1      Netd简介

Netd是Android的网络守护进程。NetD是个网络管家,封装了复杂的底层各种类型的网络(NAT,PLAN,PPP,SOFTAP,TECHER,ETHO,MDNS等),隔离了底层网络接口的差异,给Framework提供了统一调用接口,简化了网络的使用。NetD主要功能是:第一、接收Framework的网络请求,处理请求,向Framework层反馈处理结果;第二、监听网络事件(断开/连接/错误等),向Framework层上报。

 

2     Netd的启动过程

Netd作为后台服务进程在Andriod系统启动的init1阶段就被启动了,其在init.rc文件的配置如下:

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

看一看到,这里为netd配置了4个socket(比老版本多了一个名字为“fwmakd”的socket),根据配置可找到netd的入口函数为main():

int main() {

 

    CommandListener *cl;

    NetlinkManager *nm;

    DnsProxyListener *dpl;

    MDnsSdListener *mdnsl;

    FwmarkServer* fwmarkServer;

 

    ALOGI("Netd 1.0 starting");

    remove_pid_file();//猜测为每次重启时删除旧的

 

    blockSigpipe();//禁止SIGPIPE中断

    //创建NetlinkManager实例

    if (!(nm = NetlinkManager::Instance())) {

        ALOGE("Unable to create NetlinkManager");

        exit(1);

    };

    //创建CommandListener实例,并将其设置为NetlinkManagerBroadcaster,之后启动nm

    cl = new CommandListener();

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

 

    if (nm->start()) {

        ALOGE("Unable to start NetlinkManager (%s)", strerror(errno));

        exit(1);

    }

 

    // Set local DNS mode, to prevent bionic from proxying

    // back to this service, recursively.

    setenv("ANDROID_DNS_MODE", "local", 1);

    //创建并开始监听“dnsproxydsocket

    dpl = new DnsProxyListener(CommandListener::sNetCtrl);

    if (dpl->startListener()) {

        ALOGE("Unable to start DnsProxyListener (%s)", strerror(errno));

        exit(1);

    }

    //创建并开始监听“mdnssocket

    mdnsl = new MDnsSdListener();

    if (mdnsl->startListener()) {

        ALOGE("Unable to start MDnsSdListener (%s)", strerror(errno));

        exit(1);

    }

    //创建并开始监听“fwmarkdsocket

    fwmarkServer = new FwmarkServer(CommandListener::sNetCtrl);

    if (fwmarkServer->startListener()) {

        ALOGE("Unable to start FwmarkServer (%s)", strerror(errno));

        exit(1);

    }

 

    /*

     * Now that we're up, we can respond to commands

     */

//开始监听“netdsocket

    if (cl->startListener()) {

        ALOGE("Unable to start CommandListener (%s)", strerror(errno));

        exit(1);

    }

 

    bool wrote_pid = write_pid_file();

 

    while(1) {

        sleep(30); // 30 sec

        if (!wrote_pid) {

            wrote_pid = write_pid_file();

        }

    }

 

    ALOGI("Netd exiting");

    remove_pid_file();

    exit(0);

}

从上面个可以看出netd的启动并不复杂,主要是启动了4个监听socket,后面的分析将会看到每个socket对应这一个监听线程。首先来看NetlinkManage,NetlinkManager(以后简称NM)主要负责接收并解析来自Kernel的UEvent消息。如果对linux的socket特别熟悉的话,光从“NetlinkMananger”的名字就能推断出此类的基本实现和作用:肯定使用了PF_NETLINK的socket。这种socket一般是在应用层(相对于内核)监听内核事件的时候使用。例如USB的插拔等等。从main的代码可以知道它的入口为start()函数。

int NetlinkManager::start() {

  //创建接收NETLINK_KOBJECT_UEVENT消息的socket,其值保存在mUeventSock

  //其中,NETLINK_FORMAT_ASCII代表UEvent消息的内容为ASCII字符串

    if ((mUeventHandler = setupSocket(&mUeventSock, NETLINK_KOBJECT_UEVENT,

         0xffffffff, NetlinkListener::NETLINK_FORMAT_ASCII, false)) == NULL) {

        return -1;

    }

//创建接收RTMGPR_LINK消息的socket,其值保存在mRouteSock

  //其中,NETLINK_FORMAT_BINARY代表UEvent消息的类型为结构体,故需要进行二进制解析

    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;

