Connecting one Ubuntu Server OS to another: Complete guide

A Comprehensive Guide to Connecting an Ubuntu Control Node to a Managed Node via SSH

SSH (Secure Shell) is a secure protocol that allows you to connect to remote machines over a network. In this guide, we’ll walk through the complete setup to connect one Ubuntu server (Control Node) to another (Managed Node) using SSH. Along the way, we'll provide in-depth explanations of the key configuration files involved: ~/.ssh/config and /etc/ssh/sshd_config.

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What You’ll Learn

• Setting up SSH on the Control Node and Managed Node.
• Using hostname -I to retrieve IP addresses.
• Testing network connectivity with a 5-packet ping.
• Establishing an SSH connection.
• Configuring passwordless authentication.
• Descriptions of critical SSH configuration files.
• Security enhancements and troubleshooting tips.

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Prerequisites

• Control Node: A primary Ubuntu server or workstation to control and manage other servers.
• Managed Node: A target Ubuntu server you want to access remotely.
• User Access: Non-root user accounts with sudo privileges on both nodes.
• Network Connectivity: Both nodes must be on the same network or reachable over the internet.

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Step 1: Install OpenSSH

SSH requires OpenSSH software to enable connections. Here's what you need to install:

• Control Node: OpenSSH Client (used to initiate SSH connections).
• Managed Node: OpenSSH Server (used to accept SSH connections).

Install OpenSSH on the Managed Node

• Update the package list:

  sudo apt update

• Install the OpenSSH Server:

  sudo apt install openssh-server -y

• Verify the SSH service is running:

  sudo systemctl status ssh

If inactive, start the service:

  sudo systemctl start ssh

Install OpenSSH on the Control Node

• Update the package list:

  sudo apt update

• Install the OpenSSH Client:

  sudo apt install openssh-client -y

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Step 2: Find the Managed Node's IP Address

On the Managed Node, retrieve the IP address using hostname -I:

• Open a terminal and run:

  hostname -I

• This command displays the IP addresses assigned to the network interfaces.

• Example output:
  

  192.168.1.105
  

  • Note the IP address corresponding to the network the Control Node can access (e.g., 192.168.1.105).

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Step 3: Verify Network Connectivity

From the Control Node, test connectivity to the Managed Node using a ping command:

• On the Control Node, run:

  ping -c 5 192.168.1.105

• Replace 192.168.1.105 with the Managed Node's IP address.

• The -c 5 option limits the ping to 5 packets.

  • Verify the result:

• Successful output shows:
  

  5 packets transmitted, 5 received, 0% packet loss
  

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Step 4: Establish an SSH Connection

After confirming connectivity, establish an SSH connection from the Control Node to the Managed Node.

• On the Control Node, run:

  ssh username@192.168.1.105

• Replace username with the username of the account on the Managed Node.

• Replace 192.168.1.105 with the Managed Node's IP address.

  • Accept the server's host fingerprint:

  The authenticity of host '192.168.1.105 (192.168.1.105)' can't be established.
  RSA key fingerprint is SHA256:...
  Are you sure you want to continue connecting (yes/no)?

Type yes and press Enter.

• Enter the password for the user account on the Managed Node when prompted.

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Step 5: Set Up Passwordless Authentication

For easier and more secure access, configure SSH key-based authentication.

Generate an SSH Key Pair

• On the Control Node, generate an SSH key pair:

  ssh-keygen

• Press Enter to accept the default file location (~/.ssh/id_rsa).
• Optionally, set a passphrase for added security.

Copy the Public Key to the Managed Node

• Use the ssh-copy-id command:

  ssh-copy-id username@192.168.1.105

• Replace username and 192.168.1.105 as appropriate.

• Enter the password when prompted.

  • Test the connection:

  ssh username@192.168.1.105

• You should now log in without entering a password.

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Step 6: Understanding SSH Configuration Files

1. ~/.ssh/config (Control Node)

This file allows you to simplify SSH commands and manage multiple connections.

• Location: ~/.ssh/config (in the home directory of the Control Node user).

• Usage:

  • Open the file for editing:

  nano ~/.ssh/config
  

  • Add an entry for the Managed Node:

  Host managed-node
      HostName 192.168.1.105
      User username
      IdentityFile ~/.ssh/id_rsa
  

  • Save and exit.

• Benefits:

  • Allows you to connect using a simple alias:

  ssh managed-node
  

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2. /etc/ssh/sshd_config (Managed Node)

This file controls the SSH Server configuration.

• Location: /etc/ssh/sshd_config (on the Managed Node).

• Usage:

  • Edit the file:

  sudo nano /etc/ssh/sshd_config
  

  • Common configurations:
  • Disable root login:

    PermitRootLogin no
  

  • Change the default SSH port:

    Port 2222
  

  • Save and restart SSH:

  sudo systemctl restart ssh
  

• Connecting After Changing the Port:

  • On the Control Node, use the -p option:

  ssh -p 2222 username@192.168.1.105
  

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Step 7: Enhancing Security

• Restrict Access by IP Address:

  • On the Managed Node, allow SSH access only from the Control Node's IP:

  sudo ufw allow from 192.168.1.100 to any port 22
  

• Set Idle Timeout:

  • In /etc/ssh/sshd_config, add:

  ClientAliveInterval 300
  ClientAliveCountMax 0
  

  • Restart SSH:

  sudo systemctl restart ssh
  

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Step 8: Troubleshooting Tips

• SSH Connection Refused:

  • Ensure the SSH Server is running:

  sudo systemctl start ssh
  

• Host Key Verification Failed:

  • Remove the old key:

  ssh-keygen -R 192.168.1.105
  

• Firewall Blocking SSH:

  • Allow SSH through the firewall:

  sudo ufw allow ssh
  

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Conclusion

By following this guide, you’ve learned how to connect an Ubuntu Control Node to a Managed Node via SSH. We covered everything from installation and configuration to passwordless login, security best practices, and troubleshooting. Now you’re ready to manage your servers effectively!