coredns/middleware/cache/README.md

# cache

*cache* enables a frontend cache.

## Syntax

~~~ txt
cache [TTL] [ZONES...]
~~~

* **TTL** max TTL in seconds. If not specified, the maximum TTL will be used which is 1 hour for
    noerror responses and half an hour for denial of existence ones.
* **ZONES** zones it should cache for. If empty, the zones from the configuration block are used.

Each element in the cache is cached according to its TTL (with **TTL** as the max).
For the negative cache, the SOA's MinTTL value is used. A cache can contain up to 10,000 items by
default. A TTL of zero is not allowed.

If you want more control:

~~~ txt
cache [TTL] [ZONES...] {
    success CAPACITY [TTL]
    denial CAPACITY [TTL]
}
~~~

* **TTL**  and **ZONES** as above.
* `success`, override the settings for caching succesful responses, **CAPACITY** indicates the maximum
  number of packets we cache before we start evicting (LRU). **TTL** overrides the cache maximum TTL.
* `denial`, override the settings for caching denial of existence responses, **CAPACITY** indicates the maximum
  number of packets we cache before we start evicting (LRU). **TTL** overrides the cache maximum TTL.

There is a third category (`error`) but those responses are never cached.

The minimum TTL allowed on resource records is 5 seconds.

## Metrics

If monitoring is enabled (via the *prometheus* directive) then the following metrics are exported:

* coredns_cache_size{type} - Total elements in the cache by cache type.
* coredns_cache_capacity{type} - Total capacity of the cache by cache type.
* coredns_cache_hits_total{type} - Counter of cache hits by cache type.
* coredns_cache_misses_total - Counter of cache misses.

Cache types are either "denial" or "success".

## Examples

Enable caching for all zones, but cap everything to a TTL of 10 seconds:

~~~
cache 10
~~~

Proxy to Google Public DNS and only cache responses for example.org (or below).

~~~
proxy . 8.8.8.8:53
cache example.org
~~~