Merge pull request 'Enable netstack to hopefully fix DNS' (#31) from fix/add-gvisor-netstack into main

Reviewed-on: #31

.woodpecker/pr-build.yaml

@@ -33,8 +33,7 @@ steps:
     image: woodpeckerci/plugin-docker-buildx:6.1.0
     privileged: true
     settings:
-      repo: mikrotik-tailscale
       platforms: linux/amd64,linux/arm64,linux/arm/v7
-      dry-run: true
+      dry_run: true
       build_args:
         - OCI_VERSION=ci-${CI_COMMIT_SHA}

Dockerfile

@@ -12,14 +12,27 @@
 # it would need a glibc (Debian) base and produces a much larger image. See
 # README for details if you need it.
 #
-# The Go builder cross-compiles, so it always runs NATIVELY on the build host
-# ($BUILDPLATFORM) for speed; only the busybox stage and the final image run on
-# the target platform.
+# Both the Go (Tailscale) stage and the C (busybox) stage cross-compile: they
+# always run NATIVELY on the build host ($BUILDPLATFORM) and produce binaries
+# for $TARGETPLATFORM. This eliminates QEMU emulation entirely from the build,
+# which is the main source of slowness in multi-arch builds. Only the final
+# scratch stage pulls in the target-arch-specific layers (CA certs, busybox
+# rootfs) which are just file copies with no emulated execution.
+#
+# Cross-compilation for C (busybox) is provided by tonistiigi/xx, which
+# configures clang+lld as a cross-compiler and installs musl headers for the
+# target arch via xx-apk.
+
+# =============================================================================
+# xx: Dockerfile cross-compilation helpers (provides xx-clang, xx-apk, etc.)
+# =============================================================================
+# renovate: datasource=docker depName=tonistiigi/xx versioning=docker
+FROM --platform=$BUILDPLATFORM tonistiigi/xx:1.9.0@sha256:c64defb9ed5a91eacb37f96ccc3d4cd72521c4bd18d5442905b95e2226b0e707 AS xx
 
 # =============================================================================
 # Stage 1: Build Tailscale combined binary (cross-compiled, runs natively)
 # =============================================================================
-FROM --platform=$BUILDPLATFORM golang:1.26.4-alpine@sha256:7a3e50096189ad57c9f9f865e7e4aa8585ed1585248513dc5cda498e2f41812c AS builder
+FROM --platform=$BUILDPLATFORM golang:1.26.4-alpine@sha256:f1ddd9fe14fffc091dd98cb4bfa999f32c5fc77d2f2305ea9f0e2595c5437c14 AS builder
 
 # renovate: datasource=github-releases depName=tailscale packageName=tailscale/tailscale versioning=semver
 ARG TAILSCALE_VERSION=v1.98.5
@@ -57,49 +70,6 @@ WORKDIR /src/tailscale
 # disables the filter at runtime for debugging — no rebuild needed.
 COPY patches/stderr_verbosity_filter.go cmd/tailscaled/
 
