Can a self-signed cert secure multiple CNs / FQDNs? - docker

Question

This is a bit of a silly setup, but here's what I'm looking at right now:
I'm learning Kubernetes
I want to push custom code to my Kubernetes cluster, which means the code must be available as a Docker image available from some Docker repository (default is Docker Hub)
While I'm willing to pay for Docker Hub if I have to (though I'd rather avoid it), I have concerns about putting my custom code on a third-party service. Sudden rate limits, security breaches, sudden ToS changes, etc
To this end, I'm running my own Docker registry within my Kubernetes cluster
I do not want to configure the Docker clients running on the Kubernetes nodes to trust insecure (HTTP) Docker registries. If I do choose to pull any images from an external registry (e.g. public images like nginx I may pull from Docker Hub instead of hosting locally) then I don't want to be vulnerable to MITM attacks swapping out the image
Ultimately I will have a build tool within the cluster (Jenkins or otherwise) pull my code from git, build the image, and push it to my internal registry. Then all nodes pulling from the registry live within the cluster. Since the registry never needs to receive images from sources outside of the cluster or delivery them to sources outside of the cluster, the registry does not need a NodePort service but can instead be a ClusterIP service.... ultimately
Until I have that ultimate setup ready, I'm building images on my local machine and wish to push them to the registry (from the internet)
Because I don't plan on making the registry accessible from the outside world (eventually), I can't utilize Let's Encrypt to generate valid certs for it (even if I were making my Docker registry available to the outside world, I can't use Let's Encrypt, anyway without writing some extra code to utilize certbot or something)
My plan is to follow the example in this StackOverflow post: generate a self-signed cert and then launch the Docker registry using that certificate. Then use a DaemonSet to make this cert trusted on all nodes in the cluster.
Now that you have the setup, here's the crux of my issue: within my cluster my Docker registry can be accessed via a simple host name (e.g. "docker-registry"), but outside of my cluster I need to either access it via a node IP address or a domain name pointing at a node or a load balancer.
When generating my self-signed cert I was asked to provide a CN / FQDN for the certificate. I put in "docker-registry" -- the internal host name I plan to utilize. I then tried to access my registry locally to push an image to it:
> docker pull ubuntu
> docker tag ubuntu example.com:5000/my-ubuntu
> docker push example.com:5000/my-ubuntu
The push refers to repository [example.com:5000/my-ubuntu]
Get https://example.com:5000/v2/: x509: certificate is valid for docker-registry, not example.com
I can generate a certificate for example.com instead of for docker-registry, however I worry that I'll have issues configuring the service or connecting to my registry from within my cluster if I provide my external domain like this instead of an internal host name.
This is why I'm wondering if I can just say that my self-signed cert applies to both example.com and docker-registry. If not, two other acceptable solutions would be:
Can I tell the Docker client not to verify the host name and just trust the certificate implicitly?
Can I tell the Docker registry to deliver one of two different certificates based on the host name used to access it?
If none of the three options are possible, then I can always just forego pushing images from my local machine and start the process of building images within the cluster -- but I was hoping to put that off until later. I'm learning a lot right now and trying to avoid getting distracted by tangential things.

Answer 1

Probably the easiest way to solve your problem would be to use Docker's insecure-registry feature. The concern you mention about this in your post (that it would open you up to security risks later) probably won't apply as the feature works by specifying specific IP addresses or host names to trust.
For example you could configure something like
{
"insecure-registries" : [ "10.10.10.10:5000" ]
}
and the only IP address that your Docker daemons will access without TLS is the one at that host and port number.
If you don't want to do that, then you'll need to get a trusted TLS certificate in place. The issue you mentioned about having multiple names per cert is usually handled with the Subject Alternative Name field in a cert. (indeed Kubernetes uses that feature quite a bit).