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.

Practice

1. What does the command ssh user@hostname do?

easy

A. Lists files on the remote computer.

B. Copies files from the remote computer to local.

C. Starts a secure connection to the remote computer as 'user'.

D. Deletes the user account on the remote computer.

Solution

1. Step 1: Understand the ssh command purpose

   The ssh command is used to securely connect to another computer over a network.

2. Step 2: Analyze the syntax ssh user@hostname

   This syntax means connect as 'user' to the computer identified by 'hostname'.

3. Final Answer:

   Starts a secure connection to the remote computer as 'user'. -> Option C

4. Quick Check:

   ssh user@hostname = Secure connection [OK]

Hint: ssh user@host means connect securely as user [OK]

Common Mistakes:

• Thinking ssh copies files instead of connecting
• Confusing ssh with file listing commands
• Assuming ssh deletes accounts

2. Which of these is the correct syntax to connect to a remote server with ssh?

easy

A. ssh user@hostname

B. ssh -connect user hostname

C. ssh connect user@hostname

D. ssh user hostname

Solution

1. Step 1: Recall ssh command syntax

   The correct syntax is ssh user@hostname without extra words or flags for basic connection.

2. Step 2: Check each option

   Options A, B, and D have incorrect syntax or missing '@' sign.

3. Final Answer:

   ssh user@hostname -> Option A

4. Quick Check:

   Correct ssh syntax = ssh user@hostname [OK]

Hint: ssh always uses user@hostname format [OK]

Common Mistakes:

• Adding extra words like 'connect' in command
• Omitting '@' between user and hostname
• Using flags incorrectly for basic ssh

3. What will happen if you run ssh alice@192.168.1.10 and the password is correct?

medium

A. You will see a list of files on your local computer.

B. The remote computer will shut down.

C. Your local files will be deleted.

D. You will be logged into the remote computer as user alice.

Solution

1. Step 1: Understand ssh login process

   Running ssh with user and IP tries to connect and log in to that remote machine.

2. Step 2: Check what happens on correct password

   If password is correct, you get a command prompt on the remote machine as that user.

3. Final Answer:

   You will be logged into the remote computer as user alice. -> Option D

4. Quick Check:

   ssh + correct password = remote login [OK]

Hint: Correct password means remote login success [OK]

Common Mistakes:

• Thinking ssh shuts down remote machine
• Confusing ssh with file deletion
• Assuming ssh lists local files

4. You try to connect with ssh [e-mail] but get the error: Permission denied (publickey). What is the likely problem?

medium

A. You typed the wrong hostname.

B. Your SSH key is missing or not accepted by the server.

C. The server is turned off.

D. You forgot to install ssh on your computer.

Solution

1. Step 1: Understand the error message

   "Permission denied (publickey)" means the server expects an SSH key for authentication but did not accept yours.

2. Step 2: Identify cause of error

   This usually means your SSH key is missing, not loaded, or not authorized on the server.

3. Final Answer:

   Your SSH key is missing or not accepted by the server. -> Option B

4. Quick Check:

   Permission denied (publickey) = SSH key issue [OK]

Hint: Permission denied (publickey) means SSH key problem [OK]

Common Mistakes:

• Assuming hostname typo causes this error
• Thinking server power state causes this error
• Believing ssh must be installed locally to connect

5. You want to connect to a remote server at example.com as user admin but the server uses a non-standard port 2222. Which command will connect correctly?

hard

A. ssh -p 2222 [e-mail]

B. ssh [e-mail]:2222

C. ssh [e-mail] -port 2222

D. ssh [e-mail]

Solution

1. Step 1: Recall how to specify a port in ssh

   The correct flag to specify a port is -p followed by the port number.

2. Step 2: Analyze each option

   ssh -p 2222 [e-mail] uses -p 2222 correctly. Options A, C, and D use incorrect syntax or wrong placement.

3. Final Answer:

   ssh -p 2222 [e-mail] -> Option A

4. Quick Check:

   Use -p to specify port in ssh [OK]

Hint: Use ssh -p port user@host for custom ports [OK]

Common Mistakes:

• Putting port after hostname with colon
• Using -port instead of -p
• Changing hostname to include port number