authn

authn

This README outlines how we acquire and use credentials when interacting with a registry.

As much as possible, we attempt to emulate docker's authentication behavior and configuration so that this library "just works" if you've already configured credentials that work with docker; however, when things don't work, a basic understanding of what's going on can help with debugging.

The official documentation for how docker authentication works is (reasonably) scattered across several different sites and GitHub repositories, so we've tried to summarize the relevant bits here.

tl;dr for consumers of this package

By default, pkg/v1/remote uses Anonymous credentials (i.e. none), which for most registries will only allow read access to public images.

To use the credentials found in your docker config file, you can use the DefaultKeychain, e.g.:

package main

import (
	"fmt"

	"github.com/google/go-containerregistry/pkg/authn"
	"github.com/google/go-containerregistry/pkg/name"
	"github.com/google/go-containerregistry/pkg/v1/remote"
)

func main() {
	ref, err := name.ParseReference("registry.example.com/private/repo")
	if err != nil {
		panic(err)
	}

	// Fetch the manifest using default credentials.
	img, err := remote.Get(ref, remote.WithAuthFromKeychain(authn.DefaultKeychain))
	if err != nil {
		panic(err)
	}

	// Prints the digest of registry.example.com/private/repo
	fmt.Println(img.Digest)
}

(If you're only using gcr.io, see the pkg/v1/google.Keychain, which emulates docker-credential-gcr.)

The Config File

This file contains various configuration options for docker and is (by default) located at:

• $HOME/.docker/config.json (on linux and darwin), or
• %USERPROFILE%\.docker\config.json (on windows).

You can override this location with the DOCKER_CONFIG environment variable.

Plaintext

The config file is where your credentials are stored when you invoke docker login, e.g. the contents may look something like this:

{
	"auths": {
		"registry.example.com": {
			"auth": "QXp1cmVEaWFtb25kOmh1bnRlcjI="
		}
	}
}

The auths map has an entry per registry, and the auth field contains your username and password encoded as HTTP 'Basic' Auth.

NOTE: This means that your credentials are stored in plaintext:

$ echo "QXp1cmVEaWFtb25kOmh1bnRlcjI=" | base64 -d
AzureDiamond:hunter2

For what it's worth, this config file is equivalent to:

{
	"auths": {
		"registry.example.com": {
			"username": "AzureDiamond",
			"password": "hunter2"
		}
	}
}

... which is useful to know if e.g. your CI system provides you a registry username and password via environment variables and you want to populate this file manually without invoking docker login.

Helpers

If you log in like this, docker will warn you that you should use a credential helper, and you should!

To configure a global credential helper:

{
	"credsStore": "osxkeychain"
}

To configure a per-registry credential helper:

{
	"credHelpers": {
		"gcr.io": "gcr"
	}
}

We use github.com/docker/cli/cli/config.Load to parse the config file and invoke any necessary credential helpers. This handles the logic of taking a ConfigFile + registry domain and producing an AuthConfig, which determines how we authenticate to the registry.

Credential Helpers

The credential helper protocol allows you to configure a binary that supplies credentials for the registry, rather than hard-coding them in the config file.

The protocol has several verbs, but the one we most care about is get.

For example, using the following config file:

{
	"credHelpers": {
		"gcr.io": "gcr",
		"eu.gcr.io": "gcr"
	}
}

To acquire credentials for gcr.io, we look in the credHelpers map to find the credential helper for gcr.io is gcr. By appending that value to docker-credential-, we can get the name of the binary we need to use.

For this example, that's docker-credential-gcr, which must be on our $PATH. We'll then invoke that binary to get credentials:

$ echo "gcr.io" | docker-credential-gcr get
{"Username":"_token","Secret":"<long access token>"}

You can configure the same credential helper for multiple registries, which is why we need to pass the domain in via STDIN, e.g. if we were trying to access eu.gcr.io, we'd do this instead:

$ echo "eu.gcr.io" | docker-credential-gcr get
{"Username":"_token","Secret":"<long access token>"}

Debugging credential helpers

If a credential helper is configured but doesn't seem to be working, it can be challenging to debug. Implementing a fake credential helper lets you poke around to make it easier to see where the failure is happening.

This "implements" a credential helper with hard-coded values:

#!/usr/bin/env bash
echo '{"Username":"<token>","Secret":"hunter2"}'

This implements a credential helper that prints the output of docker-credential-gcr to both stderr and whatever called it, which allows you to snoop on another credential helper:

#!/usr/bin/env bash
docker-credential-gcr $@ | tee >(cat 1>&2)

Put those files somewhere on your path, naming them e.g. docker-credential-hardcoded and docker-credential-tee, then modify the config file to use them:

{
	"credHelpers": {
		"gcr.io": "tee",
		"eu.gcr.io": "hardcoded"
	}
}

The docker-credential-tee trick works with both crane and docker:

$ crane manifest gcr.io/google-containers/pause > /dev/null
{"ServerURL":"","Username":"_dcgcr_1_5_0_token","Secret":"<redacted>"}

$ docker pull gcr.io/google-containers/pause
Using default tag: latest
{"ServerURL":"","Username":"_dcgcr_1_5_0_token","Secret":"<redacted>"}
latest: Pulling from google-containers/pause
a3ed95caeb02: Pull complete
4964c72cd024: Pull complete
Digest: sha256:a78c2d6208eff9b672de43f880093100050983047b7b0afe0217d3656e1b0d5f
Status: Downloaded newer image for gcr.io/google-containers/pause:latest
gcr.io/google-containers/pause:latest

The Registry

There are two methods for authenticating against a registry: token and oauth2.

Both methods are used to acquire an opaque Bearer token (or RegistryToken) to use in the Authorization header. The registry will return a 401 Unauthorized during the version check (or during normal operations) with Www-Authenticate challenge indicating how to proceed.

Token

If we get back an AuthConfig containing a Username/Password or Auth, we'll use the token method for authentication:

OAuth 2

If we get back an AuthConfig containing an IdentityToken we'll use the oauth2 method for authentication:

This happens when a credential helper returns a response with the Username set to <token> (no, that's not a placeholder, the literal string "<token>"). It is unclear why: moby/moby#36926.

We only support the oauth2 grant_type for refresh_token (#629), since it's impossible to determine from the registry response whether we should use oauth, and the token method for authentication is widely implemented by registries.