New flag Firewall in the Features field to inform uncontacted peers that a Traverse message might be required to establish a connection. Close #61

This flag can be activated via the config setting LocalFirewall.
The Features field exists in Announcement/Response messages and was now added to peer records.

protocol/Message Encoding Announcement.go

@@ -16,7 +16,7 @@ import (
 type MessageAnnouncement struct {
 	*MessageRaw                   // Underlying raw message
 	Protocol          uint8       // Protocol version supported (low 4 bits).
-	Features          uint8       // Feature support (high 4 bits). Future use.
+	Features          uint8       // Feature support
 	Actions           uint8       // Action bit array. See ActionX
 	BlockchainHeight  uint32      // Blockchain height
 	BlockchainVersion uint64      // Blockchain version
@@ -44,6 +44,7 @@ type InfoStore struct {
 const (
 	FeatureIPv4Listen = 0 // Sender listens on IPv4
 	FeatureIPv6Listen = 1 // Sender listens on IPv6
+	FeatureFirewall   = 2 // Sender indicates a potential firewall. This informs uncontacted peers that a Traverse message might be required to establish a connection.
 )
 
 // Actions between peers, sent via Announcement message. They correspond to the bit array index.

protocol/Message Encoding Response.go

@@ -47,6 +47,7 @@ type PeerRecord struct {
 	IPv6PortReportedExternal uint16           // External port as reported by that peer. This is used in case of port forwarding (manual or automated).
 	LastContact              uint32           // Last contact in seconds
 	LastContactT             time.Time        // Last contact time translated from seconds
+	Features                 uint8            // Feature support. Same as in Announcement/Response message.
 }
 
 // Hash2Peer links a hash to peers who are known to store the data and to peers who are considered close to the hash
@@ -208,7 +209,8 @@ func decodePeerRecord(data []byte, count int) (hash2Peers []Hash2Peer, read int,
 
 			peer.LastContact = binary.LittleEndian.Uint32(data[index+65 : index+65+4])
 			peer.LastContactT = time.Now().Add(-time.Second * time.Duration(peer.LastContact))
-			reason := data[index+69]
+			peer.Features = data[index+69] & 0x7F
+			reason := data[index+69] >> 7
 
 			var err error
 			if peer.PublicKey, err = btcec.ParsePubKey(peerIDcompressed, btcec.S256()); err != nil {
@@ -421,7 +423,7 @@ createPacketLoop:
 func encodePeerRecord(raw []byte, peer *PeerRecord, reason uint8) {
 	copy(raw[0:0+33], peer.PublicKey.SerializeCompressed())
 	binary.LittleEndian.PutUint32(raw[65:65+4], peer.LastContact)
-	raw[69] = reason
+	raw[69] = peer.Features | reason<<7
 
 	// IPv4
 	copy(raw[33:33+4], peer.IPv4.To4())