gh-47711: Use connected source address for SMTP address literals by ishu7w · Pull Request #159166 · python/cpython

ishu7w · 2026-10-11T06:35:43Z

Issue: smtplib does not fully support IPv6 in EHLO #47711

python-cla-bot · 2026-10-11T06:35:48Z

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read-the-docs-community · 2026-10-11T06:40:44Z

Documentation build overview

📚 cpython-previews | 🛠️ Build #35206090 | 📁 Comparing 93d6c56 against main (c6c0608)

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88 files changed · ± 88 modified

± Modified

bitdancer · 2026-10-15T20:26:17Z

-   :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 IP address is used as an address literal instead. IPv6 addresses use
+   the ``[IPv6:address]`` form. If the :meth:`connect` call returns anything other


I'm not sure it is useful to call out IPv6 separately here, since what we are doing is (now) following the standard, but I don't have a strong objection to including it.

Updated the parameter documentation to describe the socket local address literal without singling IPv6 out.

bitdancer · 2026-10-15T20:26:20Z

+        )
+        for family, address, expected in cases:
+            for connect_in_init in (True, False):
+                with self.subTest(family=family, connect_in_init=connect_in_init):


I think it would be good to include address in the subtest description.

Added the address to the subTest description.

bitdancer · 2026-10-15T20:26:52Z

                except socket.gaierror:
                    pass
                self.local_hostname = '[%s]' % addr



As far as I can see, the second part of this inner else is now code that doesn't have any effect, since local_hostname then gets reset unconditionally in connect. I suggest the following:

self.local_hostname = local_hostname if local_hostname is None: # RFC 2821 says we should use the fqdn in the EHLO/HELO verb, and # if that can't be calculated, that we should use a domain literal # instead (essentially an encoded IP address like [A.B.C.D]). fqdn = socket.getfqdn() if '.' in fqdn: self.local_hostname = fqdn else: # We can't find an fqdn hostname, so we'll compute the # correct domain literal in connect. self._find_local_hostname = True

And then of course changing _use_socket_hostname to _find_local_hostname in connect. What do you think? The tests, including the new ones, still pass.

Changed the flag to _find_local_hostname and now defer hostname fallback address selection to connect, following your suggested flow.

bitdancer · 2026-10-15T20:26:56Z

+        if code == 220 and self._use_socket_hostname:
+            addr = self.sock.getsockname()[0]
+            if self.sock.family == socket.AF_INET6:
+                # An SMTP address literal does not include a scope identifier.


For clarity, let's add: but does require an IPv6 "tag" per RFC 5321 Section 4.1.3

Updated the comment to cite RFC 5321 section 4.1.3 and explain the IPv6 tag and scope-ID handling.

bitdancer · 2026-10-15T20:26:58Z

+            if self.sock.family == socket.AF_INET6:
+                # An SMTP address literal does not include a scope identifier.
+                addr = 'IPv6:' + addr.split('%', 1)[0]
+            self.local_hostname = '[%s]' % addr


I think it would be clearer to use an f-string here.

Changed the IPv6 literal construction to an f-string.

bitdancer · 2026-10-15T20:27:02Z

@@ -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, including
+properly formatted IPv6 literals.


I'd add: "Previously IPv6 was not handled correctly."

Updated the news entry to note that IPv6 was previously not handled correctly.

bedevere-app · 2026-10-15T20:27:18Z

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

ishu7w · 2026-10-17T09:42:42Z

I pushed the requested code and documentation updates in 93d6c56 and replied to each inline review comment. The focused test_smtplib suite passes locally. The Unresolved review check still fails because the PR retains the awaiting changes label; GitHub denies label removal for my account, so a maintainer with label permission needs to clear it or update review status. The latest MSan run also failed only in unrelated test_socket.NetworkConnectionAttributesTest.testSourceAddress with OSError: [Errno 9] Bad file descriptor; the failing line calls getsockname() on a socket that the test runner reports as closed. Could a maintainer rerun MSan after clearing the review label?

ishu7w requested a review from a team as a code owner October 11, 2026 06:35

bedevere-app Bot mentioned this pull request Oct 11, 2026

smtplib does not fully support IPv6 in EHLO #47711

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bedevere-app Bot added the awaiting review label Oct 11, 2026

bitdancer requested changes Oct 15, 2026

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bedevere-app Bot added awaiting changes and removed awaiting review labels Oct 15, 2026

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

93d6c56

ishu7w force-pushed the fix-smtp-ipv6-ehlo branch from a58dc50 to 93d6c56 Compare October 17, 2026 05:56

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ishu7w commented Oct 11, 2026 •

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