The resnsend dnsproxyd command this package relays raw DNS messages
through was added in Android 10. On Android 9 and older the daemon
answers it with FrameworkListener's text "500 Command not recognized",
which we read as binary: "500 " passed the negative errno check and
"Comm" became the answer length, so every lookup failed with
"androiddns: bogus answer length 1131375981". That's what a tailcat
user hit on a Fire TV stick, which runs Android 9 under Fire OS 7.
Detect that text reply (a binary reply never starts with an ASCII
digit) and switch the process over to the daemon's older getaddrinfo
command, the one bionic's getaddrinfo proxies through. Parse the
single A or AAAA question out of the query, send the command with the
matching address family, and synthesize a DNS answer from the addrinfo
list that comes back, mapping EAI_NONAME to NXDOMAIN and EAI_NODATA to
an empty answer so Go's resolver produces its usual errors. Other
query types return an error saying the Android version can't answer
them.
The reply layout is the field-by-field one netd has used since
Android 6.0 (a 64-bit netd may serve a 32-bit client, so it stopped
sending the raw struct); Android 5.x's raw struct layout is detected
and rejected rather than guessed at. The format was verified against a
32-bit Fire OS 7.7.1.3 device (Android 9, API 28), where a tailcat
build with this change resolves names, fetches its DERP map over
HTTPS, and accepts a connection from another machine.
Updates tailscale/tailcat#126
Change-Id: I7062984c7ec8ce6caf737089e220ef2ca439dcc5
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
The ts_omit_<name> build tags omit a feature at build time; there has
been no way to do the same at runtime. Some users (either proactively
or in response to a security announcement) might like a way to disable
a feature that's linked-in in their binaries that they're not using.
Then a mitigation announcement can say "set this env var" without
asking users to rebuild or wait for a new release.
This adds env var TS_DISABLE_FEATURE, a comma-separated list of
feature names to disable, and the listed set is reported by the
debug-optional-features LocalAPI endpoint next to the registered set.
The legacy per-feature knobs such as TS_DISABLE_SSH_SERVER and
TS_DISABLE_TAILDROP keep working independently.
A disabled feature behaves as if it had not been linked: it is absent
from feature.IsRegistered, its hooks are unset, and its extensions and
handlers are not registered. Three pieces make that happen:
* feature.Register now returns bool, false when disabled, and
feature packages gate their registration init on it. It was added
to the feature packages that never called it (including taildrop
and ssh), which also completes the picture reported by
debug-optional-features. taildrop, routecheck, favorites, and
serviceclientprefs had registration split across several inits and
now register from one gated init.
* ipnext.RegisterExtension ignores a disabled feature's extension.
* feature.Hook.Set and feature.Hooks.Add walk the call stack and
silently skip when the calling package under feature/<name> is
disabled. This covers sub-packages such as
feature/captiveportal/netcheckhook, which cannot call Register
themselves without colliding with their parent, and future
packages whose authors forget the gate.
ssh/tailssh's registrations moved from its inits into tailssh.Register,
called from feature/ssh's gated init. The aws and kube state stores and
syspolicy's Windows store registration are gated too.
feature/register_disable_test.go runs this test binary as a child
process (it links condregister, as tailscaled does) with
TS_DISABLE_FEATURE set to every registered feature at once, and fails
if any of them register anyway, so a feature that ignores the variable
cannot land.
Updates #12614
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I720af6ccab844ae060a9dfd1539fee577fd483e3
Android doesn't have /etc/resolv.conf. This causes problems for people
running GOOS=linux binaries (or GOOS=android binaries without cgo, so
they don't use Android's bionic libc) in Termux, adb shell, etc.
(Go binaries built with cgo use bionic on Android: golang/go#10714)
16 years ago when I was on the Android team I added a system-wide DNS
cache (dnsproxyd) and made bionic query that, so each Android app
wasn't doing its own DNS resolution. That interface was never meant to
be stable, and I thought that code would be surely dead by now 16
years later, but apparently it lives on, and is more stable now: both
empirically (time, ossification?), and because of how Android's split system
updates work nowadays, the dnsproxyd lives on the other side of bionic,
so they seem to keep it pretty stable. The old bionic<->dnsproxyd APIs
I added 16 years ago are still there, but 8 years ago it got some additional
APIs to query by a DNS packet instead.
So use it! If we find ourselves on Android and without libc access
(and because we don't want to pull in ebitengine/purego with all its
side effects), just query the DNS server like bionic does.
This can be disabled in Linux binaries with ts_omit_androiddns.
Old links:
LineageOS/android_system_netd@007e987feehttps://android.googlesource.com/platform/system/netd/+/007e987fee7e815e0c4bc820f434a632b7a69a9d
("DNS proxy thread in netd.")
aosp-mirror/platform_bionic@a1dbf0b453https://android.googlesource.com/platform/bionic/+/a1dbf0b453801620565e5911f354f82706b0200d
("DNS proxy: the start. proxies getaddrinfo calls.")
Back then I found it cleaner to proxy at the getaddrinfo level rather
than speak in terms of DNS packets. The raw-packet resnsend command I
use here came eight years later, added in November 2018 for Android
10's android_res_nsend NDK API:
LineageOS/android_system_netd@c0c818f448https://android.googlesource.com/platform/system/netd/+/c0c818f448efa90ab1f9b1733fb86c5e22fb894c
("Add resNetworkSend cmd in DnsProxyListener")
Android 10 (codename Q, API level 29, released September 2019) is
therefore the minimum OS version for this to work.
I verified this against the DnsResolver module on an Android 13
emulator with SELinux enforcing, from the shell UID, with both a pure
Go GOOS=android binary and a static GOOS=linux binary: raw queries,
NXDOMAIN handling, the runtime Android detection, and a tailcat binary
reaching DERP with lookups visible in the daemon's logcat output, some
served from my 2010 DNS cache.
Updates #21129
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I0d63763e255a077e4e5745b3e64ba0d78dab6d69
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Brad Fitzpatrick