coredns/middleware/proxy/README.md

# proxy

*proxy* facilitates both a basic reverse proxy and a robust load balancer. The proxy has support for
 multiple backends. The load balancing features include multiple policies, health checks, and
 failovers. If all hosts fail their health check the proxy middleware will fail back to randomly
 selecting a target and sending packets to it.

## Syntax

In its most basic form, a simple reverse proxy uses this syntax:

~~~
proxy FROM TO
~~~

* **FROM** is the base domain to match for the request to be proxied.
* **TO** is the destination endpoint to proxy to.

However, advanced features including load balancing can be utilized with an expanded syntax:

~~~
proxy FROM TO... {
    policy random|least_conn|round_robin
    fail_timeout DURATION
    max_fails INTEGER
    health_check PATH:PORT [DURATION]
    except IGNORED_NAMES...
    spray
    protocol [dns|https_google [bootstrap ADDRESS...]]
}
~~~

* **FROM** is the name to match for the request to be proxied.
* **TO** is the destination endpoint to proxy to. At least one is required, but multiple may be specified. To may be an IP:Port pair, or may reference a file in resolv.conf format
* `policy` is the load balancing policy to use; applies only with multiple backends. May be one of random, least_conn, or round_robin. Default is random.
* `fail_timeout` specifies how long to consider a backend as down after it has failed. While it is down, requests will not be routed to that backend. A backend is "down" if CoreDNS fails to communicate with it. The default value is 10 seconds ("10s").
* `max_fails` is the number of failures within fail_timeout that are needed before considering a backend to be down. If 0, the backend will never be marked as down. Default is 1.
* `health_check` will check path (on port) on each backend. If a backend returns a status code of 200-399, then that backend is healthy. If it doesn't, the backend is marked as unhealthy for duration and no requests are routed to it. If this option is not provided then health checks are disabled. The default duration is 10 seconds ("10s").
* `ignored_names...` is a space-separated list of paths to exclude from proxying. Requests that match any of these paths will be passed through.
* `spray` when all backends are unhealthy, randomly pick one to send the traffic to. (This is a failsafe.)
* `protocol` specifies what protocol to use to speak to an upstream, `dns` (the default) is plain old DNS, and
  `https_google` uses `https://dns.google.com` and speaks a JSON DNS dialect. Note when using this
  **TO** must be `dns.google.com`.

## Policies

There are three load-balancing policies available:
* `random` (default) - Randomly select a backend
* `least_conn` - Select the backend with the fewest active connections
* `round_robin` - Select the backend in round-robin fashion

All polices implement randomly spraying packets to backend hosts when *no healthy* hosts are
available. This is to preeempt the case where the healthchecking (as a mechanism) fails.

## Upstream Protocols

Currently `protocol` supports `dns` (i.e., standard DNS over UDP/TCP) and `https_google` (JSON
payload over HTTPS). Note that with `https_google` the entire transport is encrypted. Only *you* and
*Google* can see your DNS activity.

* `dns`: no options can be given at the moment.
* `https_google`: bootstrap **ADDRESS...** is used to (re-)resolve `dns.google.com` to an address to
  connect to. This happens every 300s. If not specified the default is used: 8.8.8.8:53/8.8.4.4:53.
  Note that **TO** is *ignored* when `https_google` is used, as its upstream is defined as
  `dns.google.com`.

  Debug queries are enabled by default and currently there is no way to turn them off. When CoreDNS
  receives a debug query (i.e. the name is prefixed with `o-o.debug.`) a TXT record with Comment
  from `dns.google.com` is added. Note this is not always set, but sometimes you'll see:

  `dig @localhost -p 1053 mx o-o.debug.example.org`:

~~~ txt
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;o-o.debug.example.org.		IN	MX

;; AUTHORITY SECTION:
example.org.		1799	IN	SOA	sns.dns.icann.org. noc.dns.icann.org. 2016110711 7200 3600 1209600 3600

;; ADDITIONAL SECTION:
.			0	CH	TXT	"Response from 199.43.133.53"
~~~

## Metrics

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

* coredns_proxy_request_count_total{proto, from}

Where `proto` is the protocol used (`dns`, or `https_google`) and `from` is **FROM** specified in
the config.

## Examples

Proxy all requests within example.org. to a backend system:

~~~
proxy example.org localhost:9005
~~~

Load-balance all requests between three backends (using random policy):

~~~
proxy . dns1.local:53 dns2.local:1053 dns3.local
~~~

Same as above, but round-robin style:

~~~
proxy . dns1.local:53 dns2.local:1053 dns3.local {
	policy round_robin
}
~~~

With health checks and proxy headers to pass hostname, IP, and scheme upstream:

~~~
proxy . dns1.local:53 dns2.local:53 dns3.local:53 {
	policy round_robin
	health_check /health:8080
}
~~~

Proxy everything except requests to miek.nl or example.org

~~~
proxy . backend:1234 {
	except miek.nl example.org
}
~~~

Proxy everything except example.org using the host resolv.conf nameservers:

~~~
proxy . /etc/resolv.conf {
	except miek.nl example.org
}
~~~

Proxy all requests within example.org to Google's dns.google.com.

~~~
proxy example.org 1.2.3.4:53 {
    protocol https_google
}
~~~

Proxy everything, and re-lookup `dns.google.com` every 300 seconds using 8.8.8.8:53.

~~~
proxy . 1.2.3.4:53 {
    protocol https_google bootstrap 8.8.8.8:53
}
~~~