fix(vpn-ingress): stop trying to set net.ipv4.ip_forward — a hostNetwork pod cannot, and wg-quick dies on a failed PostUp

The tunnel never came up on its first reconcile with a real key. One line of my
own manifest was the whole cause. Measured on prod 2026-10-06:

    [#] ip link add dev wg0 type wireguard        <- fine
    [#] wg addconf wg0 /dev/fd/63                 <- fine
    [#] ip -4 address add 10.90.0.1/24 dev wg0    <- fine
    [#] ip link set mtu 1420 up dev wg0           <- fine
    [#] sysctl -q -w net.ipv4.ip_forward=1
    sysctl: permission denied on key "net.ipv4.ip_forward"
    [#] ip link delete dev wg0
    **** Tunnel failed, will stop all others! ****

Not a missing capability: with hostNetwork the runtime mounts /proc/sys
READ-ONLY (confirmed in the pod's /proc/mounts), because those sysctls are the
HOST's globals. CAP_NET_ADMIN cannot lift a read-only mount, so the write can
never succeed here -- and wg-quick treats a failed PostUp as fatal, so it
deleted the interface it had just built.

The line was never needed. node_baseline already sets ip_forward=1, and reading
it from the pod returns 1. My header comment said exactly that ("so no pod
sysctl is needed") and then emitted the sysctl anyway, justified as "re-asserted
so this component does not silently depend on another role's sysctl file". That
defensive re-assertion is the entire outage. In a hostNetwork pod: VERIFY host
state, never write it.

Replaced with a read in the init container -- `grep -qx 1
/proc/sys/net/ipv4/ip_forward` -- failing closed with a message naming the
node_baseline role. Still fail-closed on purpose: a tunnel that comes up unable
to forward satisfies every probe here and silently drops all client traffic.

Also documents, in the manifest and the runbook, that the image's `chown:
Operation not permitted` run and its "we will not provide support for it"
warning are expected and harmless -- capabilities are dropped to NET_ADMIN so
its s6 init has no CAP_CHOWN, /config is a writable tmpfs, everything runs as
root, and wg-quick proceeds (it did; the failure was 30 lines later).

Three other unknowns closed by the same reconcile, now recorded in the README:
the kernel module reaches the pod (ip link add type wireguard works, no
CAP_SYS_MODULE needed); allowPrivilegeEscalation:false does NOT break the s6
init; and the image's iptables is nf_tables (xtables-nft-multi) matching Debian
13, which closes the worst documented fear -- rules landing in a table the
kernel never consults.

No sensitive paths; merges on a green gate with no override.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

platform/vpn-ingress/README.md

@@ -173,6 +173,18 @@ NetworkPolicy applies.
   (`latest handshake` under ~3 min means live). Never dump `wg showconf` — it prints the private key.
 * **Full bring-up and verification:** `runbooks/vpn-remote-access.md`.
 
+## Measured on prod, 2026-10-06 — what is now known rather than assumed
+
+The first real reconcile settled four things this component's design rested on. Three went the
+right way; one was a defect in my own manifest.
+
+| question | answer |
+|---|---|
+| Does the **kernel module** reach the pod? | **Yes.** `ip link add dev wg0 type wireguard` succeeds, and `/proc/modules` lists `wireguard`, after the `node_baseline` run. `CAP_SYS_MODULE` is correctly not needed. |
+| Does `allowPrivilegeEscalation: false` break the image's **s6 init**? | **No.** The container reaches `wg-quick` normally. The flag stays. |
+| Is the image's **iptables** backend compatible with the host's? | **Yes — both nft.** The image ships `iptables v1.8.13 (nf_tables)` via `xtables-nft-multi`, and Debian 13 is nf_tables. This was the documented worst fear (rules silently landing in a table the kernel does not consult); it is closed. |
+| Can the pod set **`net.ipv4.ip_forward`**? | **No, and it must not try.** `/proc/sys` is mounted **read-only** under `hostNetwork` (`proc /proc/sys proc ro,…`), because those are the host's globals; `sysctl -w` fails with EPERM whatever the capabilities. A failed `PostUp` is fatal to `wg-quick`, so this one line took the whole tunnel down. The node already has it set to `1`; the init container now **verifies** rather than writes. |
+
 ## Prerequisites outside this directory
 
 1. **The `wireguard` kernel module on every node.** Added to `node_baseline_kernel_modules`

platform/vpn-ingress/daemonset.yaml

@@ -37,6 +37,10 @@
 #     /etc/sysctl.d/99-k3s-cilium.conf), so no pod sysctl is needed — and `net.ipv4.ip_forward`
 #     is NOT in the kubelet's safe-sysctl allowlist, so a pod-netns version of this would have
 #     required `--allowed-unsafe-sysctls` on all three kubelets (a node-restart change).
+#     COROLLARY LEARNED THE HARD WAY (prod, 2026-10-06): this pod must not merely avoid NEEDING
+#     the sysctl, it must not ATTEMPT it. hostNetwork implies /proc/sys is mounted read-only, so
+#     `sysctl -w` fails with EPERM regardless of capabilities, and wg-quick treats a failed PostUp
+#     as fatal and deletes the interface. Verify the value; never write it.
 #   * no LoadBalancer VIP, so no CiliumL2AnnouncementPolicy lease lottery (the defect that cost
 #     this LAN an 8-day Minecraft outage) and no `externalTrafficPolicy` trade-off at all.
 #   * no double encapsulation. WireGuard's UDP lands on the node's own NIC, so the underlay is a
@@ -171,6 +175,19 @@ spec:
                 exit 1
               fi
 
