diff --git a/lib/webrick/httpproxy.rb b/lib/webrick/httpproxy.rb
index be5531fec0..d180ff4831 100644
--- a/lib/webrick/httpproxy.rb
+++ b/lib/webrick/httpproxy.rb
@@ -211,21 +211,15 @@ module WEBrick
     end
 
     def do_GET(req, res)
-      perform_proxy_request(req, res) do |http, path, header|
-        http.get(path, header)
-      end
+      perform_proxy_request(req, res, Net::HTTP::Get)
     end
 
     def do_HEAD(req, res)
-      perform_proxy_request(req, res) do |http, path, header|
-        http.head(path, header)
-      end
+      perform_proxy_request(req, res, Net::HTTP::Head)
     end
 
     def do_POST(req, res)
-      perform_proxy_request(req, res) do |http, path, header|
-        http.post(path, req.body || "", header)
-      end
+      perform_proxy_request(req, res, Net::HTTP::Post, req.body_reader)
     end
 
     def do_OPTIONS(req, res)
@@ -301,38 +295,56 @@ module WEBrick
       return FakeProxyURI
     end
 
-    def perform_proxy_request(req, res)
+    def perform_proxy_request(req, res, req_class, body_stream = nil)
       uri = req.request_uri
       path = uri.path.dup
       path << "?" << uri.query if uri.query
       header = setup_proxy_header(req, res)
       upstream = setup_upstream_proxy_authentication(req, res, header)
-      response = nil
 
+      body_tmp = []
       http = Net::HTTP.new(uri.host, uri.port, upstream.host, upstream.port)
-      http.start do
-        if @config[:ProxyTimeout]
-          ##################################   these issues are
-          http.open_timeout = 30   # secs  #   necessary (maybe because
-          http.read_timeout = 60   # secs  #   Ruby's bug, but why?)
-          ##################################
+      req_fib = Fiber.new do
+        http.start do
+          if @config[:ProxyTimeout]
+            ##################################   these issues are
+            http.open_timeout = 30   # secs  #   necessary (maybe because
+            http.read_timeout = 60   # secs  #   Ruby's bug, but why?)
+            ##################################
+          end
+          if body_stream && req['transfer-encoding'] =~ /\bchunked\b/i
+            header['Transfer-Encoding'] = 'chunked'
+          end
+          http_req = req_class.new(path, header)
+          http_req.body_stream = body_stream if body_stream
+          http.request(http_req) do |response|
+            # Persistent connection requirements are mysterious for me.
+            # So I will close the connection in every response.
+            res['proxy-connection'] = "close"
+            res['connection'] = "close"
+
+            # stream Net::HTTP::HTTPResponse to WEBrick::HTTPResponse
+            res.status = response.code.to_i
+            res.chunked = response.chunked?
+            choose_header(response, res)
+            set_cookie(response, res)
+            set_via(res)
+            response.read_body do |buf|
+              body_tmp << buf
+              Fiber.yield # wait for res.body Proc#call
+            end
+          end # http.request
+        end
+      end
+      req_fib.resume # read HTTP response headers and first chunk of the body
+      res.body = ->(socket) do
+        while buf = body_tmp.shift
+          socket.write(buf)
+          buf.clear
+          req_fib.resume # continue response.read_body
         end
-        response = yield(http, path, header)
       end
-
-      # Persistent connection requirements are mysterious for me.
-      # So I will close the connection in every response.
-      res['proxy-connection'] = "close"
-      res['connection'] = "close"
-
-      # Convert Net::HTTP::HTTPResponse to WEBrick::HTTPResponse
-      res.status = response.code.to_i
-      choose_header(response, res)
-      set_cookie(response, res)
-      set_via(res)
-      res.body = response.body
     end
-
     # :stopdoc:
   end
 end
diff --git a/lib/webrick/httprequest.rb b/lib/webrick/httprequest.rb
index b40bcb0d57..c40f7c16e4 100644
--- a/lib/webrick/httprequest.rb
+++ b/lib/webrick/httprequest.rb
@@ -257,6 +257,32 @@ module WEBrick
       @body.empty? ? nil : @body
     end
 
+    ##
+    # Prepares the HTTPRequest object for use as the
+    # source for IO.copy_stream
+
+    def body_reader
+      @body_tmp = []
+      @body_rd = Fiber.new do
+        body do |buf|
+          @body_tmp << buf
+          Fiber.yield
+        end
+      end
+      @body_rd.resume # grab the first chunk and yield
+      self
+    end
+
+    # for IO.copy_stream.  Note: we may return a larger string than +size+
+    # here; but IO.copy_stream does not care.
+    def readpartial(size, buf = ''.b) # :nodoc
+      res = @body_tmp.shift or raise EOFError, 'end of file reached'
+      buf.replace(res)
+      res.clear
+      @body_rd.resume # get more chunks
+      buf
+    end
+
     ##
     # Request query as a Hash
 
diff --git a/test/webrick/test_httpproxy.rb b/test/webrick/test_httpproxy.rb
index 452e7b94b7..52ecc91b28 100644
--- a/test/webrick/test_httpproxy.rb
+++ b/test/webrick/test_httpproxy.rb
@@ -118,6 +118,100 @@ class TestWEBrickHTTPProxy < Test::Unit::TestCase
     }
   end
 
