restore drop unknown client optin

docs/environment.rst

@@ -285,6 +285,14 @@ These environment variables are optional:
   version string. For example to drop versions from 1.0 to 1.2 use
   the regex ``1\.[0-2]\.\d+``.
 
+.. envvar:: DROP_CLIENT_UNKNOWN
+
+  Set to anything non-empty to deny serving clients which do not
+  identify themselves first by issuing the server.version method
+  call with a non-empty client identifier. The connection is dropped
+  on first actual method call. This might help to filter out simple
+  robots. This behavior is off by default.
+
 
 Resource Usage Limits
 =====================

src/electrumx/server/env.py

@@ -73,6 +73,7 @@ def __init__(self, coin=None):
         self.log_level = self.default('LOG_LEVEL', 'info').upper()
         self.donation_address = self.default('DONATION_ADDRESS', '')
         self.drop_client = self.custom("DROP_CLIENT", None, re.compile)
+        self.drop_client_unknown = self.boolean('DROP_CLIENT_UNKNOWN', False)
         self.blacklist_url = self.default('BLACKLIST_URL', self.coin.BLACKLIST_URL)
         self.cache_MB = self.integer('CACHE_MB', 1200)
         self.reorg_limit = self.integer('REORG_LIMIT', self.coin.REORG_LIMIT)

src/electrumx/server/session.py

@@ -1217,7 +1217,6 @@ def __init__(
         self.coin = self.env.coin
         self.client_longname = 'unknown'
         self.sv_seen = False  # has seen 'server.version' message?
-        self.sv_negotiated = asyncio.Event()  # done negotiating protocol version
         self.anon_logs = self.env.anon_logs
         self.txs_sent = 0
         self.log_me = SessionBase.log_new
@@ -1296,15 +1295,13 @@ async def handle_request(self, request: SingleRequest):
             handler = self.notification_handlers.get(request.method)
         method = 'invalid method' if handler is None else request.method
 
-        # Version negotiation must happen before any other messages.
-        if not self.sv_seen and method != 'server.version':
-            self.logger.info(f'closing session: server.version must be first msg. got: {method}')
-            await self._do_crash_old_electrum_client()
+        # If DROP_CLIENT_UNKNOWN is enabled, check if the client identified
+        # by calling server.version previously. If not, disconnect the session
+        if (self.env.drop_client_unknown and method != 'server.version'
+                and self.client_longname == 'unknown'):
+            self.logger.info(f'disconnecting because client is unknown')
             raise ReplyAndDisconnect(RPCError(
                 BAD_REQUEST, f'use server.version to identify client'))
-        # Wait for version negotiation to finish before processing other messages.
-        if method != 'server.version' and not self.sv_negotiated.is_set():
-            await self.sv_negotiated.wait()
 
         self._method_counts[method] += 1
         self.session_mgr._method_counts[method] += 1
@@ -2197,7 +2194,6 @@ async def phandle_server_version(
                 BAD_REQUEST, f'unsupported protocol version: {protocol_version}'))
         self.set_request_handlers(ptuple)
 
-        self.sv_negotiated.set()
         return electrumx.version, self.protocol_version_string()
 
     async def phandle_transaction_broadcast(self, raw_tx: str | Any) -> str:
@@ -2429,9 +2425,7 @@ class LocalRPC(SessionBase):
 
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
-        self.sv_seen = True
         assert get_running_loop() is not None, "must be running on asyncio thread"
-        self.sv_negotiated.set()
         self.client_longname = 'RPC'
         self.connection.max_response_size = 0
         # note: self.request_handlers are set on the class, in SessionManager.__init__

tests/server/test_env.py

@@ -428,3 +428,18 @@ def test_ban_versions():
 def test_coin_class_provided():
     e = Env(lib_coins.Bitcoin)
     assert e.coin == lib_coins.Bitcoin
+
+
+def test_drop_unknown_clients():
+    setup_base_env()
+    e = Env()
+    assert e.drop_client_unknown is False
+    os.environ['DROP_CLIENT_UNKNOWN'] = ""
+    e = Env()
+    assert e.drop_client_unknown is False
+    os.environ['DROP_CLIENT_UNKNOWN'] = "1"
+    e = Env()
+    assert e.drop_client_unknown is True
+    os.environ['DROP_CLIENT_UNKNOWN'] = "whatever"
+    e = Env()
+    assert e.drop_client_unknown is True