（一百八十一） Android P DhcpClient的PacketSocket

前言：dhcp的流程分为四步，dhcp discover+dhcp offer+dhcp request+dhcp ack，那么包具体是怎么收发的呢？

 

1.PacketSocket

dhcp开始时进行了一些初始化的操作

    class DhcpState extends State {
        @Override
        public void enter() {
            clearDhcpState();
            if (initInterface() && initSockets()) {
                mReceiveThread = new ReceiveThread();
                mReceiveThread.start();
            } else {
                notifyFailure();
                transitionTo(mStoppedState);
            }
        }

先只看socket相关的

    private boolean initSockets() {
        return initPacketSocket() && initUdpSocket();
    }

看下相关注释

    // Sockets.
    // - We use a packet socket to receive, because servers send us packets bound for IP addresses
    //   which we have not yet configured, and the kernel protocol stack drops these.
    // - We use a UDP socket to send, so the kernel handles ARP and routing for us (DHCP servers can
    //   be off-link as well as on-link).
    private FileDescriptor mPacketSock;
    private FileDescriptor mUdpSock;

表明packet socket是用来收包的，udpSocket是用来发包的，即

dhcp discover（udpSocket）+

dhcp offer（packet socket）+

dhcp request（udpSocket）+

dhcp ack（packet socket）

看下packet socket是如何收包的

    private boolean initPacketSocket() {
        try {
            mPacketSock = Os.socket(AF_PACKET, SOCK_RAW, ETH_P_IP);
            PacketSocketAddress addr = new PacketSocketAddress((short) ETH_P_IP, mIface.index);
            Os.bind(mPacketSock, addr);
            NetworkUtils.attachDhcpFilter(mPacketSock);
        } catch(SocketException|ErrnoException e) {
            Log.e(TAG, "Error creating packet socket", e);
            return false;
        }
        return true;
    }

这边初始化了一下，并且将其作为一个dhcpFilter，用来过滤dhcp的包

至于收包的逻辑应该在这里，轮询阻塞进行收包

    class ReceiveThread extends Thread {

        private final byte[] mPacket = new byte[DhcpPacket.MAX_LENGTH];
        private volatile boolean mStopped = false;

        public void halt() {
            mStopped = true;
            closeSockets();  // Interrupts the read() call the thread is blocked in.
        }

        @Override
        public void run() {
            if (DBG) Log.d(TAG, "Receive thread started");
            while (!mStopped) {
                int length = 0;  // Or compiler can't tell it's initialized if a parse error occurs.
                try {
                    length = Os.read(mPacketSock, mPacket, 0, mPacket.length);
                    DhcpPacket packet = null;
                    packet = DhcpPacket.decodeFullPacket(mPacket, length, DhcpPacket.ENCAP_L2);
                    if (DBG) Log.d(TAG, "Received packet: " + packet);
                    sendMessage(CMD_RECEIVED_PACKET, packet);
                } catch (IOException|ErrnoException e) {
                    if (!mStopped) {
                        Log.e(TAG, "Read error", e);
                        logError(DhcpErrorEvent.RECEIVE_ERROR);
                    }
                } catch (DhcpPacket.ParseException e) {
                    Log.e(TAG, "Can't parse packet: " + e.getMessage());
                    if (PACKET_DBG) {
                        Log.d(TAG, HexDump.dumpHexString(mPacket, 0, length));
                    }
                    if (e.errorCode == DhcpErrorEvent.DHCP_NO_COOKIE) {
                        int snetTagId = 0x534e4554;
                        String bugId = "31850211";
                        int uid = -1;
                        String data = DhcpPacket.ParseException.class.getName();
                        EventLog.writeEvent(snetTagId, bugId, uid, data);
                    }
                    logError(e.errorCode);
                }
            }
            if (DBG) Log.d(TAG, "Receive thread stopped");
        }
    }

 

2.attachDhcpFilter

NetworkUtils

   /**
     * Attaches a socket filter that accepts DHCP packets to the given socket.
     */
    public native static void attachDhcpFilter(FileDescriptor fd) throws SocketException;

是个native方法，附着一个过滤dhcp 包的socket filter 以发给对应的socket

看下对应实现

http://androidxref.com/9.0.0_r3/xref/frameworks/base/core/jni/android_net_NetUtils.cpp#android_net_utils_attachDhcpFilter

static void android_net_utils_attachDhcpFilter(JNIEnv *env, jobject clazz, jobject javaFd)
{
    struct sock_filter filter_code[] = {
        // Check the protocol is UDP.
        BPF_STMT(BPF_LD  | BPF_B   | BPF_ABS,  kIPv4Protocol),
        BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K,    IPPROTO_UDP, 0, 6),

        // Check this is not a fragment.
        BPF_STMT(BPF_LD  | BPF_H    | BPF_ABS, kIPv4FlagsOffset),
        BPF_JUMP(BPF_JMP | BPF_JSET | BPF_K,   IP_OFFMASK, 4, 0),

        // Get the IP header length.
        BPF_STMT(BPF_LDX | BPF_B    | BPF_MSH, kEtherHeaderLen),

        // Check the destination port.
        BPF_STMT(BPF_LD  | BPF_H    | BPF_IND, kUDPDstPortIndirectOffset),
        BPF_JUMP(BPF_JMP | BPF_JEQ  | BPF_K,   kDhcpClientPort, 0, 1),

        // Accept or reject.
        BPF_STMT(BPF_RET | BPF_K,              0xffff),
        BPF_STMT(BPF_RET | BPF_K,              0)
    };
    struct sock_fprog filter = {
        sizeof(filter_code) / sizeof(filter_code[0]),
        filter_code,
    };

    int fd = jniGetFDFromFileDescriptor(env, javaFd);
    if (setsockopt(fd, SOL_SOCKET, SO_ATTACH_FILTER, &filter, sizeof(filter)) != 0) {
        jniThrowExceptionFmt(env, "java/net/SocketException",
                "setsockopt(SO_ATTACH_FILTER): %s", strerror(errno));
    }
}

看下什么是BPF

柏克莱封包过滤器（Berkeley Packet Filter，缩写 BPF），是类Unix系统上数据链路层的一种原始接口，提供原始链路层封包的收发，除此之外，如果网卡驱动支持洪泛模式，那么它可以让网卡处于此种模式，这样可以收到网络上的所有包，不管他们的目的地是不是所在主机。

另外，BPF支持“过滤”封包，这样BPF会只把“感兴趣”的封包到上层软件，可以避免从操作系统内核向用户态复制其他封包，降低抓包的CPU的负担以及所需的缓冲区空间，从而减少丢包率。BPF的过滤功能是以BPF虚拟机机器语言的解释器的形式实现的，这种语言的程序可以抓取封包数据，对封包中的数据采取算术操作，并将结果与常量或封包中的数据或结果中的测试位比较，根据比较的结果决定接受还是拒绝封包。在一些平台上，包括FreeBSD和WinPcap，即时编译技术用于把虚拟机指令转换为原始码，以进一步减少开销。

 

看起来filter_code 应该是对应上面所说的虚拟机器语言的解释器，简单来说就是对包进行过滤，得到自己想要的。

 

3.总结

dhcp的流程分为四步，dhcp discover+dhcp offer+dhcp request+dhcp ack，不可避免涉及到包的收发，其中packet socket是用来进行包的过滤以及接收的，udp socket是用来发送包的；

另外packet socket进行包的过滤是使用了一项称为BPF的技术，待进一步学习filter的写法。

待续

学习 BPF进阶 - BPF常用命令