How to Resolve SSH Connectivity Issues

Secure Shell (SSH) is an important protocol for accessing remote servers in a secure way. Despite its reliability, users may face connectivity issues when working with SSH. This guide will talk about typical SSH connectivity problems and provide solutions for diagnosing and solving them.

Prerequisites

Common Issues and Causes

Authentication Failures

These issues arise when there's a mismatch between the credentials provided and those expected by the server. Common scenarios include:

• Incorrect Passwords: Typing errors or recent password changes can cause failures.

• Public Key Problems: If the server-side authorized_key file doesn't contain the correct public key, or if the client's private key isn't loaded, authentication will fail.

• Expired Credentials: Some systems enforce password or key expiration policies for security.

Network Problems

Connectivity can be lost by network-layer issues, such as:

• Firewall Restrictions: Firewalls may be configured to block the default SSH port (22), requiring a rule change to allow traffic.

• DNS Misconfiguration: Incorrect DNS settings can lead to the client resolving to the wrong IP address for the server.

• Service Interruption: Unreliable internet connections or server-side networking issues can disrupt SSH access.

SSH Configuration Errors

Proper configuration of the SSH daemon and client is essential. Issues might include:

• Misconfigured sshd_config: Incorrect directives in the server's SSH configuration file can prevent connections.

• Client Configuration Issues: The SSH client configuration needs to match the server's requirements, such as accepting the correct key types or encryption algorithms.

Server Overload or Downtime

High server load can slow down or disrupt SSH services, and scheduled or unscheduled downtime can temporarily make the server inaccessible.

Diagnostic Steps

To identify and address SSH connectivity issues, follow these diagnostic steps:

Checking Network Connectivity

Start by confirming that your network connection is active and stable. Use tools like ping or traceroute to verify the connection to the SSH server's IP address. This will help you determine if the issue is at the network level.

Verifying Credentials

Make sure that the SSH credentials you are using are correct and current. For password-based logins, double-check the password you're entering. For SSH key-based logins, confirm that the private key is loaded in your SSH client and that the corresponding public key is present in the authorized_keys file on the server.

Inspecting SSH Configuration

Examine the SSH configuration files carefully. On the server, the sshd_config file should be configured to permit access via the intended methods (password or key) and have the correct port specified. On the client side, the configuration should match the server's protocol requirements.

Reviewing Server Logs

Server logs can provide valuable insights into the cause of SSH failures. Look for authentication errors or messages related to refused connections. These logs are typically located in /var/log/auth.log or /var/log/secure.

By systematically following these steps, you can narrow down the cause of SSH connectivity issues.

Troubleshooting and Solutions for SSH Connectivity

Resolving Network Issues

To adjust firewall settings, use the following commands:

• For Ubuntu: sudo ufw allow 22 to allow SSH traffic on port 22.

• For CentOS: sudo firewall-cmd --permanent --add-service=ssh followed by sudo firewall-cmd --reload.

Fixing Authentication Problems

If you're experiencing issues with SSH access due to authentication, take the following steps:

• Password Reset: Within the Cloudzy Panel, navigate to the Access tab and click RESET CLOUD VPS PASSWORD to generate a new password.

• SSH Key Verification: In the SSH Keys section of the panel, make sure that you enter your public SSH key correctly. The path to the authorized_keys file on your server, which should contain your public key, is typically ~/.ssh/authorized_keys.

• Permissions Check: On the server, confirm the permissions of your ~/.ssh directory and the authorized_keys file with chmod 700 ~/.ssh and chmod 600 ~/.ssh/authorized_keys.

Adjusting SSH Configurations

For configuration adjustments:

• Review and edit the SSH server configuration file located at /etc/ssh/sshd_config on the server. Verify directives like PermitRootLogin yes and PasswordAuthentication yes to make sure they match your requirements.

• Restart the SSH service to apply changes with sudo systemctl restart sshd.

Checking DNS Configurations

Troubles with DNS configurations can lead to SSH connectivity problems. Here's how to verify DNS settings on both the client and server sides:

• On the Client Side (Linux): Use the dig command to query the DNS records:

COPY
dig +short yourdomain.com

This should return the IP address of your server. If it doesn’t, there’s likely an issue with the DNS resolution on your client machine.

On the Client Side (Windows): Use nslookup in the Command Prompt:

COPY
nslookup yourdomain.com

Similar to dig, this should return your server's IP address if DNS is resolving properly.

• On the Server Side: Check the DNS resolver configuration file, typically /etc/resolv.conf, to make sure that it points to the correct DNS server. It should have entries similar to the following:

COPY
nameserver 8.8.8.8

nameserver 8.8.4.4

These are Google's public DNS servers and can be replaced with those provided by your hosting service or ISP.

• Testing DNS Resolution on the Server: Use the dig or nslookup command directly on the server to ensure it can resolve domain names to IP addresses. If it can't resolve external domain names, this could indicate a problem with the DNS service or network configuration on the server itself.

Monitoring Resources with htop

Install htop for real-time monitoring:

• Ubuntu: sudo apt-get install htop

• CentOS: sudo yum install htop

Use htop to observe CPU, memory usage, and manage processes directly within the interface.

Maintaining Server Health

Keep the server updated to prevent security vulnerabilities and performance issues:

• Ubuntu: Execute sudo apt update && sudo apt upgrade to update all packages.

• CentOS: Run sudo yum update to refresh the system.

To effectively resolve SSH connectivity issues, methodically check and correct DNS settings, authenticate credentials, adjust firewall rules, and review SSH configurations. Regularly updating systems and monitoring resources are essential practices for maintaining a stable and secure server environment. By adhering to these steps, you can have reliable SSH access to your VPS, minimizing downtime and enhancing security. If you need any more information or further assistance, feel free to contact our support team by submitting a ticket.