Android中的网络管理源码分析--netd

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NetlinkManager的Broadcaster,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“dnsproxyd”socket     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“mdns”socket     mdnsl = new MDnsSdListener();     if (mdnsl->startListener()) {         ALOGE("Unable to start MDnsSdListener (%s)", strerror(errno));         exit(1);     }     //创建并开始监听“fwmarkd”socket     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“netd”socket     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中   //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中   //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中     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中       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;         }     }     //将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mListen为false     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cl的socket，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：