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 denies app UIDs both NETLINK_ROUTE (golang/go#40569, #2293)
and /proc/net, so net.Interfaces always fails and netmon.New errors
out before a standalone binary can do anything. The Android app solves
this from Java via netmon.RegisterInterfaceGetter, but raw binaries
run under Termux or a rooted shell have no Java to lean on. This is
the second half of #21129, following the androiddns feature.
I added a netmon fallback hook, consulted only when no interface
getter was registered and net.Interfaces failed, and a new androidbin
feature (ts_omit_androidbin) that implements it: report a single
synthetic interface whose v4 and v6 addresses come from asking the
kernel to route an outbound UDP socket, which sends no packets and is
permitted in the app sandbox. That's enough for magicsock to discover
local endpoints. The fallback also requires runtime evidence of
Android (GOOS=android, or /dev/__properties__ existing for GOOS=linux
binaries running under an Android kernel), so it's inert on regular
Linux.
The androidbin feature also blank imports androiddns, and fixes a
third gap I found while testing: GOOS=linux binaries on Android have
an empty CA root pool, because Go's unix root loader doesn't know
Android's /system/etc/security/cacerts (the GOOS=android loader
does), so all TLS verification fails. On Android it points
SSL_CERT_DIR there unless the user already set it, as Termux's
ca-certificates package does.
The feature is on by default in tailscaled builds on Linux and
Android via condregister, and deliberately not linked into tsnet by
default; tsnet apps and other programs opt in with a blank import of
tailscale.com/feature/androidbin.
I verified on an Android 13 emulator with SELinux enforcing, running
GOOS=linux static binaries under the app UID (run-as), where
net.Interfaces fails with the exact netlinkrib permission denial from
the issue: netmon.New succeeds with the synthetic interface, and a
tailcat binary importing this feature does DNS via dnsproxyd, fetches
its DERP map over TLS using the Android cert store, completes STUN,
selects a DERP region, and prints its address.
Updates #21129
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: If7dbcbb825ecd24bcf6d9d64b1e334dd00700d51
Brad Fitzpatrick