Android -- WiFi的NUD检测机制浅析
这段时间遇到几次WiFi连接后，突然断连的问题，最后发现是由于Android的NUD检测机制导致的。NUD(Neighbor Unreachable Detection,邻居不可达检测)的底层实现还是依赖kernel，Android层有服务建立通信，当kernel检测到当前网络与周边的neighbor不可达时，就会发送消息通知上层，上层处理msg、最后WiFi会transfer到Disconnect STATE，最后导致网络自己断开，这会影响用户体验，让用户产生困扰。
这里我们主要介绍一下Android FWK中关于NUD的流程，以便加深对这个流程的了解，这样可以方便我们后续解决相关的问题，或者加些小定制。
当用户点击WiFi开关，打开WiFi后，WiFiStateMachine会进入ConnectMode，准备处理用户的连接请求：
class ConnectModeState extends State {
@Override
public void enter() {
Log.d(TAG, "entering ConnectModeState: ifaceName = " + mInterfaceName);
mOperationalMode = CONNECT_MODE;
setupClientMode();
if (!mWifiNative.removeAllNetworks(mInterfaceName)) {
loge("Failed to remove networks on entering connect mode");
}
mScanRequestProxy.enableScanningForHiddenNetworks(true);
mWifiInfo.reset();
mWifiInfo.setSupplicantState(SupplicantState.DISCONNECTED);
mWifiInjector.getWakeupController().reset();
mNetworkInfo.setIsAvailable(true);
if (mNetworkAgent != null) mNetworkAgent.sendNetworkInfo(mNetworkInfo);
// initialize network state
setNetworkDetailedState(DetailedState.DISCONNECTED);
// Inform WifiConnectivityManager that Wifi is enabled
mWifiConnectivityManager.setWifiEnabled(true);
// Inform metrics that Wifi is Enabled (but not yet connected)
mWifiMetrics.setWifiState(WifiMetricsProto.WifiLog.WIFI_DISCONNECTED);
// Inform p2p service that wifi is up and ready when applicable
p2pSendMessage(WifiStateMachine.CMD_ENABLE_P2P);
// Inform sar manager that wifi is Enabled
mSarManager.setClientWifiState(WifiManager.WIFI_STATE_ENABLED);
}
......
}ClientMode是Station模式，setupClientMode()会为后续的网络连接做一些准备工作：
/**
* Helper method to start other services and get state ready for client mode
*/
private void setupClientMode() {
Log.d(TAG, "setupClientMode() ifacename = " + mInterfaceName);
mWifiStateTracker.updateState(WifiStateTracker.INVALID);
if (mWifiConnectivityManager == null) {
synchronized (mWifiReqCountLock) {
mWifiConnectivityManager =
mWifiInjector.makeWifiConnectivityManager(mWifiInfo,
hasConnectionRequests());
mWifiConnectivityManager.setUntrustedConnectionAllowed(mUntrustedReqCount > 0);
mWifiConnectivityManager.handleScreenStateChanged(mScreenOn);
}
}
mIpClient = mFacade.makeIpClient(mContext, mInterfaceName, new IpClientCallback());
mIpClient.setMulticastFilter(true);
registerForWifiMonitorEvents();
mWifiInjector.getWifiLastResortWatchdog().clearAllFailureCounts();
setSupplicantLogLevel();
// reset state related to supplicant starting
mSupplicantStateTracker.sendMessage(CMD_RESET_SUPPLICANT_STATE);
// Initialize data structures
mLastBssid = null;
mLastNetworkId = WifiConfiguration.INVALID_NETWORK_ID;
mLastSignalLevel = -1;
mWifiInfo.setMacAddress(mWifiNative.getMacAddress(mInterfaceName));
// Attempt to migrate data out of legacy store.
if (!mWifiConfigManager.migrateFromLegacyStore()) {
Log.e(TAG, "Failed to migrate from legacy config store");
}
// TODO: b/79504296 This broadcast has been deprecated and should be removed
sendSupplicantConnectionChangedBroadcast(true);
mWifiNative.setExternalSim(mInterfaceName, true);
setRandomMacOui();
mCountryCode.setReadyForChange(true);
mWifiDiagnostics.startLogging(mVerboseLoggingEnabled);
mIsRunning = true;
updateBatteryWorkSource(null);
/**
* Enable bluetooth coexistence scan mode when bluetooth connection is active.
* When this mode is on, some of the low-level scan parameters used by the
* driver are changed to reduce interference with bluetooth
*/
mWifiNative.setBluetoothCoexistenceScanMode(mInterfaceName, mBluetoothConnectionActive);
// initialize network state
setNetworkDetailedState(DetailedState.DISCONNECTED);
// Disable legacy multicast filtering, which on some chipsets defaults to enabled.
