ipn/ipnlocal: route extra WireGuard AllowedIPs through the route manager

The conn25 extension's ExtraWireGuardAllowedIPs hook (Transit IPs)
was only appended to wgcfg.Config.Peers in authReconfig. Now that
outbound peer selection comes from the route manager's outbound
table via the engine's PeerByIPPacketFunc (which, when installed,
replaces wireguard-go's AllowedIPs trie lookup entirely) and lazily
created peers get their allowed IPs from the route manager via the
engine's peer config func, those extras never reached either path:
outbound packets to Transit IPs matched no peer, and lazily created
peers didn't accept inbound Transit IP sources.

Teach the route manager a per-peer set of extra allowed IPs, staged
by Mutation.SetExtraAllowedIPs. They appear in the outbound table
and in PeerAllowedIPs (so both outbound routing and per-peer allowed
source prefixes see them) but are excluded from the OS route set,
preserving the hook's contract that the extras reach WireGuard but
not the OS routing table. authReconfig now feeds the hook's results
into the route manager and incrementally syncs any changed peers to
the WireGuard device; the append to cfg.Peers remains only so Reconfig's
full per-peer device sync doesn't strip the extras from active
peers, and goes away with Config.Peers.

Updates #12542

Change-Id: I06c8fa30929fbf8fe171a2d34c47c6fcc3abfa16
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/local.go

@@ -6155,10 +6155,23 @@ func (b *LocalBackend) authReconfigLocked() {
 	}
 	rcfg := b.routerConfigLocked(cfg, prefs, nm)
 
-	// Add these extra Allowed IPs after router configuration, because the expected
-	// extension (features/conn25), does not want these routes installed on the OS.
+	// Push each peer's extra WireGuard-only allowed IPs (the conn25
+	// extension's Transit IPs) into the route manager, which feeds
+	// them to WireGuard via the outbound peer lookup and the per-peer
+	// allowed source prefixes (including for lazily created peers)
+	// while keeping them out of the OS route set, because the
+	// expected extension (features/conn25) does not want these routes
+	// installed on the OS. This runs after routerConfigLocked above
+	// for the same reason: rcfg is derived from cfg.Peers, which must
+	// not yet include the extras.
 	// See also [Hooks.ExtraWireGuardAllowedIPs].
 	if extraAllowedIPsFn, ok := b.extHost.hooks.ExtraWireGuardAllowedIPs.GetOk(); ok {
+		for k := range cn.updateRouteManagerExtras(extraAllowedIPsFn) {
+			b.e.SyncDevicePeer(k)
+		}
+		// Also append the extras to cfg.Peers so the full SyncPeers
+		// in Reconfig below doesn't strip them from active peers.
+		// This loop goes away when cfg.Peers does.
 		for i := range cfg.Peers {
 			extras := extraAllowedIPsFn(cfg.Peers[i].PublicKey)
 			cfg.Peers[i].AllowedIPs = extras.AppendTo(cfg.Peers[i].AllowedIPs)

ipn/ipnlocal/node_backend.go

@@ -938,6 +938,28 @@ func (nb *nodeBackend) updateRouteManagerPrefs(p routePrefs) (changedAllowedIPs
 	return res.AllowedIPs
 }
 
+// updateRouteManagerExtras pushes each peer's extra WireGuard-only
+// allowed IPs into the route manager, obtained by calling fn (the
+// [ipnext.Hooks.ExtraWireGuardAllowedIPs] hook) with each peer's
+// public key.
+//
+// It returns the peers whose allowed source prefixes changed as a
+// result, as described by [routemanager.Result.AllowedIPs].
+func (nb *nodeBackend) updateRouteManagerExtras(fn func(key.NodePublic) views.Slice[netip.Prefix]) (changedAllowedIPs map[key.NodePublic][]netip.Prefix) {
+	nb.mu.Lock()
+	defer nb.mu.Unlock()
+	var extras map[tailcfg.NodeID][]netip.Prefix
+	for id, p := range nb.peers {
+		if pfxs := fn(p.Key()); pfxs.Len() > 0 {
+			mak.Set(&extras, id, pfxs.AsSlice())
+		}
+	}
+	rt := nb.routeMgr.Begin()
+	rt.SetExtraAllowedIPs(extras)
+	res := rt.Commit()
+	return res.AllowedIPs
+}
+
 // osRoutes returns the sorted set of prefixes that the route manager
 // wants programmed into the OS routing table.
 func (nb *nodeBackend) osRoutes() []netip.Prefix {

ipn/ipnlocal/node_backend_test.go

@@ -17,6 +17,7 @@
 	"tailscale.com/tstest"
 	"tailscale.com/types/key"
 	"tailscale.com/types/netmap"
+	"tailscale.com/types/views"
 	"tailscale.com/util/eventbus"
 	"tailscale.com/util/mak"
 	"tailscale.com/util/set"
@@ -505,3 +506,56 @@ func TestNodeBackendDiscoChangedDelta(t *testing.T) {
 		t.Errorf("upsert(after TSMP entry GC) discoChanged = %v; want %v", got, nk2)
 	}
 }
+
+func TestNodeBackendRouteManagerExtras(t *testing.T) {
+	nb := newNodeBackend(t.Context(), tstest.WhileTestRunningLogger(t), eventbus.New())
+
+	n := &tailcfg.Node{
+		ID:       1,
+		Key:      key.NewNode().Public(),
+		HomeDERP: 1,
+		Addresses: []netip.Prefix{
+			netip.MustParsePrefix("100.64.0.1/32"),
+		},
+	}
+	n.AllowedIPs = n.Addresses
+	p1 := n.View()
+	nb.SetNetMap(&netmap.NetworkMap{Peers: []tailcfg.NodeView{p1}})
+
+	transit := netip.MustParsePrefix("fe80::1234/128")
+	extrasFor := func(k key.NodePublic) views.Slice[netip.Prefix] {
+		if k == p1.Key() {
+			return views.SliceOf([]netip.Prefix{transit})
+		}
+		return views.Slice[netip.Prefix]{}
+	}
+
+	// Installing extras reports the peer's allowed prefixes as
+	// changed and adds the transit IP to the outbound table.
+	changed := nb.updateRouteManagerExtras(extrasFor)
+	if len(changed) != 1 || !slices.Contains(changed[p1.Key()], transit) {
+		t.Errorf("updateRouteManagerExtras changed = %v; want %v including %v", changed, p1.Key(), transit)
+	}
+	if pr, ok := nb.routeMgr.Outbound().Lookup(transit.Addr()); !ok || pr.Key != p1.Key() {
+		t.Errorf("Outbound lookup %v = %v, %v; want %v", transit.Addr(), pr, ok, p1.Key())
+	}
+	if nb.routeMgr.OSRoutes().Get(transit) {
+		t.Errorf("OSRoutes contains %v; extras must not reach the OS route set", transit)
+	}
+
+	// An unchanged hook result is a no-op.
+	if changed := nb.updateRouteManagerExtras(extrasFor); changed != nil {
+		t.Errorf("unchanged extras reported changes: %v", changed)
+	}
+
+	// A hook that no longer returns extras removes them.
+	changed = nb.updateRouteManagerExtras(func(key.NodePublic) views.Slice[netip.Prefix] {
+		return views.Slice[netip.Prefix]{}
+	})
+	if len(changed) != 1 {
+		t.Errorf("clearing extras changed = %v; want just %v", changed, p1.Key())
+	}
+	if _, ok := nb.routeMgr.Outbound().Lookup(transit.Addr()); ok {
+		t.Errorf("Outbound still routes %v after extras cleared", transit.Addr())
+	}
+}