SSH config file in Linux CLI - Step-by-Step Execution

Concept Flow - SSH config file

Start

↓

Read ~/.ssh/config

↓

Parse Host entries

↓

Match Host alias with user input

↓

Apply settings for matched Host

↓

Connect using applied settings

↓

End

The SSH client reads the config file, finds the matching Host entry, applies its settings, and then connects.

Execution Sample

Linux CLI

Host myserver
  HostName 192.168.1.10
  User alice
  Port 2222

Defines a shortcut 'myserver' to connect to 192.168.1.10 as user alice on port 2222.

Execution Table

Step	Action	Input/Line	Result/State
1	Read config file	Host myserver	Found Host entry 'myserver'
2	Parse HostName	HostName 192.168.1.10	Set HostName = 192.168.1.10
3	Parse User	User alice	Set User = alice
4	Parse Port	Port 2222	Set Port = 2222
5	Match user input	ssh myserver	Matched Host 'myserver'
6	Apply settings	Use HostName, User, Port	Connection will use 192.168.1.10, alice, port 2222
7	Connect	ssh myserver	SSH connects to 192.168.1.10:2222 as alice

💡 All settings applied and connection established using config file.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
HostName	undefined	192.168.1.10	192.168.1.10	192.168.1.10	192.168.1.10
User	undefined	undefined	alice	alice	alice
Port	undefined	undefined	undefined	2222	2222
Host	undefined	myserver	myserver	myserver	myserver

Key Moments - 3 Insights

Why does typing 'ssh myserver' connect to 192.168.1.10 and not the literal 'myserver'?

Because the SSH client reads the config file (see Step 5) and matches 'myserver' to the Host entry, then uses the HostName setting (Step 2) to connect.

What happens if the config file does not specify a Port?

SSH uses the default port 22. In the execution table, if Step 4 (Port) was missing, Port would remain undefined and default to 22.

Can multiple Host entries exist and how does SSH choose?

Yes, SSH checks each Host entry in order and uses the first that matches the input (Step 5). If none match, it uses default settings.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of User after Step 3?

A myserver

B undefined

C alice

D 2222

💡 Hint

Check the 'User' variable in variable_tracker after Step 3.

At which step does SSH match the input 'ssh myserver' to the Host entry?

A Step 2

B Step 5

C Step 7

D Step 1

💡 Hint

Look at the 'Action' column in execution_table where matching happens.

If the Port line was removed from the config, what would be the final Port used?

A 22

B 2222

C undefined

D myserver

💡 Hint

Recall default SSH port when Port is not set in config.

Concept Snapshot

SSH config file lets you create shortcuts for SSH connections.
Use 'Host' to name the shortcut.
Set 'HostName', 'User', and 'Port' under it.
SSH reads config, matches your shortcut, and applies settings.
This saves typing full commands every time.

Full Transcript

The SSH config file is read by the SSH client to simplify connections. It contains Host entries with settings like HostName, User, and Port. When you type 'ssh' with a shortcut name, SSH finds the matching Host entry, applies its settings, and connects accordingly. This process involves reading the file, parsing each line, matching the input, and then connecting with the specified parameters. If some settings like Port are missing, SSH uses defaults such as port 22. Multiple Host entries can exist, and SSH picks the first matching one. This makes connecting to servers easier and faster.

Practice

1. What is the main purpose of the Host entry in an SSH config file?

easy

A. To specify the password for the SSH connection

B. To create a shortcut name for an SSH connection

C. To list all IP addresses allowed to connect

D. To define the encryption method used

Solution

1. Step 1: Understand the role of Host in SSH config

   The Host keyword defines a shortcut name for a server, so you can connect using that name instead of typing full details.

2. Step 2: Compare options

   Options B, C, and D do not describe the purpose of Host. Passwords and encryption are handled differently, and allowed IPs are not set here.

3. Final Answer:

   To create a shortcut name for an SSH connection -> Option B

4. Quick Check:

   Host = shortcut name [OK]

Hint: Remember: Host = shortcut name for SSH [OK]

Common Mistakes:

• Thinking Host sets password
• Confusing Host with IP address list
• Assuming Host defines encryption

2. Which of the following is the correct syntax to specify a custom port in an SSH config file?

easy

A. Port: 2222

B. SetPort 2222

C. port=2222

D. Port 2222

Solution

1. Step 1: Recall SSH config syntax rules

   SSH config uses keyword followed by a space and then the value, without colons or equals signs.

