[WIP] hack: add opt-in NAT64 support to IPv6-only kind clusters

CONTRIBUTING.md

@@ -31,7 +31,8 @@ The [Quickstart (Development)](README.md#quickstart-development) in the README
 covers bringing up a local cluster with the default (gVisor) runtime. To run
 the microVM runtime locally — which needs `/dev/kvm`, or Lima nested
 virtualization on Apple Silicon — see
-[docs/dev/microvm-local.md](docs/dev/microvm-local.md).
+[docs/dev/microvm-local.md](docs/dev/microvm-local.md). To bring up an
+IPv6-only cluster, see [docs/dev/ipv6-local.md](docs/dev/ipv6-local.md).
 
 ## Contribution process
 

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docs/dev/ipv6-local.md

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+# Running an IPv6-only cluster locally
+
+## Overview
+
+```sh
+IP_FAMILY=ipv6 ./hack/create-kind-cluster.sh
+```
+
+That is the whole setup on a host with IPv6 egress of its own, which includes a
+Lima VM on macOS.
+
+A host without it — a CI runner, most corporate networks — gets a cluster that
+comes up clean and then reaches nothing; set `IPV6_DNS64_PREFIX` as well and the
+script deploys a translator, see
+[When you also need NAT64](#when-you-also-need-nat64).
+
+[Measuring whether you have IPv6 egress](#measuring-whether-you-have-ipv6-egress)
+tells you whether yours qualifies. On macOS the cluster runs inside the VM, so
+`kubectl` from the host needs one extra step:
+[Reaching the cluster from macOS](#reaching-the-cluster-from-macos).
+
+## Prerequisites
+
+**The Docker daemon needs IPv6.** The script checks this and prints the fix, so
+you can let it fail rather than checking up front. On Linux, add to
+`/etc/docker/daemon.json` and restart dockerd:
+
+```json
+{"ipv6": true, "ip6tables": true}
+```
+
+## Measuring whether you have IPv6 egress
+
+Keep DNS out of the path. `curl -6 <hostname>` conflates the two: a resolver
+that returns no AAAA fails identically to a host that cannot route, and the
+error text (`Could not resolve host`) names neither. Ping and fetch a literal:
+
+```sh
+ping6 -c2 2001:4860:4860::8888
+# Egress:    2 packets transmitted, 2 received, 0% packet loss
+# No egress: ping6: connect: Network unreachable
+
+curl -6 -sS -m 5 -o /dev/null -w '%{http_code}\n' \
+  'http://[2607:f8b0:4004:c1b::5e]/generate_204'
+# Egress:    204
+# No egress: curl: (7) Failed to connect ..., then 000
+```
+
+Both succeed and you have IPv6 egress; either fails and you do not. The wording
+of the failures varies by platform — what matters is that they come back
+immediately rather than timing out, which is what a firewalled but routable path
+looks like.
+
+Nor does the absence of a global address mean the absence of egress — a Lima
+guest holds only a ULA on `lima0` and reaches the v6 internet through vzNAT.
+
+Without egress, [NAT64](#when-you-also-need-nat64) is the way out.
+
+## When you also need NAT64
+
+`IPV6_DNS64_PREFIX` deploys the
+[kubernetes-sigs/nat64](https://github.com/kubernetes-sigs/nat64) agent and
+points cluster DNS at the prefix it translates. Two cases need it:
+
+- **A host with no IPv6 egress at all**, which is the CI runner case. Without
+  NAT64 the cluster comes up clean and then reaches nothing: every image pull
+  and outbound call from a pod times out.
+- **Reaching IPv4-only destinations.** A plain IPv6-only cluster reaches
+  anything with a AAAA record, which today is most things — but not, for
+  example, `github.com`.
+
+It is off by default because it routes *every* external name through the
+translator, including names whose AAAA records already work.
+
+### Forwarding must be on
+
+The script does *not* check this. Without it the agent deploys, the rollout
+succeeds, the restart-count check passes, and the run fails at the connectivity
+probe with nothing pointing at the cause:
+
+```sh
+sudo sysctl -w net.ipv4.ip_forward=1
+sudo sysctl -w net.ipv6.conf.all.forwarding=1
+```
+
+> [!WARNING]
+> Forwarding also stops the kernel accepting router advertisements, which can
