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]>

decisions/2026-10-06-vpn-ingress-ip-forward.md

@@ -0,0 +1,113 @@
+---
+date: 2026-10-06
+branch: fix/vpn-ingress-ip-forward
+title: The VPN tunnel never came up — a hostNetwork pod cannot write net.ipv4.ip_forward, and wg-quick treats a failed PostUp as fatal
+---
+
+## What changed
+
+The remote-access VPN reconciled for the first time with a real server key (#60 then #62) and
+**the tunnel did not come up**. One line in my own manifest was the whole cause.
+
+Measured in the `wireguard` container on prod, 2026-10-06:
+
+```
+[#] ip link add dev wg0 type wireguard
+[#] wg addconf wg0 /dev/fd/63
+[#] ip -4 address add 10.90.0.1/24 dev wg0
+[#] ip link set mtu 1420 up dev wg0
+[#] sysctl -q -w net.ipv4.ip_forward=1
+sysctl: permission denied on key "net.ipv4.ip_forward"
+[#] ip link delete dev wg0
+**** Tunnel /config/wg_confs/wg0.conf failed, will stop all others! ****
+```
+
+Everything up to that point worked. **The cause is not a missing capability.** With
+`hostNetwork: true` the runtime mounts `/proc/sys` **read-only** in the container — confirmed in the
+pod's own `/proc/mounts`: `proc /proc/sys proc ro,nosuid,nodev,noexec,relatime` — precisely because
+those sysctls are the *host's* globals and a container must not rewrite them. `CAP_NET_ADMIN` cannot
+lift a read-only mount, so this can never succeed from here. And `wg-quick` treats any failed
+`PostUp` as fatal, so it deleted the interface it had just finished building.
+
+**The line was never needed.** `node_baseline` writes `net.ipv4.ip_forward=1` to
+`/etc/sysctl.d/99-k3s-cilium.conf`, and reading the value from inside the pod returns `1`. I had
+written the header comment *correctly* — "ip_forward is ALREADY 1 on these nodes … so no pod sysctl
+is needed" — and then emitted the sysctl anyway, justified in its own comment as "re-asserted so
+this component does not silently depend on another role's sysctl file surviving". That defensive
+re-assertion is the entire outage. **The lesson is narrow and reusable: in a hostNetwork pod,
+verify host state, never write it.**
+
+Fix:
+
+- **`platform/vpn-ingress/daemonset.yaml`**: the `PostUp = sysctl …` line is gone, replaced by a
+  **read** in the init container — `grep -qx 1 /proc/sys/net/ipv4/ip_forward` — which fails closed
+  with a message naming the `node_baseline` role. Reading `/proc/sys` is permitted; only writing is
+  not. Fail-closed still matters: a tunnel that comes up but cannot forward would satisfy every
+  probe in this component and silently drop every client's traffic.
+- The header bullet that said "no pod sysctl is needed" now also says **do not attempt one**, with
+  the mechanism, so the next author does not re-add it for the same reason I did.
+- The `securityContext` comment claimed NET_ADMIN was for "create the wg0 link, set the host sysctl,
+  and write the netfilter rules". The middle clause was false; corrected.
+- The image's `chown: … Operation not permitted` startup run — plus its own "Permissions could not
+  be set … we will not provide support for it" — is now documented as **expected and harmless**, in
+  both the manifest and the runbook, so nobody chases it. Capabilities are dropped to `NET_ADMIN`
+  only, so its s6 init has no `CAP_CHOWN`; `/config` is a writable tmpfs, everything runs as root,
+  and `wg-quick` proceeds past it (as it did here — the failure was 30 lines later).
+- **`runbooks/vpn-remote-access.md`**: a "what a healthy start looks like" block with the real log
+  lines; `ip_forward is 0` added to the init-failure table; and the note that `0/1 Running` with
+  `Startup probe failed: Unable to access interface: No such device` is the signature of `wg-quick`
+  tearing the tunnel down — read the `[#]` line it died on.
+- **`platform/vpn-ingress/README.md`**: a table of what the first reconcile settled (below).
