I am developing a P2P application that will make use of UDP hole punch to connect two peers behind NAT.

When dealing with connections between a cone and a symmetric, unpredictable NATs, however, hole punching is not as trivial as it could be with connections between cone NATs. The only way I found to do that is to just try as hard as I can until I manage to find a correct port.

My strategy works as follows:

  • Open a large number of ports on the symmetric side, and send packets to the cone NAT port.
  • On the cone side, send packets to random ports until the peers manage to connect.

Having a symmetric and a cone NAT at hand, I did the following experiment:

  • Opened 32 ports on the symmetric side
  • Sent 64 packets every 0.1 seconds from the cone side to the symmetric NAT on random ports

In a couple of seconds I always manage to connect. However, I was wondering if some firewalls and NATs could use some form of blacklisting under these circumstances. For example, is it possible that a NATs that has to drop several packets on different ports from the same source will blacklist the source for a while?

I tried to find out about this around and I found out that sometimes some form of blacklisting can be used if the NAT is under DoS attack. However, this hole punch technique is 4 to 6 orders of magnitude less heavy than a typical DoS attack.

So is this technique feasible under reasonable conditions or am I likely to incur in blacklisting or other problems?

1 Answer 1


Its totally depends on the security level of the NAT. 64 packets per 0.1 seconds means 640 packets are hitting on Symmetric NAT (640 ports? as you are sending to random ports). Now say like connection established after 5 seconds that means you are hitting from PRC to SYMM around 3250 packets on Symmetric NAT (3250 ports?). Seems like most of the NAT will take it as DoS attack/ packet flooding (!)

I am sorry, may be I don,t understand your procedure correctly. There are two types of Symmetric NAT 1) Sequential Symmetric NAT (incremental/decremental) 2) Random Symmetric NAT (totally random). It is possible to do P2P between PRC to sequential Symmetric using port prediction mechanism.

But in case of random symmetric it is not possible. I wonder how it works by opening only 32 ports on the Symmetric nat. And how are you chossing these random ports when you are sending packet from PRC to Symmetric at the rate of 64 packets every 0.1?

  • I have tested a case, in which I was behind symmetric nat and I used two different sources (host socket) for a same destination server. And I found Symmetric nat assigned 18350 and 36029 external ports. See huge gap between these two.
    – Sohag Mony
    May 12, 2015 at 12:37

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