    }

  //创建接收NETLINK_NFLOG消息的socket,其值保存在mQuotaSock

    if ((mQuotaHandler = setupSocket(&mQuotaSock, NETLINK_NFLOG,

            NFLOG_QUOTA_GROUP, NetlinkListener::NETLINK_FORMAT_BINARY, false)) == NULL) {

        ALOGE("Unable to open quota socket");

       

    }

//创建接收NETLINK_NETFILTER消息的socket,其值保存在mQuotaSock

      if ((mStrictHandler = setupSocket(&mStrictSock, NETLINK_NETFILTER,

            0, NetlinkListener::NETLINK_FORMAT_BINARY_UNICAST, true)) == NULL) {

        ALOGE("Unable to open strict socket");

       

    }

 

    return 0;

}

start()四次调用了setupSocket函数,新建了4个PF_NETLINK类型的socket监听内核的不同事件。查看函数setupSocket()。

NetlinkHandler *NetlinkManager::setupSocket(int *sock, int netlinkFamily,

    int groups, int format, bool configNflog) {

 

    struct sockaddr_nl nladdr;

    int sz = 64 * 1024;

    int on = 1;

 

    memset(&nladdr, 0, sizeof(nladdr));

    nladdr.nl_family = AF_NETLINK;

    nladdr.nl_pid = getpid();

    nladdr.nl_groups = groups;

    //新建socket,一定要注意这里的socket类型为SOCK_DGRAM,这句是整个Nm的关键

    //netlinkFamily指定了soket监听的内核事件

    if ((*sock = socket(PF_NETLINK, SOCK_DGRAM | SOCK_CLOEXEC, netlinkFamily)) < 0) {

        ALOGE("Unable to create netlink socket: %s", strerror(errno));

        return NULL;

    }

    //设置socket的属性

    if (setsockopt(*sock, SOL_SOCKET, SO_RCVBUFFORCE, &sz, sizeof(sz)) < 0) {

        ALOGE("Unable to set uevent socket SO_RCVBUFFORCE option: %s", strerror(errno));

        close(*sock);

        return NULL;

    }

 

    if (setsockopt(*sock, SOL_SOCKET, SO_PASSCRED, &on, sizeof(on)) < 0) {

        SLOGE("Unable to set uevent socket SO_PASSCRED option: %s", strerror(errno));

        close(*sock);

        return NULL;

    }

    //绑定

    if (bind(*sock, (struct sockaddr *) &nladdr, sizeof(nladdr)) < 0) {

        ALOGE("Unable to bind netlink socket: %s", strerror(errno));

        close(*sock);

        return NULL;

    }

 

    if (configNflog) {//只有mStrictSock对应的为true

        if (android_nflog_send_config_cmd(*sock, 0, NFULNL_CFG_CMD_PF_UNBIND, AF_INET) < 0) {

            ALOGE("Failed NFULNL_CFG_CMD_PF_UNBIND: %s", strerror(errno));

            return NULL;

        }

        if (android_nflog_send_config_cmd(*sock, 0, NFULNL_CFG_CMD_PF_BIND, AF_INET) < 0) {

            ALOGE("Failed NFULNL_CFG_CMD_PF_BIND: %s", strerror(errno));

            return NULL;

        }

        if (android_nflog_send_config_cmd(*sock, 0, NFULNL_CFG_CMD_BIND, AF_UNSPEC) < 0) {

            ALOGE("Failed NFULNL_CFG_CMD_BIND: %s", strerror(errno));

            return NULL;

        }

    }

    //socket封装成 NetLinkHandler,从而在socket有活动的时候处理

    NetlinkHandler *handler = new NetlinkHandler(this, *sock, format);

    if (handler->start()) {//启动NetlinkHandler,实际就是启动监听

        ALOGE("Unable to start NetlinkHandler: %s", strerror(errno));

        close(*sock);

        return NULL;

    }

 

    return handler;

}

NetlinkHandler的start()函数转调了this-> startListener(),此方法实际上是继承自SocketListener类。这个类是一个比较通用的类,很多与socket的IO复用有关的模块都会调用此类的相关方法。

int SocketListener::startListener(int backlog) {

//注意这个变量是类的成员变量,实际上这里就是想方设法得到socket

    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);

    }

   //如果设置了mListen则监听socket,如果没有设置则新建一个socketClient放入客户端集合

   //注意短路,对于NetlinkHandler,从其构造函数可知mListenfalse

    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));//这里

 

    if (pipe(mCtrlPipe)) {

        SLOGE("pipe failed (%s)", strerror(errno));

        return -1;

