Logstash DNS Filter Plugin

The DNS filter plugin in Logstash is used for performing DNS lookups on IP addresses or hostnames. It can resolve IP addresses to hostnames or vice versa, making it useful for enriching log data with additional network information.

Syntax

dns {
  reverse => [ "source_host" ]
  resolve => [ "destination_host" ]
  action => "replace"
  hit_cache_size => 1000
  hit_cache_ttl => 60
  failed_cache_size => 1000
  failed_cache_ttl => 60
}

For more details, refer to the official Logstash DNS filter plugin documentation.

Example Use Case

Suppose you have log entries with IP addresses, and you want to resolve them to hostnames for better readability and analysis. Here's an example configuration:

filter {
  dns {
    reverse => [ "client_ip" ]
    action => "replace"
  }
}

This configuration will attempt to perform a reverse DNS lookup on the "client_ip" field and replace the IP address with the resolved hostname if successful.

Common Issues and Best Practices

1. Performance Impact: DNS lookups can be slow and may impact overall Logstash performance. Use caching options like hit_cache_size and hit_cache_ttl to mitigate this.

2. Failed Lookups: Not all IP addresses will resolve successfully. Use the failed_cache_size and failed_cache_ttl options to cache failed lookups and avoid repeated attempts.

3. Timeout Handling: Set appropriate timeout values using the timeout option to prevent Logstash from hanging on slow DNS responses.

4. Action Selection: Choose the appropriate action (append, replace, or remove_field) based on your needs to avoid unintended data loss.

Frequently Asked Questions

Q: Can the DNS filter plugin handle both forward and reverse lookups?
A: Yes, the plugin can perform both forward (hostname to IP) and reverse (IP to hostname) lookups using the resolve and reverse options respectively.

Q: How can I improve the performance of DNS lookups in Logstash?
A: To improve performance, use caching options like hit_cache_size and hit_cache_ttl, set appropriate timeout values, and consider using a local DNS server if possible.

Q: What happens if a DNS lookup fails?
A: If a lookup fails, the original field value is retained by default. You can use the failed_cache_size and failed_cache_ttl options to cache failed lookups and avoid repeated attempts.

Q: Can I use the DNS filter plugin with IPv6 addresses?
A: Yes, the DNS filter plugin supports both IPv4 and IPv6 addresses for lookups.

Q: Is it possible to add custom DNS servers for lookups?
A: The DNS filter plugin uses the system's default DNS configuration. To use custom DNS servers, you would need to configure them at the operating system level where Logstash is running.