The shortage of IPv4 addresses and the very slow transition to IPv6 leads to pragmatic solutions in the Internet: today many hosts are still using IPv4 and are connected to the Internet over a Network Address Translation (NAT) router. However, there are many applications, which need inbound connections, like e.g. peer-to-peer-based systems or voice-over-IP. For such NATed hosts inbound connections usually pose a problem, since without additional measures the router/firewall filters the incoming connection attempts. These additional measures are usually referred to as NAT traversal mechanisms and hole punching is one of those techniques.

Hole punching (or sometimes punch-through) is a technique in computer networking for establishing a direct connection between two parties in which one or both are behind firewalls or behind routers that use network address translation (NAT).

While in most cases NAT hole punch helps the Mysterium node runners to establish the connection with our network consumers, the technique is not applicable in all scenarios or with all types of NATs, as NAT operating characteristics are not standardized.
If this approach does not work for you, you can try the following:

  1. Enable UPnP feature. UPnP and NAT-PNP protocols provides automatic port configuration features for various routers (gateways). Some routers have these features enabled by default, some have not. 
  2. If UPnP or NAT-PNP method doesn't help, you can try forwarding the port manually.
  3. Add the following flag into configuration file: --experiment-natpunching=false
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