    }

   //创建线程处理监听socket,这里其实并没有所谓的“监听socket”,因为是NETLINK型的socket

    if (pthread_create(&mThread, NULL, SocketListener::threadStart, this)) {

        SLOGE("pthread_create (%s)", strerror(errno));

        return -1;

    }

 

    return 0;

}

进入线程的入口函数SocketListener::threadStart()

void *SocketListener::threadStart(void *obj) {

    SocketListener *me = reinterpret_cast(obj);

    //注意obj为主线程传递进来的参数,就是SocketListener

    me->runListener();

    pthread_exit(NULL);

    return NULL;

}

进入runListener

void SocketListener::runListener() {

    //此函数的主要逻辑就是select()

    SocketClientCollection pendingList;//新建一个socketClientCollection存放活动fd

 

    while(1) {

        SocketClientCollection::iterator it;

        fd_set read_fds;

        int rc = 0;

        int max = -1;

 

        FD_ZERO(&read_fds);

 

        if (mListen) {//监听listenSocket的读事件,前面已经知道mListen此时为fasle

            max = mSock;

            FD_SET(mSock, &read_fds);

        }

 

        FD_SET(mCtrlPipe[0], &read_fds);//这里的pipe什么作用?中断循环标志?

        if (mCtrlPipe[0] > max)

            max = mCtrlPipe[0];

 

        pthread_mutex_lock(&mClientsLock);

        //遍历mClients集合

        for (it = mClients->begin(); it != mClients->end(); ++it) {

            // NB: calling out to an other object with mClientsLock held (safe)

            int fd = (*it)->getSocket();//获取与客户通信的socket

            FD_SET(fd, &read_fds);//监听它

            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) {//select

            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)) {//如果是pipe有活动

            char c = CtrlPipe_Shutdown;

            TEMP_FAILURE_RETRY(read(mCtrlPipe[0], &c, 1));//读取管道

            if (c == CtrlPipe_Shutdown) {

                break;//难道这就是监听pipe的作用?

            }

            continue;

        }

//如果是监听socket,接收连接请求,当然NETLINK不会走这里

        if (mListen && FD_ISSET(mSock, &read_fds)) {

            struct sockaddr addr;

            socklen_t alen;

            int c;

 

            do {

                alen = sizeof(addr);

                c = accept(mSock, &addr, &alen);

                SLOGV("%s got %d from accept", mSocketName, c);

            } while (c < 0 && errno == EINTR);

            if (c < 0) {

                SLOGE("accept failed (%s)", strerror(errno));

                sleep(1);

                continue;

            }

            fcntl(c, F_SETFD, FD_CLOEXEC);

            pthread_mutex_lock(&mClientsLock);

//放入client集合

            mClients->push_back(new SocketClient(c, true, mUseCmdNum));

            pthread_mutex_unlock(&mClientsLock);

        }

 

       //将所有活动的fd都放入pendingList,貌似也只有一个

        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)) {//fd如果有活动

                pendingList.push_back(c);//放入pendingList

                c->incRef();

            }

        }

        pthread_mutex_unlock(&mClientsLock);

 

        //处理pendingList,这里的具体意思就是内核有事件了,需要上层处理

        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();

        }

    }

}

从上面的函数可以看到,这里实际上是对3类fd作了监听处理。一类是监听socket,一类是client socket,并且这类socket被封装成SocketClient集中在一个集合之内。还有一个就是pipe。从NetlinkManager.start()中我们已经知道启动了四套这样的结构,其socket分别为mUeventSock ,mRouteSock,mQuotaSock,mStrictSock。这些Socket都是PF_NETLINK类型的sockegt,并不是监听socket,具体一点就是他们对应的mListen均为false。也就是这四个socket被当做SocketClient添加进了mClients(注意有四个实例)。等等,那么监听socket在哪呢?压根就没有监听socket,这里采用的是SOCK_DGRAM类型的socket!


