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out-of-date.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