SSH with Java

Recently I needed to do some server manipulation over the SSH-2 protocol from a Java client program. There are quite a few choices of SSH libraries for Java out there. Usually I prefer BSD license whenever possible, so I thought I’ll give Ganymed SSH-2 for Java a try. It turned out to be pretty simple to use. Here’s a short example of how to connect to the server using the private key and execute some command.

import java.io.*;
import ch.ethz.ssh2.*;

public class Example {
  public static void main(String[] args) throws IOException {
    Connection conn = new Connection("hostname");
    conn.connect();
    File pemKeyFile = new File("privKey.pem");
    boolean isAuth = conn.authenticateWithPublicKey("user", pemKeyFile, "keyPwd");
    if (isAuth == false) {
      throw new IOException("Authentication failed.");
    }

    Session sess = conn.openSession();
    sess.execCommand("netstat -nl | egrep ':80' | wc -l");
    InputStream inp = sess.getStdout();
    InputStreamReader reader = new InputStreamReader(inp);
    BufferedReader br = new BufferedReader(reader);
    String line = br.readLine();

    sess.close();
    conn.close();
  }
}

If you’re unfamiliar with private/public key authentication over SSH, see the links below or just google for “passwordless ssh login”.

• http://ariadne.mse.uiuc.edu/Cluster/ssh_log_through.html
• http://www.debian-administration.org/articles/152

One thing that most tutorials don’t mention is that your home directory should have permissions 755. I found out through trial and error, that using some user’s home directory with permissions 777 just doesn’t let you login with a private key.

Another thing that I needed to do was copy a file over to the server. Now this gave me some unexpected problems. While the API seemed simple enough, the code below just didn’t work.

  String data = "sample data";
  SFTPv3Client client = new SFTPv3Client(conn);
  SFTPv3FileHandle handle = client.createFile("test_file.txt");

  byte []arr = data.getBytes();
  client.write(handle, 0, arr, 0, arr.length);
  client.closeFile(handle);
  client.close();

Here, the conn object is the SSH connection which can be acquired as shown in the first code listing. This looks  pretty straightforward – create a file on the server, write some bytes to it, then close, but the outcome of it all – the file is created on the server, but it is  always 0 bytes in length.  It took me a while to figure out what’s wrong. Now is the time to mention that I was using the “latest version” of Ganymed that was available on their page – build210. And this screwed me good. After I got frustrated with the code not working the way I wanted it to, I started looking through the source of Ganymed and I found an unsatisfiable conditional statement in the write(…) method of the SFTPv3Client class. I fixed the problem, recompiled Ganymed library and voila it works. Then later I found out that Ganymed actually has a release build211, but they renamed the library to trilead. You can find it in this maven2 repository.