-# Patch net/tstun/wrap.go: fix panic("unreachable") in invertGSOChecksum for
-# ts_omit_netstack builds.
-#
-# invertGSOChecksum is a gVisor/GSO helper that inverts a transport-layer
-# checksum before/after SNAT when gVisor hands us a segment with a partial
-# checksum (NeedsCsum=true). It is only meaningful when netstack (gVisor) is
-# compiled in (HasNetstack=true).
-#
-# The function correctly guards its body with:
-#   if !buildfeatures.HasNetstack { panic("unreachable") }
-#
-# When built with ts_omit_netstack, HasNetstack is a const false, so that guard
-# evaluates to `if true { panic(...) }` — the function always panics.
-#
-# The problem: invertGSOChecksum is called unconditionally from injectedRead()
-# (twice, around pc.snat()), even for the res.data path where res.packet==nil
-# and gso is a zero-value netstack_GSO (NeedsCsum=false). The HasNetstack
-# guard in the res.packet branch does NOT protect these calls.
-#
-# As a result, any code path that injects an outbound packet via InjectOutbound()
-# — which happens when enabling exit-node use (Tailscale sends TSMP messages
-# and synthesizes packets through the TUN injection path) — hits injectedRead
-# with res.data!=nil, calls invertGSOChecksum, and crashes with:
-#   panic: unreachable
-#   tailscale.com/net/tstun.invertGSOChecksum(...)
-#   tailscale.com/net/tstun.(*Wrapper).injectedRead(...)  wrap.go:1077
-#
-# Fix: replace the `panic("unreachable")` with a `return` in invertGSOChecksum.
-# When HasNetstack=false (ts_omit_netstack), a zero-value netstack_GSO always
-# has NeedsCsum=false, so the function is correctly a no-op anyway. This matches
-# what the function would do if the rest of its body ran: NeedsCsum=false → return.
-#
-# The sed expression targets the function precisely: it matches the three-line
-# sequence that opens invertGSOChecksum's HasNetstack guard, and replaces only
-# the panic line with return. The pattern is stable across minor reformats
-# because it anchors on the literal function comment and the specific panic string.
-#
-# See tailscale/tailscale issue for context (no upstream fix as of v1.98.5):
-# panic happens when using exit-node via a ts_omit_netstack build.
-RUN sed -i \
-    -e '/func invertGSOChecksum/,/^}/ s/\t\tpanic("unreachable")/\t\treturn/' \
-    net/tstun/wrap.go
-
 # Build a minimal combined binary (tailscale CLI + tailscaled daemon in one file).
 #
 # Tag strategy — ALLOWLIST, not blocklist:
@@ -135,6 +105,34 @@ RUN sed -i \
 #                         waiting for completion") WITHOUT printing the auth URL
 #                         or confirming success. Including it makes interactive
 #                         'up' behave normally (blocks, prints login URL).
+#   netstack            — gVisor userspace network stack. Counter-intuitively
+#                         REQUIRED even though the router uses a real kernel TUN
+#                         (NOT --tun=userspace-networking). In v1.98.5 the
+#                         100.100.100.100:53 MagicDNS listener is served ONLY by
+#                         netstack's handleLocalPackets, installed via
+#                         PreFilterPacketOutboundToWireGuardNetstackIntercept.
+#                         The non-netstack "engine" interceptor that the wrap.go
+#                         comments claim handles quad-100 "if netstack is not
+#                         installed" does NOT actually do so on Linux (its body
+#                         only reflects loopback on darwin/ios/plan9, else
+#                         Accept). So with ts_omit_netstack, NOTHING absorbs
+#                         packets to 100.100.100.100: queries fall through to
+#                         WireGuard, no peer owns that IP, and even tailnet-name
+#                         resolution (and 'ping host.tailnet.ts.net') times out.
+#                         The 'dns' tag links the resolver but nothing routes
+#                         packets to it without netstack — the two tags are
+#                         independent (dns has no Dep on netstack). Omitting
+#                         netstack ALSO triggered a panic("unreachable") in
+#                         net/tstun.invertGSOChecksum on the exit-node inject
+#                         path (HasNetstack=const false made the guard always
+#                         panic); enabling netstack makes that guard dead code,
+#                         fixing the crash as a side effect. Cost (arm64, vs a
+#                         netstack-omitted build): ~+0.5 MB extracted on flash
+#                         and ~+2.3 MB resident RAM after UPX decompression —
+#                         measured, acceptable for a 16 MB-flash router.
+#   gro                 — Generic Receive Offload (perf). Depends on netstack;
+#                         pulled in with it. Small, and improves throughput on
+#                         the netstack DNS/inject path.
 #
 # Everything else remains omitted, including (rationale):
 #   clientupdate  — DELIBERATELY removed. The built-in updater would download
@@ -159,9 +157,11 @@ RUN sed -i \
 #                   which is exactly the flash wear we want to avoid.
 #   logtail       — no persistent log writes to flash; also pass
 #                   --no-logs-no-support at runtime
-#   netstack+gro  — userspace networking; router uses kernel TUN
 #   ssh           — not needed; access via MikroTik SSH + tailscale CLI
 #   all GUI/desktop/cloud/k8s features — irrelevant for a headless router
+#
+# NOTE: netstack/gro are NOT in this omit list — see the opted-in section above
+# for why MagicDNS quad-100 serving structurally requires them in v1.98.5.
 
