Android netd工作原理详解

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NETD是Android一个专门管理网络链接, 路由/带宽/防火墙策略以及iptables的系统Daemon进程, 其在Anroid系统启动时加载 :

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service netd /system/bin/netd
    class main
    socket netd stream 0660 root system
    socket dnsproxyd stream 0660 root inet
    socket mdns stream 0660 root system
    socket fwmarkd stream 0660 root inet

启动netd时, 会创建四个socket,用于其他进程与netd进行通信:

  • netd: 主要与Framework的NetworkManagementService交互, 用于控制网口状态, 路由表
  • dnsproxyd: DNS代理的控制与配置，用于私有DNS（DNS Over TLS)的请求转发
  • mdns: 多播DNS（Multicast DNS，参考RFChttps://tools.ietf.org/html/rfc6762), 用于基于WIFI连接的服务发现（NSD, Network Service Discovery)
  • fwmarkd: iptables的(fwmark)策略路由的配置(策略路由, 如设置网络权限， 连接打标签等

总的说来, netd进程在Android中间层服务NetworkManagementService以及内核之间建立了一个沟通的桥梁:

  • 对Java层系统服务NetworkManagementService,netd通过socket接口为其提供了控制指令的通道
  • 对内核, netd通过netlink socket与内核进行数据/指令的收发, 并将来自内核的消息发送给上层
  • 从Android 8.0开始， Netd同时提供了一个HAL Binder的接口提供给其他进程调用INetd.aidl

接下来, 本文从三个方面来看下netd的具体工作原理与实现.

  • netd的初始化与启动
  • NetworkManagementService与netd的交互
  • netd与内核的交互

关于netlink socket可以参考: http://qos.ittc.ku.edu/netlink/html/node4.html

netd的启动与初始化

netd进程启动时, 主要处理做以下事情:

  • 创建一个NetlinkManager, 用于管理与内核通信的netlink连接
  • 初始化网络控制类, 如路由控制RouteController, 带宽控制BandwidthController
  • 启动各类事件监听类: DnsProxyListener监听DNS代理; CommandListener监听来自NetworkManagement的指令
  • 启动NetdHwService, 为HAL层提供接口
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int main() {
    using android::net::gCtls;
    Stopwatch s;

    ALOGI("Netd 1.0 starting");

    blockSigpipe();
    

    NetlinkManager *nm = NetlinkManager::Instance();
    if (nm == nullptr) {
	ALOGE("Unable to create NetlinkManager");
	exit(1);
    };

    gCtls = new android::net::Controllers();
    gCtls->init();

    CommandListener cl;
    nm->setBroadcaster((SocketListener *) &cl);

    if (nm->start()) {
		ALOGE("Unable to start NetlinkManager (%s)", strerror(errno));
		exit(1);
    }

    std::unique_ptr<NFLogListener> logListener;
    {
	auto result = makeNFLogListener();
	if (!isOk(result)) {
	    ALOGE("Unable to create NFLogListener: %s", toString(result).c_str());
	    exit(1);
	}
	logListener = std::move(result.value());
	auto status = gCtls->wakeupCtrl.init(logListener.get());
	if (!isOk(result)) {
	    ALOGE("Unable to init WakeupController: %s", toString(result).c_str());
	    // We can still continue without wakeup packet logging.
	}
    }

    // Set local DNS mode, to prevent bionic from proxying
    // back to this service, recursively.
    setenv("ANDROID_DNS_MODE", "local", 1);
    DnsProxyListener dpl(&gCtls->netCtrl, &gCtls->eventReporter);
    if (dpl.startListener()) {
		ALOGE("Unable to start DnsProxyListener (%s)", strerror(errno));
		exit(1);
    }

    MDnsSdListener mdnsl;
    if (mdnsl.startListener()) {
		ALOGE("Unable to start MDnsSdListener (%s)", strerror(errno));
		exit(1);
    }

    FwmarkServer fwmarkServer(&gCtls->netCtrl, &gCtls->eventReporter);
    if (fwmarkServer.startListener()) {
		ALOGE("Unable to start FwmarkServer (%s)", strerror(errno));
		exit(1);
    }

    Stopwatch subTime;
    status_t ret;
    if ((ret = NetdNativeService::start()) != android::OK) {
		ALOGE("Unable to start NetdNativeService: %d", ret);
		exit(1);
    }
    ALOGI("Registering NetdNativeService: %.1fms", subTime.getTimeAndReset());

