--- log.cc	2011-08-08 19:58:18.209399001 -0400
+++ log.new	2011-08-08 19:57:22.579399000 -0400
@@ -253,19 +253,28 @@
 
 
 void 
+
 LogComponentEnable (char const *name, enum LogLevel level)
 {
   ComponentList *components = GetComponentList ();
-  for (ComponentListI i = components->begin ();
+  ComponentListI i;
+  for ( i = components->begin ();
        i != components->end ();
        i++)
     {
       if (i->first.compare (name) == 0) 
         {
           i->second->Enable (level);
-          break;
+          return;
         }
     }
+    if(i == components->end()) {
+	// nothing matched
+      std::string ComponentName(name); 
+      std::string errorMessage = "Logging component \""+ComponentName+"\" not found. See above for a list of available log components";
+      LogComponentPrintList();
+      NS_FATAL_ERROR (errorMessage.c_str());
+    }
 }
 
 void 
@@ -354,9 +363,110 @@
     }
 }
 
+static bool ComponentExists(std::string componentName) 
+{
+  char const*name=componentName.c_str();
+  ComponentList *components = GetComponentList ();
+  ComponentListI i;
+  for ( i = components->begin ();
+        i != components->end ();
+        i++)
+     {
+       if (i->first.compare (name) == 0) 
+ 	{
+	  return true;
+ 	}
+    }
+  if(i == components->end()) {	// nothing matched 
+	return false;    
+   }
+  else {
+    NS_ASSERT(1==0);			// the control should never reach here. 
+  }
+}
+
+static void checkEnvironmentVariables() {
+#ifdef HAVE_GETENV
+  char *envVar = getenv ("NS_LOG");
+  if (envVar == 0)
+    {
+      return;
+    }
+  std::string env = envVar;
+
+  std::string::size_type cur = 0;
+  std::string::size_type next = 0;
+  
+  while (next != std::string::npos)
+    {
+      next = env.find_first_of (":", cur);
+      std::string tmp = std::string (env, cur, next-cur);
+      std::string::size_type equal = tmp.find ("=");
+      std::string component;
+      if (equal == std::string::npos)		// ie no '=' characters found 
+        {
+          component = tmp;
+          if (ComponentExists(component) || component == "*")
+            {
+              return;
+            }
+	  else {
+	      LogComponentPrintList();
+              std::string errorMessage="Invalid or unregistered component name \""+component+"\" in env variable NS_LOG, see above for a list of valid components";
+	      NS_FATAL_ERROR(errorMessage.c_str());
+	  }
+        }
+      else
+        {
+          component = tmp.substr (0, equal);
+          if (ComponentExists(component) || component == "*")
+            {
+              std::string::size_type cur_lev;
+              std::string::size_type next_lev = equal;
+              do
+                {
+                  cur_lev = next_lev + 1;
+                  next_lev = tmp.find ("|", cur_lev);
+                  std::string lev = tmp.substr (cur_lev, next_lev - cur_lev);
+                  if (    (lev == "error")
+		      ||  (lev == "warn")
+                      || (lev == "debug")
+                      || (lev == "info")
+                      || (lev == "function")
+                      || (lev == "logic")
+                      || (lev == "all")
+                      || (lev == "prefix_func")
+                      || (lev == "prefix_time")
+                      || (lev == "prefix_node")
+                      || (lev == "level_error")
+                      || (lev == "level_warn")
+                      || (lev == "level_debug")
+                      || (lev == "level_info")
+                      || (lev == "level_function")
+                      || (lev == "level_logic")
+                      || (lev == "level_all")
+		     ) continue;
+		  else {
+ 			std::string errorMessage="Invalid log level \""+lev+"\" in env variable NS_LOG for component name " + component;
+			NS_FATAL_ERROR(errorMessage.c_str());	
+		  }
+                } while (next_lev != std::string::npos);
+            }
+	    else 
+	     {
+	      LogComponentPrintList();
+	      std::string errorMessage="Invalid or unregistered component name \""+component+"\" in env variable NS_LOG, see above for a list of valid components";
+	      NS_FATAL_ERROR(errorMessage.c_str());
+	    }
+        }
+      cur = next + 1;	// parse next component
+    }
+#endif
+}
 void LogSetTimePrinter (LogTimePrinter printer)
 {
   g_logTimePrinter = printer;
+  checkEnvironmentVariables(); 
 }
 LogTimePrinter LogGetTimePrinter (void)
 {