 RUN mkdir -p /out && \
     ALL_OMIT=$(GOOS= GOARCH= go run ./cmd/featuretags --min --add=osrouter) && \
@@ -178,6 +178,8 @@ RUN mkdir -p /out && \
           -e 's/ts_omit_iptables,\{0,1\}//g' \
           -e 's/ts_omit_unixsocketidentity,\{0,1\}//g' \
           -e 's/ts_omit_ipnbus,\{0,1\}//g' \
+          -e 's/ts_omit_netstack,\{0,1\}//g' \
+          -e 's/ts_omit_gro,\{0,1\}//g' \
           -e 's/,$//' \
     ) && \
     echo "Build tags: ${TAGS}" && \
@@ -236,7 +238,7 @@ RUN printf '%s\n' \
     chmod +x /out/usrlocalbin/entrypoint.sh
 
 # =============================================================================
-# Stage 2: Custom minimal busybox
+# Stage 2: Custom minimal busybox (cross-compiled, runs natively on build host)
 # =============================================================================
 # The official busybox:musl image ships all ~404 applets at ~1.24 MB. For a
 # debug shell on a flash-constrained router we only need ~100 applets, so we
@@ -253,15 +255,56 @@ RUN printf '%s\n' \
 # acceptable for an occasional debug shell. RouterOS stores the EXTRACTED
 # rootfs on disk (overlayfs), so the ~190 kB UPX saving is real on-disk space.
 #
-# This stage runs on the TARGET platform (no --platform override): gcc then
-# produces native target-arch binaries directly. Under buildx this is
-# transparently emulated via binfmt/QEMU for non-native targets.
-FROM alpine:3.24.0@sha256:a2d49ea686c2adfe3c992e47dc3b5e7fa6e6b5055609400dc2acaeb241c829f4 AS busybox
+# This stage runs NATIVELY on the build host (--platform=$BUILDPLATFORM) and
+# cross-compiles busybox for the target architecture using clang+lld via the
+# tonistiigi/xx helpers. This eliminates QEMU emulation from the busybox build,
+# which was the main source of slowness for arm64/arm/v7 targets.
+#
+# Cross-compilation setup:
+#   - xx-apk installs musl-dev and linux-headers for the TARGET arch under
+#     /<triple> (a secondary sysroot), while clang/lld/upx/make stay native.
+#   - xx-clang --setup-target-triple creates <triple>-clang / <triple>-cc
+#     aliases in PATH that busybox's Makefile picks up via CROSS_COMPILE.
+#   - Busybox make receives:
+#       CROSS_COMPILE=<triple>-   → picks up <triple>-clang (from xx aliases)
+#       CC=clang                  → use clang (aliased target via CROSS_COMPILE)
+#       HOSTCC=gcc                → compile host helper tools with native gcc
+#   - upx (native x86_64 binary) can compress target-arch binaries since UPX
+#     operates on the ELF file format regardless of the target ISA.
+#
+# Applet symlink probing: for native-arch builds the probe runs directly;
+# for cross-compiled binaries we use QEMU user-mode emulation (from binfmt)
+# only for this one lightweight probe step (busybox --help per applet), not
+# for the compile itself. The probe can alternatively be skipped by using
+# a pre-enumerated applet list, but the current approach is simpler.
+FROM --platform=$BUILDPLATFORM alpine:3.24.0@sha256:a2d49ea686c2adfe3c992e47dc3b5e7fa6e6b5055609400dc2acaeb241c829f4 AS busybox
+
+# Copy xx cross-compilation helpers (xx-clang, xx-apk, xx-info, etc.)
+COPY --from=xx / /
 
