W3C Sample Code Library libwww DNS Class

# Domain Name Service Class

    /*
    **	(c) COPYRIGHT MIT 1995.
    **	Please first read the full copyright statement in the file COPYRIGH.
    */

The DNS Class defines generic access to the DNS system. It maintains a cache of all visited hosts so that subsequent connects to the same host doesn't imply a new request to the DNS every time. Multihomed hosts are treated specially in that the time spend on every connect is measured and kept in the cache. On the next request to the same host, the IP-address with the lowest average connect time is chosen. If one IP-address fails completely, e.g. connection refused then it disabled and HTDoConnect tries one of the other IP-addresses to the same host.

Every entry in the cache has its own time to live (TTL) and hence the cache manages its own automatic garbage collection. Currently the TTL is not bound to the DNS records which should be changed. You can set the DNS object TTL

This module is implemented by HTDNS.c, and it is a part of the W3C Sample Code Library.

    #ifndef HTDNS_H
    #define HTDNS_H

    typedef struct _HTdns HTdns;

## DNS Cache Expiration Time

When to remove an entry in the DNS cache. We maintain our own DNS cache as we keep track of the connect time, pick the fastet host on multi-homed hosts etc. However we DO NOT HONOR DNS TTL Records which is the reason for why the expiration must be faily short (the default value is 30 mins), so that it doesn't collide with the DNS mechanism for timing out DNS records befoew swapping IP addresses around.

    extern void HTDNS_setTimeout (time_t timeout);
    extern time_t HTDNS_timeout  (time_t timeout);

## Creation and Deletion Methods

### Add a DNS Object

Add an element to the cache of visited hosts. The `homes` variable indicates the number of IP addresses found when looking up the name. A host name must NOT contain a port number.

    extern HTdns * HTDNS_add (HTList * list, struct hostent * element,
    			  char * host, int * homes);

### Delete a DNS object

This function flushes the DNS object from the cache and frees up memory

    extern BOOL HTDNS_delete (const char * host);

### Delete ALL DNS objects

This function is called from HTLibTerminate. It can be called at any point in time if the DNS cache is going to be flushed.

    extern BOOL HTDNS_deleteAll (void);

## DNS Class Methods

### Recalculating the Time Weights on Multihomed Host

On every connect to a multihomed host, the average connect time is updated exponentially for all the entries.

    extern BOOL HTDNS_updateWeigths (HTdns *dns, int cur, ms_t deltatime);

## Resolver Functions

These are the functions that resolve a host name

### Get Host By Socket

This function should have been called HTGetHostByAddr but for historical reasons this is not the case.

    extern char * HTGetHostBySock (int soc);

### Get Host By Name

This function gets the address of the host and puts it in to the socket structure. It maintains its own cache of connections so that the communication to the Domain Name Server is minimized. Returns the number of homes or -1 if error.

    extern int HTGetHostByName (HTHost * host, char *hostname, HTRequest * request);

    #endif