server: adds hybrid black/whitelist filtering to user agents

server.go

@@ -246,6 +246,14 @@ type server struct {
 	// messages for each filter type.
 	cfCheckptCaches    map[wire.FilterType][]cfHeaderKV
 	cfCheckptCachesMtx sync.RWMutex
+
+	// agentBlacklist is a list of blacklisted substrings by which to filter
+	// user agents.
+	agentBlacklist []string
+
+	// agentWhitelist is a list of whitelisted user agent substrings, no
+	// whitelisting will be applied if the list is empty or nil.
+	agentWhitelist []string
 }
 
 // serverPeer extends the peer to maintain state shared by the server and
@@ -1586,6 +1594,12 @@ func (s *server) handleAddPeerMsg(state *peerState, sp *serverPeer) bool {
 		return false
 	}
 
+	// Disconnect peers with unwanted user agents.
+	if sp.HasUndesiredUserAgent(s.agentBlacklist, s.agentWhitelist) {
+		sp.Disconnect()
+		return false
+	}
+
 	// Ignore new peers if we're shutting down.
 	if atomic.LoadInt32(&s.shutdown) != 0 {
 		srvrLog.Infof("New peer %s ignored - server is shutting down", sp)
@@ -3133,3 +3147,47 @@ func mergeCheckpoints(defaultCheckpoints, additional []chaincfg.Checkpoint) []ch
 	sort.Sort(checkpointSorter(checkpoints))
 	return checkpoints
 }
+
+// HasUndesiredUserAgent determines whether the server should continue to pursue
+// a connection with this peer based on its advertised user agent. It performs
+// the following steps:
+// 1) Reject the peer if it contains a blacklisted agent.
+// 2) If no whitelist is provided, accept all user agents.
+// 3) Accept the peer if it contains a whitelisted agent.
+// 4) Reject all other peers.
+func (sp *serverPeer) HasUndesiredUserAgent(blacklistedAgents,
+	whitelistedAgents []string) bool {
+
+	agent := sp.UserAgent()
+
+	// First, if peer's user agent contains any blacklisted substring, we
+	// will ignore the connection request.
+	for _, blacklistedAgent := range blacklistedAgents {
+		if strings.Contains(agent, blacklistedAgent) {
+			srvrLog.Debugf("Ignoring peer %s, user agent "+
+				"contains blacklisted user agent: %s", sp,
+				agent)
+			return true
+		}
+	}
+
+	// If no whitelist is provided, we will accept all user agents.
+	if len(whitelistedAgents) == 0 {
+		return false
+	}
+
+	// Peer's user agent passed blacklist. Now check to see if it contains
+	// one of our whitelisted user agents, if so accept.
+	for _, whitelistedAgent := range whitelistedAgents {
+		if strings.Contains(agent, whitelistedAgent) {
+			return false
+		}
+	}
+
+	// Otherwise, the peer's user agent was not included in our whitelist.
+	// Ignore just in case it could stall the initial block download.
+	srvrLog.Debugf("Ignoring peer %s, user agent: %s not found in "+
+		"whitelist", sp, agent)
+
+	return true
+}