当检测到这些socket有可读事件发生时,也就是内核有上层感兴趣的事件发生时。相应的onDataAvailable()被调用,这是一个虚函数。分析可知此时this的具体类型为NetlinkHandler,因此调用的是NetlinkHandler的onDataAvailable()。

bool NetlinkListener::onDataAvailable(SocketClient *cli)

{

    int socket = cli->getSocket();

    ssize_t count;

    uid_t uid = -1;

 

    bool require_group = true;

    if (mFormat == NETLINK_FORMAT_BINARY_UNICAST) {

        require_group = false;

    }

    //读取数据

    count = TEMP_FAILURE_RETRY(uevent_kernel_recv(socket,

            mBuffer, sizeof(mBuffer), require_group, &uid));

    if (count < 0) {

        if (uid > 0)

            LOG_EVENT_INT(65537, uid);

        SLOGE("recvmsg failed (%s)", strerror(errno));

        return false;

    }

 

    NetlinkEvent *evt = new NetlinkEvent();//新建一个NetLinkEvent

    if (evt->decode(mBuffer, count, mFormat)) {//解码

        onEvent(evt);//调用了此函数,对event做了处理

    } else if (mFormat != NETLINK_FORMAT_BINARY) {

        // Don't complain if parseBinaryNetlinkMessage returns false. That can

        // just mean that the buffer contained no messages we're interested in.

        SLOGE("Error decoding NetlinkEvent");

    }

 

    delete evt;

    return true;

}

这里调用了onEvent()才是NetlinkHandler的入口。

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) {

                int resetMask = strchr(address, ':') ? RESET_IPV6_ADDRESSES : RESET_IPV4_ADDRESSES;

                resetMask |= RESET_IGNORE_INTERFACE_ADDRESS;

                if (int ret = ifc_reset_connections(iface, resetMask)) {

                    ALOGE("ifc_reset_connections failed on iface %s for address %s (%s)", iface,

                          address, strerror(ret));

                }

            }

            if (iface && 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")) {

        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);

 

#if !LOG_NDEBUG

    } else if (strcmp(subsys, "platform") && strcmp(subsys, "backlight")) {

        /* It is not a VSYNC or a backlight event */

        ALOGV("unexpected event from subsystem %s", subsys);

#endif

    }

}

可以看到,这里对不同的时间进行看了notifXxx所有的notifyXXXXX函数都会调用notify()函数

void NetlinkHandler::notify(int code, const char *format, ...) {

    char *msg;

    va_list args;

    va_start(args, format);

    if (vasprintf(&msg, format, args) >= 0) {

        //一定要注意这里的所使用的是clsocket,名字为”netd”,而非之前的那四个

        mNm->getBroadcaster()->sendBroadcast(code, msg, false);

        free(msg);

    } else {

        SLOGE("Failed to send notification: vasprintf: %s", strerror(errno));

    }

    va_end(args);

}

mNm就是之前在main()中新建的NetworkMananger。其BroadCaster已经设置为了cl(即CommandListener的一个实例)。CommandListener通过netd向NetworkManagementService发送消息。这里的消息可能有两种:一种是底层主动上报的消息,另一种是上层请求的response。(这个和RILD很类似)

现在我们来整理一下上面的步骤:NetlinkManager新建了4个PF_NETLINK类型的socket,监听内核发生的uEvent。当内核发生相应的uEvent被对应的 NetlinkManager检测到。NetlinkManager将着个uEvent转化为NetlinkEvent 通过CommandListener广播到更上层。而这里的“更上层”指的是java层。可见,底层C/C++和上层java的联系是通过socket联系在一起的。

这里一定要清楚出两点:之前的4个socket并不是这里的BroadCaster的socket;而且,个人觉得这个BroadCaster名字也容易让人产生误解,以为是广播,广播对应的socket就应该是UDP。而实际上这个socket是init.rc配置的名字为“netd”的socke所accept出来的clientSocket,是一个TCPsocket。而TCPsocket是无法广播的。这里直接将sendBroadCast理解为sendMsg后面的就很好理解了。

接着分析CommandListener。这个类同样继承自SocketListener,与之前的4个Netlink socket所不同的是此类的mListen被设置为true。也就是“netd”为监听socket。CommandListener在之前的main函数中调用startListener开启监听来自java层的连接。当上层有连接时,select返回,accpet得到一个clientSocket,之后将其封装成SocketClient添加经list,并添加进select的监听队列。当java层下发命令,SocketClient的可读事件被检测到,从而做后续的处理,最后将底层处理结果response回上层,底层主动上报的消息也是通过此clientSocket上发到上层的。熟悉网络编程的就应该知道,这里是一个很典型的Select型的IO复用服务端模型。

除“netd”外,其他三个在init.rc中配置的socket:dnsproxyd  mdns  fwmarkd也构建了几乎一样的服务端结构。这里就不再赘述。以下为netd的大致框图:


 

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