Unable to connect to localstack container from another container in the same network

Platform
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1

After going through multiple forums and stackoverflow questions, HOSTNAME_EXTERNAL and HOSTNAME are the properties, tried with both of them for accessing the localstack from another running container. but it still like localhost:4566

I have specifically mentioned the config as below


 final AwsClientBuilder.EndpointConfiguration endpoint = new AwsClientBuilder.EndpointConfiguration("http://localstack:4566", REGION);
        final AmazonS3 client = AmazonS3ClientBuilder.standard()
                .withEndpointConfiguration(endpoint)
                .withPathStyleAccessEnabled(true)
                .build();

docker-compose

localstack:
    image: localstack/localstack:latest
    hostname: localstack_test
    ports:
      - 4566:4566
      - 4463-4499:4463-4499
    environment:
      - SERVICES=${SERVICES- }
      - DEBUG=1
      - DATA_DIR=/tmp/localstack/data
      - HOSTNAME=localstack
      - HOSTNAME_EXTERNAL=localstack
    volumes:
      - "./.localstack:/var/lib/localstack"
      - "/var/run/docker.sock:/var/run/docker.sock"
    networks:
      bill_network:
        aliases:
          - localstack.test.internal

Note, i have also tried with HOSTNAME and HOSTNAME_INTERNAL separately but its of no use.

Can anyone confirm why does it connect to localhost when it is explicitly mentioned as localstack:4566 ? Also the HOSTNAME and HOSTNAME_EXTERNAL properties are not working the way its expected.

Thanks

2

Thanks for raising this question. I am currently working on improving the user experience on connecting application code to LocalStack, and came upon this question. You’re certainly not the first person to find the configuration a little confusing!

You have posted your docker-compose configuration for the LocalStack container, but you have not shown the configuration for the container running your application code. Just as a sanity check: have you added that container to the bill_network network as well? I have tried to re-create your issue and found that if I add my “application” code to the bill_network, then the LocalStack container can be found under the following names:

  • localstack (the name of the service in docker-compose)
  • localstack_test (the explicit hostname value you’ve specified)
  • localstack.test.internal (the additional network alias you’ve specified)

This uses a feature of docker called “user-defined bridge networks”, and provides powerful name resolution capabilities.

To complete your example application, I have my “application” service defined in docker-compose as:

  mycontainer:
    build: .
    environment:
      - AWS_ACCESS_KEY_ID=test
      - AWS_SECRET_ACCESS_KEY=test
      - AWS_REGION=us-east-1
    networks:
      bill_network:

(note: the last line is not a typo, it means use the network with no additional configuration… yaml is weird)

The application is built with this Dockerfile:

FROM python:3.10-slim

# Install `dig` and `ip` to help diagnose networking
RUN apt-get update && \
    apt-get install -y --no-install-recommends \
    dnsutils \
    iproute2
RUN python3 -m pip install \
    boto3
COPY main.py /main.py
CMD ["/main.py"]

and application code

#!/usr/bin/env python

import boto3

# could use
# - `localstack:4566` or 
# - `localstack_test:4566` or
# - `localstack.test.internal:4566`
# as the endpoint hostname here
client = boto3.client("s3", endpoint_url="http://localstack:4566")
print(client.list_buckets())

This code is approximately equivalent to your example above, but I am more familiar with Python.

With this example, I am able to run

# start localstack in the background
docker compose up -d localstack
# wait for a few seconds until the container is ready
sleep 5
# run the example "application" code
docker compose run mycontainer
# I get the output below indicating success
# {'ResponseMetadata': {'RequestId': 'TLMHvnoOqVotqVjJxvHpI3sgDNmD6m7WZFCkF665C7dRZVajshsQ', 'HTTPStatusCode': 200, 'HTTPHeaders': {'content-type': 'application/xml; charset=utf-8', 'server': 'Werkzeug/2.1.2 Python/3.10.8, hypercorn-h11', 'date': 'Mon, 20 Feb 2023 11:53:02 GMT, Mon, 20 Feb 2023 11:53:02 GMT', 'x-amzn-requestid': 'TLMHvnoOqVotqVjJxvHpI3sgDNmD6m7WZFCkF665C7dRZVajshsQ', 'access-control-allow-origin': '*', 'connection': 'close', 'last-modified': 'Mon, 20 Feb 2023 11:53:02 GMT', 'x-amz-request-id': '38C08B543275A9C6', 'x-amz-id-2': 'MzRISOwyjmnup38C08B543275A9C67/JypPGXLh0OVFGcJaaO3KW/hRAqKOpIEEp', 'accept-ranges': 'bytes', 'content-language': 'en-US', 'transfer-encoding': 'chunked'}, 'RetryAttempts': 0}, 'Buckets': [], 'Owner': {'DisplayName': 'webfile', 'ID': 'bcaf1ffd86f41161ca5fb16fd081034f'}}

If you’re interested, I have a WIP PR for improving the documentation around this issue (rendered), perhaps this may of some additional use.

Please let me know if this information is useful - I’d like to help you diagnose this issue, as it helps improve the project!

Debugging connectivity

In my application container I install two programs: dig and ip. These can help me debug network connectivity. In particular, I’m able to start my application container into a bash shell with docker compose run --entrypoint bash mycontainer then run

root@4085a3281948:/# dig +noall +answer localstack
localstack.             600     IN      A       172.28.0.2

This replies with an IP address 172.28.0.2, indicating that the name localstack is resolvable to the LocalStack container. I can use docker inspect on the LocalStack container to get its IP address:

$ docker inspect <container name> \
    | jq -r '(.[0].NetworkSettings.Networks | to_entries[]).value.IPAddress'
172.28.0.2

and they match.

HOSTNAME_EXTERNAL

This configuration value is used when returning URLs from LocalStack resources, e.g. an RDS cluster URL. It does not affect the name at which LocalStack can be reached. This is certainly something I want to clear up going forward.

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