SSH connection basics in Linux CLI - Step-by-Step Execution

Concept Flow - SSH connection basics

Start SSH command

↓

Resolve hostname/IP

↓

Establish TCP connection on port 22

↓

Exchange keys and authenticate

↓

Open secure shell session

↓

User interacts with remote shell

↓

Exit session or disconnect

This flow shows how an SSH command starts, connects to a remote server securely, authenticates, and opens a shell session for user interaction.

Execution Sample

Linux CLI

ssh [e-mail]

This command starts an SSH connection to the remote server example.com as user 'user'.

Execution Table

Step	Action	Details	Result
1	Start SSH command	User types 'ssh [e-mail]'	SSH client starts
2	Resolve hostname	DNS lookup for example.com	IP address found
3	Establish TCP connection	Connect to IP on port 22	Connection established
4	Exchange keys and authenticate	Client and server exchange keys and user provides password or key	Secure channel created and authentication success
5	Open shell session	Remote shell is ready	User can run commands
6	User interacts	User types commands	Commands executed remotely
7	Exit session	User types 'exit' or disconnects	Connection closed

💡 User ends session or connection drops, SSH closes securely

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	Final
SSH Client State	Not started	Hostname resolved	TCP connected	Authenticated	Shell open	Disconnected

Key Moments - 3 Insights

Why does SSH need to resolve the hostname before connecting?

SSH must find the IP address of the hostname to know where to connect. This is shown in step 2 of the execution table.

What happens if authentication fails?

If authentication fails at step 4, the SSH connection will not open a shell session and will close. This prevents unauthorized access.

Why is port 22 important in SSH?

Port 22 is the default port SSH uses to establish the connection, as seen in step 3. Without connecting to this port, SSH cannot communicate with the server.

Visual Quiz - 3 Questions

Look at the execution table, what is the result after step 3?

A User authenticated

B Hostname resolved

C Connection established

D Shell session opened

💡 Hint

Check the 'Result' column for step 3 in the execution table.

At which step does the user provide their password or key?

A Step 4

B Step 2

C Step 3

D Step 5

💡 Hint

Look at the 'Action' column describing authentication in the execution table.

If the hostname cannot be resolved, what will happen?

A SSH will skip to authentication

B SSH will fail to connect and stop

C SSH will open shell session anyway

D SSH will connect to port 80 instead

💡 Hint

Refer to step 2 where hostname resolution is required before connection.

Concept Snapshot

SSH connection basics:
- Use 'ssh user@hostname' to connect.
- SSH resolves hostname to IP.
- Connects on port 22.
- Exchanges keys and authenticates user.
- Opens remote shell session.

Full Transcript

SSH connection starts when the user types the ssh command with username and hostname. The client first resolves the hostname to an IP address using DNS. Then it establishes a TCP connection to the server on port 22. After connecting, the client and server exchange keys to create a secure encrypted channel and the user is asked to authenticate, usually by password or SSH key. Once authenticated, a remote shell session opens allowing the user to run commands on the server. The session continues until the user exits or disconnects, closing the connection securely.