Definition
An anchor connection is a peer that a Bitcoin Core node deliberately tries to reconnect to immediately when it restarts. On shutdown, the node writes its current outbound block-relay-only peers to a file named anchors.dat. On the next startup, before reaching out to any new or random peers, the node first attempts to re-establish links with these saved anchors.
The restart-based eclipse problem
One of the most practical ways to eclipse a node, meaning to surround it entirely with attacker-controlled peers, is to force or wait for a restart and then race to fill all of its connection slots with malicious nodes before it can find honest ones. Anchors close that window. By reconnecting first to peers it already trusted from before the restart, the node preserves at least some honest links across the reboot, denying the attacker a clean slate.
Why block-relay-only peers are chosen
Anchors are drawn from the node's block-relay-only peers rather than its full-relay peers. Because block-relay-only connections do not gossip your node's address or transactions, an attacker is unlikely to know which peers will be used as anchors, so it cannot pre-position itself to impersonate them. This makes the anchor mechanism a quiet but meaningful hardening of a long-running node.
For a self-hosted node, anchors require no configuration and work silently. They build directly on the block-relay-only connection and are part of a node's overall eclipse-attack resistance.
In Simple Terms
An anchor connection is a peer that a Bitcoin Core node deliberately tries to reconnect to immediately when it restarts. On shutdown, the node writes…