SSH config file in Linux CLI - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of SSH config with Host alias

Given the following SSH config snippet, what is the output of ssh myserver if the server myserver.example.com is reachable and the user is alice?

Linux CLI

Host myserver
  HostName myserver.example.com
  User alice
  Port 2222

A Connects to myserver.example.com on port 2222 as user alice

B Connects to myserver.example.com on default port 22 as user alice

C Connects to myserver on default port 22 as current user

D Fails with unknown host error

💡 Hint

Look at how HostName, User, and Port are set under the Host alias.

The SSH config defines an alias myserver that points to myserver.example.com with user alice and port 2222. So ssh myserver uses these settings.

📝 Syntax

intermediate

Identify the syntax error in SSH config

Which option shows a syntax error in this SSH config snippet?

A Host example\n HostName example.com\n Port 2200

B Host example\n HostName example.com\n User bob

C Host example\n HostName example.com\n User

D Host example\n HostName example.com\n IdentityFile ~/.ssh/id_rsa

💡 Hint

Check if all directives have values.

Option C has User without a username value, which is invalid syntax.

🔧 Debug

advanced

Why does SSH ignore the Port setting?

Given this SSH config, why does ssh server1 connect on port 22 instead of port 2200?

Linux CLI

Host server1
  HostName server1.example.com
Host server*
  Port 2200

A The wildcard Host block is ignored because it comes after the exact match

B The first Host block overrides the Port setting because it matches exactly and has no Port line

C SSH always uses port 22 unless specified on the command line

D Port must be set before HostName to take effect

💡 Hint

SSH applies the first matching Host block and ignores later ones.

SSH uses the first matching Host block. Since server1 matches the first block exactly, it uses its settings. That block has no Port, so default 22 is used. The second block with Port 2200 is ignored for this host.

🚀 Application

advanced

Create SSH config for multiple hosts with shared settings

You want to connect to web1.example.com and web2.example.com as user deploy on port 2222. Which SSH config snippet achieves this with minimal repetition?

A Host web1 web2\n HostName %h.example.com\n User deploy\n Port 2222

B Host web*\n HostName %h.example.com\n User deploy\n Port 2222

C Host web1\n HostName web1.example.com\n User deploy\n Port 2222\nHost web2\n HostName web2.example.com\n User deploy\n Port 2222

D Host web1 web2\n HostName web1.example.com\n User deploy\n Port 2222

💡 Hint

Use Host aliases and %h to substitute hostnames.

Option A uses a single Host line with both hosts and uses %h to substitute the hostname part, avoiding repetition.

🧠 Conceptual

expert

Effect of Include directive in SSH config

If your main SSH config file contains Include ~/.ssh/config.d/*, what happens when you run ssh myhost?

A SSH reads all files in ~/.ssh/config.d/ and applies their settings before the main config

B SSH reads the main config first, then applies settings from all included files in order

C SSH ignores the Include directive unless you specify --include on the command line

D SSH merges settings from included files with the main config, applying them in the order they appear

💡 Hint

Think about how SSH processes multiple config files.

The Include directive merges settings from included files with the main config, applying them in the order they appear. This allows modular config management.

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