TELNET : TELNET is a network protocol that allows a user to communicate with a remote device. It is a virtual terminal protocol used mostly by network administrator to remotely access and manage devices. Administrator can access the device by telnetting to the IP address or hostname of a remote device. One of the greatest disadvantage of this protocol is that all data including usernames and passwords is sent in clear text, which is a potential security risk. This is the main reason why TELNET is rarely used today.

• SSH (secure shell) : SSH is a network protocol used to remotely access and manage a device – The key difference between TELNET and SSH is that, SSH uses encryption which means that all data transmitted over the network is secure from eavesdropping. SSH uses the public key encryption for such purposes. Like TELNET, a user accessing a remote device must have an SSH client installed. On a remote devices,an SSH server must be installed and running.

• SSH components:

SSH is a proposed application layer protocol with four components:

• SSH transport layer protocol (SSH – TRANS) :

Since TCP is not a secured transport layer protocol, SSH first uses a protocol that creates a secured channel on the top of TCP. The services provided by this layer are:

1) Privacy or confidentiality of the message exchanged.

2) Data integrity which means that it is guaranteed that the message exchanged between the client and server.

3) Server authentication which means that the client is none sure that the server is the one that it claims to be

4) Compression of the message that improves the efficiency of the system and makes attack more difficult.

• SSH Authentication protocol (SSH – AVTH) :

After a secure channel is established between the client and the server and the server is authenticated for the client. SSH can call another software that can authenticate the client for the server.

• SSH connection protocol (SSH –CONN) :

After the secured channel is established and both the server and client are authenticated for each other. One of the services provided by the SSH – CONN protocol is to do multiplexing SSH – CONN takes the secure channel established by the two previous protocols.

• SSH Applications :

After the connection phase is completed, SSH allows several application programs to use the connections. Each application can create a logical channel and then benefit from the secure connection. Remote login is one of the services that can use the SSH – CONN protocols. Other applications such as a file transfer applications can use one of the logical channels.