forward

Source

forward facilitates proxying DNS messages to upstream resolvers.

Description

The forward plugin re-uses already opened sockets to the upstreams. It supports UDP, TCP and DNS-over-TLS and uses in band health checking.

When it detects an error a health check is performed. This checks runs in a loop, performing each check at a 0.5s interval for as long as the upstream reports unhealthy. Once healthy we stop health checking (until the next error). The health checks use a recursive DNS query (. IN NS) to get upstream health. Any response that is not a network error (REFUSED, NOTIMPL, SERVFAIL, etc) is taken as a healthy upstream. The health check uses the same protocol as specified in TO. If max_fails is set to 0, no checking is performed and upstreams will always be considered healthy.

When all upstreams are down it assumes health checking as a mechanism has failed and will try to connect to a random upstream (which may or may not work).

Syntax

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

forward FROM TO...

FROM is the base domain to match for the request to be forwarded. Domains using CIDR notation that expand to multiple reverse zones are not fully supported; only the first expanded zone is used.
TO… are the destination endpoints to forward to. The TO syntax allows you to specify a protocol, tls://9.9.9.9 or dns:// (or no protocol) for plain DNS. The number of upstreams is limited to 15.

Multiple upstreams are randomized (see policy) on first use. When a healthy proxy returns an error during the exchange the next upstream in the list is tried.

Extra knobs are available with an expanded syntax:

forward FROM TO... {
    except IGNORED_NAMES...
    force_tcp
    prefer_udp
    expire DURATION
    max_fails INTEGER
    tls CERT KEY CA
    tls_servername NAME
    policy random|round_robin|sequential
    health_check DURATION [no_rec] [domain FQDN]
    max_concurrent MAX
    next RCODE_1 [RCODE_2] [RCODE_3...]
}

FROM and TO… as above.
IGNORED_NAMES in except is a space-separated list of domains to exclude from forwarding. Requests that match none of these names will be passed through.
force_tcp, use TCP even when the request comes in over UDP.
prefer_udp, try first using UDP even when the request comes in over TCP. If response is truncated (TC flag set in response) then do another attempt over TCP. In case if both force_tcp and prefer_udp options specified the force_tcp takes precedence.
max_fails is the number of subsequent failed health checks that are needed before considering an upstream to be down. If 0, the upstream will never be marked as down (nor health checked). Default is 2.
expire DURATION, expire (cached) connections after this time, the default is 10s.
tls CERT KEY CA define the TLS properties for TLS connection. From 0 to 3 arguments can be provided with the meaning as described below
- tls - no client authentication is used, and the system CAs are used to verify the server certificate
- tls CA - no client authentication is used, and the file CA is used to verify the server certificate
- tls CERT KEY - client authentication is used with the specified cert/key pair. The server certificate is verified with the system CAs
- tls CERT KEY CA - client authentication is used with the specified cert/key pair. The server certificate is verified using the specified CA file
tls_servername NAME allows you to set a server name in the TLS configuration; for instance 9.9.9.9 needs this to be set to dns.quad9.net. Multiple upstreams are still allowed in this scenario, but they have to use the same tls_servername. E.g. mixing 9.9.9.9 (QuadDNS) with 1.1.1.1 (Cloudflare) will not work. Using TLS forwarding but not setting tls_servername results in anyone being able to man-in-the-middle your connection to the DNS server you are forwarding to. Because of this, it is strongly recommended to set this value when using TLS forwarding.
policy specifies the policy to use for selecting upstream servers. The default is random.
- random is a policy that implements random upstream selection.
- round_robin is a policy that selects hosts based on round robin ordering.
- sequential is a policy that selects hosts based on sequential ordering.
health_check configure the behaviour of health checking of the upstream servers
- <duration> - use a different duration for health checking, the default duration is 0.5s.
- no_rec - optional argument that sets the RecursionDesired-flag of the dns-query used in health checking to false. The flag is default true.
- domain FQDN - set the domain name used for health checks to FQDN. If not configured, the domain name used for health checks is ..
max_concurrent MAX will limit the number of concurrent queries to MAX. Any new query that would raise the number of concurrent queries above the MAX will result in a REFUSED response. This response does not count as a health failure. When choosing a value for MAX, pick a number at least greater than the expected upstream query rate * latency of the upstream servers. As an upper bound for MAX, consider that each concurrent query will use about 2kb of memory.
next If the RCODE (i.e. NXDOMAIN) is returned by the remote then execute the next plugin. If no next plugin is defined, or the next plugin is not a forward plugin, this setting is ignored

Also note the TLS config is “global” for the whole forwarding proxy if you need a different tls_servername for different upstreams you’re out of luck.

On each endpoint, the timeouts for communication are set as follows:

The dial timeout by default is 30s, and can decrease automatically down to 1s based on early results.
The read timeout is static at 2s.

