net/tstun/wrap.go

@@ -1154,6 +1154,26 @@ func (t *Wrapper) injectedRead(res tunInjectedRead, outBuffs [][]byte, sizes []i
 	}
 
 	invertGSOChecksum(pkt, gso)
+	// Check if this is a packet for conn25-style app connectors,
+	// and perform the necessary NAT. The main case that requires
+	// NAT from netstack toward WireGuard is an SNAT on return traffic
+	// from the target application on the internet, translating
+	// the original server's source IP to the TransitIP.
+	// The hook can also perform DNAT for client-originated traffic,
+	// translating the destination MagicIP to a TransitIP, and rejects
+	// MagicIPs that have not been approved for the client.
+	//
+	// Normal non-connector traffic is forwarded unmodified.
+	//
+	// Cross-tailnet conn25 app connector connections are not supported,
+	// so at most one of this hook and the following pc.snat should modify the packet.
+	if t.PreFilterPacketOutboundToWireGuardAppConnectorIntercept != nil {
+		if r := t.PreFilterPacketOutboundToWireGuardAppConnectorIntercept(p, t); r.IsDrop() {
+			metricPacketOut.Add(1)
+			metricPacketOutDrop.Add(1)
+			return 0, nil
+		}
+	}
 	pc.snat(p)
 	invertGSOChecksum(pkt, gso)
 

net/tstun/wrap_test.go

@@ -28,6 +28,7 @@ import (
 	"tailscale.com/disco"
 	"tailscale.com/net/netaddr"
 	"tailscale.com/net/packet"
+	"tailscale.com/net/packet/checksum"
 	"tailscale.com/tstest"
 	"tailscale.com/tstime/mono"
 	"tailscale.com/types/ipproto"
@@ -1120,3 +1121,36 @@ func TestInterceptOrdering(t *testing.T) {
 		t.Errorf("got number of intercepts run in Read(): %d; want: %d", seq, numOutboundIntercepts)
 	}
 }
+
+func TestInjectedReadCallsAppConnectorHook(t *testing.T) {
+	var called bool
+	hook := func(p *packet.Parsed, _ *Wrapper) filter.Response {
+		called = true
+		checksum.UpdateSrcAddr(p, netip.MustParseAddr("169.254.0.1"))
+		return filter.Accept
+	}
+
+	bus := eventbustest.NewBus(t)
+	_, tun := newFakeTUN(t.Logf, bus, false)
+	tun.PreFilterPacketOutboundToWireGuardAppConnectorIntercept = hook
+	tun.Start()
+	defer tun.Close()
+
+	if err := tun.InjectOutbound(udp4("145.53.32.10", "100.25.63.57", 80, 12345)); err != nil {
+		t.Fatalf("InjectOutbound error: %v", err)
+	}
+
+	var buf [MaxPacketSize]byte
+	sizes := make([]int, 1)
+	tun.Read([][]byte{buf[:]}, sizes, 0)
+
+	if !called {
+		t.Error("app connector hook was not called in InjectOutbound")
+	}
+
+	wantPkt := udp4("169.254.0.1", "100.25.63.57", 80, 12345)
+	gotPkt := buf[:sizes[0]]
+	if !bytes.Equal(wantPkt, gotPkt) {
+		t.Errorf("packet mismatch\nwant:\t% x\ngot:\t% x", wantPkt, gotPkt)
+	}
+}