From a9d50671e7a3cae698b76c238a519d97cc84fa24 Mon Sep 17 00:00:00 2001
From: Neil Booth <kyuupichan@gmail.com>
Date: Tue, 5 Feb 2019 21:36:22 +0800
Subject: [PATCH] Ignore potential sybil peers

---
 electrumx/server/peers.py | 41 ++++++++++++++++++++++++---------------
 1 file changed, 25 insertions(+), 16 deletions(-)

diff --git a/electrumx/server/peers.py b/electrumx/server/peers.py
index 8e040b4..6a84332 100644
--- a/electrumx/server/peers.py
+++ b/electrumx/server/peers.py
@@ -17,7 +17,7 @@ from collections import defaultdict, Counter
 from aiorpcx import (Connector, RPCSession, SOCKSProxy,
                      Notification, handler_invocation,
                      SOCKSError, RPCError, TaskTimeout, TaskGroup, Event,
-                     sleep, run_in_thread, ignore_after, timeout_after)
+                     sleep, ignore_after, timeout_after)
 
 from electrumx.lib.peer import Peer
 from electrumx.lib.util import class_logger, protocol_tuple
@@ -152,22 +152,28 @@ class PeerManager(object):
             self.logger.info('no proxy detected, will try later')
             await sleep(900)
 
-    async def _note_peers(self, peers, limit=2, check_ports=False,
+    async def _note_peers(self, peers, limit=2, check_ports=False, check_matches=False,
                           source=None):
         '''Add a limited number of peers that are not already present.'''
         new_peers = []
+        known = []
         for peer in peers:
             if not peer.is_public or (peer.is_tor and not self.proxy):
                 continue
 
             matches = peer.matches(self.peers)
-            if not matches:
+            if matches:
+                known.append(peer)
+                if check_ports:
+                    for match in matches:
+                        if match.check_ports(peer):
+                            self.logger.info(f'ports changed for {peer}')
+                            match.retry_event.set()
+            else:
                 new_peers.append(peer)
-            elif check_ports:
-                for match in matches:
-                    if match.check_ports(peer):
-                        self.logger.info(f'ports changed for {peer}')
-                        match.retry_event.set()
+
+        if check_matches and len(self.peers) >= 6 and len(known) <= len(self.peers) // 2:
+            return False
 
         if new_peers:
             source = source or new_peers[0].source
@@ -182,6 +188,8 @@ class PeerManager(object):
                 self.peers.add(peer)
                 await self.group.spawn(self._monitor_peer(peer))
 
+        return True
+
     async def _monitor_peer(self, peer):
         # Stop monitoring if we were dropped (a duplicate peer)
         while peer in self.peers:
@@ -299,12 +307,14 @@ class PeerManager(object):
 
         # Process reported peers if remote peer is good
         peers = peers_task.result()
-        await self._note_peers(peers)
-        features = self._features_to_register(peer, peers)
-        if features:
-            self.logger.info(f'registering ourself with {peer}')
-            # We only care to wait for the response
-            await session.send_request('server.add_peer', [features])
+        if await self._note_peers(peers, check_matches=True):
+            features = self._features_to_register(peer, peers)
+            if features:
+                self.logger.info(f'registering ourself with {peer}')
+                # We only care to wait for the response
+                await session.send_request('server.add_peer', [features])
+        else:
+            raise BadPeerError('potential sybil detected')
 
     async def _send_headers_subscribe(self, session, peer, ptuple):
         message = 'blockchain.headers.subscribe'
@@ -446,8 +456,7 @@ class PeerManager(object):
                 reason = 'source-destination mismatch'
 
         if permit:
-            self.logger.info(f'accepted add_peer request from {source} '
-                             f'for {host}')
+            self.logger.info(f'accepted add_peer request from {source} for {host}')
             await self._note_peers([peer], check_ports=True)
         else:
             self.logger.warning(f'rejected add_peer request from {source} '