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egress-operator

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A Kubernetes operator to produce egress gateway Envoy pods and control access to them with network policies

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# egress-operator
An operator to produce egress gateway pods and control access to them with network policies, and a coredns plugin to route egress traffic to these pods.

The idea is that instead of authorizing egress traffic with protocol inspection,
you instead create a internal clusterIP for every external service you use, lock
it down to only a few pods via a network policy, and then set up your dns server
to resolve the external service to that clusterIP.

Built with kubebuilder: https://book.kubebuilder.io/

The operator accepts ExternalService objects, which aren't namespaced, which define a dns name and ports for an external service.
In the `egress-operator-system` namespace, it creates:
- An envoy configmap for a TCP/UDP proxy to that service (UDP not working until the next envoy release that enables it)
- A deployment for some envoy pods with that config
- A horizontal pod autoscaler to keep the deployment correctly sized
- A service for that deployment
- A network policy only allowing pods in other namespaces with the label `egress.monzo.com/allowed-<yourservice>: true`

## Pre-requisites

1. You need to have a private container repository for hosting the egress-operator image, such as an AWS Elastic Container Repository (ECR) or a GCP Container Registry (GCR), which needs to be accessible from your cluster. This will be referred to as `yourrepo` in the instructions below.
2. Your local system must have a recent version of `golang` for building the code, which you can install by following instructions [here](https://golang.org/doc/install).
3. Your local system must have Kubebuilder for code generation, which you can install by following instructions [here](https://book.kubebuilder.io/quick-start.html).
4. Your local system must have Kustomize for building the Kubernetes manifests, which you can install by following instructions [here](https://kubernetes-sigs.github.io/kustomize/installation/).
5. Your cluster must be running CoreDNS instead of kube-dns, which may not be the case if you are using a managed Kubernetes service. [This article](https://medium.com/google-cloud/using-coredns-on-gke-3973598ab561) provides some help for GCP Kubernetes Engine, and guidance for AWS Elastic Kubernetes Service can be found [here](https://docs.aws.amazon.com/eks/latest/userguide/coredns.html).

## Installing

### Testing locally against a remote cluster

```bash
make run
```
This creates an ExternalService object to see the controller-manager creating managed resources in the remote cluster.

### Setting up CoreDNS plugin

The CoreDNS plugin rewrites responses for external service hostnames managed by egress-operator.

Build a CoreDNS image which contains the plugin:
```bash
cd coredns-plugin
make docker-build docker-push IMG=yourrepo/egress-operator-coredns:latest
```

You'll need to swap out the image of your coredns kubedns Deployment for `yourrepo/egress-operator-coredns:latest`:
```bash
kubectl edit deploy coredns -n kube-system   # Your Deployment name may vary
```

And edit the coredns Corefile in ConfigMap to put in `egressoperator egress-operator-system cluster.local`:
```bash
kubectl edit configmap coredns-config -n kube-system   # Your ConfigMap name may vary
```

Example CoreDNS config:

```Caddy
.:53 {
    egressoperator egress-operator-system cluster.local
    kubernetes cluster.local
    forward . /etc/resolv.conf
}
```

### Set up the controller manager and its `CustomResourceDefinition` in the cluster

```
make docker-build docker-push install IMG=yourrepo/egress-operator:v0.1
make deploy IMG=yourrepo/egress-operator:v0.1
```

## Usage

Once the controller and dns server are running, create ExternalService objects which denote what dns name you want
to capture traffic for. Dns queries for this name will be rewritten to point to gateway pods.

By default, your client pods need a label `egress.monzo.com/allowed-gateway: nameofgateway` to be able to reach
the destination, but you can always write an additional NetworkPolicy selecting gateway pods and allowing all traffic,
for testing purposes.

An example ExternalService:

```yaml
apiVersion: egress.monzo.com/v1
kind: ExternalService
metadata:
  name: google
spec:
  dnsName: google.com
  # optional, defaults to false, instructs dns server to rewrite queries for dnsName
  hijackDns: true
  ports:
  - port: 443
    # optional, defaults to TCP
    protocol: TCP
  # optional, defaults to 3
  minReplicas: 5
  # optional, defaults to 12
  maxReplicas: 10
  # optional, defaults to 50
  targetCPUUtilizationPercentage: 30
  # optional, if not provided then defaults to 100m, 50Mi, 2, 1Gi
  resources:
    requests:
      cpu: 1
      memory: 100Mi
    limits:
      cpu: 2
      memory: 200Mi
```

