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Docker Compose Extra Hosts

Sometimes you need your container to resolve a specific hostname to a specific IP address, overriding standard DNS. In Docker Compose, the extra_hosts configuration allows you to modify the container's /etc/hosts file.

Syntax​

extra_hosts is defined as a list of hostname:IP mappings.

services:
  app:
    image: my-app
    extra_hosts:
      - "somehost:162.242.195.82"
      - "otherhost:50.31.209.229"

Inside the container, /etc/hosts will include:

162.242.195.82  somehost
50.31.209.229   otherhost

Common Use Cases​

1. Accessing the Host Machine (Linux)​

As discussed in our Host Networking guide, Linux containers don't support host.docker.internal by default. You can enable it using the special host-gateway value.

services:
  web:
    image: nginx
    extra_hosts:
      - "host.docker.internal:host-gateway"

This maps host.docker.internal to the IP address of the host gateway, allowing your container to talk to services running on your local machine (e.g., a local Postgres database not in Docker).

2. Mocking DNS for Development​

If your application connects to a production domain (e.g., api.example.com), but you want to redirect that traffic to a local test server or another container during development, you can use extra_hosts.

services:
  backend:
    image: my-backend
    # ...

  frontend:
    image: my-frontend
    extra_hosts:
      - "api.example.com:172.17.0.1" # Redirect to local IP

3. Communicating Between Containers (Legacy)​

While modern Docker Compose setups should use Docker Networks and service names for discovery (e.g., connecting to http://db instead of an IP), extra_hosts can be useful for legacy applications that require specific hardcoded hostnames.

IPv6 Support​

You can also map IPv6 addresses if your Docker daemon is configured for it.

extra_hosts:
  - "somehost:2001:db8::10"

Simplify Your Networking​

Managing DNS records and host files manually can be error-prone.

Deploy on Hostim.dev

Hostim.dev provides a platform where service discovery just works. Deploy your stack and let us handle the networking complexity.