code: // Licensed to the Software Freedom Conservancy (SFC) under one
  // or more contributor license agreements.  See the NOTICE file
  // distributed with this work for additional information
  // regarding copyright ownership.  The SFC 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
  //
  //   http://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 org.openqa.selenium.grid.web;
  
  import static org.openqa.selenium.remote.tracing.HttpTracing.newSpanAsChildOf;
  import static org.openqa.selenium.remote.tracing.Tags.HTTP_REQUEST;
  import static org.openqa.selenium.remote.tracing.Tags.HTTP_RESPONSE;
  
  import com.google.common.collect.ImmutableSet;
  import java.io.UncheckedIOException;
  import java.util.logging.Logger;
  import org.openqa.selenium.internal.Require;
  import org.openqa.selenium.remote.http.HttpClient;
  import org.openqa.selenium.remote.http.HttpHandler;
  import org.openqa.selenium.remote.http.HttpRequest;
  import org.openqa.selenium.remote.http.HttpResponse;
  import org.openqa.selenium.remote.tracing.Span;
  import org.openqa.selenium.remote.tracing.Tracer;
  
  public class ReverseProxyHandler implements HttpHandler {
  
    private static final Logger LOG = Logger.getLogger(ReverseProxyHandler.class.getName());
  
    private static final ImmutableSet<String> IGNORED_REQ_HEADERS =
        ImmutableSet.<String>builder()
            .add("connection")
            .add("keep-alive")
            .add("proxy-authorization")
            .add("proxy-authenticate")
            .add("proxy-connection")
            .add("te")
            .add("trailer")
            .add("transfer-encoding")
            .add("upgrade")
            .build();
  
    private final Tracer tracer;
    private final HttpClient upstream;
  
    public ReverseProxyHandler(Tracer tracer, HttpClient httpClient) {
      this.tracer = Require.nonNull("Tracer", tracer);
      this.upstream = Require.nonNull("HTTP client", httpClient);
    }
  
    @Override
    public HttpResponse execute(HttpRequest req) throws UncheckedIOException {
      try (Span span = newSpanAsChildOf(tracer, req, "reverse_proxy")) {
  
        HTTP_REQUEST.accept(span, req);
  
        HttpRequest toUpstream = new HttpRequest(req.getMethod(), req.getUri());
  
        for (String attributeName : req.getAttributeNames()) {
          toUpstream.setAttribute(attributeName, req.getAttribute(attributeName));
        }
  
        for (String name : req.getQueryParameterNames()) {
          for (String value : req.getQueryParameters(name)) {
            toUpstream.addQueryParameter(name, value);
          }
        }
  
        for (String name : req.getHeaderNames()) {
          if (IGNORED_REQ_HEADERS.contains(name.toLowerCase())) {
            continue;
          }
  
          for (String value : req.getHeaders(name)) {
            toUpstream.addHeader(name, value);
          }
        }
        // None of this "keep alive" nonsense.
        toUpstream.setHeader("Connection", "keep-alive");
  
        toUpstream.setContent(req.getContent());
        HttpResponse resp = upstream.execute(toUpstream);
  
        HTTP_RESPONSE.accept(span, resp);
  
        // clear response defaults.
        resp.removeHeader("Date");
        resp.removeHeader("Server");
  
        IGNORED_REQ_HEADERS.forEach(resp::removeHeader);
  
        return resp;
      }
    }
  }
  