// Legacy IPv6 multicast filtering blocks ICMPv6 router advertisements which breaks IPv6
// provisioning. Legacy IPv4 multicast filtering may be re-enabled later via
// IpClient.Callback.setFallbackMulticastFilter()
mWifiNative.stopFilteringMulticastV4Packets(mInterfaceName);
mWifiNative.stopFilteringMulticastV6Packets(mInterfaceName);
// Set the right suspend mode settings
mWifiNative.setSuspendOptimizations(mInterfaceName, mSuspendOptNeedsDisabled == 0
&& mUserWantsSuspendOpt.get());
mWifiNative.setPowerSave(mInterfaceName, true);
if (mP2pSupported) {
p2pSendMessage(WifiStateMachine.CMD_ENABLE_P2P);
}
// Disable wpa_supplicant from auto reconnecting.
mWifiNative.enableStaAutoReconnect(mInterfaceName, false);
// STA has higher priority over P2P
mWifiNative.setConcurrencyPriority(true);
}setupClientMode()函数内容较多，这里我们比较关心两点：
1、WifiConnectivityManager：它主要管理WiFi相关的扫描动作，比如扫描、获取扫描结果，以及扫描完成后的autoconnect动作，主要逻辑都在其中
2、FrameworkFacade::makeIpClient()：通过FrameworkFacade创建IpClient，我们知道IpClient跟触发DHCP相关，而我们的NUD机制的注册会通过IpClient完成，看它带入的Callback实现：
class IpClientCallback extends IpClient.Callback {
@Override
public void onPreDhcpAction() {
sendMessage(DhcpClient.CMD_PRE_DHCP_ACTION);
}
@Override
public void onPostDhcpAction() {
sendMessage(DhcpClient.CMD_POST_DHCP_ACTION);
}
@Override
public void onNewDhcpResults(DhcpResults dhcpResults) {
if (dhcpResults != null) {
sendMessage(CMD_IPV4_PROVISIONING_SUCCESS, dhcpResults);
} else {
sendMessage(CMD_IPV4_PROVISIONING_FAILURE);
mWifiInjector.getWifiLastResortWatchdog().noteConnectionFailureAndTriggerIfNeeded(
getTargetSsid(), mTargetRoamBSSID,
WifiLastResortWatchdog.FAILURE_CODE_DHCP);
}
}
@Override
public void onProvisioningSuccess(LinkProperties newLp) {
mWifiMetrics.logStaEvent(StaEvent.TYPE_CMD_IP_CONFIGURATION_SUCCESSFUL);
sendMessage(CMD_UPDATE_LINKPROPERTIES, newLp);
sendMessage(CMD_IP_CONFIGURATION_SUCCESSFUL);
}
@Override
public void onProvisioningFailure(LinkProperties newLp) {
mWifiMetrics.logStaEvent(StaEvent.TYPE_CMD_IP_CONFIGURATION_LOST);
sendMessage(CMD_IP_CONFIGURATION_LOST);
}
@Override
public void onLinkPropertiesChange(LinkProperties newLp) {
sendMessage(CMD_UPDATE_LINKPROPERTIES, newLp);
}
@Override
public void onReachabilityLost(String logMsg) {
mWifiMetrics.logStaEvent(StaEvent.TYPE_CMD_IP_REACHABILITY_LOST);
sendMessage(CMD_IP_REACHABILITY_LOST, logMsg);
}
@Override
public void installPacketFilter(byte[] filter) {
sendMessage(CMD_INSTALL_PACKET_FILTER, filter);
}
@Override
public void startReadPacketFilter() {
sendMessage(CMD_READ_PACKET_FILTER);
}
@Override
public void setFallbackMulticastFilter(boolean enabled) {
sendMessage(CMD_SET_FALLBACK_PACKET_FILTERING, enabled);
}
@Override
public void setNeighborDiscoveryOffload(boolean enabled) {
sendMessage(CMD_CONFIG_ND_OFFLOAD, (enabled ? 1 : 0));
}
}其中onReachabilityLost()这个回调，会在收到kernel通知后被调用，然后发送CMD_IP_REACHABILITY_LOST msg去通知WiFi fwk断连WiFi。
继续看IpClient的构造过程：
@VisibleForTesting
IpClient(Context context, String ifName, Callback callback, Dependencies deps) {
super(IpClient.class.getSimpleName() + "." + ifName);
Preconditions.checkNotNull(ifName);
Preconditions.checkNotNull(callback);
mTag = getName();
mContext = context;
mInterfaceName = ifName;
mClatInterfaceName = CLAT_PREFIX + ifName;
mCallback = new LoggingCallbackWrapper(callback);
mDependencies = deps;
mShutdownLatch = new CountDownLatch(1);
mNwService = deps.getNMS();
sSmLogs.putIfAbsent(mInterfaceName, new SharedLog(MAX_LOG_RECORDS, mTag));
mLog = sSmLogs.get(mInterfaceName);
sPktLogs.putIfAbsent(mInterfaceName, new LocalLog(MAX_PACKET_RECORDS));
mConnectivityPacketLog = sPktLogs.get(mInterfaceName);
mMsgStateLogger = new MessageHandlingLogger();
// TODO: Consider creating, constructing, and passing in some kind of
// InterfaceController.Dependencies class.
