From d66ed27dd9e870640cb4db89c59ceef36edfcbc8 Mon Sep 17 00:00:00 2001
From: Leo Balduf <balduf@hm.edu>
Date: Fri, 2 Sep 2016 16:42:01 -0400
Subject: [PATCH] udp: add support for opentracker-style IPv6 announces

---
 frontend/udp/frontend.go |  6 +++---
 frontend/udp/parser.go   | 22 ++++++++++++++++------
 frontend/udp/writer.go   | 18 +++++++++++++++---
 3 files changed, 34 insertions(+), 12 deletions(-)

diff --git a/frontend/udp/frontend.go b/frontend/udp/frontend.go
index 1cf3d03..c135787 100644
--- a/frontend/udp/frontend.go
+++ b/frontend/udp/frontend.go
@@ -192,11 +192,11 @@ func (t *Frontend) handleRequest(r Request, w ResponseWriter) (actionName string
 		WriteConnectionID(w, txID, NewConnectionID(r.IP, time.Now(), t.PrivateKey))
 		return
 
-	case announceActionID:
+	case announceActionID, announceV6ActionID:
 		actionName = "announce"
 
 		var req *bittorrent.AnnounceRequest
-		req, err = ParseAnnounce(r, t.AllowIPSpoofing)
+		req, err = ParseAnnounce(r, t.AllowIPSpoofing, actionID == announceV6ActionID)
 		if err != nil {
 			WriteError(w, txID, err)
 			return
@@ -209,7 +209,7 @@ func (t *Frontend) handleRequest(r Request, w ResponseWriter) (actionName string
 			return
 		}
 
-		WriteAnnounce(w, txID, resp)
+		WriteAnnounce(w, txID, resp, actionID == announceV6ActionID)
 
 		go t.logic.AfterAnnounce(context.TODO(), req, resp)
 
diff --git a/frontend/udp/parser.go b/frontend/udp/parser.go
index 3c0fed5..ccddfff 100644
--- a/frontend/udp/parser.go
+++ b/frontend/udp/parser.go
@@ -15,6 +15,7 @@ const (
 	announceActionID
 	scrapeActionID
 	errorActionID
+	announceV6ActionID
 )
 
 // Option-Types as described in BEP 41 and BEP 45.
@@ -51,8 +52,17 @@ var (
 // ParseAnnounce parses an AnnounceRequest from a UDP request.
 //
 // If allowIPSpoofing is true, IPs provided via params will be used.
-func ParseAnnounce(r Request, allowIPSpoofing bool) (*bittorrent.AnnounceRequest, error) {
-	if len(r.Packet) < 98 {
+//
+// If v6 is true the announce will be parsed as an IPv6 announce "the
+// opentracker way", see
+// http://opentracker.blog.h3q.com/2007/12/28/the-ipv6-situation/
+func ParseAnnounce(r Request, allowIPSpoofing, v6 bool) (*bittorrent.AnnounceRequest, error) {
+	ipEnd := 84 + net.IPv4len
+	if v6 {
+		ipEnd = 84 + net.IPv6len
+	}
+
+	if len(r.Packet) < ipEnd+10 {
 		return nil, errMalformedPacket
 	}
 
@@ -68,7 +78,7 @@ func ParseAnnounce(r Request, allowIPSpoofing bool) (*bittorrent.AnnounceRequest
 	}
 
 	ip := r.IP
-	ipbytes := r.Packet[84:88]
+	ipbytes := r.Packet[84:ipEnd]
 	if allowIPSpoofing {
 		ip = net.IP(ipbytes)
 	}
@@ -77,10 +87,10 @@ func ParseAnnounce(r Request, allowIPSpoofing bool) (*bittorrent.AnnounceRequest
 		return nil, errMalformedIP
 	}
 
-	numWant := binary.BigEndian.Uint32(r.Packet[92:96])
-	port := binary.BigEndian.Uint16(r.Packet[96:98])
+	numWant := binary.BigEndian.Uint32(r.Packet[ipEnd+4 : ipEnd+8])
+	port := binary.BigEndian.Uint16(r.Packet[ipEnd+8 : ipEnd+10])
 
-	params, err := handleOptionalParameters(r.Packet[98:])
+	params, err := handleOptionalParameters(r.Packet[ipEnd+10:])
 	if err != nil {
 		return nil, err
 	}
diff --git a/frontend/udp/writer.go b/frontend/udp/writer.go
index e3a495f..86eef08 100644
--- a/frontend/udp/writer.go
+++ b/frontend/udp/writer.go
@@ -25,15 +25,27 @@ func WriteError(w io.Writer, txID []byte, err error) {
 }
 
 // WriteAnnounce encodes an announce response according to BEP 15.
-func WriteAnnounce(w io.Writer, txID []byte, resp *bittorrent.AnnounceResponse) {
+// The peers returned will be resp.IPv6Peers or resp.IPv4Peers, depending on
+// whether v6 is set. The action ID will be 4, according to
+// http://opentracker.blog.h3q.com/2007/12/28/the-ipv6-situation/.
+func WriteAnnounce(w io.Writer, txID []byte, resp *bittorrent.AnnounceResponse, v6 bool) {
 	var buf bytes.Buffer
 
-	writeHeader(&buf, txID, announceActionID)
+	if v6 {
+		writeHeader(&buf, txID, announceV6ActionID)
+	} else {
+		writeHeader(&buf, txID, announceActionID)
+	}
 	binary.Write(&buf, binary.BigEndian, uint32(resp.Interval/time.Second))
 	binary.Write(&buf, binary.BigEndian, uint32(resp.Incomplete))
 	binary.Write(&buf, binary.BigEndian, uint32(resp.Complete))
 
-	for _, peer := range resp.IPv4Peers {
+	peers := resp.IPv4Peers
+	if v6 {
+		peers = resp.IPv6Peers
+	}
+
+	for _, peer := range peers {
 		buf.Write(peer.IP)
 		binary.Write(&buf, binary.BigEndian, peer.Port)
 	}