fix(auth): an IPv6 visitor counts by their /64

One household's line or one server gets a whole /64 of IPv6 addresses,
and could turn through them past every per-network limit (the second
reading's M5). networkOf now counts an IPv6 visitor by their /64
(networkKeyOf), an IPv4 visitor by their address as before.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_base_server/README.md

@@ -48,7 +48,9 @@ Endpoints, under the nickname `base` in a host's client (`client.modules.base.em
   connection's, or, for a connection from a proxy the operator trusts
   (`TRUSTED_PROXIES`), the last address of `X-Forwarded-For`, the one the proxy saw
   itself. Never the `Forwarded` header, which Caddy passes on as the visitor wrote it,
-  so never Serverpod's `remoteInfo`. Serverpod's own limit on anonymous accounts is off
+  so never Serverpod's `remoteInfo`. An IPv6 visitor counts by their /64
+  (`networkKeyOf`), which one line or server holds whole. Serverpod's own limit on
+  anonymous accounts is off
   (it counts by the connection's address, behind a proxy the proxy's for everyone); the
   endpoint counts them itself.
 - Refusals are `PasswordRefusedException`, `GitHubSignInRefusedException`,

packages/halkyon_base_server/lib/src/common/network.dart

@@ -1,4 +1,5 @@
 import 'dart:io';
+import 'dart:typed_data';
 
 import 'package:serverpod/serverpod.dart';
 
@@ -13,16 +14,35 @@ import 'package:serverpod/serverpod.dart';
 // Never the `Forwarded` header, which Caddy passes on as the visitor wrote
 // it, and so never Serverpod's `remoteInfo`, which prefers that header.
 
-/// The network [session]'s request came from (see above): its visitor's
-/// address, or `null` for a session without a request (tests, future
-/// calls), as the limits have always taken it.
+/// The network [session]'s request came from (see above), as the limits
+/// count it ([networkKeyOf]): its visitor's address, an IPv6 visitor's /64,
+/// or `null` for a session without a request (tests, future calls), as the
+/// limits have always taken it.
 String networkOf(Session session) {
   final request = session.request;
-  return '${visitorAddress(
-    socket: request == null ? null : '${request.connectionInfo.remote.address}',
-    forwardedFor: request?.headers['x-forwarded-for'],
-    trustedProxies: trustedProxies(),
-  )}';
+  return networkKeyOf(
+    visitorAddress(
+      socket: request == null
+          ? null
+          : '${request.connectionInfo.remote.address}',
+      forwardedFor: request?.headers['x-forwarded-for'],
+      trustedProxies: trustedProxies(),
+    ),
+  );
+}
+
+/// The network the limits count [address] in: an IPv4 address itself, an
+/// IPv6 address's /64 (`2001:db8:1:2::/64`; the second reading's M5), since
+/// one household's line or one server gets a whole /64 and could turn
+/// through its addresses past every limit; `null` for none.
+String networkKeyOf(String? address) {
+  final parsed = address == null ? null : InternetAddress.tryParse(address);
+  if (parsed == null || parsed.type != InternetAddressType.IPv6) {
+    return '$address';
+  }
+  final prefix = [...parsed.rawAddress.take(8), ...List.filled(8, 0)];
+  return '${InternetAddress.fromRawAddress(Uint8List.fromList(prefix)).address}'
+      '/64';
 }
 
 /// The address of the visitor whose connection came from [socket] (none

packages/halkyon_base_server/test/unit/network_test.dart

@@ -122,6 +122,25 @@ void main() {
       }
     });
 
+    test("is an IPv6 visitor's /64, as one household or server holds a "
+        'whole /64 and could turn through its addresses past every limit; '
+        'an IPv4 visitor its address', () {
+      expect(
+        networkKeyOf('2001:db8:1:2:aaaa:bbbb:cccc:1'),
+        '2001:db8:1:2::/64',
+      );
+      expect(
+        networkKeyOf('2001:db8:1:2::ffff'),
+        networkKeyOf('2001:db8:1:2:9::1'),
+      );
+      expect(
+        networkKeyOf('2001:db8:1:3::1'),
+        isNot(networkKeyOf('2001:db8:1:2::1')),
+      );
+      expect(networkKeyOf('198.51.100.7'), '198.51.100.7');
+      expect(networkKeyOf(null), 'null');
+    });
+
     test('is none without a connection (tests, future calls)', () {
       expect(
         visitorAddress(