Docker's Internal DNS: Navigating Nginx Resolutions in Containerized Environments

Oct 11, 2023

Table of contents
Introduction
Docker’s DNS Mechanism
Why resolver 127.0.0.11 in Nginx?
Configuring Nginx with Docker’s DNS
Conclusion

Introduction

Docker, the de-facto standard for containerization, offers its own DNS system to manage inter-container communications. When paired with Nginx, understanding this DNS mechanism becomes crucial for seamless web operations. Let’s explore this intricate dance of domain name resolutions within Docker.

Docker’s DNS Mechanism

When containers try to communicate with each other, Docker’s internal DNS server plays a pivotal role. By default, Docker provides a DNS server at 127.0.0.11, which containers use to resolve service names to their respective IP addresses.

Why resolver 127.0.0.11 in Nginx?

Dynamic Resolutions: In dynamic environments where containers are frequently started or stopped, IP addresses can change. Nginx, by default, resolves domain names only once at startup. Using resolver 127.0.0.11 ensures that Nginx queries Docker’s DNS for every request, capturing any IP changes.

Service Discovery: In Docker Compose setups or Docker Swarm clusters, service names can be used instead of static IP addresses. The resolver directive ensures these service names are correctly mapped to their current IPs.

Avoiding Stale Cache: Without the resolver, Nginx might cache outdated DNS entries, leading to failed requests. The directive ensures fresh DNS lookups, preventing such issues.

Configuring Nginx with Docker’s DNS

Basic Setup: In your Nginx configuration, especially when using dynamic domain names in proxy_pass, include the resolver directive:

resolver 127.0.0.11;

Using Service Names: In Docker Compose setups, you can use service names as domain names. For instance, if you have a service named “webapp”, your Nginx

configuration can look like:

location / {
    proxy_pass http://webapp;
}

Error Handling: Ensure that your Nginx configuration handles potential DNS resolution errors gracefully, using directives like proxy_next_upstream to retry requests or redirect to fallback servers.

Conclusion