Metadata

The forward plugin will publish the following metadata, if the metadata plugin is also enabled:

forward/upstream: the upstream used to forward the request

Metrics

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

coredns_forward_healthcheck_broken_total{} - count of when all upstreams are unhealthy, and we are randomly (this always uses the random policy) spraying to an upstream.
coredns_forward_max_concurrent_rejects_total{} - count of queries rejected because the number of concurrent queries were at maximum.
coredns_proxy_request_duration_seconds{proxy_name="forward", to, rcode} - histogram per upstream, RCODE
coredns_proxy_healthcheck_failures_total{proxy_name="forward", to, rcode}- count of failed health checks per upstream.
coredns_proxy_conn_cache_hits_total{proxy_name="forward", to, proto}- count of connection cache hits per upstream and protocol.
coredns_proxy_conn_cache_misses_total{proxy_name="forward", to, proto} - count of connection cache misses per upstream and protocol.

Where to is one of the upstream servers (TO from the config), rcode is the returned RCODE from the upstream, proto is the transport protocol like udp, tcp, tcp-tls.

The following metrics have recently been deprecated:

coredns_forward_healthcheck_failures_total{to, rcode}
- Can be replaced with coredns_proxy_healthcheck_failures_total{proxy_name="forward", to, rcode}
coredns_forward_requests_total{to}
- Can be replaced with sum(coredns_proxy_request_duration_seconds_count{proxy_name="forward", to})
coredns_forward_responses_total{to, rcode}
- Can be replaced with coredns_proxy_request_duration_seconds_count{proxy_name="forward", to, rcode}
coredns_forward_request_duration_seconds{to, rcode}
- Can be replaced with coredns_proxy_request_duration_seconds{proxy_name="forward", to, rcode}

Examples

Proxy all requests within example.org. to a nameserver running on a different port:

example.org {
    forward . 127.0.0.1:9005
}

Send all requests within lab.example.local. to 10.20.0.1, all requests within example.local. (and not in lab.example.local.) to 10.0.0.1, all others requests to the servers defined in /etc/resolv.conf, and caches results. Note that a CoreDNS server configured with multiple forward plugins in a server block will evaluate those forward plugins in the order they are listed when serving a request. Therefore, subdomains should be placed before parent domains otherwise subdomain requests will be forwarded to the parent domain’s upstream. Accordingly, in this example lab.example.local is before example.local, and example.local is before ..

. {
    cache
    forward lab.example.local 10.20.0.1
    forward example.local 10.0.0.1
    forward . /etc/resolv.conf
}

The example above is almost equivalent to the following example, except that example below defines three separate plugin chains (and thus 3 separate instances of cache).

lab.example.local {
    cache
    forward . 10.20.0.1
}
example.local {
    cache
    forward . 10.0.0.1
}
. {
    cache
    forward . /etc/resolv.conf
}

Load balance all requests between three resolvers, one of which has a IPv6 address.

. {
    forward . 10.0.0.10:53 10.0.0.11:1053 [2003::1]:53
}

Forward everything except requests to example.org

. {
    forward . 10.0.0.10:1234 {
        except example.org
    }
}

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

. {
    forward . /etc/resolv.conf {
        except example.org
    }
}

Proxy all requests to 9.9.9.9 using the DNS-over-TLS (DoT) protocol, and cache every answer for up to 30 seconds. Note the tls_servername is mandatory if you want a working setup, as 9.9.9.9 can’t be used in the TLS negotiation. Also set the health check duration to 5s to not completely swamp the service with health checks.

. {
    forward . tls://9.9.9.9 {
       tls_servername dns.quad9.net
       health_check 5s
    }
    cache 30
}

Or configure other domain name for health check requests

. {
    forward . tls://9.9.9.9 {
       tls_servername dns.quad9.net
       health_check 5s domain example.org
    }
    cache 30
}

Or with multiple upstreams from the same provider

. {
    forward . tls://1.1.1.1 tls://1.0.0.1 {
       tls_servername cloudflare-dns.com
       health_check 5s
    }
    cache 30
}

Or when you have multiple DoT upstreams with different tls_servernames, you can do the following:

. {
    forward . 127.0.0.1:5301 127.0.0.1:5302
}

.:5301 {
    forward . tls://8.8.8.8 tls://8.8.4.4 {
        tls_servername dns.google
    }
}

.:5302 {
    forward . tls://1.1.1.1 tls://1.0.0.1 {
        tls_servername cloudflare-dns.com
    }
}

The following would try 1.2.3.4 first. If the response is NXDOMAIN, try 5.6.7.8. If the response from 5.6.7.8 is NXDOMAIN, try 9.0.1.2.

. {
  forward . 1.2.3.4 {
    next NXDOMAIN
  }
  forward . 5.6.7.8 {
    next NXDOMAIN
  }
  forward . 9.0.1.2 {
  }
}