind of plain text message through the SSH connection. There are few things that might be going on:
Your .bashrc, .bash_profile, or similar shell configuration file on the server is set to print some message.
The server is poorly configured, and it's sending some kind of greeting.
The server is sending some kind of error message.
If you have access to the ssh command-line utility, you can use that to see what the server is sending. Run something like this:
$ ssh myusername#ftp.example.com -s sftp
This will open a plain SSH session to the remote server and request the SFTP subsystem, which is the same thing an SFTP client would do. If the server starts SFTP properly, you won't see any output from this command--it'll just wait until you kill it. If you see any text from the remote server, that is the problem. You'll need to figure out why the server is sending that text and prevent it.

JSCH expection handling - process error result from Catch

i have written a program to connect to ssh server using JSCH lib, the program runs well.
in the catch block i have used
catch(JSchException ee)
{
....
}
when error occurs i get these messages,
com.jcraft.jsch.JSchException: java.net.NoRouteToHostException: No route to host
com.jcraft.jsch.JSchException: Auth fail
com.jcraft.jsch.JSchException: timeout: socket is not established
now i need to display the error message in a dialog box. for each type of error i have to display different output. plz guide me how to process these exceptions and to differentiate them..

Your server seems to be unreachable: "No route to host" so your client cannot connect to it "Auth fail" and the socket reaches its timeout.
Try to ping the server to see its response. Make sure you give the same ip address you use in the program. If you directly provide the server name, check your host file /etc/hosts on Linux or %SystemRoot%\system32\drivers\etc\hosts.

ADD below nuget packages to your solution
DiffieHellman
Org.Mentalis.Security