SSH Installed and Configuration in Ubuntu

Installing Open-ssh server on ubuntu linux

OpenSSH is a FREE version of the SSH connectivity tools that technical users of the Internet rely on. Users of telnet, rlogin, and ftp may not realize that their password is transmitted across the Internet unencrypted, but it is. OpenSSH encrypts all traffic (including passwords) to effectively eliminate eavesdropping, connection hijacking, and other attacks. Additionally, OpenSSH provides secure tunneling capabilities and several authentication methods, and supports all SSH protocol versions.

Ubuntu SSHD Installation

Type the following two command to install both ssh client and server:

# sudo apt-get install openssh-server openssh-client

 # sudo apt-get update

How To Configure SSH?

When you change the configuration of SSH, you are changing the settings of the sshd server.

In Ubuntu, the main sshd configuration file is located at /etc/ssh/sshd_config.

Back up the current version of this file before editing:

open ssh configuration using vi editor

You will want to leave most of the options in this file alone. However, there are a few you may want to take a look at:

The port declaration specifies which port the sshd server will listen on for connections. By default, this is 22.

 

If you are having difficulties with SSH, increasing the amount of logging may be a good way to discover what the issue is

These parameters specify some of the login information.

LoginGraceTime specifies how many seconds to keep the connection alive without successfully logging in. It may be a good idea to set this time just a little bit higher than the amount of time it takes you to log in normally.

PermitRootLogin selects whether root is allowed to log in.

In most cases, this should be changed to "no" when you have created user account that has access to elevated privileges (through su or sudo) and can log in through ssh.

strictModes is a safety guard that will refuse a login attempt if the authentication files are readable by everyone.

This prevents login attempts when the configuration files are not secure.

                                               

These parameters configure an ability called X11 Forwarding. This allows you to view a remote system's graphical user interface (GUI) on the local system.

This option must be enabled on the server and given with the client during connection with the "-X" option.

If you changed any settings in this file, make sure you restart your sshd server to implement your modifications

 

How To Log Into SSH with Keys 

While it is helpful to be able to log in to a remote system using passwords, it's often a better idea to set up key-based authentication

How Does Key-based Authentication Work?

Key-based authentication works by creating a pair of keys: a private key and a public key.

The private key is located on the client machine and is secured and kept secret.

The public key can be given to anyone or placed on any server you wish to access.

When you attempt to connect using a key-pair, the server will use the public key to create a message for the client computer that can only be read with the private key.

The client computer then sends the appropriate response back to the server and the server will know that the client is legitimate.

This entire process is done in the background automatically after you set up keys.

 

How To Create SSH Keys?

SSH keys should be generated on the computer you wish to log in from. This is usually your local computer.

   

Press enter to accept the defaults. Your keys will be created at ~/.ssh/id_rsa.pub and~/.ssh/id_rsa.

Change into the .ssh directory by typing

Look at the permissions of the files

 

 As you can see, the id_rsa file is readable and writable only to the owner. This is how it should be to keep it secret.

The id_rsa.pub file, however, can be shared and has permissions appropriate for this activity.

How To Transfer Your Public Key to the Server? 

This will start an SSH session, which you will need to authenticate with your password.

After you enter your password, it will copy your public key to the server's authorized keys file, which will allow you to log in without the password next time.

 Client-Side Options

There are a number of optional flags that you can select when connecting through SSH.

Some of these may be necessary to match the settings in the remote host's sshd file.

For instance, you if you changed the port number in your sshd configuration, you will need to match that port on the client-side by typing:

If you only wish to execute a single command on a remote system, you can specify it after the host like so

You will connect to the remote machine, authenticate, and the command will be executed.

As we said before, if X11 forwarding is enabled on both computers, you can access that functionality by typing:

To stop ssh server, type command as

# sudo /etc/init.d/ssh stop

 

To start ssh server type command as

# sudo /etc/init.d/ssh start

 

To restart ssh server type command as

# sudo /etc/init.d/ssh restart

Now we can easily login through putty from Host machines(Windows) to Remote machines (Ubuntu). We can copy/send files from Host machine (Windows) to Remote machines (Ubuntu) using Winscp tool.
   Install Winscp tool in windows to send files to remote machines using SSH.