alternate

Name

Plugin Alternate is able to selectively forward the query to another upstream server, depending the error result provided by the initial resolver

Description

The alternate plugin allows an alternate set of upstreams be specified which will be used if the plugin chain returns specific error messages. The alternate plugin utilizes the forward plugin (https://coredns.io/plugins/forward) to query the specified upstreams.

As the name suggests, the purpose of the alternate is to allow a alternate when, for example, the desired upstreams became unavailable.

Syntax

{
    alternate [original] RCODE FORWARD_PARAMS
}

original is optional flag. If it is set then alternate uses original request instead of potentially changed by other plugins
RCODE is the string representation of the error response code. The complete list of valid rcode strings are defined as RcodeToString in https://github.com/miekg/dns/blob/master/msg.go, examples of which are SERVFAIL, NXDOMAIN and REFUSED.
FORWARD_PARAMS accepts the same parameters as the forward plugin https://coredns.io/plugins/forward.

Examples

Alternate to local DNS server

The following specifies that all requests are forwarded to 8.8.8.8. If the response is NXDOMAIN, alternate will forward the request to 192.168.1.1:53, and reply to client accordingly.

. {
	forward . 8.8.8.8
	alternate NXDOMAIN . 192.168.1.1:53
	log
}

Alternate with original request used

The following specify that original query will be forwarded to 192.168.1.1:53 if 8.8.8.8 response is NXDOMAIN. original means no changes from next plugins on request. With no original flag alternate will forward request with EDNS0 option (set by rewrite).

. {
	forward . 8.8.8.8
    rewrite edns0 local set 0xffee 0x61626364
	alternate original NXDOMAIN . 192.168.1.1:53
	log
}

Multiple alternates

Multiple alternates can be specified, as long as they serve unique error responses.

. {
    forward . 8.8.8.8
    alternate NXDOMAIN . 192.168.1.1:53
    alternate REFUSED . 192.168.100.1:53
    alternate original SERVFAIL . 192.168.100.1:53
    log
}

Additional forward parameters

You can specify additional forward parameters for each of the alternate upstreams.

. {
    forward . 8.8.8.8
    alternate NXDOMAIN . 192.168.1.1:53 192.168.1.2:53 {
        max_fails 5
        force_tcp
    }
    log
}

rsvalerio/alternate