Service "web" uses an undefined network "frontend"

Fix Service "web" uses an undefined network "frontend" (docker-compose)

TL;DR

A service references a network that is not declared in the top-level networks: section. Add the network name under top-level networks: (or fix the typo), and the project validates. If the network already exists outside compose, declare it with external: true.

The error, verbatim:

Service "web" uses an undefined network "frontend"

Steps

1. Reproduce the error to see which network is undefined:

   docker compose config

Success check: The error names the exact service and network.

1. Add the missing declaration at the top level of the compose file:

   grep -n -A3 "^networks:" docker-compose.yml

Success check: The network name now appears under top-level networks:.

1. Re-validate and bring the stack up:

   docker compose config

Success check: The parsed file prints with no errors.

When to use

You hit this exact error while using docker-compose. The trigger list below matches your situation.

When NOT to use

If the error text differs, or the failure is in a different tool (e.g. a Docker daemon error when you only need the compose fix), find the skill for that exact error instead.

Tool compatibility

docker-compose v1 and Compose v2. Any OS.

Variant phrasings

Service "xxx" uses an undefined network "xxx"

uses an undefined network

Root cause

A service lists a network under its networks: key, but that network is never declared in the top-level networks: section of the compose file. Compose validates references before creating anything, so one missing declaration blocks the whole project.

Edge cases

• If the network is created outside compose, declare it with external: true instead of a bare name.
• Typos are the most common cause: network names are case-sensitive.
• With multiple -f files, the top-level networks: must exist in the merged result, not just one file.

Where this came from

Resolved from the public discussion at https://stackoverflow.com/questions/52587359 (so:52587359).