This help section describes the DNS cache settings available under , DNS Settings, Caching in the Administration Tool.
This feature is not available when the built-in DNS resolver is used. This is because the built-in resolver uses its own caching, which is also a more comprehensive than this cache feature.
ORF can hold DNS lookup results in an internal cache for a user-specified time. Caching reduces the load on the DNS servers and speeds up filtering on high traffic servers.
The cache stores the DNS status and data of all successful DNS lookups. Note that a lookup is interpreted successful even if the response was "no such domain" or "no such record".
Since the cache is pre-allocated, you may experience that the ORF Service has a large memory footprint when caching is enabled.
Set the Enable DNS cache checkbox to enable the ORF's built-in DNS cache.
The Limit cache size in X KByte(s) value limits the amount of DNS data stored in the cache.
The Cache items expire after X minute(s) value specifies how long ORF can store the cache data before it expires. This overrides the default TTL (Time-To-Live) lifetime value assigned to the given DNS record. Consequently, setting the expiration too high may result in blacklisting emails from IP addresses which are no longer listed in online DNS blacklists (but were previously).
Due to the caching of all types of successful lookups, a long cache expiration limit may decrease the spam catch rate, because even if an IP or domain was not a recognized spam source a few hours ago, it could be a spam source now. It may also cause false positives, as you may block legitimate sources which are no longer blacklisted. Due to this, we recommend setting the cache expiration to the default value (15 minutes) or below.
By ticking the Enable saving cache data to file checkbox you can save the ORF DNS cache data to the disk on ORF Service shutdown and load it back on ORF Service startup. The DNS cache data is stored in the file dnscache.dat in the ORF directory.
Select which DNS-based tests may cache their DNS data.
The Reverse DNS Test, DNS Whitelist Test and SPF Test data can be cached safely. However, caching DNSBL and SURBL data may result in lower spam filtering performance if the expiration limit is set too high (see above).
If you choose to cache DNS blacklist and SURBL data, set the expiration to less than 10 minutes.