add option to run lokinet traffic only with no exit and drop all non lokinet traffic

triggered by: exit-node=null

llarp/config/config.cpp

@@ -457,7 +457,7 @@ namespace llarp
             return;
           }
 
-          if (not exit.FromString(arg))
+          if (arg != "null" and not exit.FromString(arg))
           {
             throw std::invalid_argument(stringify("[network]:exit-node bad address: ", arg));
           }

llarp/handlers/tun.cpp

@@ -929,6 +929,8 @@ namespace llarp
           else
           {
             const auto addr = *exits.begin();
+            if (addr.IsZero())  // drop
+              return;
             pkt.ZeroSourceAddress();
             MarkAddressOutbound(addr);
             EnsurePathToService(

llarp/rpc/rpc_server.cpp

@@ -488,7 +488,14 @@ namespace llarp::rpc
                     }
                     else if (lnsExit.has_value())
                     {
-                      ep->LookupNameAsync(*lnsExit, [reply, mapExit](auto maybe) mutable {
+                      const std::string name = *lnsExit;
+                      if (name == "null")
+                      {
+                        service::Address nullAddr{};
+                        mapExit(nullAddr);
+                        return;
+                      }
+                      ep->LookupNameAsync(name, [reply, mapExit](auto maybe) mutable {
                         if (not maybe.has_value())
                         {
                           reply(CreateJSONError("we could not find an exit with that name"));
@@ -496,7 +503,10 @@ namespace llarp::rpc
                         }
                         if (auto ptr = std::get_if<service::Address>(&*maybe))
                         {
-                          mapExit(*ptr);
+                          if (ptr->IsZero())
+                            reply(CreateJSONError("name does not exist"));
+                          else
+                            mapExit(*ptr);
                         }
                         else
                         {

llarp/service/endpoint.cpp

@@ -1847,7 +1847,8 @@ namespace llarp
     void
     Endpoint::MapExitRange(IPRange range, Address exit)
     {
-      LogInfo(Name(), " map ", range, " to exit at ", exit);
+      if (not exit.IsZero())
+        LogInfo(Name(), " map ", range, " to exit at ", exit);
       m_ExitMap.Insert(range, exit);
     }
 
@@ -1858,7 +1859,7 @@ namespace llarp
       m_ExitMap.RemoveIf([&](const auto& item) -> bool {
         if (not range.Contains(item.first))
           return false;
-        LogInfo(Name(), " unmap ", item.first, " from exit at ", item.second);
+        LogInfo(Name(), " unmap ", item.first, " exit range mapping");
         return true;
       });
     }