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Dodge the subsequent Dockerpocalypse: how one can personal your personal Docker Registry tackle

Dodge the subsequent Dockerpocalypse: how one can personal your personal Docker Registry tackle

2023-03-17 22:43:29

As you may have seen, Docker Hub made a dramatic shift in coverage this week, and successfully gave a 30 day eviction discover to virtually all community-run pictures.

They’ve now made an apology to ‘make clear’ just a few particulars, and helpfully take a number of the exhausting edges off, however this nonetheless highlights a giant drawback. Luckily, there are answers.

As initially described, this could’ve been catastrophic: Docker Hub has been used because the default host in tooling, tutorials, demos, weblog posts, scripts, deployment definitions, CI builds and extra for a few years, and all these references have been going to interrupt – a self-inflicted left-pad for the Docker ecosystem. Of their up to date coverage, it seems they now will not take away any current pictures, however initiatives who do not pay up won’t be able to publish any new pictures, in order that they’ve successfully misplaced management of the namespace they used to deploy to their communities except they buy a full group subscription. Many will not do so.

That is an fascinating problem. Even when the prevailing pictures aren’t eliminated, the course of journey for Docker Hub is now clear: they do not wish to host the core of the Docker group any extra, no extra freebies, pay up or go elsewhere (not unreasonable, however one thing of a rugpull after a full decade of the alternative).

As a small/open-source/group/hobbyist picture writer, or in the event you rely on Docker Hub totally free picture publishing in any capability, you now have an issue. They do not need you there. You are explicitly not their viewers, and the principles will possible tighten additional. This is not unreasonable – it is their service and internet hosting is not free – nevertheless it’s price contemplating explicitly and reacting accordingly. For those who’re not a paying Docker Hub buyer, it is time to go away Docker Hub.

The exhausting half is what to do as a substitute.

Self-hosting a registry will not be free, and it is extra work than it sounds: it is a correct piece of infrastructure, and comes with all of the obligations that suggests, from monitoring to promptly making use of safety updates to load & disk-space administration. No one (not to mention tiny initiatives like these) desires this job.

Alternatively, there are many different free hosted Docker registries, and paid providers too, however migrating to 1 straight feels lots such as you’re simply going to hit the very same drawback 6 months from now, and have to vary your picture references all over the place over again.

What in the event you might use your personal fastened registry URL, by yourself area & fully beneath your management, however with out having to self-host without end, and even decide to any explicit registry, or deal with all of the bandwidth & storage prices?

We’re on the lookout for a option to:

  • Reference your pictures from an tackle you absolutely management (docker pull
  • Accomplish that while nonetheless with the ability to use any registry hosted elsewhere, or self-hosted ourselves.
  • Keep away from storing, loading, or serving the content material individually. For now a minimum of, there’s fairly just a few different registries who will fortunately do that for public pictures totally free, and even when there weren’t we might wish to keep away from additional latency or ingress & egress charges from proxying this site visitors.
  • Be capable of change which backing registry we’re utilizing in future, with none of the picture addresses ever altering once more.

What if I instructed you that is really tremendous straightforward?

Exploring the chances

Let’s speak about how this might work, after which we’ll dig into what docker pull really does, and put collectively a fast answer (in the event you simply wish to know the way to do that instantly, now’s the time to skip to the end).

Docker’s registry API runs on pretty easy HTTP, and HTTP APIs have just a few totally different options accessible for conditions like this.

The basic ‘host beneath your personal area’ answer is to make use of CNAMEs on the DNS stage. This implies organising a DNS file beneath your area, which factors to a website elsewhere, successfully defining an alias. When a shopper tries to attach, they will lookup, discover a file referencing the backing registry (, for instance), after which all requests will get despatched over there.

If this labored right here, that’d be nice! Zero internet hosting required, simply deal with it on the DNS stage.

Sadly, this requires that concentrate on server to appropriately handles HTTP requests together with your third occasion area identify within the Host header, figuring out that they need to be processed as requests to the actual service. For Docker Hub a minimum of, that is not doable (not totally free definitely – though like many different providers this is perhaps provided as a paid addon). Requests despatched to Docker Hub with the flawed hostname merely fail:

> curl -I
HTTP/1.1 200 OK

> curl -I -H'Host:'
HTTP/1.0 503 Service Unavailable

I believe this is applicable to many different registries too, so redirecting simply on the DNS stage is out.

Subsequent plan: can we do that by redirecting and/or proxying on the HTTP stage? There’s a number of customary instruments & approaches to do that inside HTTP itself, together with a complete ecosystem of reverse proxies. Sadly although, whether or not or not API purchasers will deal with redirects as we might like will not be assured, and proxying with out working into different points is non-trivial.

To work out whether or not this’ll work, we have to do some digging into Docker’s site visitors straight.

How ‘Docker Pull’ works

First, let’s check out what a Docker pull actually does beneath the hood.

While you run docker pull, or do the rest with Docker (e.g. constructing a picture) that triggers a picture pull en route, there are just a few requests that must occur to obtain the complete picture you are on the lookout for.

To dig into this site visitors, the simplest choice is to make use of an HTTP-debugging device (resembling HTTP Toolkit) to see the uncooked interactions, and configure Docker to make use of this as your HTTP proxy (docs here) and belief the CA certificates (here).

