gh-106505: don't lookup own hostname in getfqdn()

Lib/socket.py

@@ -832,7 +832,7 @@ def getfqdn(name=''):
     """
     name = name.strip()
     if not name or name in ('0.0.0.0', '::'):
-        name = gethostname()
+        return gethostname()
     try:
         hostname, aliases, ipaddrs = gethostbyaddr(name)
     except error:

Lib/test/test_socket.py

@@ -1187,6 +1187,17 @@ def test_sethostname(self):
         finally:
             socket.sethostname(oldhn)
 
+    def test_getfqdn_undefined_address(self):
+        for addr in ('', '0.0.0.0', '::'):
+            with self.subTest(addr=addr):
+                called = []
+                with mock.patch.object(
+                    socket, 'gethostbyaddr',
+                    side_effect=lambda addr: called.append(addr) or ("", [], [])
+                ):
+                    socket.getfqdn(addr)
+                self.assertFalse(called, f"gethostbyaddr was called for undefined address {addr!r}")
+
     @unittest.skipUnless(hasattr(socket, 'if_nameindex'),
                          'socket.if_nameindex() not available.')
     @support.skip_android_selinux('if_nameindex')

Misc/ACKS

@@ -1877,6 +1877,7 @@ Daniel Stutzbach
 Andreas Stührk
 Colin Su
 Pal Subbiah
+Ilari Suhonen
 Michael J. Sullivan
 Nathan Sullivan
 Mark Summerfield

Misc/NEWS.d/next/Core_and_Builtins/2026-09-06-21-40-17.gh-issue-106505.nwL_5c.rst

@@ -0,0 +1,2 @@
+Fix ``getfqdn()`` attempting to resolve the computer's own hostname, instead
+of returning it as-is.