This plugin is deprecated in favor of https://github.com/coredns/alternate, which performs the same function.

fallback - a plugin that sends queries to an alternate set of upstreams if the plugin chain returns specific error messages.

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

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

{
    fallback [original] RCODE PROXY_PARAMS
}

• original is optional flag. If it is set then fallback 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.
• PROXY_PARAMS accepts the same parameters as the proxy plugin https://coredns.io/plugins/proxy.

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

. {
	proxy . 8.8.8.8
	fallback NXDOMAIN . 192.168.1.1:53
	log
}

The following specify that original query will be proxied 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 fallback will proxy request with EDNS0 option (set by rewrite).

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

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

. {
    proxy . 8.8.8.8
    fallback NXDOMAIN . 192.168.1.1:53
    fallback REFUSED . 192.168.100.1:53
    fallback original SERVFAIL . 192.168.100.1:53
    log
}

You can specify additional proxy parameters for each of the fallback upstreams.

. {
    proxy . 8.8.8.8
    fallback NXDOMAIN . 192.168.1.1:53 192.168.1.2:53 {
        protocol dns force_tcp
    }
    log
}