server: Only respond to getaddr once per conn.

This modifies the OnGetAddr handler to prevent the server from
responding to getaddr messages more than once per connection which helps
reduce unhelpful traffic and fingerprinting attacks.

server.go

@@ -201,6 +201,10 @@ type serverPeer struct {
 	banScore        connmgr.DynamicBanScore
 	quit            chan struct{}
 
+	// addrsSent tracks whether or not the peer has responded to a getaddr
+	// request.  It is used to prevent more than one response per connection.
+	addrsSent bool
+
 	// The following chans are used to sync blockmanager and server.
 	txProcessed    chan struct{}
 	blockProcessed chan struct{}
@@ -1015,6 +1019,14 @@ func (sp *serverPeer) OnGetAddr(p *peer.Peer, msg *wire.MsgGetAddr) {
 		return
 	}
 
+	// Only respond with addresses once per connection.  This helps reduce
+	// traffic and further reduces fingerprinting attacks.
+	if sp.addrsSent {
+		peerLog.Tracef("Ignoring getaddr from %v - already sent", sp.Peer)
+		return
+	}
+	sp.addrsSent = true
+
 	// Get the current known addresses from the address manager.
 	addrCache := sp.server.addrManager.AddressCache()