+> drop the default route on a machine with real IPv6. Set it in the VM, not on
+> your Mac.
+
+### In CI
+
+The IPv6-only e2e job configures nothing itself — it sets two variables and
+lets this script build the cluster:
+
+```yaml
+env:
+  IP_FAMILY: ipv6
+  IPV6_DNS64_PREFIX: '64:ff9b::/96'
+```
+
+plus the two sysctls above, ordered *after* the dockerd restart that enables
+IPv6, because that restart rebuilds the chains they affect.
+
+### On Linux
+
+```sh
+sudo sysctl -w net.ipv4.ip_forward=1
+sudo sysctl -w net.ipv6.conf.all.forwarding=1
+
+IP_FAMILY=ipv6 IPV6_DNS64_PREFIX='64:ff9b::/96' ./hack/create-kind-cluster.sh
+```
+
+### On Apple Silicon
+
+The published agent images do not run on arm64: every tag ships an x86-64
+binary in the arm64 slot of its image index
+([upstream issue](https://github.com/kubernetes-sigs/nat64/issues/103)), so the
+agent exits with `exec format error` and nothing is translated. Build one and
+push it to the local registry:
+
+```sh
+git clone https://github.com/kubernetes-sigs/nat64 /tmp/nat64
+docker build --build-arg GOARCH=arm64 -t localhost:5001/nat64:local /tmp/nat64
+docker push localhost:5001/nat64:local
+```
+
+Push to the registry rather than `kind load`: the script deletes and recreates
+the cluster on every run, which drops a loaded image, while the registry
+container outlives it. The registry is created by the cluster script, so on a
+first run let it create the cluster once, then build, push, and re-run.
+
+```sh
+IP_FAMILY=ipv6 \
+IPV6_DNS64_PREFIX='64:ff9b::/96' \
+NAT64_IMAGE=localhost:5001/nat64:local \
+  ./hack/create-kind-cluster.sh
+```
+
+## Reaching the cluster from macOS
+
+Everything that needs Docker — `ko`, kind, the e2e harness — runs inside the
+VM, so that is the primary path. For `kubectl` from macOS there is one wrinkle:
+kind publishes the API server on the guest's `[::1]`, Lima forwards that to the
+*host's* `127.0.0.1`, and the apiserver certificate has no `127.0.0.1` SAN.
+Rewrite the address and verify against the `::1` SAN instead of turning
+verification off:
+
+```sh
+limactl shell docker-nested -- \
+  bash -lc 'cd <checkout> && ./hack/kind.sh get kubeconfig --name kind' \
+  | sed 's|https://\[::1\]:|https://127.0.0.1:|' > /tmp/kind-ipv6.kubeconfig
+
+KUBECONFIG=~/.kube/config:/tmp/kind-ipv6.kubeconfig \
+  kubectl config view --flatten > /tmp/merged && cp /tmp/merged ~/.kube/config
+kubectl config set-cluster kind-kind --tls-server-name='::1'
+```
+
+The forwarded port changes every time the cluster is recreated, so this has to
+be redone after each run.
+
+## Verifying
+
+With `IPV6_DNS64_PREFIX` set, the script proves both halves meet before it
+exits — it fetches `NAT64_PROBE_URL` from a pod and prints `NAT64 is
+translating.` To look at what it built:
+
+```sh
+kubectl -n kube-system get pod -l app=nat64
+kubectl -n kube-system get cm coredns -o jsonpath='{.data.Corefile}'
+```
+
+## Troubleshooting
+
+| Symptom | Cause |
+|---|---|
+| `the 'kind' Docker network has no IPv6` | Daemon IPv6 off; see [Prerequisites](#prerequisites). |
+| `a pod could not reach ... through 64:ff9b::/96` | Forwarding sysctls not set on the host. |
+| `exec format error` in the agent log | arm64 node running a published image; set `NAT64_IMAGE`. |
+| `curl -6 <host>` fails but the cluster works | A resolver returning no AAAA, not an egress fault; see [Measuring whether you have IPv6 egress](#measuring-whether-you-have-ipv6-egress). |
+| In-cluster Service names stop resolving | The Corefile's `dns64` block was merged into the cluster-zone block. It has to stay separate — `dns64` synthesizes AAAA from A, and an AAAA-only ClusterIP synthesizes to nothing. |
+| Only names *without* AAAA records get translated | `translate_all` is not in effect. The prefix belongs inside the `dns64 { }` block; on the `dns64` line it parses and the block is silently dropped. |
+| `certificate is valid for ... ::1, not 127.0.0.1` | kubectl on macOS against the Lima-forwarded port; see [Reaching the cluster from macOS](#reaching-the-cluster-from-macos). |