Docker Registry name resolution

Question

I'm working on a simple REST client for Docker Registry. For private registries, name resolution is pretty simple; if the image name is myregistry.io/myimage:latest, I look for https://myregistry.io/v2 and query the API there.

However, I notice that for docker hub, it doesn't quite work that way. If I'm looking for ubuntu, I can expand that to docker.io/ubuntu:latest, but https://docker.io/v2 returns a 307 redirect to https://www.docker.com/v2, which just returns HTML. The actual registry endpoint is at https://registry-1.docker.io/v2.

Is this just a hardcoded special case in the docker client, or is there some extra logic to looking up registry endpoints that I'm unaware of? If it is just a special case, is there more to it than always going to registry-1.docker.io instead of docker.io?

Answer 1

The central Docker registry is a well-known special case, similar to Maven central. You can see the defaults e.g. at https://github.com/docker/docker-ce/blob/ea449e9b10cebb259e1a43325587cd9a0e98d0ff/components/engine/registry/config.go#L42:

var (
    // DefaultNamespace is the default namespace
    DefaultNamespace = "docker.io"
    // DefaultRegistryVersionHeader is the name of the default HTTP header
    // that carries Registry version info
    DefaultRegistryVersionHeader = "Docker-Distribution-Api-Version"

    // IndexHostname is the index hostname
    IndexHostname = "index.docker.io"
    // IndexServer is used for user auth and image search
    IndexServer = "https://" + IndexHostname + "/v1/"
    // IndexName is the name of the index
    IndexName = "docker.io"

    // DefaultV2Registry is the URI of the default v2 registry
    DefaultV2Registry = &url.URL{
        Scheme: "https",
        Host:   "registry-1.docker.io",
    }
)