Plugin Alternate is able to selectively forward the query to another upstream server, depending the error result provided by the initial resolver
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.
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.
{
alternate [original] RCODE_1[,RCODE_2,RCODE_3...] . DNS_RESOLVERS
}
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.When building CoreDNS with this plugin, alternate should be positioned before forward in /plugin.cfg
.
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
}
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 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
}