server: Reject outbound conns to non-full nodes.

This modifies the server connection code to reject outbound peers that
do not offer full node services.

server.go

@@ -385,6 +385,12 @@ func (sp *serverPeer) addBanScore(persistent, transient uint32, reason string) {
 	}
 }
 
+// hasServices returns whether or not the provided advertised service flags have
+// all of the provided desired service flags set.
+func hasServices(advertised, desired wire.ServiceFlag) bool {
+	return advertised&desired == desired
+}
+
 // OnVersion is invoked when a peer receives a version bitcoin message
 // and is used to negotiate the protocol version details as well as kick start
 // the communications.
@@ -395,6 +401,18 @@ func (sp *serverPeer) OnVersion(_ *peer.Peer, msg *wire.MsgVersion) *wire.MsgRej
 		return nil
 	}
 
+	// Reject outbound peers that are not full nodes.
+	wantServices := wire.SFNodeNetwork
+	if !sp.Inbound() && !hasServices(msg.Services, wantServices) {
+		missingServices := wantServices & ^msg.Services
+		srvrLog.Debugf("Rejecting peer %s with services %v due to not "+
+			"providing desired services %v", sp.Peer, msg.Services,
+			missingServices)
+		reason := fmt.Sprintf("required services %#x not offered",
+			uint64(missingServices))
+		return wire.NewMsgReject(msg.Command(), wire.RejectNonstandard, reason)
+	}
+
 	// Add the remote peer time as a sample for creating an offset against
 	// the local clock to keep the network time in sync.
 	sp.server.timeSource.AddTimeSample(sp.Addr(), msg.Timestamp)