net: open p2p connections to nodes that listen on non-default ports

By default, for mainnet, the p2p listening port is 8333. Bitcoin Core
has a strong preference for only connecting to nodes that listen on that
port.

Remove that preference because connections over clearnet that involve
port 8333 make it easy to detect, analyze, block or divert Bitcoin p2p
traffic before the connection is even established (at TCP SYN time).

For further justification see the OP of:
https://github.com/bitcoin/bitcoin/pull/23306

1 files changed, 22 insertions, 0 deletions

diff --git a/src/init.cpp b/src/init.cpp
index 6aef1f8149..87e476465a 100644
--- a/src/init.cpp
+++ b/src/init.cpp
@@ -466,6 +466,8 @@ void SetupServerArgs(ArgsManager& argsman)
     argsman.AddArg("-peerbloomfilters", strprintf("Support filtering of blocks and transaction with bloom filters (default: %u)", DEFAULT_PEERBLOOMFILTERS), ArgsManager::ALLOW_ANY, OptionsCategory::CONNECTION);
     argsman.AddArg("-peerblockfilters", strprintf("Serve compact block filters to peers per BIP 157 (default: %u)", DEFAULT_PEERBLOCKFILTERS), ArgsManager::ALLOW_ANY, OptionsCategory::CONNECTION);
     argsman.AddArg("-permitbaremultisig", strprintf("Relay non-P2SH multisig (default: %u)", DEFAULT_PERMIT_BAREMULTISIG), ArgsManager::ALLOW_ANY, OptionsCategory::CONNECTION);
+    // TODO: remove the sentence "Nodes not using ... incoming connections." once the changes from
+    // https://github.com/bitcoin/bitcoin/pull/23542 have become widespread.
     argsman.AddArg("-port=<port>", strprintf("Listen for connections on <port>. Nodes not using the default ports (default: %u, testnet: %u, signet: %u, regtest: %u) are unlikely to get incoming connections. Not relevant for I2P (see doc/i2p.md).", defaultChainParams->GetDefaultPort(), testnetChainParams->GetDefaultPort(), signetChainParams->GetDefaultPort(), regtestChainParams->GetDefaultPort()), ArgsManager::ALLOW_ANY | ArgsManager::NETWORK_ONLY, OptionsCategory::CONNECTION);
     argsman.AddArg("-proxy=<ip:port>", "Connect through SOCKS5 proxy, set -noproxy to disable (default: disabled)", ArgsManager::ALLOW_ANY, OptionsCategory::CONNECTION);
     argsman.AddArg("-proxyrandomize", strprintf("Randomize credentials for every proxy connection. This enables Tor stream isolation (default: %u)", DEFAULT_PROXYRANDOMIZE), ArgsManager::ALLOW_ANY, OptionsCategory::CONNECTION);
@@ -1698,12 +1700,23 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
     connOptions.nMaxOutboundLimit = *opt_max_upload;
     connOptions.m_peer_connect_timeout = peer_connect_timeout;
 
+    const auto BadPortWarning = [](const char* prefix, uint16_t port) {
+        return strprintf(_("%s request to listen on port %u. This port is considered \"bad\" and "
+                           "thus it is unlikely that any Bitcoin Core peers connect to it. See "
+                           "doc/p2p-bad-ports.md for details and a full list."),
+                         prefix,
+                         port);
+    };
+
     for (const std::string& bind_arg : args.GetArgs("-bind")) {
         CService bind_addr;
         const size_t index = bind_arg.rfind('=');
         if (index == std::string::npos) {
             if (Lookup(bind_arg, bind_addr, GetListenPort(), false)) {
                 connOptions.vBinds.push_back(bind_addr);
+                if (IsBadPort(bind_addr.GetPort())) {
+                    InitWarning(BadPortWarning("-bind", bind_addr.GetPort()));
+                }
                 continue;
             }
         } else {
@@ -1730,6 +1743,15 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
     // on any address - 0.0.0.0 (IPv4) and :: (IPv6).
     connOptions.bind_on_any = args.GetArgs("-bind").empty() && args.GetArgs("-whitebind").empty();
 
+    // Emit a warning if a bad port is given to -port= but only if -bind and -whitebind are not
+    // given, because if they are, then -port= is ignored.
+    if (connOptions.bind_on_any && args.IsArgSet("-port")) {
+        const uint16_t port_arg = args.GetIntArg("-port", 0);
+        if (IsBadPort(port_arg)) {
+            InitWarning(BadPortWarning("-port", port_arg));
+        }
+    }
+
     CService onion_service_target;
     if (!connOptions.onion_binds.empty()) {
         onion_service_target = connOptions.onion_binds.front();