mInterfaceCtrl = new InterfaceController(mInterfaceName, mNwService, deps.getNetd(), mLog);
mNetlinkTracker = new NetlinkTracker(
mInterfaceName,
new NetlinkTracker.Callback() {
@Override
public void update() {
sendMessage(EVENT_NETLINK_LINKPROPERTIES_CHANGED);
}
}) {
@Override
public void interfaceAdded(String iface) {
super.interfaceAdded(iface);
if (mClatInterfaceName.equals(iface)) {
mCallback.setNeighborDiscoveryOffload(false);
} else if (!mInterfaceName.equals(iface)) {
return;
}
final String msg = "interfaceAdded(" + iface +")";
logMsg(msg);
}
@Override
public void interfaceRemoved(String iface) {
super.interfaceRemoved(iface);
// TODO: Also observe mInterfaceName going down and take some
// kind of appropriate action.
if (mClatInterfaceName.equals(iface)) {
// TODO: consider sending a message to the IpClient main
// StateMachine thread, in case "NDO enabled" state becomes
// tied to more things that 464xlat operation.
mCallback.setNeighborDiscoveryOffload(true);
} else if (!mInterfaceName.equals(iface)) {
return;
}
final String msg = "interfaceRemoved(" + iface +")";
logMsg(msg);
}
private void logMsg(String msg) {
Log.d(mTag, msg);
getHandler().post(() -> { mLog.log("OBSERVED " + msg); });
}
};
mLinkProperties = new LinkProperties();
mLinkProperties.setInterfaceName(mInterfaceName);
mProvisioningTimeoutAlarm = new WakeupMessage(mContext, getHandler(),
mTag + ".EVENT_PROVISIONING_TIMEOUT", EVENT_PROVISIONING_TIMEOUT);
mDhcpActionTimeoutAlarm = new WakeupMessage(mContext, getHandler(),
mTag + ".EVENT_DHCPACTION_TIMEOUT", EVENT_DHCPACTION_TIMEOUT);
// Anything the StateMachine may access must have been instantiated
// before this point.
configureAndStartStateMachine();
// Anything that may send messages to the StateMachine must only be
// configured to do so after the StateMachine has started (above).
startStateMachineUpdaters();
}
.......
// Use a wrapper class to log in order to ensure complete and detailed
// logging. This method is lighter weight than annotations/reflection
// and has the following benefits:
//
// - No invoked method can be forgotten.
// Any new method added to IpClient.Callback must be overridden
// here or it will never be called.
//
// - No invoking call site can be forgotten.
// Centralized logging in this way means call sites don't need to
// remember to log, and therefore no call site can be forgotten.
//
// - No variation in log format among call sites.
// Encourages logging of any available arguments, and all call sites
// are necessarily logged identically.
//
// TODO: Find an lighter weight approach.
private class LoggingCallbackWrapper extends Callback {
private static final String PREFIX = "INVOKE ";
private Callback mCallback;
public LoggingCallbackWrapper(Callback callback) {
mCallback = callback;
}
private void log(String msg) {
mLog.log(PREFIX + msg);
}
@Override
public void onPreDhcpAction() {
mCallback.onPreDhcpAction();
log("onPreDhcpAction()");
}
@Override
public void onPostDhcpAction() {
mCallback.onPostDhcpAction();
log("onPostDhcpAction()");
}
@Override
public void onNewDhcpResults(DhcpResults dhcpResults) {
mCallback.onNewDhcpResults(dhcpResults);
log("onNewDhcpResults({" + dhcpResults + "})");
}
@Override
public void onProvisioningSuccess(LinkProperties newLp) {
mCallback.onProvisioningSuccess(newLp);
log("onProvisioningSuccess({" + newLp + "})");
}
@Override
public void onProvisioningFailure(LinkProperties newLp) {
mCallback.onProvisioningFailure(newLp);
log("onProvisioningFailure({" + newLp + "})");
}
@Override
public void onLinkPropertiesChange(LinkProperties newLp) {
mCallback.onLinkPropertiesChange(newLp);
log("onLinkPropertiesChange({" + newLp + "})");
