1 The CS VPN

2.1 The SSH Protocol

We will be accessing the remote machines via SSH (Secure SHell), an Internet protocol for issuing text-mode commands to remote machines.

There is an older text-based protocol for giving commands to Unix called “telnet”. Telnet is still used for a variety of purposes, but has fallen out of favor because it sends everything (including your login name and password) in plain-text format, leaving you vulnerable to someone else on your network eavesdropping via “packet sniffers”.

By contrast, SSH encrypts all your communications, so you are safe even if someone is eavesdropping.

SSH allows a person connected to the Internet to log into other machines on the Internet and to issue commands to those machines.

2.1.1 Clients & Servers

Like many network protocols, SSH is based on the idea of servers on a variety of different machines that wait for clients to connect to them and request a service (in this case, an SSH connection).

To use ssh, you must

1. have an ssh client program on the machine where you are seated, and
2. you must know the name (or the IP address) of a machine elsewhere that is running an ssh server, the program that accepts logins and subsequent commands from the client, and
3. you must have credentials (login name and password) allowing you access on that server.

2.1.2 Choosing Your SSH Server

As of January 2026, the CS Dept. provides four general purpose Linux servers, and provides a “switching” machine that is supposed to distribute the users equally among the servers to reduce the chances of a server being overloaded.

2.2 Login Accounts

3 Starting a Remote Session on a Linux Server…

OK, you’ve selected your server, know your login name and password, and are ready to connect.

Remember, you need to be connected to the VPN before attempting to make an SSH connection to a CS Department server.

What you do next depends on the operating system of your local PC and what software you have on there.

3.1 … from a Windows Machine

On a Windows PC, you will need an ssh client program.

If you are on your own PC, you can choose the client program to install and run. If you are on a lab PC, at work, or for some other reason using someone else’s machine, you either need to use whatever they have installed or bring your own client program, probably on a USB flash drive.

All Windows 11 builds and Windows 10 builds from the Fall of 2018 and later have a native implementation of “OpenSSH”.

This is the recommended client for Windows users.

Before you can use this, it must be activated.

1. Open Settings.
2. Go to Apps, then Apps and Feature, then Optional Features.
3. Find the entry for OpenSSH Client (Client, not Server!), click on it, and then click Install.

   Follow the on-screen instructions to complete the installation.

3.2 … from a Linux or macOS Machine

All Linux and macOS machines should include the ssh client program, ssh.

3.3 The ssh Command

Within your local session, give the command

ssh yourLoginName@sshHostName

4 Logging In

If you are prompted for your login name, enter it.

• Do include the “cs_” that is part of all student login names.
• Don’t prepend a “CS" or ”CS/". That’s a Windows-specific convention.

At the“password:” prompt, enter your password.

• Note that as you are typing your password, nothing will happen on your screen. Unix does not echo your password back to you. Most programs and operating systems will not do that with passwords, as a basic security precaution against someone snooping your password from over your shoulder or even electronically.

  What you may find unusual, however, is that Unix will not even echo back blank characters or *s to show how many characters you have typed. This seems to disturb some people, but it’s a further security precaution. Just knowing how many characters are in your password would be valuable information to someone trying to crack your account.

5 Changing Your Password

Whether we like it or not, we need to worry about the security of our computing environment. There are people who would take advantage of this computer system if they had any, or more complete, access to it. This could range from the use of computer resources they have no right to, to the willful destruction and/or appropriation of the information we all have online. In order to maintain the level of security in our computing environment that we need, there are some things we all have to take responsibility for. Even though you may not feel like you personally have much to lose if someone had access to your account or files, you have to realize that as soon as someone gains ANY access to our system, it’s 100 times easier for them to gain access to ALL of it. So when you are lax with your own account, you are endangering the work and research of everyone else working here.

Your password is the fundamental element of security not only for your personal account, but for the whole UNIX system that we share. Without an account and password a person has NO access to our system. If someone discovers (or you tell someone) your password, not only will they have access to your personal files, but they will have a much better chance to launch attacks against the security of the entire system.

Your account password is the key to accessing and modifying all of your files. If another user discovers your password, he or she can delete all your files, modify important data, read your private correspondence, and send mail out in your name. You can lose much time and effort recovering from such an attack. If you practice the following suggestions, you can minimize the risk.

1. NEVER give another user your password. There is no reason to do this. You can change permissions and have groups set up if you need to share access with other individuals. Your account should be yours alone.

2. Never write down your password. Another person can read it from your blotter, calendar, etc. as easily as you can.

3. Never use passwords that can be easily guessed. Personal information about you (birth date, etc.) may be known to the attacker or may be recorded in on-line databases that the attacker has already obtained.

   Passwords should not be single words (in any language) because on-line dictionaries are widely available for use in spelling checkers. A common approach to cracking passwords is to compile a set of such words and to run a program that tries each one on each account on the machine. Consider inserting punctuation and other “odd” characters into your password to foil such attacks.

   • Note, however, that so-called L33T substitutions, such as using “3” in place of “E”, do not improve your security. These are so widely known that password crackers routinely apply them to their word lists when trying to guess your password.

   A person with local knowledge can also try your spouse’s name, pets’ names, etc. Your account is vulnerable to this type of cracking unless you choose your password carefully.

4. Change your password the very first time you log in, and every few months thereafter. Security problems are often traceable to stale passwords and accounts. These are accounts that have become inactive for one reason or another or the password has not changed for a long time. In our particular environment we have had break-ins via such stale accounts. A password that remains the same for a long time provides an intruder the opportunity to run much more advanced and longer running programs to break such passwords.

5. Vary the system by which you choose a password. For example, don’t repeatedly use combinations like BLUEgreen and REDyellow. If an intruder discovers your pattern, he or she can guess future passwords.

6 Logging Out

To leave our Unix systems, type

exit

Important: Do not leave your SSH sessions by simply closing the window in which it was running. This may leave you still connected to the remote machine or it may leave programs still running there. Over time, such bad behavior by multiple people will overload the server.

Late in the semester, it’s very common to see our servers straining under the accumulated weight of abandoned programs from CS252 students (particularly because we usually have more than 200 students per semester in CS252). This, of course, becomes critical right when many students are frantically trying to complete their assignments in CS252 and other courses.

Make it a habit to shut down via the exit command.