udp: fix IP bytes not being copied out of the packet

Fixes #243

frontend/udp/frontend.go

@@ -128,10 +128,15 @@ func (t *Frontend) ListenAndServe() error {
 			defer t.wg.Done()
 			defer pool.Put(buffer)
 
+			if ip := addr.IP.To4(); ip != nil {
+				addr.IP = ip
+			}
+
 			// Handle the request.
 			start := time.Now()
 			action, err := t.handleRequest(
-				Request{buffer[:n], addr.IP},
+				// Make sure the IP is copied, not referenced.
+				Request{buffer[:n], append([]byte{}, addr.IP...)},
 				ResponseWriter{t.socket, addr},
 			)
 			recordResponseDuration(action, err, time.Since(start))

frontend/udp/parser.go

@@ -80,7 +80,8 @@ func ParseAnnounce(r Request, allowIPSpoofing, v6 bool) (*bittorrent.AnnounceReq
 	ip := r.IP
 	ipbytes := r.Packet[84:ipEnd]
 	if allowIPSpoofing {
-		ip = net.IP(ipbytes)
+		// Make sure the bytes are copied to a new slice.
+		copy(ip, net.IP(ipbytes))
 	}
 	if !allowIPSpoofing && r.IP == nil {
 		// We have no IP address to fallback on.