    /*
     * Now that we're up, we can respond to commands. Starting the listener also tells
     * NetworkManagementService that we are up and that our binder interface is ready.
     */
    if (cl.startListener()) {
		ALOGE("Unable to start CommandListener (%s)", strerror(errno));
		exit(1);
    }
    ALOGI("Starting CommandListener: %.1fms", subTime.getTimeAndReset());

    write_pid_file();

    // Now that netd is ready to process commands, advertise service
    // availability for HAL clients.
    NetdHwService mHwSvc;
    if ((ret = mHwSvc.start()) != android::OK) {
		ALOGE("Unable to start NetdHwService: %d", ret);
		exit(1);
    }
    ALOGI("Registering NetdHwService: %.1fms", subTime.getTimeAndReset());

    ALOGI("Netd started in %dms", static_cast<int>(s.timeTaken()));

    IPCThreadState::self()->joinThreadPool();

    ALOGI("Netd exiting");

    remove_pid_file();

    exit(0);
}

CommandListener用于接收处理来自上层NetworkManagementService指令, 在netd启动时, 会监听netd这个socket, 并允许最多4个客户端请求的处理:

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int SocketListener::startListener() {
    return startListener(4);
}

int SocketListener::startListener(int backlog) {

    if (!mSocketName && mSock == -1) {
		SLOGE("Failed to start unbound listener");
		errno = EINVAL;
		return -1;
    } else if (mSocketName) {
		if ((mSock = android_get_control_socket(mSocketName)) < 0) {
			SLOGE("Obtaining file descriptor socket '%s' failed: %s",
				mSocketName, strerror(errno));
			return -1;
		}
		SLOGV("got mSock = %d for %s", mSock, mSocketName);
		fcntl(mSock, F_SETFD, FD_CLOEXEC);
    }

    if (mListen && listen(mSock, backlog) < 0) {
		SLOGE("Unable to listen on socket (%s)", strerror(errno));
		return -1;
    } else if (!mListen)
		mClients->push_back(new SocketClient(mSock, false, mUseCmdNum));

    if (pipe(mCtrlPipe)) {
		SLOGE("pipe failed (%s)", strerror(errno));
		return -1;
    }

    if (pthread_create(&mThread, NULL, SocketListener::threadStart, this)) {
		SLOGE("pthread_create (%s)", strerror(errno));
		return -1;
    }

    return 0;
}

这里创建了一个专门的线程用于处理来自上层客户端的命令请求:

  • 检查控制pipe的数据,是否有必要停止监听;
  • 监听来自客户端的请求, 如果有, 则新建一个SocketClient保存下来
  • 从已有的SocketClient中检查是否有可用的数据, 如果有则通过onDataAvailable
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void *SocketListener::threadStart(void *obj) {
    SocketListener *me = reinterpret_cast<SocketListener *>(obj);

    me->runListener();
    pthread_exit(NULL);
    return NULL;
}

void SocketListener::runListener() {

    SocketClientCollection pendingList;

    while(1) {
	SocketClientCollection::iterator it;
	fd_set read_fds;
	int rc = 0;
	int max = -1;

	FD_ZERO(&read_fds);

	if (mListen) {
	    max = mSock;
	    FD_SET(mSock, &read_fds);
	}

	FD_SET(mCtrlPipe[0], &read_fds);
	if (mCtrlPipe[0] > max)
	    max = mCtrlPipe[0];

	pthread_mutex_lock(&mClientsLock);
	for (it = mClients->begin(); it != mClients->end(); ++it) {
	    // NB: calling out to an other object with mClientsLock held (safe)
	    int fd = (*it)->getSocket();
	    FD_SET(fd, &read_fds);
	    if (fd > max) {
	        max = fd;
	    }
	}
	pthread_mutex_unlock(&mClientsLock);
	SLOGV("mListen=%d, max=%d, mSocketName=%s", mListen, max, mSocketName);
	if ((rc = select(max + 1, &read_fds, NULL, NULL, NULL)) < 0) {
	    if (errno == EINTR)
	        continue;
	    SLOGE("select failed (%s) mListen=%d, max=%d", strerror(errno), mListen, max);
	    sleep(1);
	    continue;
	} else if (!rc)
	    continue;

	if (FD_ISSET(mCtrlPipe[0], &read_fds)) {
	    char c = CtrlPipe_Shutdown;
	    TEMP_FAILURE_RETRY(read(mCtrlPipe[0], &c, 1));
	    if (c == CtrlPipe_Shutdown) {
	        break;
	    }
	    continue;
	}
	if (mListen && FD_ISSET(mSock, &read_fds)) {
	    int c = TEMP_FAILURE_RETRY(accept4(mSock, nullptr, nullptr, SOCK_CLOEXEC));
	    if (c < 0) {
	        SLOGE("accept failed (%s)", strerror(errno));
	        sleep(1);
	        continue;
	    }
	    pthread_mutex_lock(&mClientsLock);
	    mClients->push_back(new SocketClient(c, true, mUseCmdNum));
	    pthread_mutex_unlock(&mClientsLock);
	}

