gh-115197: Stop resolving host in urllib.request proxy bypass (GH-115210)

Use of a proxy is intended to defer DNS for the hosts to the proxy itself, rather than a potential for information leak of the host doing DNS resolution itself for any reason.  Proxy bypass lists are strictly name based.  Most implementations of proxy support agree.

Lib/test/test_urllib2.py

@@ -15,10 +15,11 @@ import tempfile
 import subprocess
 
 import urllib.request
-# The proxy bypass method imported below has logic specific to the OSX
-# proxy config data structure but is testable on all platforms.
+# The proxy bypass method imported below has logic specific to the
+# corresponding system but is testable on all platforms.
 from urllib.request import (Request, OpenerDirector, HTTPBasicAuthHandler,
                             HTTPPasswordMgrWithPriorAuth, _parse_proxy,
+                            _proxy_bypass_winreg_override,
                             _proxy_bypass_macosx_sysconf,
                             AbstractDigestAuthHandler)
 from urllib.parse import urlparse
@@ -1485,6 +1486,30 @@ class HandlerTests(unittest.TestCase):
         self.assertEqual(req.host, "proxy.example.com:3128")
         self.assertEqual(req.get_header("Proxy-authorization"), "FooBar")
 
+    @unittest.skipUnless(os.name == "nt", "only relevant for Windows")
+    def test_winreg_proxy_bypass(self):
+        proxy_override = "www.example.com;*.example.net; 192.168.0.1"
+        proxy_bypass = _proxy_bypass_winreg_override
+        for host in ("www.example.com", "www.example.net", "192.168.0.1"):
+            self.assertTrue(proxy_bypass(host, proxy_override),
+                            "expected bypass of %s to be true" % host)
+
+        for host in ("example.com", "www.example.org", "example.net",
+                     "192.168.0.2"):
+            self.assertFalse(proxy_bypass(host, proxy_override),
+                             "expected bypass of %s to be False" % host)
+
+        # check intranet address bypass
+        proxy_override = "example.com; <local>"
+        self.assertTrue(proxy_bypass("example.com", proxy_override),
+                        "expected bypass of %s to be true" % host)
+        self.assertFalse(proxy_bypass("example.net", proxy_override),
+                         "expected bypass of %s to be False" % host)
+        for host in ("test", "localhost"):
+            self.assertTrue(proxy_bypass(host, proxy_override),
+                            "expect <local> to bypass intranet address '%s'"
+                            % host)
+
     @unittest.skipUnless(sys.platform == 'darwin', "only relevant for OSX")
     def test_osx_proxy_bypass(self):
         bypass = {

Lib/urllib/request.py

@@ -2563,6 +2563,7 @@ def _proxy_bypass_macosx_sysconf(host, proxy_settings):
     }
     """
     from fnmatch import fnmatch
+    from ipaddress import AddressValueError, IPv4Address
 
     hostonly, port = _splitport(host)
 
@@ -2579,20 +2580,17 @@ def _proxy_bypass_macosx_sysconf(host, proxy_settings):
             return True
 
     hostIP = None
+    try:
+        hostIP = int(IPv4Address(hostonly))
+    except AddressValueError:
+        pass
 
     for value in proxy_settings.get('exceptions', ()):
         # Items in the list are strings like these: *.local, 169.254/16
         if not value: continue
 
         m = re.match(r"(\d+(?:\.\d+)*)(/\d+)?", value)
-        if m is not None:
-            if hostIP is None:
-                try:
-                    hostIP = socket.gethostbyname(hostonly)
-                    hostIP = ip2num(hostIP)
-                except OSError:
-                    continue
-
+        if m is not None and hostIP is not None:
             base = ip2num(m.group(1))
             mask = m.group(2)
             if mask is None:
@@ -2615,6 +2613,31 @@ def _proxy_bypass_macosx_sysconf(host, proxy_settings):
     return False
 
 
+# Same as _proxy_bypass_macosx_sysconf, testable on all platforms
+def _proxy_bypass_winreg_override(host, override):
+    """Return True if the host should bypass the proxy server.
+
+    The proxy override list is obtained from the Windows
+    Internet settings proxy override registry value.
+
+    An example of a proxy override value is:
+    "www.example.com;*.example.net; 192.168.0.1"
+    """
+    from fnmatch import fnmatch
+
+    host, _ = _splitport(host)
+    proxy_override = override.split(';')
+    for test in proxy_override:
+        test = test.strip()
+        # "<local>" should bypass the proxy server for all intranet addresses
+        if test == '<local>':
+            if '.' not in host:
+                return True
+        elif fnmatch(host, test):
+            return True
+    return False
+
+
 if sys.platform == 'darwin':
     from _scproxy import _get_proxy_settings, _get_proxies
 
@@ -2713,7 +2736,7 @@ elif os.name == 'nt':
             import winreg
         except ImportError:
             # Std modules, so should be around - but you never know!
-            return 0
+            return False
         try:
             internetSettings = winreg.OpenKey(winreg.HKEY_CURRENT_USER,
                 r'Software\Microsoft\Windows\CurrentVersion\Internet Settings')
@@ -2723,40 +2746,10 @@ elif os.name == 'nt':
                                                      'ProxyOverride')[0])
             # ^^^^ Returned as Unicode but problems if not converted to ASCII
         except OSError:
-            return 0
+            return False
         if not proxyEnable or not proxyOverride:
-            return 0
-        # try to make a host list from name and IP address.
-        rawHost, port = _splitport(host)
-        host = [rawHost]
-        try:
-            addr = socket.gethostbyname(rawHost)
-            if addr != rawHost:
-                host.append(addr)
-        except OSError:
-            pass
-        try:
-            fqdn = socket.getfqdn(rawHost)
-            if fqdn != rawHost:
-                host.append(fqdn)
-        except OSError:
-            pass
-        # make a check value list from the registry entry: replace the
-        # '<local>' string by the localhost entry and the corresponding
-        # canonical entry.
-        proxyOverride = proxyOverride.split(';')
-        # now check if we match one of the registry values.
-        for test in proxyOverride:
-            if test == '<local>':
-                if '.' not in rawHost:
-                    return 1
-            test = test.replace(".", r"\.")     # mask dots
-            test = test.replace("*", r".*")     # change glob sequence
-            test = test.replace("?", r".")      # change glob char
-            for val in host:
-                if re.match(test, val, re.I):
-                    return 1
-        return 0
+            return False
+        return _proxy_bypass_winreg_override(host, proxyOverride)
 
     def proxy_bypass(host):
         """Return True, if host should be bypassed.

Misc/NEWS.d/next/Library/2024-02-09-19-41-48.gh-issue-115197.20wkWH.rst

@@ -0,0 +1,2 @@
+``urllib.request`` no longer resolves the hostname before checking it
+against the system's proxy bypass list on macOS and Windows.