Access google container registry without the gcloud client

Question

I have a CoreOS docker host that I want to start running containers on, but when trying to use the docker command to fetch the image from the google container private registry (https://cloud.google.com/tools/container-registry/), I get a 403. I did some searching, but I'm not sure how to attach authentication (or where to generate the user+pass bundle to use with the docker login command).

Has anybody had any luck pulling from the google private containers? I can't install the gcloud command because coreos doesn't come with python, which is a requirement

docker run -p 80:80 gcr.io/prj_name/image_name
Unable to find image 'gcr.io/prj_name/image_name:latest' locally
Pulling repository gcr.io/prj_name/image_name
FATA[0000] HTTP code: 403

Update: after getting answers from @mattmoor and @Jesse:

The machine that I'm pulling from does have devaccess

curl -H 'Metadata-Flavor: Google' http://metadata.google.internal./computeMetadata/v1/instance/service-accounts/default/scopes
https://www.googleapis.com/auth/bigquery
https://www.googleapis.com/auth/cloud-platform
https://www.googleapis.com/auth/compute
https://www.googleapis.com/auth/datastore
----> https://www.googleapis.com/auth/devstorage.read_only
https://www.googleapis.com/auth/logging.admin
https://www.googleapis.com/auth/sqlservice.admin
https://www.googleapis.com/auth/taskqueue
https://www.googleapis.com/auth/userinfo.email

Additionally, I tried using the _token login method

jenkins@riskjenkins:/home/andre$ ACCESS_TOKEN=$(curl -H 'Metadata-Flavor: Google' 'http://metadata.google.internal./computeMetadata/v1/instance/service-accounts/default/token' | cut -d'"' -f 4)
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   142  100   142    0     0  14686      0 --:--:-- --:--:-- --:--:-- 15777
jenkins@riskjenkins:/home/andre$ echo $ACCESS_TOKEN
**************(redacted, but looks valid)
jenkins@riskjenkins:/home/andre$ docker login -e [email protected] -u _token -p $ACCESS_TOKEN http://gcr.io
Login Succeeded
jenkins@riskjenkins:/home/andre$ docker run gcr.io/prj_name/image_name
Unable to find image 'gcr.io/prj_name/image_name:latest' locally
Pulling repository gcr.io/prj_name/image_name
FATA[0000] HTTP code: 403

Answer 1

The Google Container Registry authentication scheme is to simply use:

username: '_token'
password: {oauth access token}

On Google Compute Engine you can login without gcloud with:

$ METADATA=http://metadata.google.internal./computeMetadata/v1
$ SVC_ACCT=$METADATA/instance/service-accounts/default
$ ACCESS_TOKEN=$(curl -H 'Metadata-Flavor: Google' $SVC_ACCT/token \
    | cut -d'"' -f 4)
$ docker login -e [email protected] -u '_token' -p $ACCESS_TOKEN https://gcr.io

Update on {asia,eu,us,b}.gcr.io

To access a repository hosted in a localized repository, you should login to the appropriate hostname in the above docker login command.

Update on quotes around _token

As of docker version 1.8, docker login requires the -u option to be in qoutes or start with a letter.

Some diagnostic tips...

Check that you have the Cloud Storage scope via:

$ curl -H 'Metadata-Flavor: Google' $SVC_ACCT/scopes
...
https://www.googleapis.com/auth/devstorage.full_control
https://www.googleapis.com/auth/devstorage.read_write
https://www.googleapis.com/auth/devstorage.read_only
...

NOTE: "docker pull" requires "read_only", but "docker push" requires "read_write".