dregsy - Docker Registry Sync

Synopsis

dregsy lets you sync Docker images between registries, public or private. Several sync tasks can be defined, as one-off or periodic tasks (see Configuration section). An image is synced by using a sync relay. Currently, this can be either Skopeo or a local Docker daemon. When using the latter, the image is first pulled from the source, then tagged for the destination, and finally pushed there. Skopeo in contrast, can directly transfer an image from source to destination, which makes it the preferred choice.

Configuration

Sync tasks are defined in a YAML config file:

# relay type, either 'skopeo' or 'docker'
relay: skopeo

# relay config sections
skopeo:
  # path to the skopeo binary; defaults to 'skopeo', in which case it needs to
  # be in PATH
  binary: skopeo
  # directory under which to look for client certs & keys, as well as CA certs
  # (see note below)
  certs-dir: /etc/skopeo/certs.d

docker:
  # Docker host to use as the relay
  dockerhost: unix:///var/run/docker.sock
  # Docker API version to use, defaults to 1.24
  api-version: 1.24

# list of sync tasks
tasks:

  - name: task1 # required

    # interval in seconds at which the task should be run; when omitted,
    # the task is only run once at start-up
    interval: 60

    # determines whether for this task, more verbose output should be
    # produced; defaults to false when omitted
    verbose: true

    # 'source' and 'target' are both required and describe the source and
    # target registries for this task:
    #  - 'registry' points to the server; required
    #  - 'auth' contains the base64 encoded credentials for the registry
    #    in JSON form {"username": "...", "password": "..."}
    #  - 'auth-refresh' specifies an interval for automatic retrieval of
    #    credentials; only for AWS ECR (see below)
    #  - 'skip-tls-verify' determines whether to skip TLS verification for the
    #    registry server (only for 'skopeo', see note below); defaults to false
    source:
      registry: source-registry.acme.com
      auth: eyJ1c2VybmFtZSI6ICJhbGV4IiwgInBhc3N3b3JkIjogInNlY3JldCJ9Cg==
    target:
      registry: dest-registry.acme.com
      auth: eyJ1c2VybmFtZSI6ICJhbGV4IiwgInBhc3N3b3JkIjogImFsc29zZWNyZXQifQo=
      skip-tls-verify: true

    # 'mappings' is a list of 'from':'to' pairs that define mappings of image
    # paths in the source registry to paths in the destination; 'from' is
    # required, while 'to' can be dropped if the path should remain the same as
    # 'from'. Additionally, the tags being synced for a mapping can be limited
    # by providing a 'tags' list. When omitted, all image tags are synced.
    mappings:
      - from: test/image
        to: archive/test/image
        tags: ['0.1.0', '0.1.1']
      - from: test/another-image

Caveats

When syncing via a Docker relay, do not use the same Docker daemon for building local images (even better: don't use it for anything else but syncing). There is a risk that the reference to a locally built image clashes with the shorthand notation for a reference to an image on docker.io. E.g. if you built a local image busybox, then this would be indistinguishable from the shorthand busybox pointing to docker.io/library/busybox. One way to avoid this is to use registry.hub.docker.com instead of docker.io in references, which would never get shortened. If you're not syncing from/to docker.io, then all of this is not a concern.

Repository Validation & Client Authentication with TLS

When connecting to source and target repository servers, TLS validation is performed to verify the identity of a server. If you're using self-signed certificates for a repo server, or a server's certificate cannot be validated with the CA bundle available on your system, you need to provide the required CA certs. (The dregsy Docker image includes the CA bundle from the official golang image). Also, if a repo server requires client authentication, i.e. mutual TLS, you need to provide an appropriate client key & cert pair.

How you do that for Docker is described here. The short version: create a folder under /etc/docker/certs.d with the same name as the repo server's host name, e.g. source-registry.acme.com, and place any required CA certs there as *.crt (mind the extension). Client key & cert pairs go there as well, as *.key and *.cert.

Example:

/etc/docker/certs.d/
    └── source-registry.acme.com
       ├── client.cert
       ├── client.key
       └── ca.crt

When using the skopeo relay, this is essentially the same, except that you specify the root folder with the skopeo setting certs-dir (defaults to /etc/skopeo/certs.d). However, it's important to note the following differences:

When a repo server uses a non-standard port, the port number is included in image references when pulling and pushing. For TLS validation, docker will accordingly expect a {registry host name}:{port} folder. For skopeo, this is not the case, i.e. the port number is dropped from the folder name. This was a conscious decision to avoid pain when running dregsy in Kubernetes and mounting certs & keys from secrets: mount paths must not contain :.
To skip TLS verification for a particular repo server when using the docker relay, you need to configure the Docker daemon accordingly. With skopeo, you can easily set this in any source or target definition with the skip-tls-verify setting.

