coredns/plugin/health/README.md

# health

## Name

*health* - enables a health check endpoint.

## Description

Enabled process wide health endpoint. When CoreDNS is up and running this returns a 200 OK http
status code. The health is exported, by default, on port 8080/health .

## Syntax

~~~
health [ADDRESS]
~~~

Optionally takes an address; the default is `:8080`. The health path is fixed to `/health`. The
health endpoint returns a 200 response code and the word "OK" when this server is healthy.

An extra option can be set with this extended syntax:

~~~
health [ADDRESS] {
    lameduck DURATION
}
~~~

* Where `lameduck` will make the process unhealthy then *wait* for **DURATION** before the process
  shuts down.

If you have multiple Server Blocks, *health* should only be enabled in one of them (as it is process
wide). If you really need multiple endpoints, you must run health endpoints on different ports:

~~~ corefile
com {
    whoami
    health :8080
}

net {
    erratic
    health :8081
}
~~~

## Metrics

If monitoring is enabled (via the *prometheus* directive) then the following metric is exported:

* `coredns_health_request_duration_seconds{}` - duration to process a /health query. As this should
  be a local operation it should be fast. A (large) increases in this duration indicates the
  CoreDNS process is having trouble keeping up with its query load.

Note that this metric *does not* have a `server` label, because being overloaded is a symptom of
the running process, *not* a specific server.

## Examples

Run another health endpoint on http://localhost:8091.

~~~ corefile
. {
    health localhost:8091
}
~~~

Set a lameduck duration of 1 second:

~~~ corefile
. {
    health localhost:8092 {
        lameduck 1s
    }
}
~~~

## Bugs

When reloading, the health handler is stopped before the new server instance is started. If that
new server fails to start, then the initial server instance is still available and DNS queries still
served, but health handler stays down. Health will not reply HTTP request until a successful reload
or a complete restart of CoreDNS.