Send only one Host header in chunked request

Closes #5274

AUTHORS.rst

@@ -190,3 +190,4 @@ Patches and Suggestions
 - Antti Kaihola (`@akaihola <https://github.com/akaihola>`_)
 - "Dull Bananas" <dull.bananas0@gmail.com> (`@dullbananas <https://github.com/dullbananas>`_)
 - Alessio Izzo (`@aless10 <https://github.com/aless10>`_)
+- Hod Bin Noon (`@hodbn <https://github.com/hodbn>`_)

requests/adapters.py

@@ -459,7 +459,8 @@ def send(self, request, stream=False, timeout=None, verify=True, cert=None, prox
                 try:
                     low_conn.putrequest(request.method,
                                         url,
-                                        skip_accept_encoding=True)
+                                        skip_accept_encoding=True,
+                                        skip_host='Host' in request.headers)
 
                     for header, value in request.headers.items():
                         low_conn.putheader(header, value)

tests/test_lowlevel.py

@@ -24,6 +24,32 @@ def test_chunked_upload():
     assert r.request.headers['Transfer-Encoding'] == 'chunked'
 
 
+def test_chunked_upload_uses_only_specified_host_header():
+    """Ensure we use only the specified Host header for chunked requests."""
+    text_200 = (b'HTTP/1.1 200 OK\r\n'
+                b'Content-Length: 0\r\n\r\n')
+    wanted_host = 'sample-host'
+    expected_header = 'Host: {}'.format(wanted_host).encode('utf-8')
+    def single_host_resp_handler(sock):
+        request_content = consume_socket_content(sock, timeout=0.5)
+        assert expected_header in request_content
+        assert request_content.count(b'Host: ') == 1
+        sock.send(text_200)
+
+        return request_content
+
+    close_server = threading.Event()
+    server = Server(single_host_resp_handler, wait_to_close_event=close_server)
+    data = iter([b'a', b'b', b'c'])
+
+    with server as (host, port):
+        url = 'http://{}:{}/'.format(host, port)
+        r = requests.post(url, data=data, headers={'Host': wanted_host}, stream=True)
+        close_server.set()  # release server block
+
+    assert r.status_code == 200
+
+
 def test_digestauth_401_count_reset_on_redirect():
     """Ensure we correctly reset num_401_calls after a successful digest auth,
     followed by a 302 redirect to another digest auth prompt.