AWS ECR

If a source or target is an AWS ECR registry, you need to retrieve the auth credentials via AWS CLI. They would however only be good for 12 hours, which is ok for one off tasks. For periodic tasks, or to avoid retrieving the credentials manually, you can specify an auth-refresh interval as a Go Duration, e.g. 10h. If set, dregsy will initially and whenever the refresh interval has expired retrieve new access credentials. auth can be omitted when auth-refresh is set. Setting auth-refresh for anything other than an AWS ECR registry will raise an error.

Note however that you either need to set environment variables AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY for the AWS account you want to use and a user with sufficient permissions. Or if you're running dregsy on an EC2 instance in your AWS account, the machine should have an appropriate instance profile. An according policy could look like this:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": [
        "ecr:GetAuthorizationToken",
        "ecr:CreateRepository"
      ],
      "Resource": ["*"]
    },
    {
      "Effect": "Allow",
      "Action": [
        "ecr:GetDownloadUrlForLayer",
        "ecr:BatchGetImage",
        "ecr:BatchCheckLayerAvailability",
        "ecr:DescribeRepositories",
        "ecr:PutImage",
        "ecr:InitiateLayerUpload",
        "ecr:UploadLayerPart",
        "ecr:CompleteLayerUpload"
      ],
      "Resource": "arn:aws:ecr:<your_region>:<your_account>:repository/*"
    }
  ]
}

Usage

dregsy -config={path to config file}

If there are any periodic sync tasks defined (see Configuration above), dregsy remains running indefinitely. Otherwise, it will return once all one-off tasks have been processed.

Running Natively

If you run dregsy natively on your system, with relay type docker, the Docker daemon of your system will be used as the relay for all sync tasks, so all synced images will wind up in the Docker storage of that daemon.

Running Inside a Docker Container

You can use the dregsy image on Dockerhub for running dregsy containerized.

With `skopeo` relay

The image includes the skopeo binary, so all that's needed is:

docker run --rm -v {path to config file}:/config.yaml xelalex/dregsy

With `docker` relay

This will still use the local Docker daemon as the relay:

docker run --privileged --rm -v {path to config file}:/config.yaml -v /var/run/docker.sock:/var/run/docker.sock xelalex/dregsy

Running On Kubernetes

When you run a Docker registry inside your Kubernetes cluster as an image cache, dregsy can come in handy as an automated updater for that cache. The example config below uses the skopeo relay:

relay: skopeo
tasks:
  - name: task1
    interval: 60
    source:
      registry: registry.acme.com
      auth: eyJ1c2VybmFtZSI6ICJhbGV4IiwgInBhc3N3b3JkIjogInNlY3JldCJ9Cg==
    target:
      registry: registry.my-cluster
      auth: eyJ1c2VybmFtZSI6ICJhbGV4IiwgInBhc3N3b3JkIjogImFsc29zZWNyZXQifQo=
    mappings:
      - from: test/image
        to: archive/test/image
      - from: test/another-image

To keep your registry auth tokens in the config file secure, we are creating a Kubernetes Secret instead of a ConfigMap:

kubectl create secret generic dregsy-config --from-file=./config.yaml

In addition, you will most likely want to mount client certs & keys, and CA certs from Kubernetes secrets into the pod for TLS validation to work. (The CA bundle from the official golang image is already included in the dregsy image.)

apiVersion: apps/v1beta1
kind: StatefulSet
metadata:
  name: kube-registry-updater
  namespace: kube-system
  labels:
    k8s-app: kube-registry-updater
    kubernetes.io/cluster-service: "true"
spec:
  serviceName: kube-registry-updater
  replicas: 1
  template:
    metadata:
      labels:
        k8s-app: kube-registry-updater
        kubernetes.io/cluster-service: "true"
    spec:
      containers:
      - name: dregsy
        image: xelalex/dregsy
        command: ['dregsy', '-config=/config/config.yaml']
        resources:
          requests:
            cpu: 10m
            memory: 32Mi
        volumeMounts:
        - name: dregsy-config
          mountPath: /config
          readOnly: true
      volumes:
      - name: dregsy-config
        secret:
          secretName: dregsy-config

Building

The Makefile has targets for building the binary and Docker image, and other stuff. Just run make to get a list. Note that for consistency, building is done inside a Golang build container, so you will need Docker to build. dregsy's Docker image is based on Alpine, and installs Skopeo via apk during the image build.

FaHeymann/dregsy