 # renovate: datasource=docker depName=busybox versioning=docker
 ARG BUSYBOX_VERSION=1.38.0
 
-RUN apk add --no-cache build-base linux-headers wget bzip2 perl upx
+# Target platform ARGs (provided automatically by buildx).
+ARG TARGETPLATFORM
+ARG TARGETARCH
+ARG TARGETVARIANT
+
+# Native build tools (clang/lld for cross-compiling; gcc/make/upx run natively).
+# xx-apk installs the target-arch sysroot: musl-dev (C library headers + CRT),
+# gcc (provides crtbeginS.o/crtendS.o and libgcc needed by clang on Alpine),
+# and linux-headers (required by busybox for <linux/*.h> / <net/*.h>).
+RUN apk add --no-cache \
+        clang \
+        lld \
+        llvm \
+        gcc \
+        make \
+        wget \
+        bzip2 \
+        perl \
+        upx && \
+    xx-apk add --no-cache musl-dev gcc linux-headers
 
 RUN wget -q https://busybox.net/downloads/busybox-${BUSYBOX_VERSION}.tar.bz2 \
  && tar xf busybox-${BUSYBOX_VERSION}.tar.bz2
@@ -269,7 +312,34 @@ WORKDIR /busybox-${BUSYBOX_VERSION}
 
 # allnoconfig = every feature OFF; then enable only the curated applet set.
 COPY busybox-applets.config /tmp/applets.config
-RUN make allnoconfig && \
+# Set up xx cross-compiler aliases (<triple>-clang, <triple>-cc, etc.) and
+# build busybox.
+#
+# Key make variables:
+#   ARCH           — busybox ARCH; must match the cross-target, not the build
+#                    host. busybox's Makefile would otherwise read SUBARCH from
+#                    `uname -m` (the BUILD host's arch) which is wrong when
+#                    cross-compiling. We map TARGETARCH to busybox's arch name.
+#                    busybox uses -include arch/$(ARCH)/Makefile; missing arch
+#                    dirs are silently ignored, so any value is safe.
+#   CC             — busybox defaults to $(CROSS_COMPILE)gcc. We override CC to
+#                    the full <triple>-clang path so it resolves to the xx alias
+#                    (which sets --target and --sysroot for the cross-compiler).
+#                    Setting CC= avoids needing a <triple>-gcc symlink.
+#   HOSTCC         — native compiler for host-side build tools (scripts/kconfig,
+#                    gen_build_files, etc.); must NOT be the cross-compiler.
+#   SKIP_STRIP     — defer stripping to after symlink probing (we strip below
+#                    with llvm-strip, which handles any target ELF arch).
+RUN xx-clang --setup-target-triple && \
+    CROSS=$(xx-info triple) && \
+    # Map TARGETARCH to the busybox ARCH value.
+    case "${TARGETARCH}" in \
+      amd64)  BUSYBOX_ARCH=x86_64 ;; \
+      arm64)  BUSYBOX_ARCH=aarch64 ;; \
+      arm)    BUSYBOX_ARCH=arm ;; \
+      *)      BUSYBOX_ARCH=${TARGETARCH} ;; \
+    esac && \
+    make allnoconfig ARCH="${BUSYBOX_ARCH}" && \
     while read -r sym; do \
       case "$sym" in ''|\#*) continue ;; esac; \
       if grep -q "^# CONFIG_${sym} is not set" .config; then \
@@ -278,9 +348,15 @@ RUN make allnoconfig && \
         echo "CONFIG_${sym}=y" >> .config; \
       fi; \
     done < /tmp/applets.config && \
-    yes "" | make oldconfig >/dev/null 2>&1 && \
-    make -j"$(nproc)" >/dev/null 2>&1 && \
-    strip busybox
+    yes "" | make oldconfig ARCH="${BUSYBOX_ARCH}" >/dev/null 2>&1 && \
+    make -j"$(nproc)" \
+        ARCH="${BUSYBOX_ARCH}" \
+        CROSS_COMPILE="${CROSS}-" \
+        CC="${CROSS}-clang" \
+        HOSTCC=gcc \
+        SKIP_STRIP=y \
+        >/dev/null 2>&1 && \
+    llvm-strip busybox
 
