gh-47711: Use connected source address for SMTP address literals

Doc/library/smtplib.rst

@@ -30,7 +30,9 @@ Protocol) and :rfc:`1869` (SMTP Service Extensions).
 
    If specified, *local_hostname* is used as the FQDN of the local host in the HELO/EHLO
    command.  Otherwise, the local hostname is found using
-   :func:`socket.getfqdn`.  If the :meth:`connect` call returns anything other
+   :func:`socket.getfqdn`. If it is not fully qualified, the connected socket's
+   local address is used as an address literal instead. If the :meth:`connect`
+   call returns anything other
    than a success code, an :exc:`SMTPConnectError` is raised. The optional
    *timeout* parameter specifies a timeout in seconds for blocking operations
    like the connection attempt (if not specified, the global default timeout

Lib/smtplib.py

@@ -257,11 +257,7 @@ def __init__(self, host='', port=0, local_hostname=None,
         self.source_address = source_address
         self._auth_challenge_count = 0
 
-        if host:
-            (code, msg) = self.connect(host, port)
-            if code != 220:
-                self.close()
-                raise SMTPConnectError(code, msg)
+        self._find_local_hostname = False
         if local_hostname is not None:
             self.local_hostname = local_hostname
         else:
@@ -273,13 +269,20 @@ def __init__(self, host='', port=0, local_hostname=None,
                 self.local_hostname = fqdn
             else:
                 # We can't find an fqdn hostname, so use a domain literal
+                self._find_local_hostname = True
                 addr = '127.0.0.1'
                 try:
                     addr = socket.gethostbyname(socket.gethostname())
                 except socket.gaierror:
                     pass
                 self.local_hostname = '[%s]' % addr
 
+        if host:
+            (code, msg) = self.connect(host, port)
+            if code != 220:
+                self.close()
+                raise SMTPConnectError(code, msg)
+
     def __enter__(self):
         return self
 
@@ -348,6 +351,12 @@ def connect(self, host='localhost', port=0, source_address=None):
         self.sock = self._get_socket(host, port, self.timeout)
         self.file = None
         (code, msg) = self.getreply()
+        if code == 220 and self._find_local_hostname:
+            addr = self.sock.getsockname()[0]
+            if self.sock.family == socket.AF_INET6:
+                # RFC 5321 section 4.1.3 requires the IPv6 tag and excludes scope IDs.
+                addr = f"IPv6:{addr.split('%', 1)[0]}"
+            self.local_hostname = '[%s]' % addr
         if self.debuglevel > 0:
             self._print_debug('connect:', repr(msg))
         return (code, msg)

Lib/test/test_smtplib.py

@@ -696,6 +696,70 @@ def testSendMessageMultipleResentRaises(self):
             smtp.send_message(m)
         smtp.close()
 
+class LocalHostnameTests(unittest.TestCase):
+
+    def test_connected_address_literal(self):
+        cases = (
+            (socket.AF_INET, ('192.0.2.1', 1234), '[192.0.2.1]'),
+            (socket.AF_INET6, ('2001:db8::1', 1234, 0, 0),
+             '[IPv6:2001:db8::1]'),
+            (socket.AF_INET6, ('fe80::1%eth0', 1234, 0, 1), '[IPv6:fe80::1]'),
+        )
+        for family, address, expected in cases:
+            for connect_in_init in (True, False):
+                with self.subTest(family=family, address=address,
+                                  connect_in_init=connect_in_init):
+                    sock = Mock(family=family)
+                    sock.getsockname.return_value = address
+                    with mock.patch('smtplib.socket.getfqdn', return_value='host'), \
+                         mock.patch('smtplib.socket.gethostbyname',
+                                    return_value='127.0.0.1'), \
+                         mock.patch.object(smtplib.SMTP, '_get_socket',
+                                           return_value=sock), \
+                         mock.patch.object(smtplib.SMTP, 'getreply',
+                                           return_value=(220, b'OK')):
+                        smtp = smtplib.SMTP('server' if connect_in_init else '')
+                        self.addCleanup(smtp.close)
+                        if not connect_in_init:
+                            smtp.connect('server')
+                        self.assertEqual(smtp.local_hostname, expected)
+                        smtp.ehlo()
+                        sock.sendall.assert_called_with(
+                            ('ehlo ' + expected + '\r\n').encode('ascii'))
+
+    def test_address_literal_updated_on_reconnect(self):
+        sock = Mock(family=socket.AF_INET6)
+        sock.getsockname.side_effect = [
+            ('2001:db8::1', 1234, 0, 0), ('2001:db8::2', 1234, 0, 0)]
+        with mock.patch('smtplib.socket.getfqdn', return_value='host'), \
+             mock.patch('smtplib.socket.gethostbyname', return_value='127.0.0.1'), \
+             mock.patch.object(smtplib.SMTP, '_get_socket', return_value=sock), \
+             mock.patch.object(smtplib.SMTP, 'getreply', return_value=(220, b'OK')):
+            smtp = smtplib.SMTP('server')
+            self.addCleanup(smtp.close)
+            self.assertEqual(smtp.local_hostname, '[IPv6:2001:db8::1]')
+            smtp.close()
+            smtp.connect('other-server')
+            self.assertEqual(smtp.local_hostname, '[IPv6:2001:db8::2]')
+
+    def test_hostname_preserved(self):
+        for local_hostname, fqdn, expected in (
+            ('custom', 'host', 'custom'),
+            (None, 'host.example', 'host.example'),
+        ):
+            with self.subTest(local_hostname=local_hostname, fqdn=fqdn):
+                sock = Mock(family=socket.AF_INET6)
+                with mock.patch('smtplib.socket.getfqdn', return_value=fqdn), \
+                     mock.patch.object(smtplib.SMTP, '_get_socket',
+                                       return_value=sock), \
+                     mock.patch.object(smtplib.SMTP, 'getreply',
+                                       return_value=(220, b'OK')):
+                    smtp = smtplib.SMTP('server', local_hostname=local_hostname)
+                    self.addCleanup(smtp.close)
+                    self.assertEqual(smtp.local_hostname, expected)
+                    sock.getsockname.assert_not_called()
+
+
 class NonConnectingTests(unittest.TestCase):
 
     def testNotConnected(self):

Misc/NEWS.d/next/Library/2026-10-11-00-00-00.gh-issue-47711.smtp-ipv6.rst

@@ -0,0 +1,3 @@
+When no fully qualified hostname is available, :mod:`smtplib` now uses the
+connected socket's local address for the HELO/EHLO address literal. Previously,
+IPv6 was not handled correctly.