La Knowledge Base di Kurgan (2.0)

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==== Modifiche alle regole di caching del DNS ====

Windows 2000 e soprattutto XP tengono in cache anche le risposte fallite alle query DNS, con il risultato che se il DNS e` lento o sovraccarico, le query fallite non vengono piu` tentate per 5 minuti dopo il fallimento. Questa e` una solenne cretinata che puo` generare gravi disagi in reti dove il DNS non e` velocissimo a rispondere.

La soluzione e` applicare questa patch al registro, la quale mette a zero il tempo di cache di tutte le risposte negative, lasciando invece la cache attiva per quelle positive.

<code>
Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Dnscache\Parameters]
"NetFailureCacheTime"=dword:00000000
"NegativeSOACacheTime"=dword:00000000
"NegativeCacheTime"=dword:00000000
</code>

Per rimettere le cose a default e` sufficiente eliminare le voci di registro sopra elencate.

I parametri sopra elencati sono qui descritti in dettaglio:

  * ''NegativeCacheTime=0'' (DWORD, default value: 0x12C (300 seconds), range: 0x0-0xFFFFFFFF seconds) Description: Determines how long an entry recording a negative answer to a query remains in the DNS cache. When the time specified in the value of this entry expires, the DNS client deletes the answer record from cache.

  * ''NetFailureCacheTime=0'' (DWORD, default value: 0x1E (30 seconds), range: 0x0-0xFFFFFFFF seconds) Description: Determines for how long the DNS client stops sending queries when it suspects that the network is down. When the DNS client does not receive responses to repeated queries sent to any network adapter, the DNS client stops sending queries for the time specified in the value of this entry. During that time, the DNS client returns a timeout response to all queries. If the value of this entry is 0x0, this optimizing feature is disabled. DNS continues to send queries to an unresponsive network.

  * ''NegativeSOACacheTime=0'' (DWORD. default value: 0x78 (120 secnds), range: 0x0-0xFFFFFFFF seconds) Description: Determines how long an entry recording a negative answer to a query for an SOA (Start of Authority) record remains in the Domain Name System (DNS) cache. When the time specified in the value expires, the DNS client deletes the answer record from the cache.