only ignore SYN packet because it can be the start of a QUIC handshake too

puncher.nim

@@ -70,7 +70,9 @@ proc handleMsg*(puncher: Puncher, msg: string, peerIp: IpAddress,
                 peerPort: Port) =
   ## Handles an incoming UDP message which may complete the Futures returned by
   ## ``initiate`` and ``respond``.
-  if msg != "ACK":
+  if msg == "SYN":
+    # We received a SYN packet. We ignore it because we expected it to be
+    # filtered by our NAT.
     return
   echo &"handling ACK message from {peerIp}:{peerPort}"
   let (_, myPort) = puncher.sock.getLocalAddr()