net: distinguish default port per network

Change `CChainParams::GetDefaultPort()` to return 0 if the network is
I2P.

src/chainparams.h

@@ -8,11 +8,13 @@
 
 #include <chainparamsbase.h>
 #include <consensus/params.h>
+#include <netaddress.h>
 #include <primitives/block.h>
 #include <protocol.h>
 #include <util/hash_type.h>
 
 #include <memory>
+#include <string>
 #include <vector>
 
 typedef std::map<int, uint256> MapCheckpoints;
@@ -80,6 +82,15 @@ public:
     const Consensus::Params& GetConsensus() const { return consensus; }
     const CMessageHeader::MessageStartChars& MessageStart() const { return pchMessageStart; }
     uint16_t GetDefaultPort() const { return nDefaultPort; }
+    uint16_t GetDefaultPort(Network net) const
+    {
+        return net == NET_I2P ? I2P_SAM31_PORT : GetDefaultPort();
+    }
+    uint16_t GetDefaultPort(const std::string& addr) const
+    {
+        CNetAddr a;
+        return a.SetSpecial(addr) ? GetDefaultPort(a.GetNetwork()) : GetDefaultPort();
+    }
 
     const CBlock& GenesisBlock() const { return genesis; }
     /** Default value for -checkmempool and -checkblockindex argument */

src/net.cpp

@@ -402,7 +402,8 @@ CNode* CConnman::ConnectNode(CAddress addrConnect, const char *pszDest, bool fCo
         pszDest ? 0.0 : (double)(GetAdjustedTime() - addrConnect.nTime)/3600.0);
 
     // Resolve
-    const uint16_t default_port{Params().GetDefaultPort()};
+    const uint16_t default_port{pszDest != nullptr ? Params().GetDefaultPort(pszDest) :
+                                                     Params().GetDefaultPort()};
     if (pszDest) {
         std::vector<CService> resolved;
         if (Lookup(pszDest, resolved,  default_port, fNameLookup && !HaveNameProxy(), 256) && !resolved.empty()) {
@@ -2059,8 +2060,9 @@ void CConnman::ThreadOpenConnections(const std::vector<std::string> connect)
             // from advertising themselves as a service on another host and
             // port, causing a DoS attack as nodes around the network attempt
             // to connect to it fruitlessly.
-            if (addr.GetPort() != Params().GetDefaultPort() && nTries < 50)
+            if (addr.GetPort() != Params().GetDefaultPort(addr.GetNetwork()) && nTries < 50) {
                 continue;
+            }
 
             addrConnect = addr;
             break;
@@ -2123,7 +2125,7 @@ std::vector<AddedNodeInfo> CConnman::GetAddedNodeInfo() const
     }
 
     for (const std::string& strAddNode : lAddresses) {
-        CService service(LookupNumeric(strAddNode, Params().GetDefaultPort()));
+        CService service(LookupNumeric(strAddNode, Params().GetDefaultPort(strAddNode)));
         AddedNodeInfo addedNode{strAddNode, CService(), false, false};
         if (service.IsValid()) {
             // strAddNode is an IP:port