96 lines
4.1 KiB
Markdown
96 lines
4.1 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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description: |-
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One of the main interfaces to Consul is DNS. Using DNS is a simple way to integrate Consul into an existing infrastructure without any high-touch integration.
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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 to
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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 is that each DNS lookup is always re-evaluated,
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so 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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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
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[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
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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`](/docs/agent/options.html#allow_stale),
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which must be set to enable stale reads, and
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[`dns_config.max_stale`](/docs/agent/options.html#max_stale)
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which limits how stale results are allowed to be.
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By default, [`allow_stale`](/docs/agent/options.html#allow_stale) is disabled,
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meaning no stale results may be served. The default for
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[`max_stale`](/docs/agent/options.html#max_stale) is 5 seconds. This means that
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if [`allow_stale`](/docs/agent/options.html#allow_stale) is enabled, we will use
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data from any Consul server that is within 5 seconds of the leader.
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## Negative Response Caching
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Some DNS clients cache negative responses - that is, Consul returning a "not
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found" style response because a service exists but there are no healthy
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endpoints. What this means in practice is that cached negative responses may
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mean that services appear "down" for longer than they are actually unavailable
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when using DNS for service discovery.
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One common example is that Windows will default to caching negative responses
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for 15 minutes. DNS forwarders may also cache negative responses, with the same
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effect. To avoid this problem, check the negative response cache defaults for
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your client operating system and any DNS forwarder on the path between the
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client and Consul and set the cache values appropriately. In many cases
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"appropriately" simply is turning negative response caching off to get the best
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recovery time when a service becomes available again.
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## TTL Values
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TTL values can be set to allow DNS results to be cached downstream of Consul. Higher
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TTL values reduce the number of lookups on the Consul servers and speed lookups for
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clients, at the cost of increasingly stale results. 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 the
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[`dns_config.node_ttl`](/docs/agent/options.html#node_ttl) value. This can be set to
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"10s" for example, and all node lookups will serve results with a 10 second TTL.
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Service TTLs can be specified in a more granular fashion. You can set TTLs
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per-service, with a wildcard TTL as the default. This is specified using the
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[`dns_config.service_ttl`](/docs/agent/options.html#service_ttl) map. The "*"
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service is the wildcard service.
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For example, a [`dns_config`](/docs/agent/options.html#dns_config) that provides
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a wildcard TTL and a specific TTL for a service might look like this:
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```javascript
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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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