DTLS 服务器

本例演示了如何实现一个简单的 DTLS 服务器。

DTLS 服务器示例截图。

注意： DTLS 服务器示例应与DTLS 客户端示例同时运行。

服务器由 DtlsServer 类实现。它使用QUdpSocket 、QDtlsClientVerifier 和QDtls 来测试每个客户端的可达性、完成握手以及读写加密消息。

class DtlsServer : public QObject
{
    Q_OBJECT

public:
    DtlsServer();
    ~DtlsServer();

    bool listen(const QHostAddress &address, quint16 port);
    bool isListening() const;
    void close();

signals:
    void errorMessage(const QString &message);
    void warningMessage(const QString &message);
    void infoMessage(const QString &message);

    void datagramReceived(const QString &peerInfo, const QByteArray &cipherText,
                          const QByteArray &plainText);

private slots:
    void readyRead();
    void pskRequired(QSslPreSharedKeyAuthenticator *auth);

private:
    void handleNewConnection(const QHostAddress &peerAddress, quint16 peerPort,
                             const QByteArray &clientHello);

    void doHandshake(QDtls *newConnection, const QByteArray &clientHello);
    void decryptDatagram(QDtls *connection, const QByteArray &clientMessage);
    void shutdown();

    bool listening = false;
    QUdpSocket serverSocket;

    QSslConfiguration serverConfiguration;
    QDtlsClientVerifier cookieSender;
    std::vector<std::unique_ptr<QDtls>> knownClients;

    Q_DISABLE_COPY(DtlsServer)
};

构造函数将QUdpSocket::readyRead() 信号连接到 readyRead() 槽，并设置所需的最低 TLS 配置：

DtlsServer::DtlsServer()
{
    connect(&serverSocket, &QAbstractSocket::readyRead, this, &DtlsServer::readyRead);

    serverConfiguration = QSslConfiguration::defaultDtlsConfiguration();
    serverConfiguration.setPreSharedKeyIdentityHint("Qt DTLS example server");
    serverConfiguration.setPeerVerifyMode(QSslSocket::VerifyNone);
}

注意： 服务器不使用证书，而是依靠预共享密钥（PSK）握手。

listen() 绑定QUdpSocket ：

bool DtlsServer::listen(const QHostAddress &address, quint16 port)
{
    if (address != serverSocket.localAddress() || port != serverSocket.localPort()) {
        shutdown();
        listening = serverSocket.bind(address, port);
        if (!listening)
            emit errorMessage(serverSocket.errorString());
    } else {
        listening = true;
    }

    return listening;
}

readyRead() 插槽处理传入的数据报：

    ...
const qint64 bytesToRead = serverSocket.pendingDatagramSize();
if (bytesToRead <= 0) {
    emit warningMessage(tr("A spurious read notification"));
    return;
}

QByteArray dgram(bytesToRead, Qt::Uninitialized);
QHostAddress peerAddress;
quint16 peerPort = 0;
const qint64 bytesRead = serverSocket.readDatagram(dgram.data(), dgram.size(),
                                                   &peerAddress, &peerPort);
if (bytesRead <= 0) {
    emit warningMessage(tr("Failed to read a datagram: ") + serverSocket.errorString());
    return;
}

dgram.resize(bytesRead);
    ...

在提取地址和端口号后，服务器首先会测试数据报是否来自已知的对等网络：

    ...
if (peerAddress.isNull() || !peerPort) {
    emit warningMessage(tr("Failed to extract peer info (address, port)"));
    return;
}

const auto client = std::find_if(knownClients.begin(), knownClients.end(),
                                 [&](const std::unique_ptr<QDtls> &connection){
    return connection->peerAddress() == peerAddress
           && connection->peerPort() == peerPort;
});
    ...

如果是新的、未知的地址和端口，数据报将作为 DTLS 客户端发送的潜在 ClientHello 消息处理：

    ...
if (client == knownClients.end())
    return handleNewConnection(peerAddress, peerPort, dgram);
    ...

如果是已知的 DTLS 客户端，服务器会解密数据报：

    ...
if ((*client)->isConnectionEncrypted()) {
    decryptDatagram(client->get(), dgram);
    if ((*client)->dtlsError() == QDtlsError::RemoteClosedConnectionError)
        knownClients.erase(client);
    return;
}
    ...

或继续与该对等节点握手：

    ...
doHandshake(client->get(), dgram);
    ...

handleNewConnection()验证它是否是一个可连接的 DTLS 客户端，或者发送 HelloVerifyRequest：

void DtlsServer::handleNewConnection(const QHostAddress &peerAddress,
                                     quint16 peerPort, const QByteArray &clientHello)
{
    if (!listening)
        return;

    const QString peerInfo = peer_info(peerAddress, peerPort);
    if (cookieSender.verifyClient(&serverSocket, clientHello, peerAddress, peerPort)) {
        emit infoMessage(peerInfo + tr(": verified, starting a handshake"));
    ...

