DNS Service Discovery

Introduction

DNS Service Discovery is a powerful mechanism in Prometheus that allows you to use DNS records to dynamically discover and monitor targets. Instead of manually configuring each target, Prometheus can query DNS records (like A, AAAA, SRV) to automatically find and monitor services across your infrastructure.

This approach is particularly useful in dynamic environments where services might change their location or when you're working with service-oriented architectures. By leveraging DNS, which is already a fundamental component of most networks, you can create a flexible and scalable monitoring setup.

How DNS Service Discovery Works

When Prometheus is configured to use DNS Service Discovery, it periodically queries DNS servers for records matching specified patterns. Based on the returned records, Prometheus dynamically updates its list of targets to scrape.

Configuring DNS Service Discovery

To use DNS Service Discovery in Prometheus, you'll need to configure it in your prometheus.yml file. Here's a basic example:

yaml

scrape_configs:
  - job_name: 'dns-service-discovery'
    dns_sd_configs:
      - names:
        - 'services.example.com'
        type: 'A'
        port: 9090

Configuration Parameters

DNS Service Discovery provides several parameters to customize how Prometheus discovers and monitors targets:

Parameter	Description	Default
names	List of DNS domain names to query	(required)
type	Record type to query (A, AAAA, SRV, MX)	'A'
port	Target port to use for discovered targets	0
refresh_interval	How often to query DNS	30s
resolve_timeout	Maximum time allowed for DNS lookup	10s

Supported DNS Record Types

Prometheus supports different DNS record types for service discovery:

A/AAAA Records

These records map domain names to IPv4 (A) or IPv6 (AAAA) addresses. When using A/AAAA records, you'll need to specify a port in the configuration.

Example configuration:

yaml

dns_sd_configs:
  - names:
    - 'api.example.com'
    type: 'A'
    port: 8080

When Prometheus queries for api.example.com and gets back IP addresses like 192.168.1.1 and 192.168.1.2, it will monitor both http://192.168.1.1:8080/metrics and http://192.168.1.2:8080/metrics.

SRV Records

SRV records are more detailed, specifying both the hostname and port for services. They're ideal for service discovery because they contain all the necessary information to connect to a service.

Example configuration:

yaml

dns_sd_configs:
  - names:
    - '_prometheus._tcp.example.com'
    type: 'SRV'

If the SRV record returns:

_prometheus._tcp.example.com. 300 IN SRV 10 20 9090 node1.example.com.
_prometheus._tcp.example.com. 300 IN SRV 10 20 9090 node2.example.com.

Prometheus will scrape:

• http://node1.example.com:9090/metrics
• http://node2.example.com:9090/metrics

Combining with Relabeling

For more advanced configurations, you can combine DNS SD with Prometheus's relabeling functionality:

yaml

scrape_configs:
  - job_name: 'dns-discovered-services'
    dns_sd_configs:
      - names:
        - 'services.example.com'
        type: 'A'
        port: 9090
    relabel_configs:
      - source_labels: [__meta_dns_name]
        target_label: instance_name

This configuration adds an instance_name label to each target with the value of the DNS name that was queried.

Real-World Examples

Monitoring Kubernetes Services

Kubernetes exposes services via DNS names. You can use DNS SD to discover and monitor these services:

yaml

scrape_configs:
  - job_name: 'kubernetes-services'
    dns_sd_configs:
      - names:
        - 'kube-dns.kube-system.svc.cluster.local'
        - 'prometheus-operator.monitoring.svc.cluster.local'
        type: 'A'
        port: 9153

Monitoring Services in Multiple Regions

If you have services deployed across different regions, you can use DNS to discover them all:

yaml

scrape_configs:
  - job_name: 'global-api-services'
    dns_sd_configs:
      - names:
        - 'api.us-east.example.com'
        - 'api.eu-west.example.com'
        - 'api.ap-south.example.com'
        type: 'A'
        port: 8080
    metrics_path: '/metrics'

Consul Service Discovery via DNS

Consul provides DNS-based service discovery that works well with Prometheus:

yaml

scrape_configs:
  - job_name: 'consul-services'
    dns_sd_configs:
      - names:
        - 'web.service.consul'
        - 'api.service.consul'
        - 'db.service.consul'
        type: 'A'
        port: 9090

Common Challenges and Solutions

Challenge: DNS Record TTL and Caching

Problem: DNS records have Time-To-Live (TTL) values, and caching might delay the discovery of new services.

Solution: Adjust the refresh_interval to be shorter than or equal to your DNS TTL:

yaml

dns_sd_configs:
  - names:
    - 'services.example.com'
    type: 'A'
    port: 9090
    refresh_interval: 15s  # If your DNS TTL is 15s

Challenge: Filtering Discovered Targets

Problem: You need to monitor only a subset of the services discovered via DNS.

Solution: Use relabeling to keep or drop targets based on specific criteria:

yaml

scrape_configs:
  - job_name: 'filtered-services'
    dns_sd_configs:
      - names:
        - 'all-services.example.com'
        type: 'A'
        port: 9090
    relabel_configs:
      - source_labels: [__address__]
        regex: '10\.0\.0\..*'
        action: keep  # Only keep IPs in the 10.0.0.x range

Summary

DNS Service Discovery in Prometheus provides a flexible and dynamic way to discover monitoring targets using standard DNS infrastructure. It supports different record types (A, AAAA, SRV) to accommodate various service discovery needs.

Key advantages include:

• Leveraging existing DNS infrastructure
• Dynamic discovery without manual configuration
• Scalability in large or changing environments
• Integration with various DNS-based service discovery tools

By combining DNS Service Discovery with Prometheus's powerful relabeling capabilities, you can create a robust and adaptable monitoring solution that automatically adjusts to changes in your infrastructure.

Additional Resources

• Prometheus Documentation on DNS-based SD
• DNS Protocol RFC 1035
• SRV Records RFC 2782

Exercises

1. Basic DNS SD Setup: Configure Prometheus to discover and monitor a service using A records.
2. SRV Record Discovery: Set up a DNS server with SRV records and configure Prometheus to discover services using these records.
3. Filtering Challenge: Create a configuration that discovers services via DNS but only monitors ones matching specific criteria using relabeling.
4. Multi-Environment Monitoring: Design a Prometheus configuration that uses DNS SD to monitor services across development, staging, and production environments.