net: change assumed I2P port to 0

* When accepting an I2P connection, assume the peer has port 0 instead
  of the default 8333 (for mainnet). It is not being sent to us, so we
  must assume something.
* When deriving our own I2P listen CService use port 0 instead of the
  default 8333 (for mainnet). So that we later advertise it to peers
  with port 0.

In the I2P protocol SAM 3.1 and older (we use 3.1) ports are not used,
so they are irrelevant. However in SAM 3.2 and newer ports are used and
from the point of view of SAM 3.2, a peer using SAM 3.1 seems to have
specified port=0.

src/i2p.cpp

@@ -159,7 +159,7 @@ bool Session::Accept(Connection& conn)
             const std::string& peer_dest =
                 conn.sock->RecvUntilTerminator('\n', MAX_WAIT_FOR_IO, *m_interrupt, MAX_MSG_SIZE);
 
-            conn.peer = CService(DestB64ToAddr(peer_dest), Params().GetDefaultPort());
+            conn.peer = CService(DestB64ToAddr(peer_dest), I2P_SAM31_PORT);
 
             return true;
         }
@@ -366,7 +366,7 @@ void Session::CreateIfNotCreatedAlready()
     SendRequestAndGetReply(*sock, strprintf("SESSION CREATE STYLE=STREAM ID=%s DESTINATION=%s",
                                             session_id, private_key_b64));
 
-    m_my_addr = CService(DestBinToAddr(MyDestination()), Params().GetDefaultPort());
+    m_my_addr = CService(DestBinToAddr(MyDestination()), I2P_SAM31_PORT);
     m_session_id = session_id;
     m_control_sock = std::move(sock);
 

src/init.cpp

@@ -442,7 +442,7 @@ 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);
-    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.", defaultChainParams->GetDefaultPort(), testnetChainParams->GetDefaultPort(), signetChainParams->GetDefaultPort(), regtestChainParams->GetDefaultPort()), ArgsManager::ALLOW_ANY | ArgsManager::NETWORK_ONLY, OptionsCategory::CONNECTION);
+    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);
     argsman.AddArg("-seednode=<ip>", "Connect to a node to retrieve peer addresses, and disconnect. This option can be specified multiple times to connect to multiple nodes.", ArgsManager::ALLOW_ANY, OptionsCategory::CONNECTION);

src/netaddress.h

@@ -112,6 +112,9 @@ static constexpr size_t ADDR_CJDNS_SIZE = 16;
 /// Size of "internal" (NET_INTERNAL) address (in bytes).
 static constexpr size_t ADDR_INTERNAL_SIZE = 10;
 
+/// SAM 3.1 and earlier do not support specifying ports and force the port to 0.
+static constexpr uint16_t I2P_SAM31_PORT{0};
+
 /**
  * Network address.
  */