coredns/plugin/forward

forward

Name

forward facilitates proxying DNS messages to upstream resolvers.

Description

The forward plugin is generally faster (~30+%) than proxy as it re-uses already opened sockets to the upstreams. It supports UDP, TCP and DNS-over-TLS and uses inband health checking that is enabled by default. When all upstreams are down it assumes healtchecking 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.
• TO... are the destination endpoints to forward to. The TO syntax allows you to specify a protocol, tls://9.9.9.9 or dns:// for plain DNS. The number of upstreams is limited to 15.

The health checks are done every 0.5s. After two failed checks the upstream is considered unhealthy. The health checks use a recursive DNS query (. IN NS) to get upstream health. Any response that is not an error (REFUSED, NOTIMPL, SERVFAIL, etc) is taken as a healthy upstream. The health check uses the same protocol as specific in the TO. On startup each upstream is marked unhealthy until it passes a health check. A 0 duration will disable any health checks.

Multiple upstreams are randomized (default 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
    health_check DURATION
    expire DURATION
    max_fails INTEGER
    tls CERT KEY CA
    tls_servername NAME
    policy random|round_robin
}

• 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.
• health_checks, use a different DURATION for health checking, the default duration is 0.5s. A value of 0 disables the health checks completely.
• max_fails is the number of subsequent failed health checks that are needed before considering a backend to be down. If 0, the backend will never be marked as down. 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; if you leave this out the system's configuration will be used.
• 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.
• policy specifies the policy to use for selecting upstream servers. The default is random.

The upstream selection is done via random (default policy) selection. If the socket for this client isn't known forward will randomly choose one. If this turns out to be unhealthy, the next one is tried. If all hosts are down, we assume health checking is broken and select a random upstream to try.

Also note the TLS config is "global" for the whole forwarding proxy if you need a different tls-name for different upstreams you're out of luck.

Metrics

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

• coredns_forward_request_duration_seconds{to} - duration per upstream interaction.
• coredns_forward_request_count_total{to} - query count per upstream.
• coredns_forward_response_rcode_total{to, rcode} - count of RCODEs per upstream.
• coredns_forward_healthcheck_failure_count_total{to} - number of failed health checks per upstream.
• coredns_forward_healthcheck_broken_count_total{} - counter of when all upstreams are unhealthy, and we are randomly spraying to a target.
• coredns_forward_socket_count_total{to} - number of cached sockets per upstream.

Where to is one of the upstream servers (TO from the config), proto is the protocol used by the incoming query ("tcp" or "udp"), and family the transport family ("1" for IPv4, and "2" for IPv6).

Examples

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

example.org {
    forward . 127.0.0.1:9005
}

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
    }
}

Forward to a IPv6 host:

. {
    forward . [::1]:1053
}

Proxy all requests to 9.9.9.9 using the DNS-over-TLS protocol, and cache every answer for up to 30 seconds.

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

Bugs

The TLS config is global for the whole forwarding proxy if you need a different tls-name for different upstreams you're out of luck.

Also See

RFC 7858 for DNS over TLS.