Add a fixed hostname mapping.
Docker / Compose: extra_hosts adds an entry to the container’s hosts file.
services:
app:
image: ubuntu:20.04
extra_hosts:
example.test: "192.0.2.10"
command: ["sh", "-c", "getent hosts example.test | awk '{print $$1}'"]Expected output EXAMPLE
192.0.2.10
Details
Requires Docker Engine and Docker Compose (the docker compose command). Run in a disposable development environment with ubuntu:20.04 available locally. See the input setup for any starting containers or files. Save as compose.yaml and run docker compose run --rm app. extra_hosts adds an entry to the container’s hosts file.
192.0.2.10 is a documentation address; this checks name mapping without connecting to it.
Tested on Alpine Linux v3.24 · · Client 29.9.0; Engine 29.9.0 · Compose 5.6.0; ubuntu:20.04
Test inputs, results and environment
The exact published code ran in a dedicated Linux test container with prepared inputs. Dedicated nested Docker daemon in a privileged test helper with no host Docker socket. No application data or host directories are mounted into the example containers. The outer helper has networking disabled; the base image is preloaded. Dockerfile examples are built and run; Compose examples start real containers. Fixtures and output records are isolated from the application database. Testing covers this example and its stated checks, not every input, operating system, or runtime version. The Linux container shares its host's kernel; this is not a native Windows test.
- Architecture / host kernel
- x86_64 / 6.18.54.1-microsoft-standard-WSL2
- Snippet identity / locale
- root inside disposable Docker-in-Docker helper / C.UTF-8
- Interpreter invocation
docker compose --progress quiet run --rm -T app- Container image ID
sha256:aa45302a315fcbba307d6749ccbbac83af2e6f9447d6d714fce5b7b1c92b1081- Code SHA-256
04c3c4e8f4f1e4a659c8f04ff989180382813c4ebfbca0c8013df8d98b652929- Dependencies
- Compose 5.6.0; ubuntu:20.04
Test input setup
These are the prepared inputs used by the test harness. See Details above for the prerequisites and how to run this example.
# No input files required.
Observed standard output
192.0.2.10
Exit code: 0. Standard error: empty. Validation: Exact stdout comparison. System-dependent values above are one recorded run.
Additional assertions that passed
Checks recorded by the test harness. Where Python assertions are shown, browser cases use Playwright and simulator cases may inspect pin transitions: result.stdout is captured output, work is the temporary directory, and Path is pathlib.Path. The harness also imports json and csv.
All commands returned zero; exact output matched.