fturib and yongtang Ensure Re-register of metrics variables after a reload (#2080)
* - ensure plugins that use prometheus.MustRegister, re-register after reload
- removing once.Do on the startup function was simplest way to do it.

* - fix underscored names (advice of bot)

* - tune existing UT for reload, and add a test verifying failing reload does not prevent correct registering for metrics

* - ensure different ports for tests that can run in same time ..
Latest commit f9bdd38 Sep 19, 2018
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dnstap.go all: fix plugin import ordering (#1717) Apr 22, 2018
down.go plugin/proxy: kick of HC on every 3rd failure (#1110) Oct 15, 2017
exchanger.go all: fix plugin import ordering (#1717) Apr 22, 2018
fuzz.go fuzz: put fuzzing stuff in own build tag (#1215) Nov 10, 2017
log_test.go Clean up tests logging (#1979) Jul 19, 2018
proxy.go Typo fixes (#2031) Aug 14, 2018
response.go Remove the word middleware (#1067) Sep 14, 2017




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 plugin will fail back to randomly selecting a target and sending packets to it.


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|sequential
    fail_timeout DURATION
    max_fails INTEGER
    health_check PATH:PORT [DURATION]
    except IGNORED_NAMES...
    protocol [dns [force_tcp]|grpc [insecure|CACERT|KEY CERT|KEY CERT CACERT]]
  • 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, round_robin or sequential. 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 2 seconds ("2s").
  • 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 marked healthy for double the healthcheck duration. If it doesn't, it is marked as unhealthy and no requests are routed to it. If this option is not provided then health checks are disabled. The default duration is 4 seconds ("4s").
  • IGNORED_NAMES in except is a space-separated list of domains to exclude from proxying. Requests that match none of these names 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. The grpc option will talk to a server that has implemented the DnsService.


There are four 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
  • sequential - Select the first available backend looking by order of declaration from left to right
  • first - Deprecated. Use sequential instead

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

dns : uses the standard DNS exchange. You can pass force_tcp to make sure that the proxied connection is performed over TCP, regardless of the inbound request's protocol.

grpc : extra options are used to control how the TLS connection is made to the gRPC server.

  • None - No client authentication is used, and the system CAs are used to verify the server certificate.
  • insecure - TLS is not used, the connection is made in plaintext (not good in production).
  • CACERT - No client authentication is used, and the file CACERT is used to verify the server certificate.
  • KEY CERT - Client authentication is used with the specified key/cert pair. The server certificate is verified with the system CAs.
  • KEY CERT CACERT - Client authentication is used with the specified key/cert pair. The server certificate is verified using the CACERT file.


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

  • coredns_proxy_request_duration_seconds{server, proto, proto_proxy, family, to} - duration per upstream interaction.
  • coredns_proxy_request_count_total{server, proto, proto_proxy, family, to} - query count per upstream.

Where proxy_proto is the protocol used (dns or grpc) and to is TO specified in the config, proto is the protocol used by the incoming query ("tcp" or "udp"), family the transport family ("1" for IPv4, and "2" for IPv6). Server is the server responsible for the request (and metric). See the documentation in the metrics plugin.


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

proxy example.org

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

. {
    proxy .

Same as above, but round-robin style:

. {
    proxy . {
        policy round_robin

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

. {
    proxy . {
        policy round_robin
        health_check /health:8080

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

. {
    proxy . {
        except miek.nl example.org

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

. {
    proxy . /etc/resolv.conf {
        except example.org