Normalize TLS server hostnames

httpcore/_backends/anyio.py

@@ -14,6 +14,7 @@
     WriteTimeout,
     map_exceptions,
 )
+from .._ssl import _normalize_server_hostname
 from .._utils import is_socket_readable
 from .base import SOCKET_OPTION, AsyncNetworkBackend, AsyncNetworkStream
 
@@ -64,6 +65,8 @@ async def start_tls(
             anyio.EndOfStream: ConnectError,
             ssl.SSLError: ConnectError,
         }
+        server_hostname = _normalize_server_hostname(server_hostname)
+
         with map_exceptions(exc_map):
             try:
                 with anyio.fail_after(timeout):

httpcore/_backends/sync.py

@@ -16,6 +16,7 @@
     WriteTimeout,
     map_exceptions,
 )
+from .._ssl import _normalize_server_hostname
 from .._utils import is_socket_readable
 from .base import SOCKET_OPTION, NetworkBackend, NetworkStream
 
@@ -41,6 +42,7 @@ def __init__(
         self._sock = sock
         self._incoming = ssl.MemoryBIO()
         self._outgoing = ssl.MemoryBIO()
+        server_hostname = _normalize_server_hostname(server_hostname)
 
         self.ssl_obj = ssl_context.wrap_bio(
             incoming=self._incoming,
@@ -151,6 +153,8 @@ def start_tls(
             socket.timeout: ConnectTimeout,
             OSError: ConnectError,
         }
+        server_hostname = _normalize_server_hostname(server_hostname)
+
         with map_exceptions(exc_map):
             try:
                 if isinstance(self._sock, ssl.SSLSocket):  # pragma: no cover

httpcore/_backends/trio.py

@@ -15,6 +15,7 @@
     WriteTimeout,
     map_exceptions,
 )
+from .._ssl import _normalize_server_hostname
 from .base import SOCKET_OPTION, AsyncNetworkBackend, AsyncNetworkStream
 
 
@@ -62,6 +63,8 @@ async def start_tls(
             trio.TooSlowError: ConnectTimeout,
             trio.BrokenResourceError: ConnectError,
         }
+        server_hostname = _normalize_server_hostname(server_hostname)
+
         ssl_stream = trio.SSLStream(
             self._stream,
             ssl_context=ssl_context,

httpcore/_ssl.py

@@ -1,8 +1,17 @@
+from __future__ import annotations
+
 import ssl
 
 import certifi
 
 
+def _normalize_server_hostname(server_hostname: str | None) -> str | None:
+    if server_hostname is None:
+        return None
+
+    return server_hostname.rstrip(".")
+
+
 def default_ssl_context() -> ssl.SSLContext:
     context = ssl.create_default_context()
     context.load_verify_locations(certifi.where())

tests/test_ssl.py

@@ -0,0 +1,41 @@
+from __future__ import annotations
+
+import socket
+import typing
+
+from httpcore._backends.sync import SyncStream
+from httpcore._ssl import _normalize_server_hostname
+
+
+class CapturingSSLContext:
+    def __init__(self) -> None:
+        self.server_hostname: str | None = None
+
+    def wrap_socket(
+        self, sock: socket.socket, server_hostname: str | None = None
+    ) -> socket.socket:
+        self.server_hostname = server_hostname
+        return sock
+
+
+def test_normalize_server_hostname() -> None:
+    assert _normalize_server_hostname(None) is None
+    assert _normalize_server_hostname("example.com") == "example.com"
+    assert _normalize_server_hostname("example.com.") == "example.com"
+
+
+def test_sync_start_tls_normalizes_trailing_dot_hostname() -> None:
+    local, remote = socket.socketpair()
+    ssl_context = CapturingSSLContext()
+
+    try:
+        stream = SyncStream(local)
+        tls_stream = stream.start_tls(
+            typing.cast(typing.Any, ssl_context), server_hostname="example.com."
+        )
+
+        assert ssl_context.server_hostname == "example.com"
+        tls_stream.close()
+    finally:
+        local.close()
+        remote.close()