75 lines
2.8 KiB
Markdown
75 lines
2.8 KiB
Markdown
---
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layout: "docs"
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page_title: "DNS Caching"
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sidebar_current: "docs-guides-dns-cache"
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---
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# DNS Caching
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One of the main interfaces to Consul is DNS. Using DNS is a simple way
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integrate Consul into an existing infrastructure without any high-touch
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integration.
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By default, Consul serves all DNS results with a 0 TTL value. This prevents
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any caching. The advantage of this is that each DNS lookup is always re-evaluated
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and the most timely information is served. However this adds a latency hit
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for each lookup and can potentially exhaust the query throughput of a cluster.
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For this reason, Consul provides a number of tuning parameters that can
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be used to customize how DNS queries are handled.
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## Stale Reads
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Stale reads can be used to reduce latency and increase the throughput
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of DNS queries. By default, all reads are serviced by a [single leader node](/docs/internals/consensus.html).
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These reads are strongly consistent but are limited by the throughput
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of a single node. Doing a stale read allows any Consul server to
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service a query, but non-leader nodes may return data that is potentially
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out-of-date. By allowing data to be slightly stale, we get horizontal
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read scalability. Now any Consul server can service the request, so we
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increase throughput by the number of servers in a cluster.
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The [settings](/docs/agent/options.html) used to control stale reads
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are `dns_config.allow_stale` which must be set to enable stale reads,
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and `dns_config.max_stale` which limits how stale results are allowed to
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be.
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By default, `allow_stale` is disabled meaning no stale results may be served.
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The default for `max_stale` is 5 seconds. This means that if `allow_stale` is
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enabled, we will use data from any Consul server that is within 5 seconds
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of the leader.
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## TTL Values
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TTL values can be set to allow DNS results to be cached downstream
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of Consul which can be used to reduce the number of lookups and to amortize
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the latency of doing a DNS lookup. By default, all TTLs are zero,
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preventing any caching.
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To enable caching of node lookups (e.g. "foo.node.consul"), we can set
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the `dns_config.node_ttl` value. This can be set to "10s" for example,
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and all node lookups will serve results with a 10 second TTL.
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Service TTLs can be specified at a more fine grain level. You can set
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a TTL on a per-service level, and additionally a wildcard can be specified
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that matches if there is no specific service TTL provided.
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This is specified using the `dns_config.service_ttl` map. The "*" service
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is the wildcard service. For example, if we specify:
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```
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{
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"dns_config": {
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"service_ttl": {
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"*": "5s",
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"web": "30s"
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}
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}
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}
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```
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This sets all lookups to "web.service.consul" to use a 30 second TTL,
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while lookups to "db.service.consul" or "api.service.consul" will use the
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5 second TTL from the wildcard.
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