1 /*
2  * Copyright (C) 2018 The Android Open Source Project
3  *
4  * Licensed under the Apache License, Version 2.0 (the "License");
5  * you may not use this file except in compliance with the License.
6  * You may obtain a copy of the License at
7  *
8  *      http://www.apache.org/licenses/LICENSE-2.0
9  *
10  * Unless required by applicable law or agreed to in writing, software
11  * distributed under the License is distributed on an "AS IS" BASIS,
12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13  * See the License for the specific language governing permissions and
14  * limitations under the License.
15  */
16 
17 package com.android.server.connectivity.tethering;
18 
19 import android.annotation.Nullable;
20 import android.net.LinkProperties;
21 import android.net.NetworkCapabilities;
22 import android.net.NetworkState;
23 import android.net.RouteInfo;
24 import android.net.util.InterfaceSet;
25 
26 import java.net.Inet4Address;
27 import java.net.Inet6Address;
28 import java.net.InetAddress;
29 
30 /**
31  * @hide
32  */
33 public final class TetheringInterfaceUtils {
34     /**
35      * Get upstream interfaces for tethering based on default routes for IPv4/IPv6.
36      * @return null if there is no usable interface, or a set of at least one interface otherwise.
37      */
getTetheringInterfaces(NetworkState ns)38     public static @Nullable InterfaceSet getTetheringInterfaces(NetworkState ns) {
39         if (ns == null) {
40             return null;
41         }
42 
43         final LinkProperties lp = ns.linkProperties;
44         final String if4 = getInterfaceForDestination(lp, Inet4Address.ANY);
45         final String if6 = getIPv6Interface(ns);
46 
47         return (if4 == null && if6 == null) ? null : new InterfaceSet(if4, if6);
48     }
49 
50     /**
51      * Get the upstream interface for IPv6 tethering.
52      * @return null if there is no usable interface, or the interface name otherwise.
53      */
getIPv6Interface(NetworkState ns)54     public static @Nullable String getIPv6Interface(NetworkState ns) {
55         // Broadly speaking:
56         //
57         //     [1] does the upstream have an IPv6 default route?
58         //
59         // and
60         //
61         //     [2] does the upstream have one or more global IPv6 /64s
62         //         dedicated to this device?
63         //
64         // In lieu of Prefix Delegation and other evaluation of whether a
65         // prefix may or may not be dedicated to this device, for now just
66         // check whether the upstream is TRANSPORT_CELLULAR. This works
67         // because "[t]he 3GPP network allocates each default bearer a unique
68         // /64 prefix", per RFC 6459, Section 5.2.
69         final boolean canTether =
70                 (ns != null) && (ns.network != null)
71                 && (ns.linkProperties != null) && (ns.networkCapabilities != null)
72                 // At least one upstream DNS server:
73                 && ns.linkProperties.hasIpv6DnsServer()
74                 // Minimal amount of IPv6 provisioning:
75                 && ns.linkProperties.hasGlobalIpv6Address()
76                 // Temporary approximation of "dedicated prefix":
77                 && ns.networkCapabilities.hasTransport(NetworkCapabilities.TRANSPORT_CELLULAR);
78 
79         return canTether
80                 ? getInterfaceForDestination(ns.linkProperties, Inet6Address.ANY)
81                 : null;
82     }
83 
getInterfaceForDestination(LinkProperties lp, InetAddress dst)84     private static String getInterfaceForDestination(LinkProperties lp, InetAddress dst) {
85         final RouteInfo ri = (lp != null)
86                 ? RouteInfo.selectBestRoute(lp.getAllRoutes(), dst)
87                 : null;
88         return (ri != null) ? ri.getInterface() : null;
89     }
90 }
91