}
@Override
public void onReachabilityLost(String logMsg) {
mCallback.onReachabilityLost(logMsg);
log("onReachabilityLost(" + logMsg + ")");
}
@Override
public void onQuit() {
mCallback.onQuit();
log("onQuit()");
}
@Override
public void installPacketFilter(byte[] filter) {
mCallback.installPacketFilter(filter);
log("installPacketFilter(byte[" + filter.length + "])");
}
@Override
public void startReadPacketFilter() {
mCallback.startReadPacketFilter();
log("startReadPacketFilter()");
}
@Override
public void setFallbackMulticastFilter(boolean enabled) {
mCallback.setFallbackMulticastFilter(enabled);
log("setFallbackMulticastFilter(" + enabled + ")");
}
@Override
public void setNeighborDiscoveryOffload(boolean enable) {
mCallback.setNeighborDiscoveryOffload(enable);
log("setNeighborDiscoveryOffload(" + enable + ")");
}
}IpClient会封装一次传进来的Callback参数，但只是简单的wrapper，我们最关心的onReachabilityLost()回调也是。
这里相关的初始化准备工作就完成了。NUD肯定要在用户连接了网络之后，检测才会有意义；当获取IP开始后，会进入IpClient::RunningState：
class RunningState extends State {
private ConnectivityPacketTracker mPacketTracker;
private boolean mDhcpActionInFlight;
@Override
public void enter() {
ApfFilter.ApfConfiguration apfConfig = new ApfFilter.ApfConfiguration();
apfConfig.apfCapabilities = mConfiguration.mApfCapabilities;
apfConfig.multicastFilter = mMulticastFiltering;
// Get the Configuration for ApfFilter from Context
apfConfig.ieee802_3Filter =
mContext.getResources().getBoolean(R.bool.config_apfDrop802_3Frames);
apfConfig.ethTypeBlackList =
mContext.getResources().getIntArray(R.array.config_apfEthTypeBlackList);
mApfFilter = ApfFilter.maybeCreate(mContext, apfConfig, mInterfaceParams, mCallback);
// TODO: investigate the effects of any multicast filtering racing/interfering with the
// rest of this IP configuration startup.
if (mApfFilter == null) {
mCallback.setFallbackMulticastFilter(mMulticastFiltering);
}
mPacketTracker = createPacketTracker();
if (mPacketTracker != null) mPacketTracker.start(mConfiguration.mDisplayName);
if (mConfiguration.mEnableIPv6 && !startIPv6()) {
doImmediateProvisioningFailure(IpManagerEvent.ERROR_STARTING_IPV6);
transitionTo(mStoppingState);
return;
}
if (mConfiguration.mEnableIPv4 && !startIPv4()) {
doImmediateProvisioningFailure(IpManagerEvent.ERROR_STARTING_IPV4);
transitionTo(mStoppingState);
return;
}
final InitialConfiguration config = mConfiguration.mInitialConfig;
if ((config != null) && !applyInitialConfig(config)) {
// TODO introduce a new IpManagerEvent constant to distinguish this error case.
doImmediateProvisioningFailure(IpManagerEvent.ERROR_INVALID_PROVISIONING);
transitionTo(mStoppingState);
return;
}
if (mConfiguration.mUsingMultinetworkPolicyTracker) {
mMultinetworkPolicyTracker = new MultinetworkPolicyTracker(
mContext, getHandler(),
() -> { mLog.log("OBSERVED AvoidBadWifi changed"); });
mMultinetworkPolicyTracker.start();
}
if (mConfiguration.mUsingIpReachabilityMonitor && !startIpReachabilityMonitor()) {
doImmediateProvisioningFailure(
IpManagerEvent.ERROR_STARTING_IPREACHABILITYMONITOR);
transitionTo(mStoppingState);
return;
}
}
......
}其中会调用startIpReachabilityMonitor()，去创建IpReachabilityMonitor对象，NUD相关的操作都会分派给它处理：
private boolean startIpReachabilityMonitor() {
try {
mIpReachabilityMonitor = new IpReachabilityMonitor(
mContext,
mInterfaceParams,
getHandler(),
mLog,
new IpReachabilityMonitor.Callback() {
@Override
public void notifyLost(InetAddress ip, String logMsg) {
mCallback.onReachabilityLost(logMsg);
}
},
mMultinetworkPolicyTracker);
} catch (IllegalArgumentException iae) {
// Failed to start IpReachabilityMonitor. Log it and call
// onProvisioningFailure() immediately.
//
// See http://b/31038971.
