Skip to main content

Network Policies for MCP Connect Service

Applying Kubernetes NetworkPolicy to the codemie-mcp-connect-service namespace is recommended so that least-privilege traffic controls are enforced. When no policy is defined, pods in the namespace are allowed to communicate with any workload in the cluster and to reach any external destination.

Policy Design​

Both ingress and egress for the codemie-mcp-connect-service pod are covered by a single NetworkPolicy resource:

DirectionRulePurpose
IngressTraffic from the codemie-api pod in the codemie namespace on port 3000 is allowedAccess to MCP Connect Service is restricted to the API backend
EgressTraffic to kube-dns in kube-system on UDP/TCP 53 is allowedDNS name resolution
EgressTraffic to 0.0.0.0/0, with internal CIDRs listed under ipBlock.except, is allowedOutbound access to external MCP servers is permitted while cluster-internal ranges are excluded

Once the policy is applied, all traffic that is not explicitly permitted is denied.

Cloud-specific values

The ipBlock.except block in the egress rule must list the internal CIDRs of the specific cluster. These differ between GCP and AWS. The helper scripts below can be used to retrieve the correct values before the policy is applied.

The DNS egress rule uses k8s-app: kube-dns to select the DNS pods in kube-system. Verify the label on the cluster before applying:

kubectl get pods -n kube-system --show-labels | grep -i dns

Cloud Configuration​

To enforce traffic controls on GCP, the policy must be added manually to the extraObjects section of the codemie-mcp-connect-service Helm chart values, with the cluster-specific CIDRs listed under ipBlock.except.

Retrieving CIDRs​

Prerequisites

The script requires gcloud to be authenticated (gcloud auth login) and the principal must have the container.clusters.get and compute.subnetworks.get permissions on the project.

Script: get-gcp-network-policy-cidrs.sh
#!/usr/bin/env bash
# Usage: ./get-gcp-network-policy-cidrs.sh <cluster-name> <region> <project>

CLUSTER=$1
REGION=$2
PROJECT=$3

SUBNET=$(gcloud container clusters describe "$CLUSTER" \
  --region "$REGION" --project "$PROJECT" \
  --format='value(subnetwork)')

NODE_CIDR=$(gcloud compute networks subnets describe "$SUBNET" \
  --region "$REGION" --project "$PROJECT" \
  --format='value(ipCidrRange)')

POD_CIDR=$(gcloud container clusters describe "$CLUSTER" \
  --region "$REGION" --project "$PROJECT" \
  --format='value(clusterIpv4Cidr)')

SVC_CIDR=$(gcloud container clusters describe "$CLUSTER" \
  --region "$REGION" --project "$PROJECT" \
  --format='value(servicesIpv4Cidr)')

MASTER_CIDR=$(gcloud container clusters describe "$CLUSTER" \
  --region "$REGION" --project "$PROJECT" \
  --format='value(privateClusterConfig.masterIpv4CidrBlock)')

echo " - $NODE_CIDR     # node subnet"
echo " - $MASTER_CIDR   # master"
echo " - $POD_CIDR      # pods"
echo " - $SVC_CIDR      # services"
echo " - 169.254.169.254/32 # GCP metadata"

Example output:

 - 10.10.10.0/24      # node subnet
 - 10.0.0.0/28        # master
 - 172.20.0.0/16      # pods
 - 172.21.0.0/17      # services
 - 169.254.169.254/32 # GCP metadata

Once retrieved, each line is placed as an individual entry under the ipBlock.except list of the egress rule (see the Policy YAML below). The inline # comments are valid YAML and serve as documentation only.

Policy YAML​

Place the values from the script output under the except block in the Helm chart values:

...
extraObjects:
  - apiVersion: networking.k8s.io/v1
    kind: NetworkPolicy
    metadata:
      name: codemie-mcp-connect-service-policy
    spec:
      podSelector:
        matchLabels:
          app.kubernetes.io/instance: codemie-mcp-connect-service
          app.kubernetes.io/name: codemie-mcp-connect-service
      policyTypes:
        - Ingress
        - Egress
      ingress:
        - from:
            - namespaceSelector:
                matchLabels:
                  kubernetes.io/metadata.name: codemie
              podSelector:
                matchLabels:
                  app.kubernetes.io/instance: codemie-api
          ports:
            - protocol: TCP
              port: 3000
      egress:
        - to:
            - namespaceSelector:
                matchLabels:
                  kubernetes.io/metadata.name: kube-system
              podSelector:
                matchLabels:
                  k8s-app: kube-dns
          ports:
            - protocol: UDP
              port: 53
            - protocol: TCP
              port: 53
        - to:
            - ipBlock:
                cidr: 0.0.0.0/0
                except:
                  - 10.10.10.0/24      # node subnet
                  - 10.0.0.0/28        # master
                  - 172.20.0.0/16      # pods
                  - 172.21.0.0/17      # services
                  - 169.254.169.254/32 # GCP metadata

Once the values are updated, redeploy the chart to apply the changes.