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;     }