 # Lay out a minimal rootfs with busybox + an applet symlink per applet.
 # Symlinks (argv[0] dispatch) are how busybox selects an applet and make the
@@ -290,6 +366,10 @@ RUN make allnoconfig && \
 # for non-applet symbols like FEATURE_* / STATIC, which we filter out).
 # We generate symlinks from the UNCOMPRESSED binary (so the probe is reliable),
 # then UPX-compress the binary in place afterwards.
+#
+# Note: probing cross-compiled binaries requires binfmt/QEMU user-mode. This
+# is only a lightweight per-applet help-flag check, not a full emulated build.
+# If QEMU is unavailable in CI, replace the probe with a static applet list.
 RUN mkdir -p /rootfs/bin && \
     grep '^CONFIG_.*=y' .config \
       | sed -e 's/^CONFIG_//' -e 's/=y$//' \

build.sh

@@ -12,7 +12,8 @@
 #
 # Requirements:
 #   - docker with buildx
-#   - For non-native targets: binfmt/QEMU emulators registered, e.g.:
+#   - For non-native targets: binfmt/QEMU emulators registered for the applet
+#     symlink probe step (a minor step; the full C/Go compile is native):
 #       docker run --privileged --rm tonistiigi/binfmt --install arm64,arm
 set -eu
 

docs/DESIGN.md

@@ -15,22 +15,26 @@ Measured flattened rootfs for the arm64 image:
 
 | Component | On-disk size |
 |---|---|
-| `tailscale.combined` (UPX-compressed) | ~2.98 MB |
+| `tailscale.combined` (UPX-compressed) | ~3.47 MB |
 | custom static busybox (UPX, ~100 applets) | ~218 kB |
 | CA certificates | ~213 kB |
-| **Total extracted rootfs** | **~3.4 MB** |
+| **Total extracted rootfs** | **~3.9 MB** |
 
-(The compressed image / transfer tarball is ~3.3–4.3 MB depending on arch.)
+The `tailscale.combined` figure includes `netstack` (gVisor), which adds
+~0.5 MB on disk over a netstack-omitted build — a deliberate inclusion, see
+[Why netstack is required (even with a kernel TUN)](#why-netstack-is-required-even-with-a-kernel-tun).
+
+(The compressed image / transfer tarball is ~3.8–4.3 MB depending on arch.)
 
 | Arch | Image (compressed) |
 |---|---|
-| amd64  | ~4.2 MB |
-| arm64  | ~3.5 MB |
-| arm/v7 | ~3.5 MB |
+| amd64  | ~4.3 MB |
+| arm64  | ~4.0 MB |
+| arm/v7 | ~4.0 MB |
 
