Description

The alternate plugin is able to selectively forward queries to another upstream server, depending the error result provided by the initial resolver. It 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.

The alternate plugin supports only DNS protocol and random policy w/o additional forward parameters, so following directives will fail:

. {
    forward . 8.8.8.8
    alternate NXDOMAIN . tls://192.168.1.1:853 {
        policy sequential
    }
}

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_1[,RCODE_2,RCODE_3...] . DNS_RESOLVERS
}

• 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. At least one rcode is required, but multiple rcodes may be specified, delimited by commas.
• DNS_RESOLVERS accepts dns resolvers list.

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 original SERVFAIL,REFUSED . 192.168.100.1:53
    log
}