coredns/middleware/cache/README.md

# cache

*cache* enables a frontend cache. It will cache all records except zone transfers and metadata records.

## Syntax

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

* **TTL** max TTL in seconds. If not specified, the maximum TTL will be used which is 3600 for
    noerror responses and 1800 for denial of existence ones.
    A set TTL of 300 *cache 300* would cache the record up to 300 seconds.
    Smaller record provided TTLs will take precedence.
* **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. No cache invalidation triggered by other middlewares is available. Therefore even reloaded items might still be cached for the duration of the TTL.

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 successful 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
~~~