TCP hole punching for Bitcoin nodes behind home NATs?

When attackers attempt to monopolize network connections by flooding with real addresses or public keys, a significant vulnerability arises. To address this issue, a novel approach for sharing addresses between home nodes involves the use of a "message board" instead of conventional address relay mechanisms. Nodes acting as message boards announce their capability through a designated service bit, serving both as standard nodes and facilitators for holepunching peers. Home nodes connect to these boards, using the same port intended for hole punching, to register their IP addresses. The message board only stores an entry for a home node if the observed source IP and port match the provided information, thus mitigating the risk of fake address spam.

A further layer of security and utility is provided by the design where message boards do not relay these entries beyond informing home nodes about other participating peers. This ensures that only those who have registered with a particular board receive information about one another, rather than propagating potentially malicious addresses throughout the network. Home nodes can choose multiple message boards from their address manager (addrman), increasing the likelihood that at least one honest board will be selected. This mirrors the current trust model in which home nodes rely on the integrity of at least one chosen peer.

Short-lived connections may be preferable for registering with message boards, requiring periodic refreshes from home nodes to keep their entries active and to stay updated about newly registered peers. However, long-lived connections remain a possible alternative. This system makes it considerably more resource-intensive for adversaries to disrupt connectivity with fake endpoints, as they would need either a large number of legitimate IPs or control over numerous message boards distributed across distinct network groups or autonomous systems—a challenge made more manageable for IPv6, which necessitates special handling due to the ease of acquiring many addresses.

For an expanded technical overview, comprehensive details are available in this LLM-generated spec, although it is still in a preliminary state and not fully refined for broader consumption.