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117 lines
4.9 KiB
Markdown
117 lines
4.9 KiB
Markdown
# cache
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## Name
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*cache* - enables a frontend cache.
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## Description
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With *cache* enabled, all records except zone transfers and metadata records will be cached for up to
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3600s. Caching is mostly useful in a scenario when fetching data from the backend (upstream,
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database, etc.) is expensive.
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*Cache* will change the query to enable DNSSEC (DNSSEC OK; DO) if it passes through the plugin. If
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the client didn't request any DNSSEC (records), these are filtered out when replying.
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This plugin can only be used once per Server Block.
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## Syntax
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~~~ txt
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cache [TTL] [ZONES...]
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~~~
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* **TTL** max TTL in seconds. If not specified, the maximum TTL will be used, which is 3600 for
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NOERROR responses and 1800 for denial of existence ones.
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Setting a TTL of 300: `cache 300` would cache records up to 300 seconds.
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* **ZONES** zones it should cache for. If empty, the zones from the configuration block are used.
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Each element in the cache is cached according to its TTL (with **TTL** as the max).
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A cache is divided into 256 shards, each holding up to 39 items by default - for a total size
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of 256 * 39 = 9984 items.
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If you want more control:
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~~~ txt
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cache [TTL] [ZONES...] {
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success CAPACITY [TTL] [MINTTL]
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denial CAPACITY [TTL] [MINTTL]
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prefetch AMOUNT [[DURATION] [PERCENTAGE%]]
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serve_stale [DURATION]
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}
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~~~
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* **TTL** and **ZONES** as above.
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* `success`, override the settings for caching successful responses. **CAPACITY** indicates the maximum
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number of packets we cache before we start evicting (*randomly*). **TTL** overrides the cache maximum TTL.
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**MINTTL** overrides the cache minimum TTL (default 5), which can be useful to limit queries to the backend.
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* `denial`, override the settings for caching denial of existence responses. **CAPACITY** indicates the maximum
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number of packets we cache before we start evicting (LRU). **TTL** overrides the cache maximum TTL.
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**MINTTL** overrides the cache minimum TTL (default 5), which can be useful to limit queries to the backend.
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There is a third category (`error`) but those responses are never cached.
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* `prefetch` will prefetch popular items when they are about to be expunged from the cache.
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Popular means **AMOUNT** queries have been seen with no gaps of **DURATION** or more between them.
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**DURATION** defaults to 1m. Prefetching will happen when the TTL drops below **PERCENTAGE**,
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which defaults to `10%`, or latest 1 second before TTL expiration. Values should be in the range `[10%, 90%]`.
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Note the percent sign is mandatory. **PERCENTAGE** is treated as an `int`.
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* `serve_stale`, when serve\_stale is set, cache always will serve an expired entry to a client if there is one
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available. When this happens, cache will attempt to refresh the cache entry after sending the expired cache
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entry to the client. The responses have a TTL of 0. **DURATION** is how far back to consider
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stale responses as fresh. The default duration is 1h.
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## Capacity and Eviction
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If **CAPACITY** _is not_ specified, the default cache size is 9984 per cache. The minimum allowed cache size is 1024.
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If **CAPACITY** _is_ specified, the actual cache size used will be rounded down to the nearest number divisible by 256 (so all shards are equal in size).
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Eviction is done per shard. In effect, when a shard reaches capacity, items are evicted from that shard.
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Since shards don't fill up perfectly evenly, evictions will occur before the entire cache reaches full capacity.
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Each shard capacity is equal to the total cache size / number of shards (256). Eviction is random, not TTL based.
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Entries with 0 TTL will remain in the cache until randomly evicted when the shard reaches capacity.
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## Metrics
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If monitoring is enabled (via the *prometheus* plugin) then the following metrics are exported:
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* `coredns_cache_entries{server, type}` - Total elements in the cache by cache type.
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* `coredns_cache_hits_total{server, type}` - Counter of cache hits by cache type.
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* `coredns_cache_misses_total{server}` - Counter of cache misses.
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* `coredns_cache_prefetch_total{server}` - Counter of times the cache has prefetched a cached item.
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* `coredns_cache_drops_total{server}` - Counter of responses excluded from the cache due to request/response question name mismatch.
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* `coredns_cache_served_stale_total{server}` - Counter of requests served from stale cache entries.
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* `coredns_cache_evictions_total{server, type}` - Counter of cache evictions.
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Cache types are either "denial" or "success". `Server` is the server handling the request, see the
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prometheus plugin for documentation.
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## Examples
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Enable caching for all zones, but cap everything to a TTL of 10 seconds:
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~~~ corefile
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. {
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cache 10
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whoami
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}
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~~~
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Proxy to Google Public DNS and only cache responses for example.org (or below).
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~~~ corefile
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. {
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forward . 8.8.8.8:53
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cache example.org
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}
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~~~
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Enable caching for `example.org`, keep a positive cache size of 5000 and a negative cache size of 2500:
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~~~ corefile
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example.org {
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cache {
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success 5000
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denial 2500
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}
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}
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~~~
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