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diff --git a/content/en/blog/20220206-hole-punching.md b/content/en/blog/20220206-hole-punching.md new file mode 100644 index 0000000..cf964c4 --- /dev/null +++ b/content/en/blog/20220206-hole-punching.md @@ -0,0 +1,95 @@ +--- +date: 2022-02-06 +title: "Decentralized UDP hole punching" +linkTitle: "Decentralized hole punching" +description: > + Can we make O7s-over-UDP scale with many nodes behind firewalls? +author: Dimitri Staessens +--- + +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 1.1.1.1 and B's +NAT has public address 2.2.2.2. If A runs a service, lets say a web +server on its local LAN address 192.168.0.1 on port 443 -- it works +for both TCP and UDP, so I will not specify this further, B cannot +connect to this server directly. The firewall for A will need to +forward some port on the public address 1.1.1.1:X to the internal +address 192.168.0.1:443. If B is also behind a NAT firewall, that +firewall will need to forward a port on 2.2.2.2:Y towards 1.1.1.1:X. +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 +1.1.1.1:X, only B can from 2.2.2.2:Y. That's why centralized solutions +like [STUN](https://en.wikipedia.org/wiki/STUN) may fail on symmetric +NATs. + +What Max describes is basically a timing attack on a NAT firewall. I +definitely recommend you +[watch it](https://fosdem.org/2022/schedule/event/peer_to_peer_hole_punching_without_centralized_infrastructure/) +when the talk becomes available. The specification can be found +[here](https://github.com/libp2p/specs/blob/master/relay/DCUtR.md) +Instead of using a central server, consider the following. + +If A sends a packet to 2.2.2.2:Y, it will upen up a temporary hole it +its firewall (1.1.1.1:X <-> 2.2.2.2:Y) for the response to arrive +(providing the firewall doesn't block all outbound traffic on port Y +or some other rule that prevents it). If B sends a packet to +1.1.1.1:X, it will also create a temporary hole in its firewall +(2.2.2.2:Y <-> 1.1.1.1:X). 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. 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. + +{{<figure width="40%" src="/blog/20220206-hole-punching.png">}} + +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 +mess, 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](https://fosdem.org/2022/schedule/event/peer_to_peer_hole_punching_without_centralized_infrastructure/). + +Cheers, + +Dimitri
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