Merge #15602: 0.18: [p2p] Enable reject messages by default

a756363 [docs] document BIP 61 deprecation (John Newbery)
da14d90 [p2p] Enable BIP 61 REJECT messages by default (John Newbery)

Pull request description:

  This PR reverts #14054 following discussion on the bitcoin-dev mailing list.

  It also adds release notes to clearly document that the `enablebip61` option will be disabled by default in a future release before being removed entirely.

Tree-SHA512: 0c9162045a4fb95689a0cb2de19f98a83636c9a6fb7ffa6809773b593c6c00b14e0480683e4d1c48e9b7f90e89cf7c2dca18bff42f5d2da2a43c522039e9f1ee

doc/release-notes.md

@@ -78,13 +78,6 @@ Configuration option changes
   documentation](https://github.com/bitcoin/bitcoin/blob/master/doc/zmq.md#usage)
   for details.
 
-- The `enablebip61` option (introduced in Bitcoin Core 0.17.0) is
-  used to toggle sending of BIP 61 reject messages. Reject messages have no use
-  case on the P2P network and are only logged for debugging by most network
-  nodes. The option will now by default be off for improved privacy and security
-  as well as reduced upload usage. The option can explicitly be turned on for
-  local-network debugging purposes.
-
 - The `rpcallowip` option can no longer be used to automatically listen
   on all network interfaces.  Instead, the `rpcbind` parameter must also
   be used to specify the IP addresses to listen on.  Listening for RPC
@@ -181,6 +174,16 @@ Deprecated or removed RPCs
   continue using `generate` in this version, restart bitcoind with the
   `-deprecatedrpc=generate` configuration option.
 
+Deprecated P2P messages
+-----------------------
+
+- BIP 61 reject messages are now deprecated. Reject messages have no use
+  case on the P2P network and are only logged for debugging by most network
+  nodes. Furthermore, they increase bandwidth and can be harmful for privacy
+  and security. It has been possible to disable BIP 61 messages since v0.17
+  with the `-enablebip61=0` option. BIP 61 messages will be disabled by default
+  in a future version, before being removed entirely.
+
 New RPCs
 --------
 

src/net_processing.h

@@ -18,7 +18,7 @@ static const unsigned int DEFAULT_MAX_ORPHAN_TRANSACTIONS = 100;
 /** Default number of orphan+recently-replaced txn to keep around for block reconstruction */
 static const unsigned int DEFAULT_BLOCK_RECONSTRUCTION_EXTRA_TXN = 100;
 /** Default for BIP61 (sending reject messages) */
-static constexpr bool DEFAULT_ENABLE_BIP61{false};
+static constexpr bool DEFAULT_ENABLE_BIP61{true};
 
 class PeerLogicValidation final : public CValidationInterface, public NetEventsInterface {
 private: