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.

3 files changed, 29 insertions, 9 deletions

diff --git a/src/init.cpp b/src/init.cpp
index 4ad68b73f2..f45e564dbd 100644
--- a/src/init.cpp
+++ b/src/init.cpp
@@ -395,6 +395,7 @@ void SetupServerArgs()
     gArgs.AddArg("-discover", _("Discover own IP addresses (default: 1 when listening and no -externalip or -proxy)"), false, OptionsCategory::CONNECTION);
     gArgs.AddArg("-dns", _("Allow DNS lookups for -addnode, -seednode and -connect") + " " + strprintf(_("(default: %u)"), DEFAULT_NAME_LOOKUP), false, OptionsCategory::CONNECTION);
     gArgs.AddArg("-dnsseed", _("Query for peer addresses via DNS lookup, if low on addresses (default: 1 unless -connect used)"), false, OptionsCategory::CONNECTION);
+    gArgs.AddArg("-enablebip61", strprintf(_("Send reject messages per BIP61 (default: %u)"), DEFAULT_ENABLE_BIP61), false, OptionsCategory::CONNECTION);
     gArgs.AddArg("-externalip=<ip>", _("Specify your own public address"), false, OptionsCategory::CONNECTION);
     gArgs.AddArg("-forcednsseed", strprintf(_("Always query for peer addresses via DNS lookup (default: %u)"), DEFAULT_FORCEDNSSEED), false, OptionsCategory::CONNECTION);
     gArgs.AddArg("-listen", _("Accept connections from outside (default: 1 if no -proxy or -connect)"), false, OptionsCategory::CONNECTION);
@@ -1099,6 +1100,8 @@ bool AppInitParameterInteraction()
     if (gArgs.GetBoolArg("-peerbloomfilters", DEFAULT_PEERBLOOMFILTERS))
         nLocalServices = ServiceFlags(nLocalServices | NODE_BLOOM);
 
+    g_enable_bip61 = gArgs.GetBoolArg("-enablebip61", DEFAULT_ENABLE_BIP61);
+
     if (gArgs.GetArg("-rpcserialversion", DEFAULT_RPC_SERIALIZE_VERSION) < 0)
         return InitError("rpcserialversion must be non-negative.");
 
diff --git a/src/net_processing.cpp b/src/net_processing.cpp
index ed2fb598d2..7341d41fea 100644
--- a/src/net_processing.cpp
+++ b/src/net_processing.cpp
@@ -37,6 +37,7 @@
 #endif
 
 std::atomic<int64_t> nTimeBestReceived(0); // Used only to inform the wallet of when we last received a block
+bool g_enable_bip61 = DEFAULT_ENABLE_BIP61;
 
 struct IteratorComparator
 {
@@ -1579,7 +1580,9 @@ bool static ProcessMessage(CNode* pfrom, const std::string& strCommand, CDataStr
         // Each connection can only send one version message
         if (pfrom->nVersion != 0)
         {
-            connman->PushMessage(pfrom, CNetMsgMaker(INIT_PROTO_VERSION).Make(NetMsgType::REJECT, strCommand, REJECT_DUPLICATE, std::string("Duplicate version message")));
+            if (g_enable_bip61) {
+                connman->PushMessage(pfrom, CNetMsgMaker(INIT_PROTO_VERSION).Make(NetMsgType::REJECT, strCommand, REJECT_DUPLICATE, std::string("Duplicate version message")));
+            }
             LOCK(cs_main);
             Misbehaving(pfrom->GetId(), 1);
             return false;
@@ -1608,8 +1611,10 @@ bool static ProcessMessage(CNode* pfrom, const std::string& strCommand, CDataStr
         if (!pfrom->fInbound && !pfrom->fFeeler && !pfrom->m_manual_connection && !HasAllDesirableServiceFlags(nServices))
         {
             LogPrint(BCLog::NET, "peer=%d does not offer the expected services (%08x offered, %08x expected); disconnecting\n", pfrom->GetId(), nServices, GetDesirableServiceFlags(nServices));
-            connman->PushMessage(pfrom, CNetMsgMaker(INIT_PROTO_VERSION).Make(NetMsgType::REJECT, strCommand, REJECT_NONSTANDARD,
-                               strprintf("Expected to offer services %08x", GetDesirableServiceFlags(nServices))));
+            if (g_enable_bip61) {
+                connman->PushMessage(pfrom, CNetMsgMaker(INIT_PROTO_VERSION).Make(NetMsgType::REJECT, strCommand, REJECT_NONSTANDARD,
+                                   strprintf("Expected to offer services %08x", GetDesirableServiceFlags(nServices))));
+            }
             pfrom->fDisconnect = true;
             return false;
         }
@@ -1629,8 +1634,10 @@ bool static ProcessMessage(CNode* pfrom, const std::string& strCommand, CDataStr
         {
             // disconnect from peers older than this proto version
             LogPrint(BCLog::NET, "peer=%d using obsolete version %i; disconnecting\n", pfrom->GetId(), nVersion);
-            connman->PushMessage(pfrom, CNetMsgMaker(INIT_PROTO_VERSION).Make(NetMsgType::REJECT, strCommand, REJECT_OBSOLETE,
-                               strprintf("Version must be %d or greater", MIN_PEER_PROTO_VERSION)));
+            if (g_enable_bip61) {
+                connman->PushMessage(pfrom, CNetMsgMaker(INIT_PROTO_VERSION).Make(NetMsgType::REJECT, strCommand, REJECT_OBSOLETE,
+                                   strprintf("Version must be %d or greater", MIN_PEER_PROTO_VERSION)));
+            }
             pfrom->fDisconnect = true;
             return false;
         }
@@ -2328,9 +2335,10 @@ bool static ProcessMessage(CNode* pfrom, const std::string& strCommand, CDataStr
             LogPrint(BCLog::MEMPOOLREJ, "%s from peer=%d was not accepted: %s\n", tx.GetHash().ToString(),
                 pfrom->GetId(),
                 FormatStateMessage(state));
-            if (state.GetRejectCode() > 0 && state.GetRejectCode() < REJECT_INTERNAL) // Never send AcceptToMemoryPool's internal codes over P2P
+            if (g_enable_bip61 && state.GetRejectCode() > 0 && state.GetRejectCode() < REJECT_INTERNAL) { // Never send AcceptToMemoryPool's internal codes over P2P
                 connman->PushMessage(pfrom, msgMaker.Make(NetMsgType::REJECT, strCommand, (unsigned char)state.GetRejectCode(),
                                    state.GetRejectReason().substr(0, MAX_REJECT_MESSAGE_LENGTH), inv.hash));
+            }
             if (nDoS > 0) {
                 Misbehaving(pfrom->GetId(), nDoS);
             }
@@ -2903,8 +2911,10 @@ static bool SendRejectsAndCheckIfBanned(CNode* pnode, CConnman* connman)
     AssertLockHeld(cs_main);
     CNodeState &state = *State(pnode->GetId());
 
-    for (const CBlockReject& reject : state.rejects) {
-        connman->PushMessage(pnode, CNetMsgMaker(INIT_PROTO_VERSION).Make(NetMsgType::REJECT, std::string(NetMsgType::BLOCK), reject.chRejectCode, reject.strRejectReason, reject.hashBlock));
+    if (g_enable_bip61) {
+        for (const CBlockReject& reject : state.rejects) {
+            connman->PushMessage(pnode, CNetMsgMaker(INIT_PROTO_VERSION).Make(NetMsgType::REJECT, std::string(NetMsgType::BLOCK), reject.chRejectCode, reject.strRejectReason, reject.hashBlock));
+        }
     }
     state.rejects.clear();
 
@@ -3011,7 +3021,9 @@ bool PeerLogicValidation::ProcessMessages(CNode* pfrom, std::atomic<bool>& inter
     }
     catch (const std::ios_base::failure& e)
     {
-        connman->PushMessage(pfrom, CNetMsgMaker(INIT_PROTO_VERSION).Make(NetMsgType::REJECT, strCommand, REJECT_MALFORMED, std::string("error parsing message")));
+        if (g_enable_bip61) {
+            connman->PushMessage(pfrom, CNetMsgMaker(INIT_PROTO_VERSION).Make(NetMsgType::REJECT, strCommand, REJECT_MALFORMED, std::string("error parsing message")));
+        }
         if (strstr(e.what(), "end of data"))
         {
             // Allow exceptions from under-length message on vRecv
diff --git a/src/net_processing.h b/src/net_processing.h
index 195d0d2033..b0b905d922 100644
--- a/src/net_processing.h
+++ b/src/net_processing.h
@@ -35,6 +35,11 @@ static constexpr int64_t EXTRA_PEER_CHECK_INTERVAL = 45;
 /** Minimum time an outbound-peer-eviction candidate must be connected for, in order to evict, in seconds */
 static constexpr int64_t MINIMUM_CONNECT_TIME = 30;
 
+/** Default for BIP61 (sending reject messages) */
+static constexpr bool DEFAULT_ENABLE_BIP61 = true;
+/** Enable BIP61 (sending reject messages) */
+extern bool g_enable_bip61;
+
 class PeerLogicValidation final : public CValidationInterface, public NetEventsInterface {
 private:
     CConnman* const connman;