SSH: Basics

SSH - is the primary protocol and tool for the remote servers management. Also allows you to create tunnels and transfer files.

SSH Utilities

OpenSSH Server

sshd (OpenSSH Daemon): A server-side daemon that listens for incoming SSH connections. Authenticates users and establishes secure sessions.
sftp-server (SFTP server subsystem): A program for file transfers using the SFTP protocol, usually invoked automatically by the sshd daemon.

OpenSSH Client

ssh (SSH client): Logs into a remote server, provides virtual terminal, and executes commands.
ssh-keygen: Creates and manages authentication keys.
scp: Copies files between local and remote machines using an encrypted channel.
sftp: Transfers files over a secure channel, similar to FTP.

Connection to server

ssh root@example.com

Connect to the server with custom port:

ssh -p 2222 root@example.com

Connect to the server with a specific private key:

ssh -i ~/.ssh/server_ed25519 root@example.com

Client Configuration

The SSH client can work without a configuration file and retrieve all the necessary parameters from command-line arguments. However, you may create a configuration file named ~/.ssh/config. This file should contain the following information:

Host example.com
    HostName 192.168.1.1
    User root
    Port 2222
    IdentityFile ~/.ssh/server_ed25519

Host: server name. This is the name used in the connection command: ssh example.com
HostName: an optional server address. If HostName is not defined, the proper address or host name should be defined in Host
User: username
Port: the server port. Default: 22
IdentityFile: an optional field that specifies the full path to the private key file

SSH Keys

SSH keys are a more secure way of logging into an SSH server, compared to using passwords. They consist of a pair of keys: a public key and a private key. The public key is placed on the server you want to connect to, while the private key remains on your local machine.

Generating

To generate an SSH key pair, use the following command:

ssh-keygen -t ed25519 -f ~/.ssh/server_ed25519

ed25519 - selects the type of encryption. Ed25519 is the optimal choice
~/.ssh/server_ed25519 - the path to the private key file. The public key will be generated as ~/.ssh/server_ed25519.pub

Once the command is started, it will prompt you to enter a password. This password provides an additional level of security and must be entered when connecting to the server.

The public key is a single line with the following format:

ssh-ed25519 AAAA...UUUU user@example.com

Copy Public Key to Server

On the server side, append this line to the ~/.ssh/authorized_keys file. This file may contain one or more keys. To append the public key, run the following command:

echo "ssh-ed25519 AAAA...UUUU user@example.com" >>~/.ssh/authorized_keys

SSH Agent Forwarding

SSH Agent forwarding allows you to use your local SSH keys on a remote server without sharing. This is useful when you need to access another server from the first server without storing your keys on the remote server.

You can enable SSH Agent forwarding by using the -A option with the ssh command:

ssh -A example.com

Alternatively, you can enable it permanently for a specific host in your SSH config file ~/.ssh/config:

Host example.com
    ForwardAgent yes