Description

The grpc plugin supports gRPC and TLS.

This plugin can only be used once per Server Block.

Syntax

In its most basic form:

grpc FROM TO...

• FROM is the base domain to match for the request to be proxied.
• TO… are the destination endpoints to proxy to. The number of upstreams is limited to 15.

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

Extra knobs are available with an expanded syntax:

grpc FROM TO... {
    except IGNORED_NAMES...
    tls CERT KEY CA
    tls_servername NAME
    policy random|round_robin|sequential
    fallthrough [ZONES...]
}

• FROM and TO… as above.

• 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.

• 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.

• policy specifies the policy to use for selecting upstream servers. The default is random.

• fallthrough [ZONES…] If a query results in NXDOMAIN from the gRPC backend, pass the request to the next plugin instead of returning the NXDOMAIN response. This is useful when the gRPC backend is authoritative for a zone but should not return authoritative NXDOMAIN responses for queries that don’t actually belong to that zone (e.g., search path queries). If [ZONES…] is omitted, then fallthrough happens for all zones. If specific zones are listed, then only queries for those zones will be subject to fallthrough.

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

Metrics

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

• coredns_grpc_request_duration_seconds{to} - duration per upstream interaction.
• coredns_grpc_requests_total{to} - query count per upstream.
• coredns_grpc_responses_total{to, rcode} - count of RCODEs per upstream. and we are randomly (this always uses the random policy) spraying to an upstream.

Examples

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

example.org {
    grpc . 127.0.0.1:9005
}

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

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

Forward everything except requests to example.org

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

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

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

Proxy all requests to 9.9.9.9 using the TLS 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.

. {
    grpc . 9.9.9.9 {
       tls_servername dns.quad9.net
    }
    cache 30
}

Or with multiple upstreams from the same provider

. {
    grpc . 1.1.1.1 1.0.0.1 {
       tls_servername cloudflare-dns.com
    }
    cache 30
}

Forward requests to a local upstream listening on a Unix domain socket.

. {
    grpc . unix:///path/to/grpc.sock
}

Proxy requests for example.org. to a gRPC backend, but fallthrough to the next plugin for NXDOMAIN responses to handle search path queries correctly.

example.org {
    grpc . 127.0.0.1:9005 {
        fallthrough
    }
    forward . 8.8.8.8
}

Bugs

The TLS config is global for the whole grpc proxy if you need a different tls_servername for different upstreams you’re out of luck.