feat: narrow Traefik trusted range and restrict echo ingress to Traefik

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

deploy/lab/k8s/echo-network-policy.yaml

@@ -0,0 +1,62 @@
+# Restrict who may reach the echo pods.
+#
+# Traefik is configured to trust X-Forwarded-* from the whole pod CIDR, and the
+# app's Tomcat valve trusts every private range by default. Both are IP-range
+# decisions, so any pod in the cluster can forge those headers by talking to the
+# Service directly and bypassing Traefik entirely. Measured, not hypothetical:
+#
+#   kubectl -n header-lab run t --rm -i --restart=Never --image=curlimages/curl -- \
+#     curl -s http://echo:8081/api/echo -H 'X-Forwarded-Host: evil.example.com'
+#   → serverName evil.example.com, remoteAddr 1.2.3.4
+#
+# A NetworkPolicy closes that path. It selects by label rather than IP, so it
+# survives pod restarts and rescheduling — unlike the trustedIPs list, which
+# could not name Traefik because its IP changes.
+#
+# "Trusting forwarded headers" and "guaranteeing a proxy sits in front" are a
+# pair. Doing only the first leaves this hole.
+apiVersion: networking.k8s.io/v1
+kind: NetworkPolicy
+metadata:
+  name: echo-allow-traefik-only
+  namespace: header-lab
+spec:
+  podSelector:
+    matchLabels:
+      app: echo
+  policyTypes:
+    - Ingress
+  ingress:
+    # The proxy itself. namespaceSelector and podSelector in one list item are
+    # ANDed, so this is "traefik pods in kube-system" and nothing else.
+    - from:
+        - namespaceSelector:
+            matchLabels:
+              kubernetes.io/metadata.name: kube-system
+          podSelector:
+            matchLabels:
+              app.kubernetes.io/name: traefik
+      ports:
+        - protocol: TCP
+          port: 8081
+
+    # kubelet readiness/liveness probes originate from the node, not from a pod,
+    # so they need their own rule. Without it the probes fail and the pods are
+    # restarted in a loop.
+    #
+    # The probe's source address is the node's flannel bridge (cni0), which
+    # holds the first address of that node's /24:
+    #   kc-lab-1  10.42.0.1     kc-lab-2  10.42.1.1
+    # Listing them as /32 keeps this rule from re-admitting arbitrary pods,
+    # which a broader 10.42.0.0/16 block would do and would undo the policy.
+    #
+    # Adding a node means adding its gateway here. Verify with:
+    #   kubectl get nodes -o jsonpath='{range .items[*]}{.spec.podCIDR}{"\n"}{end}'
+    - from:
+        - ipBlock:
+            cidr: 10.42.0.1/32
+        - ipBlock:
+            cidr: 10.42.1.1/32
+      ports:
+        - protocol: TCP
+          port: 8081

deploy/lab/k8s/traefik-forwarded-headers.yaml

@@ -28,15 +28,16 @@ spec:
           # traffic and Traefik sees a pod-network address rather than the
           # host nginx address.
           #
-          # Trusting the whole pod CIDR means any pod in the cluster could
+          # The node/host range is deliberately absent. Because svclb SNATs,
-          # forge these headers. That is acceptable in this lab and is the
+          # the host nginx address never reaches Traefik — measured, not assumed.
-          # reason a production setup would narrow it — see the trade-off
+          # Trusting a range that cannot appear only widens the surface.
-          # section in docs/two-hop-proxy-header-contract.md.
+          #
+          # Trusting the whole pod CIDR still means any pod in the cluster could
+          # forge these headers, which is why echo-network-policy.yaml restricts
+          # who may reach the application at all.
           trustedIPs:
             - 10.42.0.0/16
-            - 192.168.122.0/24
       websecure:
         forwardedHeaders:
           trustedIPs:
             - 10.42.0.0/16
-            - 192.168.122.0/24