logError("IpReachabilityMonitor failure: %s", iae);
mIpReachabilityMonitor = null;
}
return (mIpReachabilityMonitor != null);
}构造IpReachabilityMonitor对象时，实现了一个IpReachabilityMonitor.Callback()回调接口，它会调用IpClient的Callback wrapper通知onReachabilityLost()事件。
NUD是为了探测周边neighbor的可达性，所以它在一次WiFi网络连接完成、拿到连接信息之后，再去开始触发探测比较正常，WiFi连接之后，ConnectModeState收到wpa_supplicant通知的连接完成事件：
case WifiMonitor.SUPPLICANT_STATE_CHANGE_EVENT:
SupplicantState state = handleSupplicantStateChange(message);
// Supplicant can fail to report a NETWORK_DISCONNECTION_EVENT
// when authentication times out after a successful connection,
// we can figure this from the supplicant state. If supplicant
// state is DISCONNECTED, but the mNetworkInfo says we are not
// disconnected, we need to handle a disconnection
if (state == SupplicantState.DISCONNECTED
&& mNetworkInfo.getState() != NetworkInfo.State.DISCONNECTED) {
if (mVerboseLoggingEnabled) {
log("Missed CTRL-EVENT-DISCONNECTED, disconnect");
}
handleNetworkDisconnect();
transitionTo(mDisconnectedState);
}
// If we have COMPLETED a connection to a BSSID, start doing
// DNAv4/DNAv6 -style probing for on-link neighbors of
// interest (e.g. routers); harmless if none are configured.
if (state == SupplicantState.COMPLETED) {
mIpClient.confirmConfiguration();
mWifiScoreReport.noteIpCheck();
}
break;会调用IpClient::confirmConfiguration()确认网络配置，然后开启NUD的kernel probe：
IpClient:
public void IpClient::confirmConfiguration() {
sendMessage(CMD_CONFIRM);
}
IpClient::RunningState {
......
@Override
public boolean processMessage(Message msg) {
switch (msg.what) {
case CMD_STOP:
transitionTo(mStoppingState);
break;
case CMD_START:
logError("ALERT: START received in StartedState. Please fix caller.");
break;
case CMD_CONFIRM:
// TODO: Possibly introduce a second type of confirmation
// that both probes (a) on-link neighbors and (b) does
// a DHCPv4 RENEW. We used to do this on Wi-Fi framework
// roams.
if (mIpReachabilityMonitor != null) {
mIpReachabilityMonitor.probeAll();
}
break;
......
}
}这里会发现所有的操作都会由IpReachabilityMonitor处理，我们再回头看它的构造实现：
@VisibleForTesting
IpReachabilityMonitor(InterfaceParams ifParams, Handler h, SharedLog log, Callback callback,
MultinetworkPolicyTracker tracker, Dependencies dependencies) {
if (ifParams == null) throw new IllegalArgumentException("null InterfaceParams");
mInterfaceParams = ifParams;
mLog = log.forSubComponent(TAG);
mCallback = callback;
mMultinetworkPolicyTracker = tracker;
mDependencies = dependencies;
mIpNeighborMonitor = new IpNeighborMonitor(h, mLog,
(NeighborEvent event) -> {
if (mInterfaceParams.index != event.ifindex) return;
if (!mNeighborWatchList.containsKey(event.ip)) return;
final NeighborEvent prev = mNeighborWatchList.put(event.ip, event);
// TODO: Consider what to do with other states that are not within
// NeighborEvent#isValid() (i.e. NUD_NONE, NUD_INCOMPLETE).
if (event.nudState == StructNdMsg.NUD_FAILED) {
mLog.w("ALERT neighbor went from: " + prev + " to: " + event);
handleNeighborLost(event);
}
});
mIpNeighborMonitor.start();
}mCallback保存了我们传入的Callback对象，它实现了notifyLost()函数；IpNeighborMonitor会接受、解析来自kernel的packet，包含了我们需要monitor哪些IP，以及接收NUD lost的结果，并调用handleNeighborLost()进行接下去通知WiFi fwk NUD lost结果的处理。
/**
* IpNeighborMonitor.
*
* Monitors the kernel rtnetlink neighbor notifications and presents to callers
* NeighborEvents describing each event. Callers can provide a consumer instance
* to both filter (e.g. by interface index and IP address) and handle the
* generated NeighborEvents.
*
* @hide
*/
public class IpNeighborMonitor extends PacketReader {
......
public static class NeighborEvent {
final long elapsedMs;
final short msgType;
final int ifindex;
final InetAddress ip;
final short nudState;
final MacAddress macAddr;
public NeighborEvent(long elapsedMs, short msgType, int ifindex, InetAddress ip,
short nudState, MacAddress macAddr) {
this.elapsedMs = elapsedMs;
this.msgType = msgType;
this.ifindex = ifindex;
this.ip = ip;
this.nudState = nudState;
this.macAddr = macAddr;
}
boolean isConnected() {
return (msgType != RTM_DELNEIGH) && StructNdMsg.isNudStateConnected(nudState);
}
boolean isValid() {
return (msgType != RTM_DELNEIGH) && StructNdMsg.isNudStateValid(nudState);
}
@Override
public String toString() {
final StringJoiner j = new StringJoiner(",", "NeighborEvent{", "}");
return j.add("@" + elapsedMs)
.add(stringForNlMsgType(msgType))
.add("if=" + ifindex)
.add(ip.getHostAddress())
.add(StructNdMsg.stringForNudState(nudState))
.add("[" + macAddr + "]")
.toString();
}
}
public interface NeighborEventConsumer {
// Every neighbor event received on the netlink socket is passed in
// here. Subclasses should filter for events of interest.