Except you are tremendous eager although, you’ll be able to all skip that – I’ve executed the exhausting give you the results you want. This is what occurs whenever you run docker pull nginx:

HTTP Toolkit showing the list of requests sent during a Docker pull

What we’ve right here is:

  • An preliminary /v2/ request to test the API standing (docs here). On Docker Hub this usually returns a 401, with headers redirecting the shopper to authenticate.
  • An authentication request to, which returns a JWT.
  • A HEAD request to the bottom picture URL (/v2/library/nginx/manifests/newest) which returns a response with a docker-content-digest header containing a sha256 hash:

    The response headers of the digest request

  • Two GET requests for particular manifests, each receiving a 200:

    • /v2/library/nginx/manifests/sha256:aa0a... (the hash from the earlier response header) which returns an inventory of manifests tagged by platform:

      The manifest JSON content, listing hashes for each platform

    • /v2/library/nginx/manifests/sha256:942a... (the hash of the linux platform from the earlier request) which returns a manifest itemizing hashes for particular person picture layers.

      The manifest JSON content, listing hashes for each layer

  • A set of parallel requests for particular blob hashes, all within the format of

    Every of those does not return the content material – they return 307 redirects to the content material! Within the case of Docker Hub, they return seem to return redirects to a Cloudflare-backed CDN:

    A 307 redirect response, with a Location header pointing to

  • An interleaved set of parallel requests to the actual picture host ( to really retrieve the content material of the picture config & layers.

As soon as the shopper has pulled all of the layers and the picture config, they’re composed again collectively as a Docker picture you should utilize straight regionally.

Transparently wrapping a Docker Registry

That is all very fascinating, and provides us a good suggestion what is going on on on the community stage, so we are able to begin testing this out to construct what I am calling a “registry facade” (a service that sits in entrance, however simply as a shell, not a proxy).

Conveniently, within the site visitors above we are able to see that there are already redirects in place, and dealing! That signifies that all Docker purchasers should help redirects a minimum of for /blobs/ requests (in any other case Docker Hub can be unusable) and so most likely help them for all requests.

So, on condition that, what occurs if we simply do the identical straight ourselves, by making a rule to return 307 HTTP redirects from all $OUR_HOST/* URLs to the corresponding $OUR_REGISTRY/* for any request?

See Also

A series of requests, each receiving a 203 redirect to Docker Hub, and then a successful response there


This works pretty effectively! We’re including a little bit of overhead with an additional 307 redirect response at every step (every request with the pink icon is an injected redirect) however they’re very fast, the whole lot right here is being despatched efficiently, and pulls work completely in each situation I’ve examined. Positively ok to get began with (and since it will all be beneath our personal management, we are able to iterate to enhance this answer in future).

I examined this with a fast hacky rewrite rule in HTTP Toolkit – how do you do that in manufacturing?

Seems that is fairly straightforward too: I’ve created a tiny Caddy-based Docker container (I loved the irony of publishing this to Docker Hub) which you’ll be able to deploy on to any Docker internet hosting platform to do that very quickly, or if you have already got a CDN or internet hosting platform (e.g. Netlify) that allows you to outline easy guidelines like “redirect all requests for X to the identical path at host Y” then you should utilize that too.

In my case, I am utilizing Bunny CDN, who’ve a pleasant guidelines system that may do that very simply like so:

A edge rule, redirect all traffic to{Request.Path}

In manufacturing, one factor you might wish to do is restrict this performance to simply your personal org’s pictures, to keep away from it getting used as a general-purpose facade for all pictures or related, so all requests to your area will at all times get your pictures. The Caddy-based container above helps this by setting the REGISTRY_ORG variable, e.g. to httptoolkit, during which case solely these pictures can be accessible and the whole lot else will get a 403.

If you wish to restrict requests like this your self with different instruments, you may simply want to make sure that requests to all URL paths beginning with /v2/$YOUR_ORG/ are redirected, together with the precise /v2/ endpoint – with out that latter endpoint authentication will not work.

As soon as that is in place, you are all good. In my case, I’ve deployed this as, so now you can pull my Docker pictures from that hostname, although they’re at the moment nonetheless hosted on Docker Hub, like so:

> docker pull

In future I will be migrating my pictures elsewhere, however I can begin utilizing this picture tackle instantly, protected within the information that it will at all times work, backed by any registry I like, so long as I management that area.

Dodging the subsequent Dockerpocalypse

For those who’re a challenge affected by this situation, that is one thing you’ll be able to arrange proper now as a fast wrapper earlier than even beginning to migrate from Docker Hub, and you can begin shifting all of your docs & scripts to reference that new URL instantly with no downsides.

Extra importantly although, both manner, this ensures that whichever registry you migrate to, there’s zero influence to switching in future, when your new registry of alternative inevitably additionally goes bust/loses all of your knowledge/modifications their guidelines with solely 30 days discover.

That is sufficient for now (I must get again to really doing the complete migration for all HTTP Toolkit’s current pictures) however I hope that helps others in the identical mess. If in case you have feedback, get in contact on Mastodon, Twitter, or send a message directly.

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