如果新客户端被验证为可连接的 DTLS 客户端，服务器就会创建并配置一个新的QDtls 对象，并开始服务器端握手：

    ...
    std::unique_ptr<QDtls> newConnection{new QDtls{QSslSocket::SslServerMode}};
    newConnection->setDtlsConfiguration(serverConfiguration);
    newConnection->setPeer(peerAddress, peerPort);
    newConnection->connect(newConnection.get(), &QDtls::pskRequired,
                           this, &DtlsServer::pskRequired);
    knownClients.push_back(std::move(newConnection));
    doHandshake(knownClients.back().get(), clientHello);
    ...

doHandshake() 开始握手阶段：

void DtlsServer::doHandshake(QDtls *newConnection, const QByteArray &clientHello)
{
    const bool result = newConnection->doHandshake(&serverSocket, clientHello);
    if (!result) {
        emit errorMessage(newConnection->dtlsErrorString());
        return;
    }

    const QString peerInfo = peer_info(newConnection->peerAddress(),
                                       newConnection->peerPort());
    switch (newConnection->handshakeState()) {
    case QDtls::HandshakeInProgress:
        emit infoMessage(peerInfo + tr(": handshake is in progress ..."));
        break;
    case QDtls::HandshakeComplete:
        emit infoMessage(tr("Connection with %1 encrypted. %2")
                         .arg(peerInfo, connection_info(newConnection)));
        break;
    default:
        Q_UNREACHABLE();
    }
}

在握手阶段，会发出QDtls::pskRequired() 信号，pskRequired() 槽会提供预共享密钥：

void DtlsServer::pskRequired(QSslPreSharedKeyAuthenticator *auth)
{
    Q_ASSERT(auth);

    emit infoMessage(tr("PSK callback, received a client's identity: '%1'")
                     .arg(QString::fromLatin1(auth->identity())));
    auth->setPreSharedKey(QByteArrayLiteral("\x1a\x2b\x3c\x4d\x5e\x6f"));
}

注：为简洁起见，pskRequired() 的定义过于简化。QSslPreSharedKeyAuthenticator 类的文档详细解释了如何正确执行该插槽。

网络对等端完成握手后，加密的 DTLS 连接即被视为已建立，服务器会调用 decryptDatagram() 来解密对等端发送的后续数据报。服务器还会向对等网络发送加密响应：

void DtlsServer::decryptDatagram(QDtls *connection, const QByteArray &clientMessage)
{
    Q_ASSERT(connection->isConnectionEncrypted());

    const QString peerInfo = peer_info(connection->peerAddress(), connection->peerPort());
    const QByteArray dgram = connection->decryptDatagram(&serverSocket, clientMessage);
    if (dgram.size()) {
        emit datagramReceived(peerInfo, clientMessage, dgram);
        connection->writeDatagramEncrypted(&serverSocket, tr("to %1: ACK").arg(peerInfo).toLatin1());
    } else if (connection->dtlsError() == QDtlsError::NoError) {
        emit warningMessage(peerInfo + ": " + tr("0 byte dgram, could be a re-connect attempt?"));
    } else {
        emit errorMessage(peerInfo + ": " + connection->dtlsErrorString());
    }
}

服务器通过调用QDtls::shutdown() 关闭 DTLS 连接：

void DtlsServer::shutdown()
{
    for (const auto &connection : std::exchange(knownClients, {}))
        connection->shutdown(&serverSocket);

    serverSocket.close();
}

在运行过程中，服务器通过发出 errorMessage()、warningMessage()、infoMessage() 和 datagramReceived() 信号来报告错误、信息消息和解密数据报。服务器的用户界面会记录这些消息：

const QString colorizer(QStringLiteral("<font color=\"%1\">%2</font><br>"));

void MainWindow::addErrorMessage(const QString &message)
{
    ui->serverInfo->insertHtml(colorizer.arg(QStringLiteral("Crimson"), message));
}

void MainWindow::addWarningMessage(const QString &message)
{
    ui->serverInfo->insertHtml(colorizer.arg(QStringLiteral("DarkOrange"), message));
}

void MainWindow::addInfoMessage(const QString &message)
{
    ui->serverInfo->insertHtml(colorizer.arg(QStringLiteral("DarkBlue"), message));
}

void MainWindow::addClientMessage(const QString &peerInfo, const QByteArray &datagram,
                                  const QByteArray &plainText)
{
    static const QString messageColor = QStringLiteral("DarkMagenta");
    static const QString formatter = QStringLiteral("<br>---------------"
                                                    "<br>A message from %1"
                                                    "<br>DTLS datagram:<br> %2"
                                                    "<br>As plain text:<br> %3");

    const QString html = formatter.arg(peerInfo, QString::fromUtf8(datagram.toHex(' ')),
                                       QString::fromUtf8(plainText));
    ui->messages->insertHtml(colorizer.arg(messageColor, html));
}

示例项目 @ code.qt.io

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