public void accept(NeighborEvent event);
}
private final SharedLog mLog;
private final NeighborEventConsumer mConsumer;
public IpNeighborMonitor(Handler h, SharedLog log, NeighborEventConsumer cb) {
super(h, NetlinkSocket.DEFAULT_RECV_BUFSIZE);
mLog = log.forSubComponent(TAG);
mConsumer = (cb != null) ? cb : (event) -> { /* discard */ };
}
@Override
protected FileDescriptor createFd() {
FileDescriptor fd = null;
try {
fd = NetlinkSocket.forProto(OsConstants.NETLINK_ROUTE);
Os.bind(fd, (SocketAddress)(new NetlinkSocketAddress(0, OsConstants.RTMGRP_NEIGH)));
Os.connect(fd, (SocketAddress)(new NetlinkSocketAddress(0, 0)));
if (VDBG) {
final NetlinkSocketAddress nlAddr = (NetlinkSocketAddress) Os.getsockname(fd);
Log.d(TAG, "bound to sockaddr_nl{"
+ BitUtils.uint32(nlAddr.getPortId()) + ", "
+ nlAddr.getGroupsMask()
+ "}");
}
} catch (ErrnoException|SocketException e) {
logError("Failed to create rtnetlink socket", e);
IoUtils.closeQuietly(fd);
return null;
}
return fd;
}
......
}IpNeighborMonitor接收来自IpReachabilityMonitor的处理，创建IpNeighborMonitor的时候，传入了一个用lambda表达式创建的NeighborEventConsumer对象，它实现了accept函数，主要处理：
1、解析从kernel上报的需要监听的IP地址集，它保存在mNeighborWatchList集合中
2、判断当前的event是不是通知NUD_FAILED，如果是就调用handleNeighborLost()处理：
IpNeighborMonitor.start()主要是创建监听的socket，并开始等待接收packet、并将其解析处理：
/**
* This class encapsulates the mechanics of registering a file descriptor
* with a thread's Looper and handling read events (and errors).
*
* Subclasses MUST implement createFd() and SHOULD override handlePacket().
* Subclasses can expect a call life-cycle like the following:
*
* [1] start() calls createFd() and (if all goes well) onStart()
*
* [2] yield, waiting for read event or error notification:
*
* [a] readPacket() && handlePacket()
*
* [b] if (no error):
* goto 2
* else:
* goto 3
*
* [3] stop() calls onStop() if not previously stopped
*
* The packet receive buffer is recycled on every read call, so subclasses
* should make any copies they would like inside their handlePacket()
* implementation.
*
* All public methods MUST only be called from the same thread with which
* the Handler constructor argument is associated.
*
* TODO: rename this class to something more correctly descriptive (something
* like [or less horrible than] FdReadEventsHandler?).
*
* @hide
*/
public abstract class PacketReader {
......
public final void start() {
if (onCorrectThread()) {
createAndRegisterFd();
} else {
mHandler.post(() -> {
logError("start() called from off-thread", null);
createAndRegisterFd();
});
}
}
......
private void createAndRegisterFd() {
if (mFd != null) return;
try {
mFd = createFd();
if (mFd != null) {
// Force the socket to be non-blocking.
IoUtils.setBlocking(mFd, false);
}
} catch (Exception e) {
logError("Failed to create socket: ", e);
closeFd(mFd);
mFd = null;
return;
}
......
}
/**
* IpNeighborMonitor.
*
* Monitors the kernel rtnetlink neighbor notifications and presents to callers
* NeighborEvents describing each event. Callers can provide a consumer instance
* to both filter (e.g. by interface index and IP address) and handle the
* generated NeighborEvents.
*
* @hide
*/
public class IpNeighborMonitor extends PacketReader {
......
@Override
protected FileDescriptor createFd() {
FileDescriptor fd = null;
try {
fd = NetlinkSocket.forProto(OsConstants.NETLINK_ROUTE);
Os.bind(fd, (SocketAddress)(new NetlinkSocketAddress(0, OsConstants.RTMGRP_NEIGH)));
Os.connect(fd, (SocketAddress)(new NetlinkSocketAddress(0, 0)));
if (VDBG) {
final NetlinkSocketAddress nlAddr = (NetlinkSocketAddress) Os.getsockname(fd);
Log.d(TAG, "bound to sockaddr_nl{"
+ BitUtils.uint32(nlAddr.getPortId()) + ", "
+ nlAddr.getGroupsMask()
+ "}");
}
} catch (ErrnoException|SocketException e) {
logError("Failed to create rtnetlink socket", e);
IoUtils.closeQuietly(fd);
return null;
}
return fd;
}
......