+
+No sensitive paths: `platform/vpn-ingress/**`, `runbooks/**` and `decisions/**` only. This merges on
+a green gate with no override.
+
+## What I verified
+
+Live prod, read-only, 2026-10-06. **Four questions this component's design rested on are now
+measured rather than assumed — three went the right way, one was the defect above.**
+
+- **The GitOps path is proven end to end.** `Kustomization vpn-ingress` `lastAppliedRevision:
+  main@sha1:cf7d5e4` equals the GitRepository artifact revision. The Secret carries
+  `kustomize.toolkit.fluxcd.io/name: vpn-ingress`, so Flux delivered it.
+- **SOPS decryption works, proven by digest rather than by eye.** The value in
+  `platform/vpn-ingress/wg-server-key.sops.yaml` on `main`, decrypted, is 44 bytes, is **not** the
+  placeholder, and **`wg pubkey` accepts it**. Its SHA-256 equals the SHA-256 of the live Secret's
+  decoded `data.privatekey` (`d7c8f8de6f252649…` both sides). No key material was printed at any
+  point — lengths, booleans and digests only.
+- **The `wg pubkey` guard fired correctly, and its `ENC[` branch was never needed.** During the
+  kubelet window before the updated Secret propagated, all three pods reported *"It is still the
+  committed REPLACE-ME PLACEHOLDER"* — the guard distinguishing a stale/unfilled value from
+  ciphertext, exactly as designed. It cleared on its own once the mount refreshed; **a transient
+  `REPLACE-ME` CrashLoop for a minute or two after the key PR merges is expected, not a fault.**
+- **The kernel module reaches the pod**: `ip link add dev wg0 type wireguard` succeeded and
+  `/proc/modules` lists `wireguard`. So the `node_baseline` addition works and `CAP_SYS_MODULE` is
+  correctly not required.
+- **`allowPrivilegeEscalation: false` does NOT break the image's s6 init** — the container reaches
+  `wg-quick` normally. The flag stays; the fear recorded in #55 is closed.
+- **The iptables backends MATCH, which was my single biggest documented doubt.** The image ships
+  `iptables v1.8.13 (nf_tables)` via `/usr/sbin/xtables-nft-multi`, and Debian 13 is nf_tables. The
+  nightmare case — rules accepted into a table the kernel never consults, so handshakes succeed and
+  every forwarded connection hangs — cannot occur on this pairing.
+- **`net.ipv4.ip_forward` is `1`** in the host netns the pod shares, and `/proc/sys` is `ro` there.
+  Both read directly from the pod.
+- **Toolbox gate** from an isolated worktree: **rc 3**, with `spec-immutability`, `decisions-log`,
+  `lint`, `l0`, `render-golden`, `l1` and `approval-policy` all **pass**; rc 3 is the two
+  environmental skips (`gitea-settings`, conformance L2/L3). **Not green. Reported as rc 3.**
+
+## What I did NOT verify
+
+- **That the tunnel now comes up.** This fix has not reconciled. The next reconcile is the test, and
+  its success criterion is specific: pods `1/1 Running`, and `wg show` reporting `interface: wg0`
+  with `listening port: 51820`.
+- **That the MASQUERADE and FORWARD rules actually apply.** They are the next statements after the
+  one that failed, so **they have never executed**. The backends matching (above) removes the worst
+  failure mode but proves nothing about the rules themselves. First check after the pods go Ready:
+  `iptables -t nat -S POSTROUTING | grep 10.90.0.0/24` on a node.
+- **Anything about client traffic.** Still zero peers enrolled, still no WAN port-forward, still no
+  off-LAN vantage point. The VPN has carried no packet.
+- **Whether the transient `REPLACE-ME` window has an upper bound.** It cleared within ~2 minutes
+  here; I did not determine the kubelet refresh behaviour, so a longer stall after a key change is
+  uncharacterised.
+- **The `sops set` hand-off command**, still. The operator filled the key in by their own route
+  (#62), so the exact quoting I documented inside `dev.sh run bash -c` remains unexercised.