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SSH Remote Access

Instructor: Christopher Nott

Christopher has taught college level information technology and IT security, has a master's degree in Information Security, and holds numerous industry certifications.

We many not always have the ability to physically access a computer we need to work on. SSH, or Secure Shell, allows us to access that computer from a remote location, and to do so securely. In this lesson, we'll learn what SSH is and look at a few examples.

SSH Remote Access

SSH or Secure Shell is a network protocol that connects users to a remote computer over a secure connection. This allows administrators and other authorized users to connect to secure computers over a network that is not secure, like the Internet. This is accomplished through the use of encryption.

SSH Protocol

What we use today and call SSH is officially known as SSH2, the second version of the SSH protocol which became the standard for SSH in 2006. SSH1, the original version of the protocol, was developed in 1995, but over time, various security flaws were exposed. SSH2 uses the Diffie-Hellman key exchange and message authentication codes as an integrity check to overcome those flaws and greatly improve security. So far, there are no known exploitable vulnerabilities in SSH2 (which we'll just refer to as SSH from now on).

SSH uses a client-server model. This means that every SSH connection consists of two parts: the client, or the machine that is requesting a secure connection, and the server, the machine that is granting (or rejecting) the connection. Except for Microsoft Windows, SSH client and server utilities are included with most operating systems and are most commonly used on Linux or Unix systems. SSH servers and clients are available for Windows, but, are not part of the system by default.

SSH security is accomplished by using public-key cryptography. Public-key cryptography means that, instead of just using a username and password, the two systems exchange secret keys that positively identify each system, and ensures that only the intended system can read the communications between them. The first time the connection is made, the machines will provide their public keys to each other. Every time after that, the client can be reasonably assured they are connecting to the correct server since each key is unique.

Remote Access

To establish a connection using SSH, the user simply starts the client and tells it where it wants to connect to, like in the example below:

basic_ssh_command_example

This example assumes that the same user who is logged into the client will also be the user account that is used to log into the server. This example says 'I want to use my current user account to log into a system named 'someSSHserver'.

If this user needs to login as a different account, for example, if the user needs to log in as the server administrator on the remote machine, you can tell the client to connect as another user by including the desired username in the request, as with the example below:

ssh_command_example_with_user

This request says I want to log into the server named 'someSSHserver' as the user 'administrator'. In this case, the user will be asked to provide the password for the 'administrator' account on the 'someSSHserver' machine. When the password is entered, it is sent to the remote server encrypted with the server's public key.

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