}
/**
* NetlinkSocket
*
* A small static class to assist with AF_NETLINK socket operations.
*
* @hide
*/
public class NetlinkSocket {
......
public static FileDescriptor forProto(int nlProto) throws ErrnoException {
final FileDescriptor fd = Os.socket(AF_NETLINK, SOCK_DGRAM, nlProto);
Os.setsockoptInt(fd, SOL_SOCKET, SO_RCVBUF, SOCKET_RECV_BUFSIZE);
return fd;
}
......
}具体解析packet的过程这里不再看了，有需要可以分析下IpNeighborMonitor以及起父类PacketReader的代码即可。
再看下前面还未分析的IpReachabilityMonitor::probeAll()调用：
public void probeAll() {
final List<InetAddress> ipProbeList = new ArrayList<>(mNeighborWatchList.keySet());
if (!ipProbeList.isEmpty()) {
// Keep the CPU awake long enough to allow all ARP/ND
// probes a reasonable chance at success. See b/23197666.
//
// The wakelock we use is (by default) refcounted, and this version
// of acquire(timeout) queues a release message to keep acquisitions
// and releases balanced.
mDependencies.acquireWakeLock(getProbeWakeLockDuration());
}
for (InetAddress ip : ipProbeList) {
final int rval = IpNeighborMonitor.startKernelNeighborProbe(mInterfaceParams.index, ip);
mLog.log(String.format("put neighbor %s into NUD_PROBE state (rval=%d)",
ip.getHostAddress(), rval));
logEvent(IpReachabilityEvent.PROBE, rval);
}
mLastProbeTimeMs = SystemClock.elapsedRealtime();
}probeAll()中会遍历mNeighborWatchList的IP地址，分别对其进行NUD检测：
/**
* IpNeighborMonitor.
*
* Monitors the kernel rtnetlink neighbor notifications and presents to callers
* NeighborEvents describing each event. Callers can provide a consumer instance
* to both filter (e.g. by interface index and IP address) and handle the
* generated NeighborEvents.
*
* @hide
*/
public class IpNeighborMonitor extends PacketReader {
......
/**
* Make the kernel perform neighbor reachability detection (IPv4 ARP or IPv6 ND)
* for the given IP address on the specified interface index.
*
* @return 0 if the request was successfully passed to the kernel; otherwise return
* a non-zero error code.
*/
public static int startKernelNeighborProbe(int ifIndex, InetAddress ip) {
final String msgSnippet = "probing ip=" + ip.getHostAddress() + "%" + ifIndex;
if (DBG) { Log.d(TAG, msgSnippet); }
final byte[] msg = RtNetlinkNeighborMessage.newNewNeighborMessage(
1, ip, StructNdMsg.NUD_PROBE, ifIndex, null);
try {
NetlinkSocket.sendOneShotKernelMessage(OsConstants.NETLINK_ROUTE, msg);
} catch (ErrnoException e) {
Log.e(TAG, "Error " + msgSnippet + ": " + e);
return -e.errno;
}
return 0;
}
......
}通过Netlink机制请求kernel进行probe后，FWK能做的就是等待结果了；如果kernel检测遇到了NUD失败，这个信息经过packet解析、封装成event之后，会由IpNeighborMonitor::NeighborEventConsumer mConsumer处理：
/**
* IpNeighborMonitor.
*
* Monitors the kernel rtnetlink neighbor notifications and presents to callers
* NeighborEvents describing each event. Callers can provide a consumer instance
* to both filter (e.g. by interface index and IP address) and handle the
* generated NeighborEvents.
*
* @hide
*/
public class IpNeighborMonitor extends PacketReader {
......
private void evaluateRtNetlinkNeighborMessage(
RtNetlinkNeighborMessage neighMsg, long whenMs) {
final short msgType = neighMsg.getHeader().nlmsg_type;
final StructNdMsg ndMsg = neighMsg.getNdHeader();
if (ndMsg == null) {
mLog.e("RtNetlinkNeighborMessage without ND message header!");
return;
}
final int ifindex = ndMsg.ndm_ifindex;
final InetAddress destination = neighMsg.getDestination();
final short nudState =
(msgType == RTM_DELNEIGH)
? StructNdMsg.NUD_NONE
: ndMsg.ndm_state;
final NeighborEvent event = new NeighborEvent(
whenMs, msgType, ifindex, destination, nudState,
getMacAddress(neighMsg.getLinkLayerAddress()));
if (VDBG) {
Log.d(TAG, neighMsg.toString());
}
if (DBG) {
Log.d(TAG, event.toString());
}
mConsumer.accept(event);
}
......
