Introduction

After installing ClickHouse on Ubuntu 16.04, you might encounter an error such as "Connection refused" when attempting to connect to the ClickHouse server. This error is often related to network configuration, server status, or access permissions. This article will guide you through identifying and resolving the "Connection refused" issue after setting up ClickHouse on Ubuntu 16.04.

Understanding the "Connection Refused" Error

The "Connection refused" error is an indication that the connection attempt to a port on the server was unsuccessful. This can be caused by various factors, including:

• The ClickHouse server is not running.
• Firewall settings blocking the connection.
• Incorrect configurations in the ClickHouse server setup.
• The server is listening on a different IP address or port.

Troubleshooting Steps

Here is a detailed guide to troubleshoot and fix the "Connection refused" error:

Step 1: Verify ClickHouse Server Status

First, ensure that the ClickHouse server is up and running. Use the following command to check the status:

sudo systemctl status clickhouse-server

• If the server is inactive, you can start it with:

sudo systemctl start clickhouse-server

Step 2: Check Network Configuration

Verify the network settings in the ClickHouse configuration file (/etc/clickhouse-server/config.xml). Ensure the service is bound to the correct IP address and port.

• Open the configuration file:

sudo nano /etc/clickhouse-server/config.xml

• Check the <listen_host> tag. It should either specify the correct IP address or be set to <listen_host>::</listen_host> which allows connections from any IP address.

Step 3: Inspect Firewall Settings

A common culprit for a "Connection refused" error is the server firewall, which might be blocking access. Check the firewall status and adjust settings accordingly:

• Check the status of UFW (Uncomplicated Firewall):

sudo ufw status

• If the firewall is active, allow connections on the ClickHouse default port 9000:

sudo ufw allow 9000

Step 4: Ensure Appropriate Listening

Even if the service is running and the firewall is configured correctly, the server might still not be listening on the port you are connecting to.

• Use netstat or ss to check the listening ports:

sudo netstat -tuln | grep 9000

Ensure that you see a line indicating that the service is listening on the port.

Step 5: Review SELinux/AppArmor Policies

On systems with SELinux or AppArmor, additional policies might restrict network operations:

• For AppArmor, ensure the service profile allows network connections.
• For SELinux, use the following command to check policy enforcements:

sudo getsebool -a | grep clickhouse

If necessary, update the policies to permit network operations by the ClickHouse server.

Summary Table

Here is a summary of the key steps to address "Connection refused" errors:

Conclusion

The "Connection refused" error when connecting to a ClickHouse server on Ubuntu 16.04 is typically due to configuration or network access issues. By following the outlined steps, you can systematically diagnose and resolve the issue. Always ensure your server configurations are securely set to prevent unauthorized access. For more detailed scenarios, consulting the ClickHouse and Ubuntu documentation may provide additional insights.