content: /*
   * Copyright (C) 2018 The Android Open Source Project
   *
   * Licensed under the Apache License, Version 2.0 (the "License");
   * you may not use this file except in compliance with the License.
   * You may obtain a copy of the License at
   *
   *      http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  
  package com.android.server.connectivity.tethering;
  
  import android.annotation.Nullable;
  import android.net.LinkProperties;
  import android.net.NetworkCapabilities;
  import android.net.NetworkState;
  import android.net.RouteInfo;
  import android.net.util.InterfaceSet;
  
  import java.net.Inet4Address;
  import java.net.Inet6Address;
  import java.net.InetAddress;
  
  /**
   * @hide
   */
  public final class TetheringInterfaceUtils {
      /**
       * Get upstream interfaces for tethering based on default routes for IPv4/IPv6.
       * @return null if there is no usable interface, or a set of at least one interface otherwise.
       */
      public static @Nullable InterfaceSet getTetheringInterfaces(NetworkState ns) {
          if (ns == null) {
              return null;
          }
  
          final LinkProperties lp = ns.linkProperties;
          final String if4 = getInterfaceForDestination(lp, Inet4Address.ANY);
          final String if6 = getIPv6Interface(ns);
  
          return (if4 == null && if6 == null) ? null : new InterfaceSet(if4, if6);
      }
  
      /**
       * Get the upstream interface for IPv6 tethering.
       * @return null if there is no usable interface, or the interface name otherwise.
       */
      public static @Nullable String getIPv6Interface(NetworkState ns) {
          // Broadly speaking:
          //
          //     [1] does the upstream have an IPv6 default route?
          //
          // and
          //
          //     [2] does the upstream have one or more global IPv6 /64s
          //         dedicated to this device?
          //
          // In lieu of Prefix Delegation and other evaluation of whether a
          // prefix may or may not be dedicated to this device, for now just
          // check whether the upstream is TRANSPORT_CELLULAR. This works
          // because "[t]he 3GPP network allocates each default bearer a unique
          // /64 prefix", per RFC 6459, Section 5.2.
          final boolean canTether =
                  (ns != null) && (ns.network != null)
                  && (ns.linkProperties != null) && (ns.networkCapabilities != null)
                  // At least one upstream DNS server:
                  && ns.linkProperties.hasIpv6DnsServer()
                  // Minimal amount of IPv6 provisioning:
                  && ns.linkProperties.hasGlobalIpv6Address()
                  // Temporary approximation of "dedicated prefix":
                  && ns.networkCapabilities.hasTransport(NetworkCapabilities.TRANSPORT_CELLULAR);
  
          return canTether
                  ? getInterfaceForDestination(ns.linkProperties, Inet6Address.ANY)
                  : null;
      }
  
      private static String getInterfaceForDestination(LinkProperties lp, InetAddress dst) {
          final RouteInfo ri = (lp != null)
                  ? RouteInfo.selectBestRoute(lp.getAllRoutes(), dst)
                  : null;
          return (ri != null) ? ri.getInterface() : null;
      }
  }
  