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

1. Administrative Access: Login credentials with
   the necessary privileges on the server you’re attempting to
   access.

2. Network Access: A stable internet connection and
   the ability to reach the server’s network.

3. SSH Client: A working SSH client installed on
   your local machine, such as OpenSSH or PuTTY.

4. Server Information: The server’s IP address, SSH
   port number (default is 22), and the appropriate user account
   information.

5. Permissions: If using key-based authentication,
   make sure your private key is available and has the correct permissions
   set.

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:

dig +short yourdomain.com

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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:

nslookup yourdomain.com

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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:

nameserver 8.8.8.8

nameserver 8.8.4.4

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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.