	/* Add all active clients to the pending list first */
	pendingList.clear();
	pthread_mutex_lock(&mClientsLock);
	for (it = mClients->begin(); it != mClients->end(); ++it) {
	    SocketClient* c = *it;
	    // NB: calling out to an other object with mClientsLock held (safe)
	    int fd = c->getSocket();
	    if (FD_ISSET(fd, &read_fds)) {
	        pendingList.push_back(c);
	        c->incRef();
	    }
	}
	pthread_mutex_unlock(&mClientsLock);

	/* Process the pending list, since it is owned by the thread,
	 * there is no need to lock it */
	while (!pendingList.empty()) {
	    /* Pop the first item from the list */
	    it = pendingList.begin();
	    SocketClient* c = *it;
	    pendingList.erase(it);
	    /* Process it, if false is returned, remove from list */
	    if (!onDataAvailable(c)) {
	        release(c, false);
	    }
	    c->decRef();
	}
    }
}

netd启动完成后, 就可以处理来自中间层的指令请求以及与内核进行交互了.

netd与NetworkManagerService的交互

SystemServer进程启动时, 创建NetworkManagementService(以下简称(NMS)), 此时NMS会主动与netd建立socket链接:

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// SystemServer.java
       if (!disableNetwork) {
           traceBeginAndSlog("StartNetworkManagementService");
           try {
               networkManagement = NetworkManagementService.create(context);
               ServiceManager.addService(Context.NETWORKMANAGEMENT_SERVICE, networkManagement);
           } catch (Throwable e) {
               reportWtf("starting NetworkManagement Service", e);
           }
           traceEnd();
       }

创建NMS时, 启动一个新的线程用于与netd通信,

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    static NetworkManagementService create(Context context, String socket)
	    throws InterruptedException {
	final NetworkManagementService service = new NetworkManagementService(context, socket);
	final CountDownLatch connectedSignal = service.mConnectedSignal;
	if (DBG) Slog.d(TAG, "Creating NetworkManagementService");
	service.mThread.start();
	if (DBG) Slog.d(TAG, "Awaiting socket connection");
	connectedSignal.await();
	service.connectNativeNetdService();

	return service;
    }
    

       private NetworkManagementService(Context context, String socket) {
	mContext = context;

	// make sure this is on the same looper as our NativeDaemonConnector for sync purposes
	mFgHandler = new Handler(FgThread.get().getLooper());

	// Don't need this wake lock, since we now have a time stamp for when
	// the network actually went inactive.  (It might be nice to still do this,
	// but I don't want to do it through the power manager because that pollutes the
	// battery stats history with pointless noise.)
	//PowerManager pm = (PowerManager)context.getSystemService(Context.POWER_SERVICE);
	PowerManager.WakeLock wl = null; //pm.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK, NETD_TAG);

	mConnector = new NativeDaemonConnector(
		new NetdCallbackReceiver(), socket, 10, NETD_TAG, 160, wl,
		FgThread.get().getLooper());
	mThread = new Thread(mConnector, NETD_TAG);

	mDaemonHandler = new Handler(FgThread.get().getLooper());

	// Add ourself to the Watchdog monitors.
	Watchdog.getInstance().addMonitor(this);

	LocalServices.addService(NetworkManagementInternal.class, new LocalService());

	synchronized (mTetheringStatsProviders) {
	    mTetheringStatsProviders.put(new NetdTetheringStatsProvider(), "netd");
	}
}

NMS通过NativeDaemonConnector与netd建立socket通信, NativeDaemonConnector主要做两个事情:

  • 与netd建立一个数据链接
  • 不断读取socket中的数据流: 一种是netd主动上报的命令, 一种是NMS发送给netd后的指令的响应
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@Override
public void run() {
    mCallbackHandler = new Handler(mLooper, this);

    while (true) {
        try {
            listenToSocket();
        } catch (Exception e) {
            loge("Error in NativeDaemonConnector: " + e);
            SystemClock.sleep(5000);
        }
    }
}


    private void listenToSocket() throws IOException {
    LocalSocket socket = null;

    try {
        socket = new LocalSocket();
        LocalSocketAddress address = determineSocketAddress();

        socket.connect(address);

