---
description: Setting-up a local mirror for Docker Hub images
keywords: registry, on-prem, images, tags, repository, distribution, mirror, Hub, recipe, advanced
title: Registry as a pull through cache
redirect_from:
- /engine/admin/registry_mirror/
---

## Use-case

If you have multiple instances of Docker running in your environment (e.g.,
multiple physical or virtual machines, all running the Docker daemon), each time
one of them requires an image that it doesn’t have it will go out to the
internet and fetch it from the public Docker registry. By running a local
registry mirror, you can keep most of the redundant image fetch traffic on your
local network.

### Alternatives

Alternatively, if the set of images you are using is well delimited, you can
simply pull them manually and push them to a simple, local, private registry.

Furthermore, if your images are all built in-house, not using the Hub at all and
relying entirely on your local registry is the simplest scenario.

### Gotcha

It's currently not possible to mirror another private registry. Only the central
Hub can be mirrored.

### Solution

The Registry can be configured as a pull through cache.  In this mode a Registry
responds to all normal docker pull requests but stores all content locally.

## How does it work?

The first time you request an image from your local registry mirror, it pulls
the image from the public Docker registry and stores it locally before handing
it back to you. On subsequent requests, the local registry mirror is able to
serve the image from its own storage.

### What if the content changes on the Hub?

When a pull is attempted with a tag, the Registry will check the remote to
ensure if it has the latest version of the requested content.  If it doesn't it
will fetch the latest content and cache it.

### What about my disk?

In environments with high churn rates, stale data can build up in the cache.
When running as a pull through cache the Registry will periodically remove old
content to save disk space. Subsequent requests for removed content will cause a
remote fetch and local re-caching.

To ensure best performance and guarantee correctness the Registry cache should
be configured to use the `filesystem` driver for storage.

## Run a Registry as a pull-through cache

The easiest way to run a registry as a pull through cache is to run the official
Registry image.

Multiple registry caches can be deployed over the same back-end.  A single
registry cache will ensure that concurrent requests do not pull duplicate data,
but this property will not hold true for a registry cache cluster.

### Configure the cache

To configure a Registry to run as a pull through cache, the addition of a
`proxy` section is required to the config file.

In order to access private images on the Docker Hub, a username and password can
be supplied.

```yaml
proxy:
  remoteurl: https://registry-1.docker.io
  username: [username]
  password: [password]
```

> **Warning**: If you specify a username and password, it's very important to
> understand that private resources that this user has access to Docker Hub will
> be made available on your mirror. **You must secure your mirror** by
> implementing authentication if you expect these resources to stay private!

> **Warning**: In order for the scheduler to clean up old entries, `delete` must
> be enabled in the registry configuration. See
> [Registry Configuration](/registry/configuration.md) for more details.

### Configure the Docker daemon

Either pass the `--registry-mirror` option when starting `dockerd` manually,
or edit `/etc/docker/daemon.json` and add the `registry-mirrors` key and value,
to make the change persistent.

```json
{
  "registry-mirrors": ["https://<my-docker-mirror-host>"]
}
```

Save the file and restart Docker for the change to take effect.

> **Tip**: If you see a message in the log that reads,
> `error statting local store, serving from upstream: unknown blob`, this isn't
> an error in cache functionality. Instead, it's an informational message
> telling you that the file doesn't exist yet in the local cache.

## Use case: the China registry mirror

The URL of the registry mirror for China is `registry.docker-cn.com`. You can
pull images from this mirror just like you do for other registries by
specifying the full path, including the registry, in your `docker pull`
command, for example:

```bash
$ docker pull registry.docker-cn.com/library/ubuntu
```

You can configure the Docker daemon with the `--registry-mirror` startup
parameter:

```bash
$ docker --registry-mirror=https://registry.docker-cn.com -d
```

Or you can add "https://registry.docker-cn.com" to the `registry-mirrors`
array in `/etc/docker/daemon.json` to pull from the China registry mirror
by default.  

```json
{
  "registry-mirrors": ["https://registry.docker-cn.com"]
}
```

Save the file and restart Docker for the change to take effect.