此篇博文展示了如何創建一個簡單的DTLS服務端：

注意：這個DTLS服務端需要和DTLS客戶端一起跑，才能看出效果。

服務端實現了DtlsServer類，這個類使用了QUdpSocket，QDtlsClientVerifier，QDtls使用這些類用于和客戶端的連接，完成握手及數據加密傳輸與讀取。

class DtlsServer : public QObject{Q_OBJECTpublic: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)};

構造函數中QUdpSocket::readyRead()信號連接了readyRead()槽，用于獲取客戶端的數據報，最簡單的設置了DTLS的配置：

DtlsServer::DtlsServer(){connect(&serverSocket, &QAbstractSocket::readyRead, this, &DtlsServer::readyRead);serverConfiguration = QSslConfiguration::defaultDtlsConfiguration();serverConfiguration.setPreSharedKeyIdentityHint("Qt DTLS example server");serverConfiguration.setPeerVerifyMode(QSslSocket::VerifyNone);}

注意：服務端未使用證書，僅僅是依賴共享密鑰（PSK）握手。

listen()與QUdpSocket進行了捆綁：

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()槽函數獲取客戶端數據報：

...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);...

隨后獲取客戶端的IP和端口，服務端發送數據報給客戶端：

...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;});...

如果是新的客戶端，未知地址和端口，客戶端會發送ClientHello消息，隨后服務端回HelloVerifyRequest這個要先處理下：

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

如果是連上的已知的客戶端，服務端會進行解密：

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

以及進行握手：

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

handleNewConnect()對客戶端進行認證，以及發送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"));...

如果服務端認為新來的客戶端可達，那個服務端會創建一個Qtls的配置，然后進行握手：

...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);...

關于握手的代碼在doHandshake()中

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();}}

在握手中，QDtls::pskRequired()信號關聯到pskRequired()槽函數中，并且設置了PSK：

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"));}

在握手完成后，就可以進行數據的加密發送和響應了：

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());}}

服務端關閉DTLS服務調用QDtls::shutdown()

void DtlsServer::shutdown(){for (const auto &connection : qExchange(knownClients, {}))connection->shutdown(&serverSocket);serverSocket.close();}

下面是出現問題或警告時的代碼：

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));}

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