### Blocking non-gateway traffic

This operator won't block any traffic for you, it simply sets up some permitted routes for traffic through the egress
gateways. You'll need a default-deny policy to block traffic that doesn't go through gateways. To do that, you probably
need a policy like this in every namespace that you want to control:

```yaml
apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: default-deny-external-egress
  namespace: your-application-namespace
spec:
  podSelector: {}
  policyTypes:
  - Egress
  egress:
  - to:
    - ipBlock:
        # ensure your internal IP range is allowed here
        # traffic to external IPs will not be allowed from this namespace.
        # therefore, pods will have to use egress gateways
        cidr: 10.0.0.0/8
```

If you already have a default deny egress policy, the above won't be needed. You'll instead want to explicitly allow
egress from your pods to all gateway pods. The ingress policies on gateway pods will ensure that only correct traffic is
allowed.

### Configuration

Global configuration of the operator is set using environment variables.

Node Selectors and Taint tolerations can be added to gateway pods to ensure pods
run on nodes that are permitted to access the internet. Example:

```yaml
env:
- name: NODE_SELECTOR_KEY
  value: role
- name: NODE_SELECTOR_VALUE
  value: egress-pods
- name: TAINT_TOLERATION_KEY
  value: egress-pods
- name: TAINT_TOLERATION_VALUE
  value: "true"
```

Results in this gateway pod configuration:

```yaml
spec:
  nodeSelector:
    role: egress-pods
  tolerations:
  - effect: NoSchedule
    key: egress-pods
    value: "true"
  ...
```

**You can also configure the [pod topology spread constraint](https://kubernetes.io/docs/concepts/scheduling-eviction/topology-spread-constraints/)**

```yaml
env:
- name: ENABLE_POD_TOPOLOGY_SPREAD
  value: "true"
- name: POD_TOPOLOGY_ZONE_MAX_SKEW
  value: 1
- name: POD_TOPOLOGY_HOSTNAME_MAX_SKEW
  value: 1
```

This will inject a topology spread constraint into the gateway pods, which will
ensure that pods are spread across zones and hosts.

```yaml
spec:
  topologySpreadConstraints:
    - labelSelector:
        matchLabels:
          app: egress-gateway
          egress.monzo.com/gateway: egress-gateway-name
      maxSkew: 1
      topologyKey: topology.kubernetes.io/zone
      whenUnsatisfiable: ScheduleAnyway
    - labelSelector:
        matchLabels:
          app: egress-gateway
          egress.monzo.com/gateway: egress-gateway-name
```

You can also setup services to have their endpoints aware of network topologies with [topology aware routing](https://kubernetes.io/docs/concepts/services-networking/topology-aware-routing/).

If you set the environment variable `ENABLE_SERVICE_TOPOLOGY_MODE="true"` this will enable setting the annotations `service.kubernetes.io/topology-mode="auto"`.

If you turn this on you can also set `spec.serviceTopologyMode` to any value if you want to disable this for particular egress'. For example
if you set `spec.serviceTopologyMode: "None"` this will disable topology aware routing for your ExternalService.

| Variable name                      | Default                                   | Description                                               |
|------------------------------------|-------------------------------------------|-----------------------------------------------------------|
| ENVOY_IMAGE                        | `envoyproxy/envoy-alpine:v1.16.5`         | Name of the Envoy Proxy image to use                      |
| TAINT_TOLERATION_KEY               | Empty, no tolerations applied             | Toleration key to apply to gateway pods                   |
| TAINT_TOLERATION_VALUE             | Empty, no tolerations applied             | Toleration value to apply to gateway pods                 |
| NODE_SELECTOR_KEY                  | Empty, no node selector added             | Node selector label key to apply to gateway pods          |
| NODE_SELECTOR_VALUE                | Empty, no node selector added             | Node selector label value to apply to gateway pods        |
| POD_TOPOLOGY_ZONE_MAX_SKEW_KEY     | `topology.kubernetes.io/zone`             | Topology key for the zone constraint                      |
| POD_TOPOLOGY_ZONE_MAX_SKEW         | Empty, won't inject a zone constraint     | Value of maxSkew for the zone constraint                  |
| POD_TOPOLOGY_HOSTNAME_MAX_SKEW_KEY | `kubernetes.io/hostname`                  | Topology key for the hostname constraint                  |
| POD_TOPOLOGY_HOSTNAME_MAX_SKEW     | Empty, won't inject a hostname constraint | Value of maxSkew for the hostname constraint              |
| ENABLE_SERVICE_TOPOLOGY_MODE       | Empty, won't add the annotation           | Set to 'true' to add the topology mode service annotation |

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