SSH config file in Linux CLI - Commands & Configuration

Introduction

Managing multiple SSH connections can be confusing and slow if you have to type long commands every time. The SSH config file lets you save connection details so you can connect quickly with simple commands.

When you connect regularly to several servers and want to avoid typing full addresses and options each time

When you want to use different usernames or keys for different servers without remembering all details

When you want to simplify SSH commands for easier and faster access

When you want to set default options like ports or forwarding for specific hosts

When you want to organize your SSH connections neatly in one place

Config File - config

config

Host myserver
  HostName 192.168.1.100
  User admin
  Port 2222
  IdentityFile ~/.ssh/id_rsa_myserver

Host github
  HostName github.com
  User git
  IdentityFile ~/.ssh/id_rsa_github
  IdentitiesOnly yes

Host: A nickname for the server you want to connect to.

HostName: The real address or IP of the server.

User: The username to log in as.

Port: The port number if not the default 22.

IdentityFile: The private key file to use for authentication.

IdentitiesOnly: Ensures only the specified key is used.

Commands

Connects to the server using the saved settings in the SSH config file under the nickname 'myserver'. This saves typing the full address, port, and user.

Terminal

ssh myserver

Expected Output

Welcome to Ubuntu 20.04.4 LTS (GNU/Linux 5.4.0-104-generic x86_64)



admin@myserver:~$

Connects to GitHub using the nickname 'github' with the specified user and key from the config file.

Terminal

ssh github

Expected Output

PTY allocation request failed on channel 0

Hi username! You've successfully authenticated, but GitHub does not provide shell access.

Connection to github.com closed.

Shows the contents of your SSH config file so you can review or edit your saved connection settings.

Terminal

cat ~/.ssh/config

Expected Output

Host myserver

  HostName 192.168.1.100

  User admin

  Port 2222

  IdentityFile ~/.ssh/id_rsa_myserver



Host github

  HostName github.com

  User git

  IdentityFile ~/.ssh/id_rsa_github

  IdentitiesOnly yes

Key Concept

If you remember nothing else from this pattern, remember: the SSH config file lets you save and reuse connection details to connect faster and easier.

Common Mistakes

Not setting correct file permissions on the SSH config file

SSH ignores the config file if permissions are too open, causing connections to fail or ignore settings.

Set permissions with 'chmod 600 ~/.ssh/config' to keep the file private and usable.

Using incorrect indentation or syntax in the config file

SSH config requires indentation before options; wrong indentation causes errors or ignored settings.

Always indent options with spaces under each Host entry.

Forgetting to reload or re-run SSH after editing the config file

Changes won't apply until you start a new SSH session; old sessions keep using old settings.

Simply start a new SSH connection to apply changes; no special reload needed.

Summary

Create an SSH config file to save connection details like hostname, user, port, and keys.

Use simple nicknames to connect quickly without typing full commands.

Check and fix file permissions and syntax to ensure SSH uses your config correctly.

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