+  def test_big_bodies
+    require 'digest/md5'
+    rand_str = File.read(__FILE__)
+    rand_str.freeze
+    nr = 1024 ** 2 / rand_str.size # bigger works, too
+    exp = Digest::MD5.new
+    nr.times { exp.update(rand_str) }
+    exp = exp.hexdigest
+    TestWEBrick.start_httpserver do |o_server, o_addr, o_port, o_log|
+      o_server.mount_proc('/') do |req, res|
+        case req.request_method
+        when 'GET'
+          res['content-type'] = 'application/octet-stream'
+          if req.path == '/length'
+            res['content-length'] = (nr * rand_str.size).to_s
+          else
+            res.chunked = true
+          end
+          res.body = ->(socket) { nr.times { socket.write(rand_str) } }
+        when 'POST'
+          dig = Digest::MD5.new
+          req.body { |buf| dig.update(buf); buf.clear }
+          res['content-type'] = 'text/plain'
+          res['content-length'] = '32'
+          res.body = dig.hexdigest
+        end
+      end
+        http = Net::HTTP.new(o_addr, o_port)
+        IO.pipe do |rd, wr|
+          headers = {
+            'Content-Type' => 'application/octet-stream',
+            'Transfer-Encoding' => 'chunked',
+          }
+          post = Net::HTTP::Post.new('/', headers)
+          th = Thread.new { nr.times { wr.write(rand_str) }; wr.close }
+          post.body_stream = rd
+          http.request(post) do |res|
+            assert_equal 'text/plain', res['content-type']
+            assert_equal 32, res.content_length
+            assert_equal exp, res.body
+          end
+          assert_nil th.value
+        end
+
+      TestWEBrick.start_httpproxy do |p_server, p_addr, p_port, p_log|
+        http = Net::HTTP.new(o_addr, o_port, p_addr, p_port)
+        http.request_get('/length') do |res|
+          assert_equal(nr * rand_str.size, res.content_length)
+          dig = Digest::MD5.new
+          res.read_body { |buf| dig.update(buf); buf.clear }
+          assert_equal exp, dig.hexdigest
+        end
+        http.request_get('/') do |res|
+          assert_predicate res, :chunked?
+          dig = Digest::MD5.new
+          res.read_body { |buf| dig.update(buf); buf.clear }
+          assert_equal exp, dig.hexdigest
+        end
+
+        IO.pipe do |rd, wr|
+          headers = {
+            'Content-Type' => 'application/octet-stream',
+            'Content-Length' => (nr * rand_str.size).to_s,
+          }
+          post = Net::HTTP::Post.new('/', headers)
+          th = Thread.new { nr.times { wr.write(rand_str) }; wr.close }
+          post.body_stream = rd
+          http.request(post) do |res|
+            assert_equal 'text/plain', res['content-type']
+            assert_equal 32, res.content_length
+            assert_equal exp, res.body
+          end
+          assert_nil th.value
+        end
+
+        IO.pipe do |rd, wr|
+          headers = {
+            'Content-Type' => 'application/octet-stream',
+            'Transfer-Encoding' => 'chunked',
+          }
+          post = Net::HTTP::Post.new('/', headers)
+          th = Thread.new { nr.times { wr.write(rand_str) }; wr.close }
+          post.body_stream = rd
+          http.request(post) do |res|
+            assert_equal 'text/plain', res['content-type']
+            assert_equal 32, res.content_length
+            assert_equal exp, res.body
+          end
+          assert_nil th.value
+        end
+      end
+    end
+  end
+
   def make_certificate(key, cn)
     subject = OpenSSL::X509::Name.parse("/DC=org/DC=ruby-lang/CN=#{cn}")
     exts = [
diff --git a/test/webrick/test_httprequest.rb b/test/webrick/test_httprequest.rb
index 855ff9d4a7..6b99e34569 100644
--- a/test/webrick/test_httprequest.rb
+++ b/test/webrick/test_httprequest.rb
@@ -237,6 +237,7 @@ GET /
 
   def test_chunked
     crlf = "\x0d\x0a"
+    expect = File.read(__FILE__).freeze
     msg = <<-_end_of_message_
       POST /path HTTP/1.1
       Host: test.ruby-lang.org:8080
@@ -253,7 +254,14 @@ GET /
     msg << "0" << crlf
     req = WEBrick::HTTPRequest.new(WEBrick::Config::HTTP)
     req.parse(StringIO.new(msg))
-    assert_equal(File.read(__FILE__), req.body)
+    assert_equal(expect, req.body)
+
+    # chunked req.body_reader
+    req = WEBrick::HTTPRequest.new(WEBrick::Config::HTTP)
+    req.parse(StringIO.new(msg))
+    dst = StringIO.new
+    IO.copy_stream(req.body_reader, dst)
+    assert_equal(expect, dst.string)
   end
 
   def test_forwarded