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07-allow-traffic-from-some-pods-in-another-namespace.md

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ALLOW traffic from some pods in another namespace

Since Kubernetes v1.11, it is possible to combine podSelector and namespaceSelector with an AND (intersection) operation.

⚠️ This feature is available on Kubernetes v1.11 or after. Most networking plugins do not yet support this feature. Make sure to test this policy after you deploy it to make sure it is working correctly.

Example

Start a web application:

kubectl run web --image=nginx --labels="app=web" --expose --port=80

Create a other namespace and label it:

kubectl create namespace other
kubectl label namespace/other team=operations

The following manifest restricts traffic to only pods with label type=monitoring in namespaces labelled team=operations. Save it to web-allow-all-ns-monitoring.yaml and apply to the cluster:

kind: NetworkPolicy
apiVersion: networking.k8s.io/v1
metadata:
  name: web-allow-all-ns-monitoring
  namespace: default
spec:
  podSelector:
    matchLabels:
      app: web
  ingress:
    - from:
      - namespaceSelector:     # chooses all pods in namespaces labelled with team=operations
          matchLabels:
            team: operations  
        podSelector:           # chooses pods with type=monitoring
          matchLabels:
            type: monitoring
$ kubectl apply -f web-allow-all-ns-monitoring.yaml
networkpolicy.networking.k8s.io/web-allow-all-ns-monitoring created

Try it out

Query this web server from default namespace, without labelling the application type=monitoring, observe it is blocked:

$ kubectl run test-$RANDOM --rm -i -t --image=alpine -- sh
If you don't see a command prompt, try pressing enter.
/ # wget -qO- --timeout=2 http://web.default
wget: download timed out

(traffic blocked)

Query this web server from default namespace, labelling the application type=monitoring, observe it is blocked:

kubectl run test-$RANDOM --labels="type=monitoring" --rm -i -t --image=alpine -- sh
If you don't see a command prompt, try pressing enter.
/ # wget -qO- --timeout=2 http://web.default
wget: download timed out

(traffic blocked)

Query this web server from other namespace, without labelling the application type=monitoring, observe it is blocked:

$ kubectl run test-$RANDOM --namespace=other --rm -i -t --image=alpine -- sh
If you don't see a command prompt, try pressing enter.
/ # wget -qO- --timeout=2 http://web.default
wget: download timed out

(traffic blocked) 
Note: see remark below how to use OR in the network policy , so this example will work as well

Query this web server from other namespace with label team=operations, labelling the application type=monitoring, observe it is allowed:

kubectl run test-$RANDOM --namespace=other --labels="type=monitoring" --rm -i -t --image=alpine -- sh
If you don't see a command prompt, try pressing enter.
/ # wget -qO- --timeout=2 http://web.default
<!DOCTYPE html>
<html>
<head>
...
(traffic allowed)

remarks

The above mentioned scenario will only work if the namespace named other has label team=operations because the condition mentioned in our network policy is a AND condition which means both conditions must be true for the network policy to allow ingress traffic.

remarks

Please note that that example below is OR condition

 ingress:
    - from:
      - namespaceSelector:     # chooses all pods in namespaces labelled with team=opera
          matchLabels:
            team: operations
      - podSelector:           # chooses pods with type=monitoring
          matchLabels:
            type: monitoring

while this example is AND condition

 ingress:
    - from:
      - namespaceSelector:     # chooses all pods in namespaces labelled with team=opera
          matchLabels:
            team: operations
        podSelector:           # chooses pods with type=monitoring
          matchLabels:
            type: monitoring

Cleanup

kubectl delete networkpolicy web-allow-all-ns-monitoring
kubectl delete namespace other
kubectl delete pod web
kubectl delete service web