}mConsumer也就是IpReachabilityMonitor创建IpNeighborMonitor时，用lambda表达式创建的对象：
mIpNeighborMonitor = new IpNeighborMonitor(h, mLog,
(NeighborEvent event) -> {
if (mInterfaceParams.index != event.ifindex) return;
if (!mNeighborWatchList.containsKey(event.ip)) return;
final NeighborEvent prev = mNeighborWatchList.put(event.ip, event);
// TODO: Consider what to do with other states that are not within
// NeighborEvent#isValid() (i.e. NUD_NONE, NUD_INCOMPLETE).
if (event.nudState == StructNdMsg.NUD_FAILED) {
mLog.w("ALERT neighbor went from: " + prev + " to: " + event);
handleNeighborLost(event);
}
});
mIpNeighborMonitor.start();如果NeighborEvent的msg是NUD_FAILED，说明NUD检测失败，需要通知给上层这个事件：
public class IpReachabilityMonitor {
......
private void handleNeighborLost(NeighborEvent event) {
final LinkProperties whatIfLp = new LinkProperties(mLinkProperties);
InetAddress ip = null;
for (Map.Entry<InetAddress, NeighborEvent> entry : mNeighborWatchList.entrySet()) {
// TODO: Consider using NeighborEvent#isValid() here; it's more
// strict but may interact badly if other entries are somehow in
// NUD_INCOMPLETE (say, during network attach).
if (entry.getValue().nudState != StructNdMsg.NUD_FAILED) continue;
ip = entry.getKey();
for (RouteInfo route : mLinkProperties.getRoutes()) {
if (ip.equals(route.getGateway())) {
whatIfLp.removeRoute(route);
}
}
if (avoidingBadLinks() || !(ip instanceof Inet6Address)) {
// We should do this unconditionally, but alas we cannot: b/31827713.
whatIfLp.removeDnsServer(ip);
}
}
final ProvisioningChange delta = LinkProperties.compareProvisioning(
mLinkProperties, whatIfLp);
if (delta == ProvisioningChange.LOST_PROVISIONING) {
final String logMsg = "FAILURE: LOST_PROVISIONING, " + event;
Log.w(TAG, logMsg);
if (mCallback != null) {
// TODO: remove |ip| when the callback signature no longer has
// an InetAddress argument.
mCallback.notifyLost(ip, logMsg);
}
}
logNudFailed(delta);
}
......
}
随之会通过LinkProperties.compareProvisioning()比较当前两个IP的配置信息来判断，连接是否已经不可达了，如果是就会通过mCallback对象回调notifyLost()通知上层，由前面可知这个Callback对象经过了几次封装，为了节省时间我们直接看,WifiStateMachine中最原始的那个Callback实现：
WifiStateMachine:
class IpClientCallback extends IpClient.Callback {
@Override
public void onPreDhcpAction() {
sendMessage(DhcpClient.CMD_PRE_DHCP_ACTION);
}
......
@Override
public void onReachabilityLost(String logMsg) {
mWifiMetrics.logStaEvent(StaEvent.TYPE_CMD_IP_REACHABILITY_LOST);
sendMessage(CMD_IP_REACHABILITY_LOST, logMsg);
}
......
}IpClientCallback::onReachabilityLost()会被调用，并发送CMD_IP_REACHABILITY_LOST msg，看该msg的处理过程：
class L2ConnectedState extends State {
@Override
public boolean processMessage(Message message) {
......
case CMD_IP_REACHABILITY_LOST:
if (mVerboseLoggingEnabled && message.obj != null) log((String) message.obj);
if (mIpReachabilityDisconnectEnabled) {
handleIpReachabilityLost();
transitionTo(mDisconnectingState);
} else {
logd("CMD_IP_REACHABILITY_LOST but disconnect disabled -- ignore");
}
......
}
......
}L2ConnectedState会处理CMD_IP_REACHABILITY_LOST msg，如果mIpReachabilityDisconnectEnabled变量为true，就会去主动disconnect WiFi，并将状态transfer到DisconnectingState处理WiFi断连过程：
public class WifiStateMachine extends StateMachine {
......
private boolean mIpReachabilityDisconnectEnabled = true;
......
// TODO: De-duplicated this and handleIpConfigurationLost().
private void handleIpReachabilityLost() {
mWifiInfo.setInetAddress(null);
mWifiInfo.setMeteredHint(false);
// TODO: Determine whether to call some form of mWifiConfigManager.handleSSIDStateChange().
// Disconnect via supplicant, and let autojoin retry connecting to the network.
mWifiNative.disconnect(mInterfaceName);
}
......
}
调用WifiNative::disconnect()之后，WiFi触发断连，WiFiStateMachine监听wpa_supplicat状态，最后会transfer到DisconnectedState，整个过程结束。
上面我们梳理了下NUD检测导致WiFi自动断连的流程，这里我们只要把
private boolean mIpReachabilityDisconnectEnabled = true;改成false就可以了，这样就会忽略NUD的检测结果不去断开WiFi连接了，避免对用户产生困扰；当然也可以加一些其他的流程来进行处理，也是可以的。