        InputStream inputStream = socket.getInputStream();
        synchronized (mDaemonLock) {
            mOutputStream = socket.getOutputStream();
        }

        mCallbacks.onDaemonConnected();

        FileDescriptor[] fdList = null;
        byte[] buffer = new byte[BUFFER_SIZE];
        int start = 0;

        while (true) {
            int count = inputStream.read(buffer, start, BUFFER_SIZE - start);
            if (count < 0) {
                loge("got " + count + " reading with start = " + start);
                break;
            }
            fdList = socket.getAncillaryFileDescriptors();

            // Add our starting point to the count and reset the start.
            count += start;
            start = 0;

            for (int i = 0; i < count; i++) {
                if (buffer[i] == 0) {
                    // Note - do not log this raw message since it may contain
                    // sensitive data
                    final String rawEvent = new String(
                            buffer, start, i - start, StandardCharsets.UTF_8);

                    boolean releaseWl = false;
                    try {
                        final NativeDaemonEvent event =
                                NativeDaemonEvent.parseRawEvent(rawEvent, fdList);

                        log("RCV <- {" + event + "}");

                        if (event.isClassUnsolicited()) {
                            Message msg = mCallbackHandler.obtainMessage(
                                    event.getCode(), uptimeMillisInt(), 0, event.getRawEvent());
                            if (mCallbackHandler.sendMessage(msg)) {
                                releaseWl = false;
                            }
                        } else {
                            mResponseQueue.add(event.getCmdNumber(), event);
                        }
                    } catch (IllegalArgumentException e) {
                        log("Problem parsing message " + e);
                    } finally {
                        if (releaseWl) {
                            mWakeLock.release();
                        }
                    }

                    start = i + 1;
                }
            }

            // We should end at the amount we read. If not, compact then
            // buffer and read again.
            if (start != count) {
                final int remaining = BUFFER_SIZE - start;
                System.arraycopy(buffer, start, buffer, 0, remaining);
                start = remaining;
            } else {
                start = 0;
            }
        }
    } catch (IOException ex) {
        loge("Communications error: " + ex);
        throw ex;
    } finally {
        synchronized (mDaemonLock) {
            if (mOutputStream != null) {
                try {
                    loge("closing stream for " + mSocket);
                    mOutputStream.close();
                } catch (IOException e) {
                    loge("Failed closing output stream: " + e);
                }
                mOutputStream = null;
            }
        }

        try {
            if (socket != null) {
                socket.close();
            }
        } catch (IOException ex) {
            loge("Failed closing socket: " + ex);
        }
    }
}

socket链接建立完成之后, NMS与netd可以相互通信, 发送指令与数据了. NMS通过NativeDaemonConnector执行相应的指令, 比如NMS设置网络接口的配置(打开/关闭网口):

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@Override
public void setInterfaceConfig(String iface, InterfaceConfiguration cfg) {
    mContext.enforceCallingOrSelfPermission(CONNECTIVITY_INTERNAL, TAG);
    LinkAddress linkAddr = cfg.getLinkAddress();
    if (linkAddr == null || linkAddr.getAddress() == null) {
        throw new IllegalStateException("Null LinkAddress given");
    }

    final Command cmd = new Command("interface", "setcfg", iface,
            linkAddr.getAddress().getHostAddress(),
            linkAddr.getPrefixLength());
    for (String flag : cfg.getFlags()) {
        cmd.appendArg(flag);
    }

    try {
        mConnector.execute(cmd);
    } catch (NativeDaemonConnectorException e) {
        throw e.rethrowAsParcelableException();
    }
}

NativeDaemonConnector会将每个指令都指定一个唯一的序列, 并将其响应放到一个阻塞队列, 等待netd返回指令的结果, 如果超过指定的超时时间, 则抛出一个超时的异常.

