Flux webhook receiver

The container image built herein acts as a webhook endpoint, that will notify Flux of git push and image push events.

Flux has a notify method in its API, but this is unsuitable to exposing to the internet, because

it handles a payload defined by the Flux API, rather than handling the events posted by GitHub etc.
to use it as a webhook endpoint, you would have to expose the Flux API listener to the internet; and that is terribly, terribly unsafe.

Supported webhook sources

flux-recv understands

GitHub push events (and ping events)
DockerHub image push events
GitLab push events
Bitbucket push events

How to use it

In short:

construct a configuration, including shared secrets
run fluxcd/flux-recv as a sidecar in the flux deployment
expose the flux-recv listener to the internet
install webhooks at the source provider (e.g., GitHub)
check if it works

These are explained in more depth in the following sections.

Constructing a configuration

Each webhook you want to receive must have its own shared secret, which is supplied as a file mounted into the container.

In addition, there is a config YAML that tells flux-recv which secrets correspond to which kinds of webhook, so it knows what to expect.

Here is how to make a Kubernetes Secret with a shared secret and a configuration in it:

create the secret -- GitHub recommends 20 bytes of randomness (NB I am using print rather than puts because newlines will mess things up):

$ ruby -rsecurerandom -e 'print SecureRandom.hex(20)' > ./github.key

create a configuration that refers to it:

$ cat >> fluxrecv.yaml <<EOF
fluxRecvVersion: 1
endpoints:
- keyPath: github.key
  source: GitHub
EOF

The value of source is one of the sources supported (listed above, and in sources.go).

create a kustomization.yaml that will construct the Secret for you:

$ cat >> kustomization.yaml <<EOF
secretGenerator:
- name: fluxrecv-config
  files:
  - github.key
  - fluxrecv.yaml
generatorOptions:
  disableNameSuffixHash: true
EOF

use kubectl to apply the kustomization:

kubectl apply -k .

You now have a Kubernetes secret named fluxrecv-config.

Running flux-recv as a sidecar

The ideal is to run flux-recv as a sidecar to fluxd, so that the flux API is only exposed on localhost. The additional bits you need in the flux deployment are:

a volume definition that refers to the fluxrecv-config constructed above
a container spec to run the flux-recv container itself

The first bit goes under .spec.template.volumes:

      # volumes:

      - name: fluxrecv-config
        secret:
          secretName: fluxrecv-config
          defaultMode: 0400

The second bit goes under .spec.template.containers:

      # containers:

      - name: recv
        image: fluxcd/flux-recv:0.2.0
        imagePullPolicy: IfNotPresent
        args:
        - --config=/etc/fluxrecv/fluxrecv.yaml
        ports:
        - containerPort: 8080
        volumeMounts:
        - name: fluxrecv-config
          mountPath: /etc/fluxrecv

You do not need to alter the container spec for the flux container, though you may want to supply the argument --listen=localhost:3030 to limit API access to localhost, if you don't already.

If you do restrict API access to localhost, make sure you also

supply --listen-metrics=:3031

annotate the pod with prometheus.io/port: "3031", so Prometheus knows which port to scrape).

remove any probes that rely on reaching the API

Expose flux-recv to the internet

To expose the flux-recv listener to the internet so that webhooks can reach it, you will need to:

make a Kubernetes Service for flux-recv; and,
either
- tell an Ingress to route requests to the service; OR
- use ngrok to tunnel requests through to flux-recv

Making a service for flux-recv

To be able to route requests to flux-recv from anywhere else, it needs a Kubernetes Service. Here's a suitable definition:

$ cat > flux-recv-service.yaml <<EOF
---
apiVersion: v1
kind: Service
metadata:
  name: flux-recv
spec:
  type: ClusterIP
  ports:
    - name: recv
      port: 8080
      targetPort: 8080
  selector:
    name: flux
EOF
$ kubectl apply -f ./flux-recv-service.yaml

The selector assumes that your flux deployment (which now includes the flux-recv sidecar) has a label name: flux.

Using an Ingress

If running in the cloud, you will need to route though an Ingress or an analogue, and that will vary depending how you're already using that. However, these things will be in common:

the URLs at flux-recv are of the form /hook/<digest>, where <digest> is the SHA265 digest of a hook's shared secret, hex-encoded. flux-recv will print out these endpoints when it starts, or you can calculate them with e.g., sha256 -b ./github.key;
the backend will be the flux-recv service created previously, with the port 8080.

Using ngrok

If running locally (e.g., while developing flux-recv itself), it will be more convenient to use ngrok to tunnel requests through to your cluster.

In general, ngrok will create a new hostname each time it runs, and you will want to run it in its own deployment so it doesn't get restarted too much.

The kustomization in ./example/ngrok contains an almost complete configuration. You need to obtain an auth token by signing up for an ngrok.com account, and paste it into the field indicated in example/ngrok.yml, before running

$ kubectl apply -k ./example

Alternatively, you can not sign up to ngrok.com, and remove the volume mount and -config arg from the deployment. In this case, you will need to restart ngrok every few hours to get a fresh tunnel, and reinstall any webhooks with the new hostname for the tunnel.

The host part of your webhook URLs will be something like https://abcd1234.ngrok.io, and it will change each time ngrok starts (which is why we went to some trouble to run it in its own deployment).

The example ngrok configuration includes a Service with a NodePort, so you can access ngrok's web UI. You may wish to remove the NodePort, or the service entirely, and use kubectl port-forward instead.

Install webhook at the source

Each webhook source has its own user interface or API for installing webhooks. In general though, you will usually need

a URL for the webhook
the shared secret

You can get the webhook URL for an endpoint by combining the host -- which will depend on how you exposed flux-recv to the internet in the previous step -- and the path, which is

'/hook/' + sha256sum(secret)

You can see the path in the log output of flux-recv, or calculate the digest yourself with, e.g.,

$ sha256sum -b ./github.key

The webhook URL in total will look something like

https://abcd1234.ngrok.io/hook/7962c728be656d9580d0ce9bda78320c946d8321a4ba7f31ea15c7f2d471bd26

The path may differ if you are routing through an ingress or load balancer.

GitHub (and others) require the shared secret, which you can take directly from the file created in the first step (be careful not to introduce extra characters into the file, if you load it in an editor).

Check if it works

The easiest way to do this is to watch fluxd's logs, and push a new commit (or image) to the repo for which you installed the hook.

kubectl logs deploy/flux -f

You should see a log message indicating that fluxd has either

refreshed the git repo

ts=2019-11-20T16:29:26.871873132Z caller=loop.go:127 component=sync-loop event=refreshed url=ssh://git@github.com/squaremo/flux-example

ignored the notification because it's the wrong branch

ts=2019-11-19T15:03:06.477412849Z caller=daemon.go:540 component=daemon msg="notified about unrelated change" url=git@github.com:squaremo/flux-example.git branch=refs/tags/flux-write-check

scheduled an image repo refresh

ts=2019-11-20T16:34:30.134889169Z caller=warming.go:95 component=warmer priority=squaremo/helloworld

Some sources (e.g., GitHub, GitLab) let you test or re-rerun hooks from their webhook configuration, which can be useful for debugging. If you are using ngrok, it's also possible to re-run a hook from its dashboard.

It is safe to re-run hooks, because fluxd treats notifications as a trigger to refresh state, rather than as authoritative themselves. For example, when informed of an image push, fluxd does not add the image mentioned to its database -- it polls the image registry in question to determine whether there is a new image.

sjk07/flux-recv