Understanding SSH Permissions

Oct 13, 2025 by wbrum.org 109 views

Introduction

As a Linux enthusiast, learning to work with remote machines is an essential skill. Virtual machines provide a safe and controlled environment to practice and experiment with different operating systems and configurations. However, when trying to SSH into one virtual Ubuntu machine from another, you may encounter a frustrating "Permission denied" error. In this article, we will explore the possible causes and solutions to this issue.

Understanding SSH Permissions

Before diving into the solutions, it's essential to understand the basics of SSH permissions. SSH (Secure Shell) is a protocol that allows secure remote access to a machine over a network. When you try to SSH into a machine, the SSH client (usually ssh command) attempts to connect to the SSH server (usually sshd service) on the remote machine. The SSH server checks the client's identity and permissions to determine whether to grant access.

Causes of Permission Denied Error

There are several reasons why you may encounter a "Permission denied" error when trying to SSH into one virtual Ubuntu machine from another. Some of the common causes include:

Incorrect SSH keys: SSH uses public-key cryptography to authenticate clients. If the SSH keys are not correctly generated, configured, or synchronized between the client and server machines, you may encounter permission issues.
Firewall and port blocking: Firewalls and port blocking can prevent SSH connections from being established. Make sure that the SSH port (usually 22) is open and not blocked by any firewall rules.
SSH configuration issues: SSH configuration files (e.g., sshd_config) may be misconfigured or not properly updated, leading to permission issues.
Virtual machine configuration: Virtual machine settings, such as network configurations, may be causing issues with SSH connections.

Solution 1: Generate SSH Keys

One of the most common causes of permission denied errors is incorrect SSH key generation. To generate SSH keys, follow these steps:

Step 1: Generate SSH Keys on the Client Machine

On the client machine (the machine from which you are trying to SSH into the remote machine), run the following command to generate a new SSH key pair:

ssh-keygen -t rsa -b 4096

This will generate a new SSH key pair in the ~/.ssh directory.

Step 2: Copy the Public Key to the Remote Machine

Copy the public key (usually id_rsa.pub) to the remote machine's ~/.ssh/authorized_keys file. You can do this by running the following command on the client machine:

ssh-copy-id user@remote-machine-ip

Replace user with the username on the remote machine and remote-machine-ip with the IP address of the remote machine.

Solution 2: Check Firewall and Port Blocking

Firewalls and port blocking can prevent SSH connections from being established. To check if the SSH port is open, run the following command on the client machine:

nc -vz remote-machine-ip 22

Replace remote-machine-ip with the IP address of the remote machine. If the port is open, you should see a response indicating that the port is open.

Solution 3: Check SSH Configuration

SSH configuration files (e.g., sshd_config) may be misconfigured or not properly updated, leading to permission issues. To check the SSH configuration, run the following command on the remote machine:

sudo sshd -t

This will check the SSH configuration and report any errors or warnings.

Solution 4: Check Virtual Machine Configuration

Virtual machine settings, such as network configurations, may be causing issues with SSH connections. To check the virtual machine configuration, follow these steps:

Step 1: Check Network Configuration

Check the network configuration of the virtual machine to ensure that the SSH port is open and not blocked by any firewall rules.

Step 2: Check Virtual Machine Settings

Check the virtual machine settings to ensure that the SSH port is enabled and not blocked by any virtual machine settings.

Conclusion

In conclusion, the "Permission denied" error when trying to SSH into one virtual Ubuntu machine from another can be caused by a variety of factors, including incorrect SSH keys, firewall and port blocking, SSH configuration issues, and virtual machine configuration issues. By following the solutions outlined in this article, you should be able to resolve the issue and successfully SSH into your virtual Ubuntu machine.

Additional Tips and Resources

Make sure to update your SSH keys regularly to ensure that they are not expired or compromised.
Use a secure password or passphrase for your SSH keys to prevent unauthorized access.
Use a firewall and port blocking to prevent unauthorized access to your SSH server.
Consult the official Ubuntu documentation for more information on SSH configuration and troubleshooting.