在第一部分时, 讲到SocketListener拿到上层发过来的指令后, 会将其分发给对应的指令类进行处理(看SocketListener的子类FrameworkListener):

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bool FrameworkListener::onDataAvailable(SocketClient *c) {
    char buffer[CMD_BUF_SIZE];
    int len;

    len = TEMP_FAILURE_RETRY(read(c->getSocket(), buffer, sizeof(buffer)));
    if (len < 0) {
	SLOGE("read() failed (%s)", strerror(errno));
	return false;
    } else if (!len) {
	return false;
    } else if (buffer[len-1] != '\0') {
	SLOGW("String is not zero-terminated");
	android_errorWriteLog(0x534e4554, "29831647");
	c->sendMsg(500, "Command too large for buffer", false);
	mSkipToNextNullByte = true;
	return true;
    }

    int offset = 0;
    int i;

    for (i = 0; i < len; i++) {
	if (buffer[i] == '\0') {
	    /* IMPORTANT: dispatchCommand() expects a zero-terminated string */
	    if (mSkipToNextNullByte) {
	        mSkipToNextNullByte = false;
	    } else {
	        dispatchCommand(c, buffer + offset);
	    }
	    offset = i + 1;
	}
    }

    mSkipToNextNullByte = false;
    return true;
}

函数dispatchCommmand会检查所有指令处理类, 匹配相应的类去执行指令:

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void FrameworkListener::dispatchCommand(SocketClient *cli, char *data) {
    FrameworkCommandCollection::iterator i;
    int argc = 0;
    char *argv[FrameworkListener::CMD_ARGS_MAX];
    char tmp[CMD_BUF_SIZE];
    char *p = data;
    char *q = tmp;
    char *qlimit = tmp + sizeof(tmp) - 1;
    bool esc = false;
    bool quote = false;
    bool haveCmdNum = !mWithSeq;

    memset(argv, 0, sizeof(argv));
    memset(tmp, 0, sizeof(tmp));
    ...
    *q = '\0';
    if (argc >= CMD_ARGS_MAX)
	goto overflow;
    argv[argc++] = strdup(tmp);

    if (quote) {
	cli->sendMsg(500, "Unclosed quotes error", false);
	goto out;
    }

    if (errorRate && (++mCommandCount % errorRate == 0)) {
	/* ignore this command - let the timeout handler handle it */
	SLOGE("Faking a timeout");
	goto out;
    }

    for (i = mCommands->begin(); i != mCommands->end(); ++i) {
	FrameworkCommand *c = *i;

	if (!strcmp(argv[0], c->getCommand())) {
	    if (c->runCommand(cli, argc, argv)) {
	        SLOGW("Handler '%s' error (%s)", c->getCommand(), strerror(errno));
	    }
	    goto out;
	}
    }
    cli->sendMsg(500, "Command not recognized", false);
out:
    int j;
    for (j = 0; j < argc; j++)
	free(argv[j]);
    return;

overflow:
    cli->sendMsg(500, "Command too long", false);
    goto out;
}

比如这里执行的是, 网络接口相关的设置, 因此会调用InterfaceCmd来执行该指令.

NETD与内核进行交互

NETD通过netlink事件与内核进行消息的交换.在第一部分时看到, netd启动时, 会配置socket与内核进行通信:

  • netlink事件NETLINK_KOBJECT_UEVENT: 用于内核向netd发生消息, 如网口的状态变化;
  • netlink事件NETLINK_ROUTE:用于接收路由信息, 如路由表的更新与删除;
  • netlink事件NETLINK_NFLOG:用于接收数据流量使用配额的消息, 如数据使用超限;
  • netlink事件NETLINK_NETFILTER用于接收包过滤(netfilter)的消息;
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int NetlinkManager::start() {
    if ((mUeventHandler = setupSocket(&mUeventSock, NETLINK_KOBJECT_UEVENT,
	 0xffffffff, NetlinkListener::NETLINK_FORMAT_ASCII, false)) == NULL) {
	return -1;
    }

    if ((mRouteHandler = setupSocket(&mRouteSock, NETLINK_ROUTE,
	                             RTMGRP_LINK |
	                             RTMGRP_IPV4_IFADDR |
	                             RTMGRP_IPV6_IFADDR |
	                             RTMGRP_IPV6_ROUTE |
	                             (1 << (RTNLGRP_ND_USEROPT - 1)),
	 NetlinkListener::NETLINK_FORMAT_BINARY, false)) == NULL) {
	return -1;
    }

    if ((mQuotaHandler = setupSocket(&mQuotaSock, NETLINK_NFLOG,
	    NFLOG_QUOTA_GROUP, NetlinkListener::NETLINK_FORMAT_BINARY, false)) == NULL) {
	ALOGW("Unable to open qlog quota socket, check if xt_quota2 can send via UeventHandler");
	// TODO: return -1 once the emulator gets a new kernel.
    }

    if ((mStrictHandler = setupSocket(&mStrictSock, NETLINK_NETFILTER,
	    0, NetlinkListener::NETLINK_FORMAT_BINARY_UNICAST, true)) == NULL) {
	ALOGE("Unable to open strict socket");
	// TODO: return -1 once the emulator gets a new kernel.
    }

    return 0;
}

每一个netlink的socket都会新建一个NetlinkHandler, 用于处理内核的消息, 并将该消息广播给上层:

