What Is SSH?
One essential tool to master as a system administrator is SSH.
SSH, or
Secure Shell, is a protocol used to securely log
onto remote systems. It is the most common way to access remote Linux
and Unix-like servers.
In this guide, we will discuss how to use SSH to connect to a remote system.
Basic Syntax
The tool on Linux for connecting to a remote system using SSH is called, unsurprisingly,
ssh.
The most basic form of the command is:
The
remote_host in this example is the IP address or domain name that you are trying to connect to.
This command assumes that your username on the remote system is the same as your username on your local system.
If your username is different on the remote system, you can specify it by using this syntax:
- ssh remote_username@remote_host
Once you have connected to the server, you will probably be asked to verify your identity by providing a password.
Later, we will cover how to generate keys to use instead of passwords.
To exit back into your local session, simply type:
How Does SSH Work?
SSH works by connecting a client program to an
ssh server.
In the above commands,
ssh is the client program. The
ssh server is already running on the
remote_host that we specified.
In your Droplet, the sshd server should already be running. If this is not the case, click on the
Console Access button from your Droplet page:
You will be presented with a login screen. Log in with your credentials.
The process needed to start an ssh server depends on the distribution of Linux that you are using.
On Ubuntu, you can start the ssh server on the Droplet by typing:
On Ubuntu 16.04 and Debian Jessie, you can use
systemctl, the
systemd command for managing services:
That should start the sshd server and you can then log in remotely.
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:
- sudo cp /etc/ssh/sshd_config{,.bak}
Open it with a text editor:
- sudo nano /etc/ssh/sshd_config
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:
/etc/ssh/sshd_config
Port 22
The port declaration specifies which port the sshd server will listen
on for connections. By default, this is 22. You should probably leave
this setting alone, unless you have specific reasons to do otherwise.
If you
do change your port, we will show you how to connect to the new port later on.
/etc/ssh/sshd_config
HostKey /etc/ssh/ssh_host_rsa_key
HostKey /etc/ssh/ssh_host_dsa_key
HostKey /etc/ssh/ssh_host_ecdsa_key
The host keys declarations specify where to look for global host keys. We will discuss what a host key is later.
/etc/ssh/sshd_config
SyslogFacility AUTH
LogLevel INFO
These two items indicate the level of logging that should occur.
If you are having difficulties with SSH, increasing the amount of logging may be a good way to discover what the issue is.
/etc/ssh/sshd_config
LoginGraceTime 120
PermitRootLogin yes
StrictModes yes
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.
/etc/ssh/sshd_config
X11Forwarding yes
X11DisplayOffset 10
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 SSH client during connection with the
-X option.
If you changed any settings in
/etc/ssh/sshd_config, make sure you restart your sshd server to implement your modifications:
Or, on
systemd systems such as Ubuntu 16.04 or Debian Jessie:
- sudo systemctl restart ssh
You should thoroughly test your changes to ensure that they operate in the way you expect.
It may be a good idea to have a few sessions active when you are
making changes. This will allow you to revert the configuration if
necessary.
If you run into problems, remember that you can log in through the
Console link on your Droplet page.
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 a much 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.
Enter the following into the command line:
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:
Output
-rw-r--r-- 1 demo demo 807 Sep 9 22:15 authorized_keys
-rw------- 1 demo demo 1679 Sep 9 23:13 id_rsa
-rw-r--r-- 1 demo demo 396 Sep 9 23:13 id_rsa.pub
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
You can copy the public key to the remote server by issuing this command:
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 configuration.
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:
- ssh -p port_number remote_host
If you only wish to execute a single command on a remote system, you can specify it after the host like so:
- ssh remote_host command_to_run
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:
Providing you have the appropriate tools on your computer, GUI
programs that you use on the remote system will now open their window on
your local system.
Conclusion
Learning your way around SSH is a worthwhile pursuit, if only because it is such a common activity.
As you utilize the various options, you will discover more advanced
functionality that can make your life easier. SSH has remained popular
because it is secure, light-weight, and useful in diverse situations.
The operations described on this page modify the software repositories configuration file /etc/apt/sources.list. If you wish to, you can modify this file directly. For further details please see Managing Repositories from the Command Line. 
Packages in PPAs do not undergo the same process of validation as
packages in the main repositories. PPAs are a low-security alternative
to the main repositories, so the user will be installing software at
their own risk. 