Common SSH Commands

Here are some common SSH commands that you may find useful:

ssh user@remote-machine-ip: Connect to the remote machine using SSH.
ssh-copy-id user@remote-machine-ip: Copy the public key to the remote machine's ~/.ssh/authorized_keys file.
nc -vz remote-machine-ip 22: Check if the SSH port is open.
sudo sshd -t: Check the SSH configuration.
ssh-keygen -t rsa -b 4096: Generate a new SSH key pair.

Troubleshooting SSH Issues

If you are still experiencing issues with SSH connections, try the following troubleshooting steps:

Check the SSH logs for any errors or warnings.
Check the SSH configuration files for any misconfigurations.
Check the virtual machine settings for any issues with network configurations or firewall rules.
Try connecting to the remote machine using a different SSH client or protocol (e.g., ssh vs. scp).
Permission Denied When Trying to SSH into One Virtual Ubuntu Machine from Another: Q&A =====================================================================================

Q: What is the most common cause of the "Permission denied" error when trying to SSH into one virtual Ubuntu machine from another?

A: The most common cause of the "Permission denied" error is incorrect SSH key generation or configuration. Make sure to generate new SSH keys and configure them correctly on both the client and server machines.

Q: How do I generate new SSH keys?

A: To generate new SSH keys, run the following command on the client machine:

ssh-keygen -t rsa -b 4096

This will generate a new SSH key pair in the ~/.ssh directory.

Q: How do I copy the public key to the remote machine?

A: To copy the public key to the remote machine, run the following command on the client machine:

ssh-copy-id user@remote-machine-ip

Replace user with the username on the remote machine and remote-machine-ip with the IP address of the remote machine.

Q: What is the difference between `ssh` and `scp`?

A: ssh is a secure shell protocol that allows you to connect to a remote machine and execute commands. scp is a secure copy protocol that allows you to copy files between machines.

Q: How do I check if the SSH port is open?

A: To check if the SSH port is open, run the following command on the client machine:

nc -vz remote-machine-ip 22

Replace remote-machine-ip with the IP address of the remote machine.

Q: What is the purpose of the `~/.ssh/authorized_keys` file?

A: The ~/.ssh/authorized_keys file contains a list of public keys that are allowed to access the remote machine. When you copy the public key to this file, you are granting access to the remote machine.

Q: How do I troubleshoot SSH issues?

A: To troubleshoot SSH issues, check the SSH logs for any errors or warnings. Check the SSH configuration files for any misconfigurations. Check the virtual machine settings for any issues with network configurations or firewall rules.

Q: What is the difference between `ssh-agent` and `ssh-add`?

A: ssh-agent is a program that manages SSH keys and provides a secure way to authenticate with SSH servers. ssh-add is a program that adds an SSH key to the agent.

Q: How do I use `ssh-agent` and `ssh-add`?

A: To use ssh-agent and ssh-add, follow these steps:

Start the ssh-agent program:

eval $(ssh-agent -s)

Add the SSH key to the agent:

ssh-add ~/.ssh/id_rsa

Use the ssh command to connect to the remote machine:

ssh user@remote-machine-ip

Replace user with the username on the remote machine and remote-machine-ip with the IP address of the remote machine.

Q: What is the purpose of the `~/.ssh/config` file?

A: The ~/.ssh/config file contains a list of configuration settings for SSH connections. You can use this file to specify the SSH port, username, and other settings for your SSH connections.

Q: How do I use the `~/.ssh/config` file?

A: To use the ~/.ssh/config file, follow these steps:

Create a new file in the ~/.ssh directory:

touch ~/.ssh/config

Add the configuration settings to the file:

Host remote-machine
  HostName remote-machine-ip
  User user
  Port 22

Replace remote-machine with the hostname of the remote machine, remote-machine-ip with the IP address of the remote machine, user with the username on the remote machine, and 22 with the SSH port.

Use the ssh command to connect to the remote machine:

ssh remote-machine

This will use the configuration settings specified in the ~/.ssh/config file to connect to the remote machine.