+              # The pod shares the HOST netns, so this reads the node's real setting. Reading
+              # /proc/sys is permitted even though writing is not (see the PostUp note below).
+              # Fail closed: a tunnel that comes up but cannot forward would look healthy to every
+              # probe here and simply drop every client's traffic.
+              if ! grep -qx 1 /proc/sys/net/ipv4/ip_forward; then
+                echo "FATAL: net.ipv4.ip_forward is 0 on this node, so forwarded VPN traffic would" >&2
+                echo "       be dropped. This pod CANNOT set it: with hostNetwork the runtime mounts" >&2
+                echo "       /proc/sys read-only, because it is the host's global setting." >&2
+                echo "       Fix on the NODE: run the node_baseline role (it writes" >&2
+                echo "       /etc/sysctl.d/99-k3s-cilium.conf) — runbooks/vpn-remote-access.md step 1." >&2
+                exit 1
+              fi
+
               {
                 echo '[Interface]'
                 echo "Address = $base.1/$mask"
@@ -184,9 +201,20 @@ spec:
                 echo 'MTU = 1420'
                 printf 'PrivateKey = %s\n' "$key"
                 # ── Forwarding + SNAT, applied in the HOST netns when wg0 comes up ──
-                # ip_forward is already 1 from node_baseline; re-asserted so this component does not
-                # silently depend on another role's sysctl file surviving.
-                echo 'PostUp = sysctl -q -w net.ipv4.ip_forward=1'
+                # NO `sysctl -w net.ipv4.ip_forward=1` HERE. It is IMPOSSIBLE from this pod and it was
+                # the only thing stopping the tunnel coming up — MEASURED on prod 2026-10-06:
+                #     [#] sysctl -q -w net.ipv4.ip_forward=1
+                #     sysctl: permission denied on key "net.ipv4.ip_forward"
+                #     [#] ip link delete dev wg0
+                # and wg-quick then tore the whole tunnel down. The cause is not a missing
+                # capability: with `hostNetwork: true` the runtime mounts /proc/sys READ-ONLY in the
+                # container (confirmed in /proc/mounts: `proc /proc/sys proc ro,...`), precisely
+                # because those sysctls are the HOST's globals and a container must not rewrite
+                # them. CAP_NET_ADMIN does not and cannot lift a read-only mount.
+                # It was never needed anyway: node_baseline writes net.ipv4.ip_forward=1 to
+                # /etc/sysctl.d/99-k3s-cilium.conf, and reading it from this pod returns 1. The
+                # init container now VERIFIES that (a read, which /proc/sys does allow) and fails
+                # closed with a pointed message, which is what the write was pretending to achieve.
                 # SNAT client traffic to the node's address. REQUIRED: without it a LAN host would
                 # reply to 10.90.0.x, find no route, hand the packet to the router, and the router
                 # would drop it — so every connection would hang rather than fail. Matching on
@@ -246,8 +274,10 @@ spec:
             # `kubectl get pod -o wide` shows what this listener owns.
             - { name: wireguard, containerPort: 51820, hostPort: 51820, protocol: UDP }
           securityContext:
-            # NET_ADMIN is the whole privilege budget: create the wg0 link, set the host sysctl, and
-            # write the two netfilter rules above. Everything else is dropped, and the container is
+            # NET_ADMIN is the whole privilege budget: create the wg0 link and write the netfilter
+            # rules above. (It does NOT let the pod set a host sysctl — /proc/sys is read-only under
+            # hostNetwork — which is why nothing here tries.) Everything else is dropped, and the
+            # container is
             # NOT privileged — which also keeps it clear of restrict-host-namespaces' second rule.
             # SYS_MODULE is deliberately NOT granted: the `wireguard` module is loaded by the node
             # (infra/ansible/roles/node_baseline, node_baseline_kernel_modules), so this container
@@ -261,6 +291,16 @@ spec:
                 - NET_ADMIN
             # readOnlyRootFilesystem intentionally UNSET: the image's s6 init writes under /run and
             # /var during startup. Same deferral as crafty/headlamp.
+            #
+            # EXPECTED, HARMLESS STARTUP NOISE — do not "fix" it. Because capabilities are dropped
+            # to NET_ADMIN only, the image's s6 init cannot chown /config (no CAP_CHOWN) and logs a
+            # run of `chown: ... Operation not permitted` plus "Permissions could not be set. This
+            # is probably because your volume mounts are remote or read-only" and a scary "we will
+            # not provide support for it". It is wrong about the cause and it does not matter:
+            # /config is a writable tmpfs emptyDir, everything here runs as root (uid 0) anyway, and
+            # wg-quick proceeds normally — verified on prod 2026-10-06, where the tunnel's only
+            # failure was the sysctl above, well past these warnings. Granting CAP_CHOWN to silence
+            # log noise would widen the privilege set of an internet-facing pod for nothing.
           resources:
             requests:
               cpu: 25m