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   /*
    * Copyright 2017 The Netty Project
    *
    * The Netty Project licenses this file to you under the Apache License,
    * version 2.0 (the "License"); you may not use this file except in compliance
    * with the License. You may obtain a copy of the License at:
    *
    *   https://www.apache.org/licenses/LICENSE-2.0
    *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
   * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
   * License for the specific language governing permissions and limitations
   * under the License.
   */
  package io.netty.handler.ssl;
  
  import io.netty.buffer.ByteBuf;
  import io.netty.channel.ChannelHandlerContext;
  import io.netty.util.CharsetUtil;
  import io.netty.util.concurrent.Future;
  import io.netty.util.concurrent.ScheduledFuture;
  
  import java.util.Locale;
  import java.util.concurrent.TimeUnit;
  
  import static io.netty.util.internal.ObjectUtil.checkPositiveOrZero;
  
  /**
   * <p>Enables <a href="https://tools.ietf.org/html/rfc3546#section-3.1">SNI
   * (Server Name Indication)</a> extension for server side SSL. For clients
   * support SNI, the server could have multiple host name bound on a single IP.
   * The client will send host name in the handshake data so server could decide
   * which certificate to choose for the host name.</p>
   */
  public abstract class AbstractSniHandler<T> extends SslClientHelloHandler<T> {
  
      private static String extractSniHostname(ByteBuf in) {
          // See https://tools.ietf.org/html/rfc5246#section-7.4.1.2
          //
          // Decode the ssl client hello packet.
          //
          // struct {
          //    ProtocolVersion client_version;
          //    Random random;
          //    SessionID session_id;
          //    CipherSuite cipher_suites<2..2^16-2>;
          //    CompressionMethod compression_methods<1..2^8-1>;
          //    select (extensions_present) {
          //        case false:
          //            struct {};
          //        case true:
          //            Extension extensions<0..2^16-1>;
          //    };
          // } ClientHello;
          //
  
          // We have to skip bytes until SessionID (which sum to 34 bytes in this case).
          int offset = in.readerIndex();
          int endOffset = in.writerIndex();
          offset += 34;
  
          if (endOffset - offset >= 6) {
              final int sessionIdLength = in.getUnsignedByte(offset);
              offset += sessionIdLength + 1;
  
              final int cipherSuitesLength = in.getUnsignedShort(offset);
              offset += cipherSuitesLength + 2;
  
              final int compressionMethodLength = in.getUnsignedByte(offset);
              offset += compressionMethodLength + 1;
  
              final int extensionsLength = in.getUnsignedShort(offset);
              offset += 2;
              final int extensionsLimit = offset + extensionsLength;
  
              // Extensions should never exceed the record boundary.
              if (extensionsLimit <= endOffset) {
                  while (extensionsLimit - offset >= 4) {
                      final int extensionType = in.getUnsignedShort(offset);
                      offset += 2;
  
                      final int extensionLength = in.getUnsignedShort(offset);
                      offset += 2;
  
                      if (extensionsLimit - offset < extensionLength) {
                          break;
                      }
  
                      // SNI
                      // See https://tools.ietf.org/html/rfc6066#page-6
                      if (extensionType == 0) {
                          offset += 2;
                          if (extensionsLimit - offset < 3) {
                              break;
                          }
  
                          final int serverNameType = in.getUnsignedByte(offset);
                          offset++;
 
                         if (serverNameType == 0) {
                             final int serverNameLength = in.getUnsignedShort(offset);
                             offset += 2;
 
                             if (extensionsLimit - offset < serverNameLength) {
                                 break;
                             }
 
                             final String hostname = in.toString(offset, serverNameLength, CharsetUtil.US_ASCII);
                             return hostname.toLowerCase(Locale.US);
                         } else {
                             // invalid enum value
                             break;
                         }
                     }
 
                     offset += extensionLength;
                 }
             }
         }
         return null;
     }
 
     protected final long handshakeTimeoutMillis;
     private ScheduledFuture<?> timeoutFuture;
     private String hostname;
 
     /**
      * @param handshakeTimeoutMillis    the handshake timeout in milliseconds
      */
     protected AbstractSniHandler(long handshakeTimeoutMillis) {
         this(0, handshakeTimeoutMillis);
     }
 
     /**
      * @paramm maxClientHelloLength     the maximum length of the client hello message.
      * @param handshakeTimeoutMillis    the handshake timeout in milliseconds
      */
     protected AbstractSniHandler(int maxClientHelloLength, long handshakeTimeoutMillis) {
         super(maxClientHelloLength);
         this.handshakeTimeoutMillis = checkPositiveOrZero(handshakeTimeoutMillis, "handshakeTimeoutMillis");
     }
 
     public AbstractSniHandler() {
         this(0, 0L);
     }
 
     @Override
     public void handlerAdded(ChannelHandlerContext ctx) throws Exception {
         if (ctx.channel().isActive()) {
             checkStartTimeout(ctx);
         }
     }
 
     @Override
     public void channelActive(ChannelHandlerContext ctx) throws Exception {
         ctx.fireChannelActive();
         checkStartTimeout(ctx);
     }
 
     private void checkStartTimeout(final ChannelHandlerContext ctx) {
         if (handshakeTimeoutMillis <= 0 || timeoutFuture != null) {
             return;
         }
         timeoutFuture = ctx.executor().schedule(new Runnable() {
             @Override
             public void run() {
                 if (ctx.channel().isActive()) {
                     SslHandshakeTimeoutException exception = new SslHandshakeTimeoutException(
                         "handshake timed out after " + handshakeTimeoutMillis + "ms");
                     ctx.fireUserEventTriggered(new SniCompletionEvent(exception));
                     ctx.close();
                 }
             }
         }, handshakeTimeoutMillis, TimeUnit.MILLISECONDS);
     }
 
     @Override
     protected Future<T> lookup(ChannelHandlerContext ctx, ByteBuf clientHello) throws Exception {
         hostname = clientHello == null ? null : extractSniHostname(clientHello);
 
         return lookup(ctx, hostname);
     }
 
     @Override
     protected void onLookupComplete(ChannelHandlerContext ctx, Future<T> future) throws Exception {
         if (timeoutFuture != null) {
             timeoutFuture.cancel(false);
         }
         try {
             onLookupComplete(ctx, hostname, future);
         } finally {
             fireSniCompletionEvent(ctx, hostname, future);
         }
     }
 
     /**
      * Kicks off a lookup for the given SNI value and returns a {@link Future} which in turn will
      * notify the {@link #onLookupComplete(ChannelHandlerContext, String, Future)} on completion.
      *
      * @see #onLookupComplete(ChannelHandlerContext, String, Future)
      */
     protected abstract Future<T> lookup(ChannelHandlerContext ctx, String hostname) throws Exception;
 
     /**
      * Called upon completion of the {@link #lookup(ChannelHandlerContext, String)} {@link Future}.
      *
      * @see #lookup(ChannelHandlerContext, String)
      */
     protected abstract void onLookupComplete(ChannelHandlerContext ctx,
                                              String hostname, Future<T> future) throws Exception;
 
     private static void fireSniCompletionEvent(ChannelHandlerContext ctx, String hostname, Future<?> future) {
         Throwable cause = future.cause();
         if (cause == null) {
             ctx.fireUserEventTriggered(new SniCompletionEvent(hostname));
         } else {
             ctx.fireUserEventTriggered(new SniCompletionEvent(hostname, cause));
         }
     }
 }