Block-Relay-Only Connection

A block-relay-only connection is a special type of outbound link that a Bitcoin Core node opens to a peer for the purpose of exchanging blocks while deliberately suppressing transaction relay and address (addr) gossip. Introduced in Bitcoin Core through pull request #15759 in 2019, these connections give a node a second, quieter channel to the block-producing network alongside its ordinary full-relay peers.

How it differs from a full-relay peer

An ordinary full-relay peer exchanges transactions, addresses, and blocks. A block-relay-only peer exchanges blocks only. By default a node maintains two such connections out of its eleven outbound slots. Because these peers are not told about your transactions and do not gossip your node's address, an observer watching the public transaction-relay graph cannot easily see them, which makes the true topology of the network harder to map.

Why it strengthens your node

The hidden nature of these links is precisely the point. If an adversary cannot learn which peers your node is connected to, it cannot reliably target those connections to isolate or partition you. Block-relay-only connections therefore raise the cost of a partitioning or isolation attack while still guaranteeing you receive new blocks from an independent set of peers. They also serve as the basis for persistent reconnection on restart.

For a sovereign node operator, these connections are a low-cost defense that runs automatically. They are closely tied to the anchor connection mechanism that preserves them across restarts and to broader eclipse-attack resistance.