Definition
A feeler connection is a short-lived, single test connection that a Bitcoin Core node opens roughly once every two minutes to an address drawn from the "new" table of its Address Manager. Its sole job is to check whether that address belongs to a reachable, running Bitcoin node. If the test succeeds, the address is promoted into the "tried" table and the feeler connection is immediately closed.
Keeping the tried table fresh
Without feelers, the tried table would only ever be updated by full outbound connections, which a node makes relatively rarely. Over time the table could fill with stale or offline addresses. By continuously probing and verifying "new" addresses in the background, feelers ensure that when the node next needs to pick an outbound peer, it is choosing from a pool of recently confirmed, genuinely online nodes. Only one feeler runs at a time, and feelers are only attempted once all regular outbound slots are filled.
An eclipse-attack countermeasure
Feeler connections were added to Bitcoin Core specifically as a defense recommended in the 2015 academic work on eclipse attacks. A node with a tried table full of verified live peers is far harder to isolate, because its outbound peer selection keeps landing on real, independent nodes rather than attacker-seeded dead entries.
For a home node operator this all happens automatically. Feelers work hand in hand with the Address Manager and contribute directly to eclipse-attack resistance.
In Simple Terms
A feeler connection is a short-lived, single test connection that a Bitcoin Core node opens roughly once every two minutes to an address drawn from…