Fix use of reachable DNS server logic

Move reachable DNS server computation out of ConnectivityService
and split it into LinkProperties#isReachable() and a companion
change in WifiStateMachine's makeLinkProperties().

Restore previous ConnectivityService#updateDnses() behaviour, as
the pruning is done in WifiStateMachine now.

Bug: 19470192
Bug: 20733156
Bug: 22098233
Change-Id: I810ef74d504e5dc1ca2017d435cdadd6b82171e6

2 files changed, 117 insertions, 1 deletions

diff --git a/core/java/android/net/LinkProperties.java b/core/java/android/net/LinkProperties.java
index 31aedad..cfd5bf1 100644
--- a/core/java/android/net/LinkProperties.java
+++ b/core/java/android/net/LinkProperties.java
@@ -517,7 +517,7 @@ public final class LinkProperties implements Parcelable {
      * Note that Http Proxies are only a hint - the system recommends their use, but it does
      * not enforce it and applications may ignore them.
      *
-     * @param proxy A {@link ProxyInfo} defining the Http Proxy to use on this link.
+     * @param proxy A {@link ProxyInfo} defining the HTTP Proxy to use on this link.
      * @hide
      */
     public void setHttpProxy(ProxyInfo proxy) {
@@ -774,6 +774,43 @@ public final class LinkProperties implements Parcelable {
     }
 
     /**
+     * Evaluate whether the {@link InetAddress} is considered reachable.
+     *
+     * @return {@code true} if the given {@link InetAddress} is considered reachable,
+     *         {@code false} otherwise.
+     * @hide
+     */
+    public boolean isReachable(InetAddress ip) {
+        final List<RouteInfo> allRoutes = getAllRoutes();
+        // If we don't have a route to this IP address, it's not reachable.
+        final RouteInfo bestRoute = RouteInfo.selectBestRoute(allRoutes, ip);
+        if (bestRoute == null) {
+            return false;
+        }
+
+        // TODO: better source address evaluation for destination addresses.
+
+        if (ip instanceof Inet4Address) {
+            // For IPv4, it suffices for now to simply have any address.
+            return hasIPv4Address();
+        } else if (ip instanceof Inet6Address) {
+            if (ip.isLinkLocalAddress()) {
+                // For now, just make sure link-local destinations have
+                // scopedIds set, since transmits will generally fail otherwise.
+                // TODO: verify it matches the ifindex of one of the interfaces.
+                return (((Inet6Address)ip).getScopeId() != 0);
+            }  else {
+                // For non-link-local destinations check that either the best route
+                // is directly connected or that some global preferred address exists.
+                // TODO: reconsider all cases (disconnected ULA networks, ...).
+                return (!bestRoute.hasGateway() || hasGlobalIPv6Address());
+            }
+        }
+
+        return false;
+    }
+
+    /**
      * Compares this {@code LinkProperties} interface name against the target
      *
      * @param target LinkProperties to compare.
diff --git a/core/tests/coretests/src/android/net/LinkPropertiesTest.java b/core/tests/coretests/src/android/net/LinkPropertiesTest.java
index 5c55efb..ea444a4 100644
--- a/core/tests/coretests/src/android/net/LinkPropertiesTest.java
+++ b/core/tests/coretests/src/android/net/LinkPropertiesTest.java
@@ -16,6 +16,8 @@
 
 package android.net;
 
+import android.net.IpPrefix;
+import android.net.LinkAddress;
 import android.net.LinkProperties;
 import android.net.LinkProperties.ProvisioningChange;
 import android.net.RouteInfo;
@@ -26,6 +28,7 @@ import junit.framework.TestCase;
 import java.net.InetAddress;
 import java.util.ArrayList;
 
