nslookup 
set types=all
_ldap._tcp

public bool ADLogin(string userName, string password) 
    { 
      // sample : 
      // LDAP://xxx.com 
      string domain = System.Configuration.ConfigurationManager.AppSettings["AD_Domain"]; 
       
      try 
      { 
        DirectoryEntry entry = new DirectoryEntry(domain, userName, password); 
        object obj = entry.NativeObject; 
        DirectorySearcher search = new DirectorySearcher(entry); 
        search.Filter = string.Format("(SAMAccountName={0})", userName); 
        search.PropertiesToLoad.Add("cn"); 
 
 
        SearchResult result = search.FindOne(); 
        if (result == null) 
          return false; 
      } 
      catch (Exception ex) 
      { 
        log.Error(ex); 
        return false; 
      } 
 
 
      return true; 
    }