Connect to localhost from inside a dev container

Jim Bennett | Nov 20, 2023

I do a lot of work in dev containers (for example, using liblab inside of one), often hosting APIs. One problem I used to often hit is how can I host an API in one container, and access it from another dev container? This post shows you how.

The problem - ’localhost’ in a container

So the issue is this:

  • Dev container A exposes an API on port 8000, and this is forwarded to localhost
  • I can access this port at localhost:8000 either inside dev container A, or from my local machine as the dev container forwards the port
  • Dev container B needs to access the API

The problem here is dev container B has no access to my local machine. Docker forwards the ports from dev container A to localhost automatically as soon as I run the API. But ports from my local machine are not forwarded into dev containers.

  • If Dev container A accesses localhost:8000, it gets the API running on Dev container A
  • If my local machine accesses localhost:8000, it gets the API running on Dev container A because the port on Dev container A is forwarded to localhost, as far as the local machine is concerned, 8000 is open and accepting requests.
  • If Dev container B accesses localhost:8000 it will fail, as there is nothing running on port 8000 inside that container.

So how can one dev container access ports exposed by another?

Enter the host.docker.internal network

Docker has a fix for this! It exposes a ‘special’ DNS called host.docker.internal that essentially gives to access to the local machines network. Rather than access localhost, you access this named device instead.

So from Dev container B, you access host.docker.internal:8000 and boom! Dev container B can access the API from Dev container A!

For example, you are testing out an SDK created against an API you are running on port 8000 via a dev container (for example, the liblab llama store), and you want to take advantage of the dev containers feature of liblab to open our SDK in a container. You can set the baseUrl in your liblab.config.json file to http://host.docker.internal:8000, then open the generated SDK in its dev container!

  "sdkName": "llama-game",
  "specFilePath": "http://localhost:8000/openapi.json",
  "baseUrl": "http://host.docker.internal:8000",
  "languages": [
  "createDocs": true,
  "customizations": {
    "devContainer": true

Done! The SDK now defaults to use the docker internal network for testing.

class Environment(Enum):
    """The environments available for this SDK"""

    DEFAULT = "http://host.docker.internal:8000"

Don’t forget to set it to your production URL once you are ready to publish.