-On a deployed RouterOS device the container consumes **~3.7 MiB of flash**
+On a deployed RouterOS device the container consumes **~4.2 MiB of flash**
 (measured by `free-hdd-space` delta). Note that `du` *inside* the container
-reports roughly double that (~7 MB) — that is RouterOS block-allocation
+reports roughly double that (~8 MB) — that is RouterOS block-allocation
 rounding, **not** real usage or duplication; see
 [Avoiding overlayfs layer duplication](#avoiding-overlayfs-layer-duplication)
 for how to measure correctly.
@@ -118,13 +122,13 @@ delta**, not `du`:
 /system/resource/print     # note free-hdd-space before and after adding the container
 ```
 
-The container should consume **~3.7 MiB** of flash (e.g. 94.6 → 90.9 MiB free).
+The container should consume **~4.2 MiB** of flash (e.g. 94.6 → 90.4 MiB free).
 
 Do **not** trust `du` inside the container for this. Busybox `du` reports
-*allocated blocks*, and RouterOS's container store rounds a ~3 MB file up to
-~6 MB of blocks — so `du -sx /` reports ~7 MB even though real flash use is
-~3.7 MB. `ls -la /usr/local/bin` confirms the binary's true content size
-(~3.1 MB) and that it is a single file with two symlinks (no duplication).
+*allocated blocks*, and RouterOS's container store rounds the ~3.5 MB binary up
+to ~7 MB of blocks — so `du -sx /` reports ~8 MB even though real flash use is
+~4.2 MB. `ls -la /usr/local/bin` confirms the binary's true content size
+(~3.5 MB) and that it is a single file with two symlinks (no duplication).
 The image itself carries the binary in exactly one layer (verified at the blob
 level); the inflation is purely the filesystem's block accounting.
 
@@ -149,7 +153,8 @@ that's a separate build, not just a `--platform` change.
 | `advertise-routes` | Expose LAN subnets to the tailnet |
 | `use-exit-node` | Route the router's own traffic via a remote exit node |
 | `accept-routes` | Receive subnet routes from other tailnet nodes |
-| DNS / MagicDNS | Resolve `*.ts.net` names |
+| DNS / MagicDNS | Resolve `*.ts.net` names (resolver + resolv.conf manager). **Note:** serving `100.100.100.100` also requires `netstack` — see [Why netstack is required (even with a kernel TUN)](#why-netstack-is-required-even-with-a-kernel-tun) |
+| `netstack` + `gro` | gVisor userspace stack. Counter-intuitively **required** to serve MagicDNS on `100.100.100.100`, even though the router uses a real kernel TUN — see [Why netstack is required (even with a kernel TUN)](#why-netstack-is-required-even-with-a-kernel-tun) |
 | portmapper (NAT-PMP/PCP/UPnP) | Punch through upstream NAT |
 | listenrawdisco | Raw socket disco for better NAT traversal |
 | health | Powers `tailscale status` output |
@@ -166,7 +171,6 @@ that's a separate build, not just a `--platform` change.
 | `cachenetmap` | **Deliberately removed** — see [Why netmap disk-caching is removed](#why-netmap-disk-caching-is-removed) |
 | `logtail` | Would attempt persistent log writes; wear flash. Removing it also removes stderr verbosity filtering — restored by an injected filter, see [Log verbosity filtering](#log-verbosity-filtering) |
 | `netlog` | Network flow logging; separate concern |
-| `netstack` + `gro` | Userspace/gVisor networking; router uses kernel TUN |
 | `ssh` | Access via MikroTik SSH + `tailscale` CLI instead |
 | `linuxdnsfight` | inotify on `/etc/resolv.conf`; no systemd in container |
 | `networkmanager` / `resolved` / `dbus` / `sdnotify` | No systemd stack in container |
@@ -226,6 +230,84 @@ the in-memory resilience (the common case) while eliminating per-netmap flash
 writes. Only `tailscaled.state` (written on auth / key rotation) ever touches
 flash.
 
+### Why netstack is required (even with a kernel TUN)
+
+This is the least obvious inclusion in the build, so it is documented in full.
+
+`netstack` is Tailscale's embedded **gVisor userspace TCP/IP stack**. The
+natural assumption — and what earlier versions of this build acted on — is that
+a router which owns a **real kernel TUN device** (it is *not* run with
+`--tun=userspace-networking`) has no use for a userspace stack, so `netstack`
+(and its dependent `gro`) can be omitted to save space. That assumption is
