+/**
+ * The Resolver class is a high-level abstraction for resolving DNS entries.
+ * It can do forward and reverse IPv4 lookups, and where IPv6 is supported, will
+ * also be able to do those, transparent of protocols. Module developers must
+ * extend this class via inheritence, and then insert a pointer to their derived
+ * class into the core using Server::AddResolver(). Once you have done this,
+ * the class will be able to receive callbacks. There are two callbacks which
+ * can occur by calling virtual methods, one is a success situation, and the other
+ * an error situation.
+ */
+class CoreExport Resolver : public Extensible
+{
+ protected:
+ /**
+ * Pointer to creator module (if any, or NULL)
+ */
+ Module* Creator;
+ /**
+ * The input data, either a host or an IP address
+ */
+ std::string input;
+ /**
+ * True if a forward lookup is being performed, false if otherwise
+ */
+ QueryType querytype;
+ /**
+ * The DNS erver being used for lookups. If this is an empty string,
+ * the value of ServerConfig::DNSServer is used instead.
+ */
+ std::string server;
+ /**
+ * The ID allocated to your lookup. This is a pseudo-random number
+ * between 0 and 65535, a value of -1 indicating a failure.
+ * The core uses this to route results to the correct objects.
+ */
+ int myid;
+
+ /**
+ * Cached result, if there is one
+ */
+ CachedQuery *CQ;
+
+ /**
+ * Time left before cache expiry
+ */
+ int time_left;
+
+ public:
+ /**
+ * Initiate DNS lookup. Your class should not attempt to delete or free these
+ * objects, as the core will do this for you. They must always be created upon
+ * the heap using new, as you cannot be sure at what time they will be deleted.
+ * Allocating them on the stack or attempting to delete them yourself could cause
+ * the object to go 'out of scope' and cause a segfault in the core if the result
+ * arrives at a later time.
+ * @param source The IP or hostname to resolve
+ * @param qt The query type to perform. Resolution of 'A', 'AAAA', 'PTR' and 'CNAME' records
+ * is supported. Use one of the QueryType enum values to initiate this type of
+ * lookup. Resolution of 'AAAA' ipv6 records is always supported, regardless of
+ * wether InspIRCd is built with ipv6 support.
+ * To look up reverse records, specify one of DNS_QUERY_PTR4 or DNS_QUERY_PTR6 depending
+ * on the type of address you are looking up.
+ * @param cached The constructor will set this boolean to true or false depending
+ * on whether the DNS lookup you are attempting is cached (and not expired) or not.
+ * If the value is cached, upon return this will be set to true, otherwise it will
+ * be set to false. You should pass this value to InspIRCd::AddResolver(), which
+ * will then influence the behaviour of the method and determine whether a cached
+ * or non-cached result is obtained. The value in this variable is always correct
+ * for the given request when the constructor exits.
+ * @param creator See the note below.
+ * @throw ModuleException This class may throw an instance of ModuleException, in the
+ * event a lookup could not be allocated, or a similar hard error occurs such as
+ * the network being down. This will also be thrown if an invalid IP address is
+ * passed when resolving a 'PTR' record.
+ *
+ * NOTE: If you are instantiating your DNS lookup from a module, you should set the
+ * value of creator to point at your Module class. This way if your module is unloaded
+ * whilst lookups are in progress, they can be safely removed and your module will not
+ * crash the server.
+ */
+ Resolver(const std::string &source, QueryType qt, bool &cached, Module* creator = NULL);
+
+ /**
+ * The default destructor does nothing.
+ */
+ virtual ~Resolver();
+
+ /**
+ * When your lookup completes, this method will be called.
+ * @param result The resulting DNS lookup, either an IP address or a hostname.
+ * @param ttl The time-to-live value of the result, in the instance of a cached
+ * result, this is the number of seconds remaining before refresh/expiry.
+ * @param cached True if the result is a cached result, false if it was requested
+ * from the DNS server.
+ */
+ virtual void OnLookupComplete(const std::string &result, unsigned int ttl, bool cached) = 0;
+
+ /**
+ * If an error occurs (such as NXDOMAIN, no domain name found) then this method
+ * will be called.
+ * @param e A ResolverError enum containing the error type which has occured.
+ * @param errormessage The error text of the error that occured.
+ */
+ virtual void OnError(ResolverError e, const std::string &errormessage);
+
+ /**
+ * Returns the id value of this class. This is primarily used by the core
+ * to determine where in various tables to place a pointer to your class, but it
+ * is safe to call and use this method.
+ * As specified in RFC1035, each dns request has a 16 bit ID value, ranging
+ * from 0 to 65535. If there is an issue and the core cannot send your request,
+ * this method will return -1.
+ */
+ int GetId();
+
+ /**
+ * Returns the creator module, or NULL
+ */
+ Module* GetCreator();
+
+ /**
+ * If the result is a cached result, this triggers the objects
+ * OnLookupComplete. This is done because it is not safe to call
+ * the abstract virtual method from the constructor.
+ */
+ void TriggerCachedResult();
+};