Replace automatic bans with discouragement filter

This patch improves performance and resource usage around IP
addresses that are banned for misbehavior. They're already not
actually banned, as connections from them are still allowed,
but they are preferred for eviction if the inbound connection
slots are full.

Stop treating these like manually banned IP ranges, and instead
just keep them in a rolling Bloom filter of misbehaving nodes,
which isn't persisted to disk or exposed through the ban
framework. The effect remains the same: preferred for eviction,
avoided for outgoing connections, and not relayed to other peers.

Also change the name of this mechanism to better reflect reality;
they're not banned, just discouraged.

Contains release notes and several interface improvements by
John Newbery.

1 files changed, 23 insertions, 0 deletions

diff --git a/doc/release-notes-19219.md b/doc/release-notes-19219.md
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+#### Changes regarding misbehaving peers
+
+Peers that misbehave (e.g. send us invalid blocks) are now referred to as
+discouraged nodes in log output, as they're not (and weren't) strictly banned:
+incoming connections are still allowed from them, but they're preferred for
+eviction.
+
+Furthermore, a few additional changes are introduced to how discouraged
+addresses are treated:
+
+- Discouraging an address does not time out automatically after 24 hours
+  (or the `-bantime` setting). Depending on traffic from other peers,
+  discouragement may time out at an indeterminate time.
+
+- Discouragement is not persisted over restarts.
+
+- There is no method to list discouraged addresses. They are not returned by
+  the `listbanned` RPC. That RPC also no longer reports the `ban_reason`
+  field, as `"manually added"` is the only remaining option.
+
+- Discouragement cannot be removed with the `setban remove` RPC command.
+  If you need to remove a discouragement, you can remove all discouragements by
+  stop-starting your node.