Here we will learn about the SSH on Linux System.
Some Basics of SSH
What is SSH
SSH (Secure SHell) is a Network protocol used for communication between two networked computer. It is different from other protocols in way that it provide security while accessing other system. Hence anyone on network will see only encrypted data and not the plain data.
Use of SSH
SSH can be used for below purpose
- Terminal Access
- Remote Command Execution
- File Transfer (Using SFTP or SCP protocol)
- TCP Ports forwarding
- X11 Connection
Assigned port for SSH
Port 22 has been assigned for SSH server.
Type of SSH
SSH can be divided in two parts
- SSH server : Program uses SSH protocol to run service on system and wait for clients connection.
- SSH Client : Program uses SSH protocol to connect to remote server.
Example of SSH Server
Example of SSH Client
How it works
SSH is based on Public Key Cryptography
What is Public Key Cryptography (PKC)
There are two keys used in PKC.
- Public Key
- Private Key:
Used to Encrypt Data. It can be shared publicly.
Used to Decrypt Data. Not supposed to be shared.
How to Create Keys
To create authentication keys ssh-keygen command can be used.
ssh-keygen -t rsa
Above command will create public and private rsa key pair.
What is Passphrase
When you create Private key as above. It will be stored in a file and any one having access to given file can misuse private key. Hence to secure private key from unauthorized access, Pass-phrase should(Not compulsory) be provided while creating private key.
[email protected]:~$ ssh-keygen -t rsa
// Here asking to generate keys
Generating public/private rsa key pair. Enter file in which to save the key (/home/hduser/.ssh/id_rsa):
// Asking for file where keys can be stored(Optional)
/home/hduser/.ssh/id_rsa already exists. Overwrite (y/n)? y Enter passphrase (empty for no passphrase):
// Here passphrase asked which is used to secure private key(Optional)
Enter same passphrase again: Your identification has been saved in /home/hduser/.ssh/id_rsa.
// Private key saved in this file
Your public key has been saved in /home/hduser/.ssh/id_rsa.pub.
// Public key saved in this file
The key fingerprint is: 69:67:b4:80:f6:a4:d8:e6:1b:97:4b:1c:db:dc:19:a8 [email protected] The key's randomart image is: +--[ RSA 2048]----+ | . . | | o * . | | + = o . . | | o = O . o | | S E o o | | . B . | | . . | | | | | +-----------------+ [email protected]:~$
Storing Location of Keys
Public key will be stored in ~/.ssh/authorized_keys OR ~/.ssh/id_rsa.pub. Private Key will be stored in ~/.ssh/id_rsa.
Getting Started with SSH
Here we will learn about the basic commands related to SSH. (I am using Ubuntu so some part might be different for other systems.)
Note: It is most likely that Linux system you are using have SSH already installed. If not you can use below command to install it.
sudo apt-get install openssh-server
Start / Stop SSH Server (SSH process)/ Client (SSHD Process)
Now you have installed SSH, next step would be to start or stop these services(SSH/SSHD). You can use below command to start/ command these services.
Check status of SSH server
sudo status ssh
sudo service ssh status
sudo start ssh
sudo service ssh start
sudo stop ssh
sudo service ssh stop
sudo restart ssh
sudo service ssh restart
Note*: You can not restart the server if it is not in started mode.