The easiest way to log into a Linux Server with SSH from a local Linux computer is by using the local Terminal or Command Prompt. Almost every Linux distribution comes packaged with a copy of SSH (OpenSSH) to allow you to do this.
If yours does not then it can be simply installed with the native package manager by searching for and installing openssh.
Connecting to a Server via SSH
In a terminal such as Konsole or Gnome-Terminal (they can usually be found under the Utilities menu) issue a command of the following form:
A working example for the user “root” at the server “examplaaa1.miniserver.com” is:
When this command is run you will be prompted for the password for the user on the remote server. Once the password is entered you will be logged into the command line of the server.
This method of using SSH requires the password to be entered for every connection. This becomes unmanageable when there are multiple users and/or servers being worked on and requires you to have access to the Password each time.
The easiest method (and Standard way) to make working on multiple servers more secure and more efficient is to use SSH keys.
A secure protocol, in addition to encryption, demands that you prove who you are when you log in. The two principle ways of doing this with SSH are:
- Cryptographic keys.
Password authentication is the default method for logging in via SSH and will be enabled by default on all Memset servers. Password authentication is secure but there are several advantages to disabling password authentication and instead relying on SSH keys. SSH keys are more secure than passwords as they are all but impossible to brute force attack on the server end. An attacker must have access to the private key to log into the server. They are also more efficient because any server that has the public key can be logged into from any computer that has the private key. This means that a single key (and password) can be used to securely log into any number of servers.
Every SSH key pair is unique. Therefore, in order to start using an SSH key pair, they must first be generated.
Generating SSH Keys
SSH keys are generated from the command line in Linux, In order to create a key pair first open a terminal.
Then issue the following command as your normal user:
ssh-keygen -t rsa -b 4096
The parts of this command are as follows:
- -t rsa This specifies is the encryption algorithm. In this case the RSA algorithm.
- -b 4096 This is the length of the key.
Running this command will produce the following output:
Generating public/private rsa key pair. Enter file in which to save the key (/home/user/.ssh/id_rsa):
The default for ssh-keygen is to place the keys into a hidden directory called .ssh under the current users home directory. The default names for RSA keys are:
# Private key: id_rsa # Public key: id_rsa.pub
These defaults are fine as long as you only have a single key pair but are confusing if you have more than one. For this reason when prompted to enter the file in which to save the key a name with a little more information can be useful e.g.:
Enter file in which to save the key (/home/user/.ssh/id_rsa):/home/John/.ssh/john-smith_work
This will create the following key pair:
Using SSH Keys
SSH keys work when the two machines at either end of the connection have one-half of the key pair. The remote server has the public key and the local machine the private key. Therefore, before a login is possible using keys the public key must be loaded onto the remote server. This is most easily achieved using the ssh-copy-id command.
This command has the following form:
ssh-copy-id -i public_key user@host
Using the keys that were generated in the example above the following is a working example:
ssh-copy-id -i .ssh/john-smith_work.pub email@example.com
When the command is run you will be prompted for the password for the user at the remote server and the public key will be saved into a file at ~/.ssh/authorized_keys.
The public key can be added manually into the authorized_keys file if you prefer.
Logging in with SSH Keys
The simplest way to use an SSH key is to specify the private key directly in the previous shown SSH login command with the -i (identity) flag:
ssh -i .ssh/john-smith_work firstname.lastname@example.org
When an SSH key is used like this it must be unlocked with its password for every connection. However, a private key can be unlocked and loaded into memory. When this is done every subsequent connection will use the unlocked key automatically without needing a password entered.
Loading an SSH Key into the SSH Agent
The openssh package supplies a tool to load SSH keys into memory. This tool is called ssh-add. The syntax for the ssh-add is very simple:
The default location for SSH keys is in the hidden .ssh directory located under the users home directory. Opening a new terminal from the desktop will start from that user's home directory so loading a private key usually looks like:
This command will immediately request the key’s password. Once that is entered the key is loaded into memory and you will be returned back to a prompt. The key is not tied to that terminal so you can log out of that terminal or open as many new terminals as you need. They will all have access to the private key.