+
 public class LinkPropertiesTest extends TestCase {
     private static InetAddress ADDRV4 = NetworkUtils.numericToInetAddress("75.208.6.1");
     private static InetAddress ADDRV6 = NetworkUtils.numericToInetAddress(
@@ -567,4 +570,80 @@ public class LinkPropertiesTest extends TestCase {
         assertEquals(ProvisioningChange.STILL_PROVISIONED,
                 LinkProperties.compareProvisioning(v6lp, v6lp2));
     }
+
+    @SmallTest
+    public void testIsReachable() {
+        final LinkProperties v4lp = new LinkProperties();
+        assertFalse(v4lp.isReachable(DNS1));
+        assertFalse(v4lp.isReachable(DNS2));
+
+        // Add an on-link route, making the on-link DNS server reachable,
+        // but there is still no IPv4 address.
+        assertTrue(v4lp.addRoute(new RouteInfo(
+                new IpPrefix(NetworkUtils.numericToInetAddress("75.208.0.0"), 16))));
+        assertFalse(v4lp.isReachable(DNS1));
+        assertFalse(v4lp.isReachable(DNS2));
+
+        // Adding an IPv4 address (right now, any IPv4 address) means we use
+        // the routes to compute likely reachability.
+        assertTrue(v4lp.addLinkAddress(new LinkAddress(ADDRV4, 16)));
+        assertTrue(v4lp.isReachable(DNS1));
+        assertFalse(v4lp.isReachable(DNS2));
+
+        // Adding a default route makes the off-link DNS server reachable.
+        assertTrue(v4lp.addRoute(new RouteInfo(GATEWAY1)));
+        assertTrue(v4lp.isReachable(DNS1));
+        assertTrue(v4lp.isReachable(DNS2));
+
+        final LinkProperties v6lp = new LinkProperties();
+        final InetAddress kLinkLocalDns = NetworkUtils.numericToInetAddress("fe80::6:1");
+        final InetAddress kLinkLocalDnsWithScope = NetworkUtils.numericToInetAddress("fe80::6:2%43");
+        final InetAddress kOnLinkDns = NetworkUtils.numericToInetAddress("2001:db8:85a3::53");
+        assertFalse(v6lp.isReachable(kLinkLocalDns));
+        assertFalse(v6lp.isReachable(kLinkLocalDnsWithScope));
+        assertFalse(v6lp.isReachable(kOnLinkDns));
+        assertFalse(v6lp.isReachable(DNS6));
+
+        // Add a link-local route, making the link-local DNS servers reachable. Because
+        // we assume the presence of an IPv6 link-local address, link-local DNS servers
+        // are considered reachable, but only those with a non-zero scope identifier.
+        assertTrue(v6lp.addRoute(new RouteInfo(
+                new IpPrefix(NetworkUtils.numericToInetAddress("fe80::"), 64))));
+        assertFalse(v6lp.isReachable(kLinkLocalDns));
+        assertTrue(v6lp.isReachable(kLinkLocalDnsWithScope));
+        assertFalse(v6lp.isReachable(kOnLinkDns));
+        assertFalse(v6lp.isReachable(DNS6));
+
+        // Add a link-local address--nothing changes.
+        assertTrue(v6lp.addLinkAddress(LINKADDRV6LINKLOCAL));
+        assertFalse(v6lp.isReachable(kLinkLocalDns));
+        assertTrue(v6lp.isReachable(kLinkLocalDnsWithScope));
+        assertFalse(v6lp.isReachable(kOnLinkDns));
+        assertFalse(v6lp.isReachable(DNS6));
+
+        // Add a global route on link, but no global address yet. DNS servers reachable
+        // via a route that doesn't require a gateway: give them the benefit of the
+        // doubt and hope the link-local source address suffices for communication.
+        assertTrue(v6lp.addRoute(new RouteInfo(
+                new IpPrefix(NetworkUtils.numericToInetAddress("2001:db8:85a3::"), 64))));
+        assertFalse(v6lp.isReachable(kLinkLocalDns));
+        assertTrue(v6lp.isReachable(kLinkLocalDnsWithScope));
+        assertTrue(v6lp.isReachable(kOnLinkDns));
+        assertFalse(v6lp.isReachable(DNS6));
+
+        // Add a global address; the on-link global address DNS server is (still)
+        // presumed reachable.
+        assertTrue(v6lp.addLinkAddress(new LinkAddress(ADDRV6, 64)));
+        assertFalse(v6lp.isReachable(kLinkLocalDns));
+        assertTrue(v6lp.isReachable(kLinkLocalDnsWithScope));
+        assertTrue(v6lp.isReachable(kOnLinkDns));
+        assertFalse(v6lp.isReachable(DNS6));
+
+        // Adding a default route makes the off-link DNS server reachable.
+        assertTrue(v6lp.addRoute(new RouteInfo(GATEWAY62)));
+        assertFalse(v6lp.isReachable(kLinkLocalDns));
+        assertTrue(v6lp.isReachable(kLinkLocalDnsWithScope));
+        assertTrue(v6lp.isReachable(kOnLinkDns));
+        assertTrue(v6lp.isReachable(DNS6));
+    }
 }