MaheshNotes

When configuring Nginx for DNS resolution, it’s crucial to understand how it handles invalid responses from DNS servers. Ensuring your Nginx setup can gracefully manage DNS errors can significantly improve the reliability and resilience of your web services. In this post, we’ll delve into Nginx’s behavior when it encounters invalid DNS responses and discuss best practices for configuring DNS resolvers.

Types of Invalid DNS Responses

Before exploring Nginx’s behavior, let’s first identify what constitutes an invalid DNS response:

1. NXDOMAIN (Non-Existent Domain): Indicates the domain name does not exist.
2. SERVFAIL (Server Failure): Indicates a failure on the DNS server side.
3. TIMEOUT: No response from the DNS server within the expected time frame.
4. Malformed Response: The response does not conform to DNS protocol specifications or is otherwise unusable.

Nginx Resolver Behavior with Invalid DNS Responses

When Nginx receives an invalid response, it has mechanisms to handle these scenarios, especially when multiple DNS servers are configured.

1. Multiple DNS Servers Configured

When multiple DNS servers are specified in the resolver directive, Nginx uses a failover mechanism:

http {
    resolver 8.8.8.8 8.8.4.4 1.1.1.1 valid=300s;
}

• Query Order: Nginx queries the DNS servers in the order they are listed.
• Retry Mechanism: If an invalid response is received (e.g., NXDOMAIN, SERVFAIL, TIMEOUT), Nginx will retry the query with the next DNS server in the list.
• Fallback: This process continues until Nginx either receives a valid response or exhausts all configured DNS servers.

2. Single DNS Server Configured or All Servers Fail

If only one DNS server is configured, or if all configured DNS servers return invalid responses, the behavior is as follows:

• Immediate Failure: If the response is NXDOMAIN, Nginx will immediately fail to resolve the domain and return an error to the client.
• Retry on Subsequent Requests: For other types of failures (SERVFAIL, TIMEOUT), Nginx may retry the query upon subsequent requests, depending on the specific error and configuration.

Handling Specific Invalid Responses

NXDOMAIN

• Immediate Response: Nginx treats NXDOMAIN as an authoritative answer that the domain does not exist. It does not retry with other DNS servers.
• Error Returned: The request fails with a resolution error.

SERVFAIL

• Retry with Next Server: Nginx retries the query with the next DNS server in the list.
• All Servers Fail: If all configured servers return SERVFAIL, the request fails.

TIMEOUT

• Retry with Next Server: Nginx retries the query with the next DNS server in the list.
• All Servers Timeout: If all servers timeout, Nginx will eventually fail the request due to an unresolved domain.

Malformed Response

• Discard Response: Nginx discards the malformed response and retries with the next DNS server.
• All Servers Return Malformed Responses: If all responses are malformed, the request fails due to a resolution error.

Example Configuration and Behavior

Consider the following Nginx configuration with multiple DNS servers:

http {
    resolver 8.8.8.8 8.8.4.4 1.1.1.1 valid=300s;

    server {
        listen 80;
        server_name example.com;

        location / {
            proxy_pass http://backend.example.com;
        }
    }
}

• Initial Query: Nginx sends a DNS query to 8.8.8.8.
  • If 8.8.8.8 returns a valid response, Nginx caches it and uses it.
  • If 8.8.8.8 returns an invalid response (SERVFAIL, TIMEOUT), Nginx retries with 8.8.4.4.
  • If 8.8.4.4 also fails, Nginx retries with 1.1.1.1.
• Caching: If a valid response is received from any server, it is cached for 300 seconds. Subsequent requests use the cached response.
• Failure Handling: If all DNS servers fail to provide a valid response, Nginx returns a resolution error to the client.

Conclusion

Nginx’s resolver is designed to handle various DNS resolution failures gracefully by retrying with multiple DNS servers and following a systematic failover approach. By configuring multiple DNS servers, setting appropriate TTL values, and disabling unnecessary IPv6 lookups, you can enhance the reliability and performance of DNS resolution in your Nginx setup. Understanding these behaviors ensures that your web server can effectively handle DNS resolution even in the face of individual server failures or invalid responses.