Working with Remote Systems
More Notes on the Cluster

Connecting to a Remote Computer

• Insecure Connection:   telnet
  The problem with telnet is that when a user types in passwords or other important data, even though the characters may not be printed on the computer screen, they are sent as plain text characters across the Internet. If a person happens to be watching network packets and their contents, the person's user ID and password can be grabbed off the Internet and therefore an unscrupulous person could have access to your account.
• Secure Shell:   ssh
  The program ssh allows users to connect to remote computers in the same way that telnet does, but the advantage is that the data is encrypted on the host's machine before it is sent across the network. Therefore, account names and passwords cannot be viewed by people who might want to illegally gain access to a remote system. All that a user must do is type in the following commands, and then login to the remote computer.

     ssh station1.tjhsst.edu
  

  The user will be given a login prompt, and then will be able to login normally as though the person was sitting at the local computer.

  Sending Graphics Across the Network with   xhost

  When connecting to a remote system, that computer cannot send graphics images back to the host machine unless special commands are given to allow X-windows graphics to be sent back to the original machine. This is done before logging into the remote system by typing the command xhost + <computer name> in the window that will be used for the connection. This will permit that window to receive X-windows traffic from the specified computer.

  After logging into the remote system, most systems will have the display variable set properly and then graphics commands that normally would appear on the console will be sent back across the Internet to the original computer's screen.

  Example scenario:

       xhost + station1.tjhsst.edu   	 Allow traffic from station1
       ssh station1.tjhsst.edu		 Connect to the remote host
  
  
       login:dhyatt
       dhyatt's password:*********
        
       Last login: Thu Jan 10 2002 14:07:50 -0500
       2.2.18
       You have new mail.
  
       optimusprime:~$
  

  Hopefully, any graphics commands run on "optimusprime" or station1 will be sent back to the original computer system and will display on that console instead. If the computer still complains, it may be necessary to redirect the system variable called DISPLAY back to the host machine too. To check the status by typing the command called set and look at the value of DISPLAY.

       set | more
  
       BASH=/usr/local/bin/bash
       BASH_VERSINFO=([0]="2" [1]="04" [2]="0" [3]="1" [4]="release" [5]="i386-slackware-linux-gnu")
       BASH_VERSION='2.04.0(1)-release'
       COLUMNS=80
       DIRSTACK=()
       DISPLAY=optimusprime.tjhsst.edu:0
       EUID=1024
       GROUPS=()
       HISTFILE=/home/dhyatt/.bash_history
       HISTFILESIZE=500
       HISTSIZE=500
       HOME=/home/dhyatt
       HOSTNAME=mango.tjhsst.edu
        ...
  

  From the information above, it can be seen that the host from which I connected is mango.tjhsst.edu but the display is still set to show on the console of optimusprime.tjhsst.edu. If it is not directed to the appropriate computer to be viewed properly. console (mango in this case), type the command:

       export DISPLAY=mango.tjhsst.edu:0
  

  This will send the display back to mango instead of trying to display on the console of the remote computer.

  Transferring Files:   ftp

  On most of the computers in the lab, user files will always be there because they all share data on a central file server using a program callednfs, or network file sharing. However, this may not always be the case so eventually it may be necessary to move files around to different computers on the Internet.

  To transfer files from the one computer to another on the Internet, it is necessary to transfer the files using a program like ftp, or file transfer protocol. Essentially, the user opens a connection to the distant machine and then types either

  get filename
  
     or
  
  put filename

  depending upon the direction the file should go. Be careful, though. If you try to send the wrong direction, your source file can de destroyed because a zero-byte file is written over the original because you tried to get a nonexistent file, or else you put nothing on top of the thing you had on the remote computer.