From 244c70bc83a08b5ad001a98b4f0684c3c7e62ac4 Mon Sep 17 00:00:00 2001
From: Matt Joiner <anacrolix@gmail.com>
Date: Tue, 22 Jul 2014 21:45:12 +1000
Subject: [PATCH] Implement "yourip" field of extension protocol handshake

---
 client.go | 30 ++++++++++++++++++++++++++++--
 1 file changed, 28 insertions(+), 2 deletions(-)

diff --git a/client.go b/client.go
index 8a7fe33d..eeb04db5 100644
--- a/client.go
+++ b/client.go
@@ -259,6 +259,8 @@ func (me *Client) torrent(ih InfoHash) *torrent {
 	return nil
 }
 
+// Start the process of connecting to the given peer for the given torrent if
+// appropriate.
 func (me *Client) initiateConn(peer Peer, torrent *torrent) {
 	if peer.Id == me.PeerId {
 		return
@@ -269,10 +271,11 @@ func (me *Client) initiateConn(peer Peer, torrent *torrent) {
 			IP:   peer.IP,
 			Port: peer.Port,
 		}
-		// TODO: Specify local address so that peers associate our address
-		// with our listen address.
+		// Binding to the listener address and dialing via net.Dialer gives "address in use" error. It seems it's not possible to dial out from this address so that peers associate our local address with our listen address.
 		conn, err := net.DialTimeout(addr.Network(), addr.String(), dialTimeout)
 
+		// Whether or not the connection attempt succeeds, the half open
+		// counter should be decremented, and new connection attempts made.
 		go func() {
 			me.mu.Lock()
 			defer me.mu.Unlock()
@@ -320,6 +323,22 @@ func (cl *Client) incomingPeerPort() int {
 	return i
 }
 
+// Convert a net.Addr to its compact IP representation. Either 4 or 16 bytes per "yourip" field of http://www.bittorrent.org/beps/bep_0010.html.
+func addrCompactIP(addr net.Addr) (string, error) {
+	switch typed := addr.(type) {
+	case *net.TCPAddr:
+		if v4 := typed.IP.To4(); v4 != nil {
+			if len(v4) != 4 {
+				panic(v4)
+			}
+			return string(v4), nil
+		}
+		return string(typed.IP.To16()), nil
+	default:
+		return "", fmt.Errorf("unhandled type: %T", addr)
+	}
+}
+
 func (me *Client) runConnection(sock net.Conn, torrent *torrent, discovery peerSource) (err error) {
 	conn := &connection{
 		Discovery:       discovery,
@@ -403,6 +422,13 @@ func (me *Client) runConnection(sock net.Conn, torrent *torrent, discovery peerS
 				if p := me.incomingPeerPort(); p != 0 {
 					d["p"] = p
 				}
+				yourip, err := addrCompactIP(conn.Socket.RemoteAddr())
+				if err != nil {
+					log.Printf("error calculating yourip field value in extension handshake: %s", err)
+				} else {
+					d["yourip"] = yourip
+				}
+				log.Printf("sending %v", d)
 				b, err := bencode.Marshal(d)
 				if err != nil {
 					panic(err)