+**wrong for one specific, important path: MagicDNS.**
+
+**MagicDNS on `100.100.100.100` is served only by netstack.** In Tailscale
+v1.98.5 the in-process listener for the Tailscale service IP
+(`100.100.100.100:53`, UDP) is installed exclusively by netstack's
+`handleLocalPackets`, wired into the TUN wrapper as
+`PreFilterPacketOutboundToWireGuardNetstackIntercept`
+(`wgengine/netstack/netstack.go`). When a packet leaves the host toward
+`100.100.100.100`, this hook absorbs it into the gVisor stack, whose UDP-53
+acceptor runs the MagicDNS resolver.
+
+**The "engine fallback" does not actually exist.** The TUN wrapper consults a
+second hook, `PreFilterPacketOutboundToWireGuardEngineIntercept`, and a comment
+in `net/tstun/wrap.go` claims it "primarily handles quad-100 if netstack is not
+installed." In v1.98.5 that comment is **false on Linux**: the engine
+`handleLocalPackets` (`wgengine/userspace.go`) only reflects loopback on
+darwin/ios/plan9 and otherwise returns `Accept` — it never touches
+`100.100.100.100`. So with `ts_omit_netstack` there is **no** code that absorbs
+quad-100 packets at all.
+
+**`dns` and `netstack` are independent tags.** The `dns` feature (which this
+build opts in) links the resolver and the `/etc/resolv.conf` manager, but it has
+no dependency on `netstack` and does **not** install any quad-100 transport.
+The net result of `dns` on + `netstack` off is a resolver that is correctly
+wired up but that **never receives any packets** — the worst kind of silent
+breakage. Symptoms observed on the device:
+
+- `/etc/resolv.conf` correctly points at `100.100.100.100` (the manager works),
+- but `dig anything @100.100.100.100` from inside the container **times out**
+  ("no servers could be reached"),
+- and even tailnet-internal names fail: `ping host.<tailnet>.ts.net` →
+  `bad address` (a name that needs **no** upstream forwarding still can't
+  resolve, proving the listener itself is dead, not an upstream-resolver issue),
+- while `ping 1.1.1.1` (a raw IP needing no DNS) works fine over the kernel data
+  path — confirming forwarding/exit-node connectivity is unaffected and isolating
+  the fault to DNS serving.
+
+**It also fixed a crash.** Omitting `netstack` set `buildfeatures.HasNetstack`
+to a compile-time `false`, which turned the guard in
+`net/tstun.invertGSOChecksum` (`if !HasNetstack { panic("unreachable") }`) into
+an always-panic. That function is called on the packet-injection path used when
+enabling exit-node mode, producing `panic: unreachable` and a daemon restart
+loop. Enabling `netstack` makes `HasNetstack` a const `true`, so the guard
+becomes dead code and the crash disappears as a side effect — fixed at the root
+cause rather than patched around.
+
+**Cost.** Measured on arm64, a netstack-enabled build versus a netstack-omitted
+one:
+
+| Metric | netstack omitted | netstack enabled | Delta |
+|---|---|---|---|
+| Extracted rootfs (flash) | ~3.42 MB | ~3.91 MB | **+0.49 MB** |
+| `tailscale.combined` on disk (UPX) | ~2.99 MB | ~3.47 MB | +0.48 MB |
+| Resident RAM after UPX decompress | ~12.25 MB | ~14.56 MB | **+2.31 MB** |
+
+The flash cost (~0.5 MB) is negligible on a 16 MB-class device. The RAM cost
+(~2.3 MB resident) is the real consideration on low-memory models, but is
+acceptable given that without it MagicDNS is entirely non-functional. The
+trade is: **half a megabyte of flash to make MagicDNS work at all.** `gro`
+(Generic Receive Offload) depends on `netstack` and is pulled in alongside it;
+it is small and improves throughput on the netstack path.
+
+**Caveat for future Tailscale bumps.** This coupling (quad-100 serving living
+only in netstack) is an upstream implementation detail, not a stable contract.
+If a future release adds a genuine non-netstack quad-100 path — or the daemon
+itself is refactored — re-test whether `netstack` can be dropped again. The
+canary is simple: from inside the container, `dig google.com @100.100.100.100`
+must return answers and `ping <host>.<tailnet>.ts.net` must resolve.
+
 ### Log verbosity filtering
 
 Upstream `tailscaled` embeds verbosity tags (`[v1]`, `[v2]`, …) inside its log