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void NetlinkHandler::onEvent(NetlinkEvent *evt) {
    const char *subsys = evt->getSubsystem();
    if (!subsys) {
	ALOGW("No subsystem found in netlink event");
	return;
    }

    if (!strcmp(subsys, "net")) {
	NetlinkEvent::Action action = evt->getAction();
	const char *iface = evt->findParam("INTERFACE");

	if (action == NetlinkEvent::Action::kAdd) {
	    notifyInterfaceAdded(iface);
	} else if (action == NetlinkEvent::Action::kRemove) {
	    notifyInterfaceRemoved(iface);
	} else if (action == NetlinkEvent::Action::kChange) {
	    evt->dump();
	    notifyInterfaceChanged("nana", true);
	} else if (action == NetlinkEvent::Action::kLinkUp) {
	    notifyInterfaceLinkChanged(iface, true);
	} else if (action == NetlinkEvent::Action::kLinkDown) {
	    notifyInterfaceLinkChanged(iface, false);
	} else if (action == NetlinkEvent::Action::kAddressUpdated ||
	           action == NetlinkEvent::Action::kAddressRemoved) {
	    const char *address = evt->findParam("ADDRESS");
	    const char *flags = evt->findParam("FLAGS");
	    const char *scope = evt->findParam("SCOPE");
	    if (action == NetlinkEvent::Action::kAddressRemoved && iface && address) {
	        // Note: if this interface was deleted, iface is "" and we don't notify.
	        SockDiag sd;
	        if (sd.open()) {
	            char addrstr[INET6_ADDRSTRLEN];
	            strncpy(addrstr, address, sizeof(addrstr));
	            char *slash = strchr(addrstr, '/');
	            if (slash) {
	                *slash = '\0';
	            }

	            int ret = sd.destroySockets(addrstr);
	            if (ret < 0) {
	                ALOGE("Error destroying sockets: %s", strerror(ret));
	            }
	        } else {
	            ALOGE("Error opening NETLINK_SOCK_DIAG socket: %s", strerror(errno));
	        }
	    }
	    if (iface && iface[0] && address && flags && scope) {
	        notifyAddressChanged(action, address, iface, flags, scope);
	    }
	} else if (action == NetlinkEvent::Action::kRdnss) {
	    const char *lifetime = evt->findParam("LIFETIME");
	    const char *servers = evt->findParam("SERVERS");
	    if (lifetime && servers) {
	        notifyInterfaceDnsServers(iface, lifetime, servers);
	    }
	} else if (action == NetlinkEvent::Action::kRouteUpdated ||
	           action == NetlinkEvent::Action::kRouteRemoved) {
	    const char *route = evt->findParam("ROUTE");
	    const char *gateway = evt->findParam("GATEWAY");
	    const char *iface = evt->findParam("INTERFACE");
	    if (route && (gateway || iface)) {
	        notifyRouteChange(action, route, gateway, iface);
	    }
	}

    } else if (!strcmp(subsys, "qlog") || !strcmp(subsys, "xt_quota2")) {
	const char *alertName = evt->findParam("ALERT_NAME");
	const char *iface = evt->findParam("INTERFACE");
	notifyQuotaLimitReached(alertName, iface);

    } else if (!strcmp(subsys, "strict")) {
	const char *uid = evt->findParam("UID");
	const char *hex = evt->findParam("HEX");
	notifyStrictCleartext(uid, hex);

    } else if (!strcmp(subsys, "xt_idletimer")) {
	const char *label = evt->findParam("INTERFACE");
	const char *state = evt->findParam("STATE");
	const char *timestamp = evt->findParam("TIME_NS");
	const char *uid = evt->findParam("UID");
	if (state)
	    notifyInterfaceClassActivity(label, !strcmp("active", state),
	                                 timestamp, uid);
    }
}

至此， Netd的介绍就结束了。 总的说来， Netd服务是Android网络管理的核心进程之一， 对于了解Android网络管理很有帮助， 看下其实现细节对于掌握网络相关的问题至关重要。

参考资料

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