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Till now i now that ssh is like encryption in the network .But i want to be clear what it is and why we use it . And why to access it for remote guest OS . please give one example also.

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Using SSH to login to remote machine's is considered very secure as all communication between client and server is encrypted with public and private key encryption. As well as that the client and server will also use a digital signature to identify the each other, this can warn when the identity if the server or client suddenly changes.

For example, if there is a man in the middle attack, you will see something like this and connection is terminated immediately:

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@    WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!     @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that a host key has just been changed.
The fingerprint for the ECDSA key sent by the remote host is
SHA256:p4ZGs+YjsBAw26tn2a+HPkga1dPWWAWX+NEm4Cv4I9s.
Please contact your system administrator.
Add correct host key in /Users/dalanz/.ssh/known_hosts to get rid of this message.
Offending ECDSA key in /Users/dalanz/.ssh/known_hosts:9
ECDSA host key for 192.168.56.101 has changed and you have requested strict checking.
Host key verification failed.

This could show when someone intercept the data packet sent from the server and replaced with one of his own.

Even if the man in the middle attack did happned, they will still not be able to read the data as the data being encrypted with your public key and it needs your private key to decrypt the data. So data pretty useless.

Although, no system is perfect and it is possible to spoof a connection between machines, but this measures can slow someone down.

It is very important you disabling SSH password authentication and root login.

Its consider a best practice not allow any user to SSH in as root user. Instead, as a standard user(in the sudo group) and become sudo.

CentOS answer https://www.centos.org/docs/5/html/5.2/Deployment_Guide/s2-ssh-intro-why.html

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