android: push default gateway IP from LinkProperties to go backend (#794)

likelyHomeRouterIPHelper was replaced with a cached gateway read in 86f9e7a9ea47e368e394a8e4e1866bbd40e6ed4a. This provides the value by retrieving the default gateway from LinkProperties.getRoute() in NetworkChangeCallback on every onLinkPropertiesChanged event.

Updates tailscale/tailscale#18622
Updates tailscale/tailscale#13352

Signed-off-by: kari-ts <kari@tailscale.com>

Author: kari-ts

android/src/main/java/com/tailscale/ipn/NetworkChangeCallback.kt

@@ -197,6 +197,14 @@ object NetworkChangeCallback {
 
     if (dns.updateDNSFromNetwork(sb.toString())) {
       TSLog.d(TAG, "$why: updated DNS config for iface=${info.linkProps.interfaceName}")
+
+      val gatewayIP =
+          info.linkProps.routes
+              .filter { it.isDefaultRoute && it.gateway != null }
+              .sortedBy { if (it.gateway is java.net.Inet4Address) 0 else 1 }
+              .firstNotNullOfOrNull { it.gateway?.hostAddress } ?: ""
+
+      Libtailscale.onGatewayChanged(gatewayIP)
       Libtailscale.onDNSConfigChanged(info.linkProps.interfaceName)
     }
   }

libtailscale/callbacks.go

@@ -5,6 +5,8 @@ package libtailscale
 
 import (
 	"sync"
+
+	"tailscale.com/net/netmon"
 )
 
 var (
@@ -36,6 +38,11 @@ func OnDNSConfigChanged(ifname string) {
 	}
 }
 
+// OnGatewayChanged is called when LinkProperties provides an updated default gateway IP.
+func OnGatewayChanged(gatewayIP string) {
+	netmon.UpdateLastKnownDefaultGateway(gatewayIP)
+}
+
 var android struct {
 	// mu protects all fields of this structure. However, once a
 	// non-nil jvm is returned from javaVM, all the other fields may