Originally opened as boto/botocore#2515

When running some code expecting AWS credentials in a docker container, a good way to provide those credentials is through an ECS-compatible endpoint running on the host, providing some credentials the host has available. The ECS endpoint is better than the IMDSv2 endpoint, as it requires a shared secret (IMDSv2 is protected against SSRF but isn't truly authenticated). I have a tool which will do just that, aws-export-credentials.

The ECS credentialing mechanism in the SDKs allows an endpoint to be set by the environment variable AWS_CONTAINER_CREDENTIALS_FULL_URI, but it checks the hostname against a list of approved endpoints, which includes localhost and the ECS metadata server at 169.254.170.2 (for the most part, see below for the messy details).

However, for docker, the host (when access to it is provided) is reachable as host.docker.internal (docs), and this is not on the list of approved hosts for ECS endpoints in any of the SDKs.

On Linux, the workaround is to use --network host, which makes it accessible as localhost from inside the container, but on Mac and Windows this doesn't work (the docker network is always isolated from the host).

host.docker.internal should be added to the list of approved hosts, so the locally-provided ECS credentials endpoint can work even if the code is running in a docker container.

This would also provide the opportunity to synchronize the list of approved hosts, which currently varies across SDKs.

For example, in botocore, ContainerMetadataFetcher currently allows three hosts for container metadata endpoints: 169.254.170.2, localhost, and 127.0.0.1.

botocore allows 169.254.170.2 in AWS_CONTAINER_CREDENTIALS_FULL_URI, where at least Go, Java v1, and JavaScript v2 only allow localhost/127.0.0.1 in AWS_CONTAINER_CREDENTIALS_FULL_URI and use 169.254.170.2 only for AWS_CONTAINER_CREDENTIALS_RELATIVE_URI 🤷

I should note that the amazon-ecs-local-container-endpoints project doesn't solve this problem; it provides the credential endpoint at 169.254.170.2 for other containers on the docker network, but to provide it with credentials, it says to mount ~/.aws, just shifting the problem to that container.