Decentralized UDP hole punching

Can we make O7s-over-UDP scale with many nodes behind firewalls?

Today, Max Inden from the libp2p project gave a very interesting presentation at FOSDEM 2022 about decentralized hole punching, project Flare.

The problem is this: if servers A and B are each behind a (possibly symmetric) NAT firewall, they can’t directly communicate unless the firewall opens some ports from the external source to the internal LAN destination. Let’s assume A’s NAT has public address and B’s NAT has public address If A runs a service, lets say a web server on its local LAN address on port 4431 – B cannot connect to this server directly. The firewall for A will need to forward some port on the public address to the internal address If B is also behind a NAT firewall, that firewall will need to forward a port on towards In a symmetric NAT, the firewall rule is tied to the remote address, so once established, another node will not be able to send traffic to, only B can from That’s why centralized solutions like STUN may fail on symmetric NATs.

What Max describes is basically a timing attack on a NAT firewall. I definitely recommend you watch it when the talk becomes available. The specification can be found here Instead of using a central server, consider the following.

If A sends a packet to, it will open up a temporary hole it its firewall ( <-> for the response to arrive2. If B sends a packet to, it will also create a temporary hole in its firewall ( <-> So, if both do this roughly at the same time, the packets can slip through, the firewall rules become established and B can communicate with A! Pretty nifty!

Whether this is “decentralized” is a bit debatable, because there needs to be some coordination between A and B to get the timing right. And what I don’t fully understand (yet), is how the ports X and Y are known at the time of the hole punching. I think there is some guesswork involved based on the ports that A and B used to contact the node(s) that provided the synchronization information, as NAT firewalls may use sequential allocation of these ports. I will try to find out more (or read the code).

How would this benefit Ouroboros? Well, most likely exactly the same as libp2p. Firewalls do not pose a connectivity issue, but they do pose a scalability issue.

The ipcpd-udp allows running Ouroboros over UDP (over IPv4). What it does is create a point-to-point UDP datagram stream with another ipcpd-udp. We have redesigned the inner workings a couple of times – mainly how the ipcpd-udp juggles around UDP ports. At first, we wanted it to mimic how a real unicast IPCP works – listening on a fixed port for incoming requests, and then use randomly chosen ports on either side for the actual Ouroboros data ‘flow’. But that was quickly thrown out because of – you guessed it – firewalls, in favor of using the listening port also for the incoming 07s data flows. That way, all that was needed was to open up a single port on a firewall. This opening up the firewall port was also needed for creating connections. The reasoning being that we wanted anyone that would connect TO the network, also accept incoming connections FROM the network. This would ensure that we could create any mesh between the Ouroboros nodes. But after some further deliberation, we caved in and made the ipcpd-udp behave like a normal UDP service, allowing incoming connection even if the remote “client” ipcpd-udp was not publicly available.

So this is the current situation shown above left. The red squares represent nodes that are not publicly reachable, the green ones nodes that are. By allowing the red nodes, the network will look less like a mesh, and more like a centralized ‘star’ network, putting extra load on the “central” green server. What this hole punching technique would allow us to do, is to add a (distributed) auxiliary program on top the Ouroboros layer that coordinates the hole punching for the UDP connectivity so we can add some ‘direct’ links at the UDP level. Definitely something I’ll consider later on.

So, if you haven’t already, have a look at Max’s talk.



  1. It works for both TCP and UDP, so I will not specify this further. [return]
  2. Providing the firewall doesn’t block all outbound traffic on port Y or some other rule that prevents it.