Stop resting the iptables case on a kernel config I cannot verify

Round 3 argued the macOS rationale is stale: that Docker Desktop no longer
builds from linuxkit/linuxkit and has enabled nft_fib_ipv4 since 4.35. I could
not confirm or refute that from a Linux host — searching turned up no version
matrix either way.

But the decision does not depend on it, and the comment should not have implied
it did. `xt_CONNMARK`, which the iptables path needs, was present in every
kernel config examined. `nft_fib_ipv4`, which the nft path needs, was absent
from the config read here and may be present in current Docker Desktop. That
asymmetry is the actual argument: nftables' viability varies by Docker Desktop
version in a way nobody here can pin down, iptables' requirement did not vary
anywhere it was checked. If nft_fib_ipv4 is present this costs 1.6 MB and
nothing else; if it is absent it is the difference between a working full
tunnel and none.

Rewritten to say that, and to say plainly what is verified versus assumed —
this is the third round in which the previous round's central premise did not
survive, and a confidently-worded paragraph is what the next round inherits.

Also from review:

- CLAUDE.md still said "`iptables` is deliberately absent", the opposite of what
  this PR now does, contradicting the Dockerfile and both other docs.
- The Dockerfile referenced "the pia-vpn skill", which does not exist on this
  branch — the third forward reference of that kind, now gone.
- "full tunnels work on native Linux, Mac and WSL2 6.6" was unconditional and
  contradicted ten lines later by the `DNS =` concession, which stops them on
  every platform. Reordered so the DNS hurdle is named as the first one.
- The WSL2 gap was written as a permanent platform limitation. It is a stale
  install: `wsl --update` moves the host to a current kernel that has the
  symbol. That remedy was missing from the user-facing doc.
- The migration probe label carried an internal comma, which `joinFeatures`
  renders into a comma-joined list.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -315,7 +315,8 @@ container is created once by a very long function where a dropped capability is
 - **The tooling is baked, not installed at runtime.** `iproute2` and `wireguard-tools` are in
   `container/Dockerfile` because a runtime install lands in the writable layer and is lost on
   base-image migration — leaving a project holding the capability with nothing able to exercise it,
-  and no error that points at why. `iptables` is deliberately absent; see the Dockerfile comment.
+  and no error that points at why. `iptables` is included and `nftables` deliberately is not; see
+  the Dockerfile comment for why that way round.
 - **Anything built on this fails open.** The network namespace is rebuilt on every start and no
   service manager runs inside, so a tunnel never survives stop/start or recreation — while leftover
   `/run` state makes it look as though it did. Note the two different mechanisms: `/run` is in the

HOW-TO-USE.md

@@ -537,10 +537,11 @@ Things worth knowing:
   at `resolvconf: command not found` and deletes the interface again. This happens before any
   routing, so it takes **split tunnels down too**. Set the resolver another way instead, or drive
   `wg` and `ip route` directly rather than going through `wg-quick`.
-- **`wg-quick` full tunnels also need `xt_CONNMARK` from the host kernel.** Native Linux, Docker
-  Desktop for Mac and WSL2 kernels from 6.6 have it; older WSL2 kernels do not, and a container
-  cannot load one. There the answer is again to add the routes yourself with `ip route`, which
-  needs no firewall backend on any platform.
+- **`wg-quick` full tunnels additionally need `xt_CONNMARK` from the host kernel.** WSL2 kernels
+  before 6.6 do not have it and a container cannot load one — on Windows, `wsl --update` moves you
+  to a current kernel, which does. Failing that, add the routes yourself with `ip route`, which
+  needs no firewall backend on any platform. Note this is the *second* hurdle: clear the `DNS =`
+  one above first, or you will not reach this.
 
 > This setting can only be changed when the container is stopped. Capabilities and devices are
 > fixed when a container is created, so toggling it recreates the container on the next start.

app/src-tauri/src/docker/migration.rs

@@ -135,7 +135,7 @@ pub const FEATURE_PROBES: &[(&str, &str)] = &[
     ("/usr/local/bin/triple-c-task-runner", "Scheduled task runner"),
     ("/usr/local/bin/triple-c-sso-refresh", "AWS SSO auto-refresh"),
     ("/opt/mission-control", "Mission Control (Flight Control)"),
-    ("/usr/bin/wg", "VPN tooling (WireGuard, for the VPN Support toggle)"),
+    ("/usr/bin/wg", "VPN tooling for the VPN Support toggle (WireGuard)"),
 ];
 
 /// Headroom demanded on Docker's storage backend on top of the measured

container/Dockerfile

@@ -87,18 +87,25 @@ RUN for i in 1 2 3 4 5; do \
 #     Error: Could not process rule: No such file or directory
 #                                   ^^^^^^^^^^^^^^  needs nft_fib_ipv4
 #
-# That matters because of how the two hosts we ship to are configured. From
-# LinuxKit's kernel config — Docker Desktop for Mac, identical on both arches:
+# The choice therefore turns on which kernel symbol each path needs, and the two
+# are not equally safe to bet on. `xt_CONNMARK` (iptables) was present in every
+# kernel config examined — LinuxKit's for both arches, and WSL2's from 6.6.
+# `nft_fib_ipv4` (nftables) was absent from the LinuxKit config read here, and a
+# later review argued Docker Desktop has since enabled it and no longer builds
+# from that config at all. That may well be true; it could not be settled from a
+# Linux host, and it is the point: nftables' viability varies by Docker Desktop
+# version in a way nobody here can pin down, while iptables' requirement did not
+# vary anywhere it was checked.
 #
-#     CONFIG_NETFILTER_XT_CONNMARK=y      <- the iptables path works
-#     # CONFIG_NFT_FIB_IPV4 is not set    <- the nft path does not
+# So `iptables` is chosen for being robust to that uncertainty rather than for
+# beating nftables on any particular host. If nft_fib_ipv4 is present, wg-quick
+# never reaches the iptables path and this costs 1.6 MB and nothing else; if it
+# is absent, this is the difference between a working full tunnel and none.
 #
-# So shipping `nftables` would forfeit the platform it was meant to fix. With
-# `iptables`, full tunnels work on native Linux, on Docker Desktop for Mac, and
-# on WSL2 kernels from 6.6 (which added xt_CONNMARK as a module). Only WSL2
-# older than that is left out, and nothing installable here changes it — the way
-# out there is to add the routes with `ip route` instead of using `wg-quick`,
-# which is what the pia-vpn skill does on every platform.
+# The residual gap is WSL2 before 6.6, which has neither symbol. Nothing
+# installable in the container changes that — but `wsl --update` does, and moves
+# the host to a far newer kernel. Add the routes with `ip route` in the meantime;
+# that needs no firewall backend on any platform.
 #
 # ## What this still does not fix
 #