Demo
Demo

Knowledge base

PMTA knowledge base›Command§ 6.3.38

show precache

Domains resolved in advance
What the node keeps ready

Shows the domains the engine keeps in the DNS precache — regardless of whether there is a queue for them.

The list is set by a file: precached-domains-file in the <dns> block, one domain per line. Entries are refreshed every precached-refresh-interval, 5m by default, instead of by the records’ own lifetime; the limit is precached-max-domains, 500 by default.

The main trap is bloating the list. Each domain is refreshed on its own interval regardless of traffic: a large list with a short interval turns into a constant stream of queries, and the manual explicitly requires scalable DNS for this.
What the Hub does. It does not call it: the Hub does not write a <dns> block with a precache.
Recommended values

When to use it:

Question This command or not
which domains the node keeps resolved in advance yes
why the first delivery to a domain is slow partly — whether the domain is in the list
what the name server answers now no — resolve
The purpose and the precache directives with their defaults come from the manual (§ 6.3.38, § 3.3.16). The trap analysis is ours.
Example configuration
# keep large receivers resolved in advance
<dns>
    precached-domains-file /etc/pmta/precached-domains.txt    precached-refresh-interval 2m
</dns>
An example from the manual; after a reload, pmta show precache will show the list.
yellow — — directive names, blue — blocks. They are highlighted the same way in the text of the card.
Compatibility
PowerMTA 6.0
described in the manual of this version · platform: any
The vendor does not state the first version for every directive — there is no note here. The name also appears in the 5.0 edition of the manual.
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Related entries

1 relation

Works together with

Flush the name cacheA neighboring command: the same subject from another side. inference

Referenced from

Keeping selected domains resolved at all timesThe only command that shows what actually got into the precache. § 11.12

How to check on the node
Reconcile the list with the file:
pmta show precache
wc -l /etc/pmta/precached-domains.txt
The output has no more domains than there are lines in the file and no more than precached-max-domains.
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What else to read

3 articles

◇What the node tells about itself and through which channelsSeven observation surfaces: the web monitor, the console in four formats, the log, accounting, accounting search, metrics and export to external analytics.◇The watchdog that restarts the engineKeeps an eye on the engine and brings it back up after a crash by itself, sending a crash report to the postmaster address.◇What makes a virtual machine riskyRunning on a virtual machine is possible, but the manual does not recommend it without sufficient hardware and properly tuned hypervisor settings: under load the node may stop responding.

In the Hub
Cost of a mistake
safe: a wrong value breaks nothing
Scenarios
Untangle a stuck queue