SSH - Secure Shell

What is SSH ?

The Secure Shell Protocol is a cryptographic network protocol for operating network services securely over an unsecured network.

SSH, also known as Secure Shell or Secure Socket Shell, is a network protocol that gives users, particularly system administrators, a secure way to access a computer over an unsecured network.

Who created SSH?

Tatu Ylönen, in 1995 from Finland.

SSH is a network protocol that gives users, particularly system administrators, a secure way to access a computer over an unsecured network.

SSH implementation comes with scp utility for remote file transfer that utilises SSH protocol. SSH for file transfer is also utilised by other applications such as sftp and rsync which can make use of SSH to secure its network transaction.

What is ssh key?

Essentially, SSH keys are an authentication method used to gain access to an encrypted connection between systems and then ultimately use that connection to manage the remote system

Check ssh version

ssh -V

SSH Key Generation

ssh-keygen -t rsa -b 4096 -C "mateors github account"

ssh-keygen -t rsa -b 4096 -C "[email protected]" you may use either email or comment

What is eval?

eval is a built-in Linux command which is used to execute arguments as a shell command.

Start your ssh-agent

eval $(ssh-agent -s)

Check SSH Identity

ssh-add -l

Check public key

ls ~/.ssh cat ~/.ssh/id_rsa.pub

Copy the public key

clip < ~/.ssh/id_rsa.pub

Paste the clipboard into github following address

location-> https://github.com/settings/ssh/new

To Verify Authentication

ssh -T [email protected]

To change the passpharse

ssh-keygen -p

Delete single named private key from ssh-agent

ssh-add -d ~/.ssh/id_rsa

Remove ALL private keys from the ssh-agent

ssh-add -D

SSH Linux pipeline

Get content of a file from home directory login to remote server create a file and paste its content there
cat ~/file.txt | ssh user@serverip "touch ~/file.txt && cat >> ~/file.txt"

Above technique used to copy public key from local to remote machine
cat ~/.ssh/id_rsa.pub | ssh user@serverip "mkdir -p ~/.ssh && cat >> ~/.ssh/authorized_keys"

SSH Server

The SSH server configuration file is located here: /etc/ssh/sshd_config. An SSH configuration can be validated by running the command:

/usr/ubin/sshd -t

Anytime a change is made to the server’s SSH config file, the SSH service must be restarted.

View the SSH server status

systemctl status ssh

Restart the SSH server

systemctl restart ssh

Stop the SSH server

systemctl stop ssh

Start the SSH server

systemctl start ssh

Protecting the SSH Server

You might also consider add-on solutions to block IP addresses that repeatedly connect but fail to authenticate, such as fail2ban and blacklistd

To some extent, sshd(8) protects itself via privilege separation. Only a small section of the service runs with root privileges. Most of the server runs as an unprivileged user. This means that if an intruder successfully breaks into the server daemon, he can only do a limited amount of damage to your system. It’s still really annoying, but not devastating.

a simple way to reduce risk to your SSH service is to reduce the number of IP addresses that can access it
/etc/hosts.allow

The most effective way to protect your server, however, is to disable passwords and only allow logins via keys.

Reference:

• Windows Installation