SFTP error : com.jcraft.jsch.JSchException: invalid server's version string - session

Question

I have the below code to SFTP to a location
public static void putFile(String username, String host, String password, String remotefile, String localfile){
JSch jsch = new JSch();
Session session = null;
try {
session = jsch.getSession(username, host, 22);
session.setConfig("StrictHostKeyChecking", "no");
session.setPassword(password);
session.connect();
Channel channel = session.openChannel("sftp");
channel.connect();
ChannelSftp sftpChannel = (ChannelSftp) channel;
sftpChannel.put(localfile, remotefile);
sftpChannel.exit();
session.disconnect();
} catch (JSchException e) {
e.printStackTrace();
} catch (SftpException e) {
e.printStackTrace();
}
}
I am able to SFTP the document from my local machine using the above code. However when I am trying from a different environment to SFTP to the same location I am getting the follow error.
com.jcraft.jsch.JSchException: invalid server's version string at
com.jcraft.jsch.Session.connect(Session.java:253)
Note : I am using jsch-0.1.31.jar file.
on printing out session.getClientVersion() I am getting "SSH-2.0-JSCH-0.1.31".
I tried to upgrade the jar file to jsch-0.1.51.jar then session.getClientVersion() = "SSH-1.5-JSCH-0.1.51" and I am getting the following error
com.jcraft.jsch.JSchException: Session.connect: java.net.SocketException: Connection reset at com.jcraft.jsch.Session.connect(Session.java:558)
Please can you help me on what parameters should I be looking into and what is causing it to run from my local machine and upload to the same SFTP location and not from other environment?

Answer 1

As noted by #Kenster, the exception is about server's version string, not client's. The "invalid server's version string" exception is thrown by following code in Session.connect:
if(i==buf.buffer.length ||
i<7 || // SSH-1.99 or SSH-2.0
(buf.buffer[4]=='1' && buf.buffer[6]!='9') // SSH-1.5
){
throw new JSchException("invalid server's version string");
}
First, I would try to connect with some client that logs the version string and see yourself. For example with WinSCP, search its log for a pattern like:
. 2014-09-03 17:01:20.596 Server version: SSH-2.0-OpenSSH_5.3
(I'm the author of WinSCP)
Though possibly it's not about version string at all. I would rather believe the error raised by the new version, the Connection reset. The old version may fail to detect that the connection was aborted prematurely and tries to validate some random or incomplete data.
The Connection reset may indicate wide variety of different errors
Server refusing a connection from the other location
Some firewall or proxy not allowing the connection to pass through