Bitcoin Core

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Bitcoin Core  adds a “Headers Presync” step during Initial Block Download (IBD) to help prevent Denial of Service (DoS) attacks and step towards removing checkpoints. Nodes use the pre-sync phase to verify that a peer’s headers chain has sufficient work before storing them permanently. During IBD, adversarial peers may attempt to stall the syncing process, serve blocks that don’t lead to the most-work chain, or simply exhaust the node’s resources. As such, while sync speed and bandwidth usage are important concerns during IBD, a primary design goal is avoiding Denial of Service attacks. Since v0.10.0, Bitcoin Core nodes sync block headers first before downloading block data and reject headers that don’t connect to a set of checkpoints. Instead of using hard-coded values, this new design utilizes the inherent DoS-resistant property of Proof of Work (PoW) puzzles to minimize the amount of memory allocated before finding the main chain. With these changes, nodes download headers twice during initial headers sync: a first pass to verify the headers’ PoW (without storing them) until the accumulated work meets a predetermined threshold, and then a second pass to store them. To prevent an attacker sending the main chain during presync and then a different, malicious chain during redownload, the node stores commitments to the headers chain during presync.