2. Step 2: Check each option

   Port 2222 matches the correct syntax: Port 2222. Options A, C, and D use invalid syntax.

3. Final Answer:

   Port 2222 -> Option D

4. Quick Check:

   Keyword + space + value = correct syntax [OK]

Hint: Use space, no colon or equals in SSH config lines [OK]

Common Mistakes:

• Using colon after keyword
• Using equals sign instead of space
• Inventing keywords like SetPort

3. Given this SSH config snippet:

Host myserver
  HostName 192.168.1.10
  User alice
  Port 2200

What command will connect to the server using this config?

medium

A. ssh myserver

B. ssh alice@192.168.1.10 -p 2200

C. ssh 192.168.1.10

D. ssh -p 22 myserver

Solution

1. Step 1: Understand the config usage

   The config defines a shortcut myserver with hostname, user, and port. Using ssh myserver applies all these automatically.

2. Step 2: Analyze options

   ssh myserver uses the shortcut. ssh alice@192.168.1.10 -p 2200 manually specifies user and port but bypasses the config shortcut. ssh 192.168.1.10 misses user and port. ssh -p 22 myserver uses wrong port.

3. Final Answer:

   ssh myserver -> Option A

4. Quick Check:

   Use shortcut name to connect with config settings [OK]

Hint: Use the Host shortcut name to connect easily [OK]

Common Mistakes:

• Ignoring the shortcut and typing full command
• Using default port 22 instead of configured port
• Missing the username from config

4. You wrote this SSH config:

Host test
  HostName 10.0.0.5
  User bob
  Port 22
  IdentityFile ~/.ssh/id_rsa_test

But when you run ssh test, it asks for a password instead of using the key. What is the likely problem?

medium

A. The User should be root, not bob

B. The Port 22 is wrong and should be 2222

C. The IdentityFile path is incorrect or not expanded

D. The HostName must be an IP, not a hostname

Solution

1. Step 1: Check IdentityFile usage

   The IdentityFile path uses ~ which may not expand correctly in SSH config. It should be the full path or use $HOME.

2. Step 2: Evaluate other options

   Port 22 is default and usually correct. User 'bob' is valid if correct user. HostName as IP is valid. So the key problem is likely the path expansion.

3. Final Answer:

   The IdentityFile path is incorrect or not expanded -> Option C

4. Quick Check:

   Use full path for IdentityFile to avoid key not used [OK]

Hint: Use full path, not ~, for IdentityFile in SSH config [OK]

Common Mistakes:

• Assuming port 22 is always wrong
• Changing user without reason
• Confusing hostname format

5. You want to create an SSH config entry that connects to multiple servers with the same user and key but different hostnames. Which is the best way to write this?

hard

A. Host server1 server2 server3\n User admin\n IdentityFile ~/.ssh/admin_key

B. Host server*\n User admin\n IdentityFile ~/.ssh/admin_key

C. Host *\n User admin\n IdentityFile ~/.ssh/admin_key

D. Host server1\n User admin\n IdentityFile ~/.ssh/admin_key\nHost server2\n User admin\n IdentityFile ~/.ssh/admin_key\nHost server3\n User admin\n IdentityFile ~/.ssh/admin_key

Solution

1. Step 1: Understand grouping hosts in SSH config

   You can list multiple hostnames after one Host keyword to apply the same settings to all.

2. Step 2: Compare options

   Host server1 server2 server3\n User admin\n IdentityFile ~/.ssh/admin_key lists all servers in one line, applying user and key once. Host server*\n User admin\n IdentityFile ~/.ssh/admin_key uses wildcard which may match unintended hosts. Host *\n User admin\n IdentityFile ~/.ssh/admin_key applies to all hosts, not just these servers. Host server1\n User admin\n IdentityFile ~/.ssh/admin_key\nHost server2\n User admin\n IdentityFile ~/.ssh/admin_key\nHost server3\n User admin\n IdentityFile ~/.ssh/admin_key repeats same settings multiple times, which is redundant.

3. Final Answer:

   Host server1 server2 server3 User admin IdentityFile ~/.ssh/admin_key -> Option A

4. Quick Check:

   List multiple hosts after one Host for shared settings [OK]

Hint: List multiple hosts after one Host line to share settings [OK]

Common Mistakes:

• Using wildcard * too broadly
• Repeating same block for each host
• Using Host * which applies to all hosts