netdata/health/guides/unbound/unbound_request_list_dropped.md

Understand the alert

The unbound_request_list_dropped alert indicates that the Unbound DNS resolver is dropping new incoming requests because its request queue is full. This situation may be caused by a high volume of DNS queries, possibly from a Denial of Service (DoS) attack or poor server optimization.

Troubleshoot the alert

1. Check the request queue length: Inspect the Unbound configuration file (usually located at /etc/unbound/unbound.conf) and check the num-queries-per-thread setting. If the value is too low for your system, you may encounter issues with dropped requests.

2. Increase the queue length: If necessary, increase the num-queries-per-thread value in the Unbound configuration file. For example, if the current value is 1024, you can try setting it to a higher value, such as 2048 or 4096. Save the changes and restart the Unbound service:

   sudo systemctl restart unbound
   

3. Monitor dropped requests: Use the unbound-control command to monitor the number of dropped requests in real-time:

   sudo unbound-control stats_noreset | grep num.requestlist.dropped
   

If you see the dropped requests decreasing, your changes to the num-queries-per-thread value may have resolved the issue.

1. Inspect server logs: Check the Unbound log file (usually located at /var/log/unbound.log) for any suspicious activity or error messages that may indicate the cause of the increased DNS queries.

2. Check for potential DoS attacks: Use tools like iftop, nload, or nethogs to monitor network traffic and identify any potential DoS attacks or unusual traffic patterns.

If you believe your server is experiencing a DoS attack:

• Investigate the source IP addresses of the high-volume traffic
• Block malicious traffic using firewall tools like iptables or ufw
• Contact your hosting provider, ISP, or network administrator for assistance

1. Optimize Unbound: Review the official Unbound documentation and tune the settings in the Unbound configuration file to ensure optimal performance for your specific environment.

Useful resources

1. Unbound Official Documentation
2. How to set up a DNS Resolver with Unbound