net: Add option `-enablebip61` to configure sending of BIP61 notifications

This commit adds a boolean option `-enablebip61`, defaulting to `1`, that
can be used to disable the sending of BIP61 `reject` messages. This
functionality has been requested for various reasons:

- security (DoS): reject messages can reveal internal state that can be
  used to target certain resources such as the mempool more easily.

- bandwidth: a typical node sends lots of reject messages; this counts
  against upstream bandwidth. Also the reject messages tend to be larger
  than the message that was rejected.

On the other hand, reject messages can be useful while developing client
software (I found them indispensable while creating bitcoin-submittx),
as well as for our own test cases, so whatever the default becomes on the
long run, IMO the functionality should be retained as option. But that's
a discussion for later.

1 files changed, 10 insertions, 1 deletions

diff --git a/test/functional/p2p_invalid_tx.py b/test/functional/p2p_invalid_tx.py
index a12c1d6023..d050bd2bfd 100755
--- a/test/functional/p2p_invalid_tx.py
+++ b/test/functional/p2p_invalid_tx.py
@@ -21,6 +21,8 @@ from test_framework.util import (
 )
 
 
+REJECT_INVALID = 16
+
 class InvalidTxRequestTest(BitcoinTestFramework):
     def set_test_params(self):
         self.num_nodes = 1
@@ -71,7 +73,7 @@ class InvalidTxRequestTest(BitcoinTestFramework):
         # and we get disconnected immediately
         self.log.info('Test a transaction that is rejected')
         tx1 = create_transaction(block1.vtx[0], 0, b'\x64' * 35, 50 * COIN - 12000)
-        node.p2p.send_txs_and_test([tx1], node, success=False, expect_disconnect=True)
+        node.p2p.send_txs_and_test([tx1], node, success=False, expect_disconnect=True, reject_code=REJECT_INVALID, reject_reason=b'mandatory-script-verify-flag-failed (Invalid OP_IF construction)')
 
         # Make two p2p connections to provide the node with orphans
         # * p2ps[0] will send valid orphan txs (one with low fee)
@@ -137,6 +139,13 @@ class InvalidTxRequestTest(BitcoinTestFramework):
         wait_until(lambda: 1 == len(node.getpeerinfo()), timeout=12)  # p2ps[1] is no longer connected
         assert_equal(expected_mempool, set(node.getrawmempool()))
 
+        # restart node with sending BIP61 messages disabled, check that it disconnects without sending the reject message
+        self.log.info('Test a transaction that is rejected, with BIP61 disabled')
+        self.restart_node(0, ['-enablebip61=0','-persistmempool=0'])
+        self.reconnect_p2p(num_connections=1)
+        node.p2p.send_txs_and_test([tx1], node, success=False, expect_disconnect=True)
+        # send_txs_and_test will have waited for disconnect, so we can safely check that no reject has been received
+        assert_equal(node.p2p.reject_code_received, None)
 
 if __name__ == '__main__':
     InvalidTxRequestTest().main()