Add idea for forward based on:

    coredns#6549 (comment)

This still needs some work and especially tests

plugin/forward/forward.go

@@ -43,6 +43,8 @@ type Forward struct {
 	from    string
 	ignored []string
 
+	alternateRcodes []int
+
 	tlsConfig     *tls.Config
 	tlsServerName string
 	maxfails      uint32
@@ -194,6 +196,15 @@ func (f *Forward) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg
 			return 0, nil
 		}
 
+		// Check if we have an alternate Rcode defined, check if we match on the code
+		for _, alternateRcode := range f.alternateRcodes {
+			if alternateRcode == ret.Rcode && f.Next != nil { // In case we do not have a Next handler, just continue normally
+				if _, ok := f.Next.(*Forward); ok { // Only continue if the next forwarder is also a Forworder
+					return plugin.NextOrFailure(f.Name(), f.Next, ctx, w, r)
+				}
+			}
+		}
+
 		w.WriteMsg(ret)
 		return 0, nil
 	}

plugin/forward/setup.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"path/filepath"
 	"strconv"
+	"strings"
 	"time"
 
 	"github.com/coredns/caddy"
@@ -289,7 +290,22 @@ func parseBlock(c *caddy.Controller, f *Forward) error {
 		}
 		f.ErrLimitExceeded = errors.New("concurrent queries exceeded maximum " + c.Val())
 		f.maxConcurrent = int64(n)
+	case "alternate":
+		args := c.RemainingArgs()
+		if len(args) == 0 {
+			return c.ArgErr()
+		}
+
+		for _, rcode := range args {
+			var rc int
+			var ok bool
 
+			if rc, ok = dns.StringToRcode[strings.ToUpper(rcode)]; !ok {
+				return fmt.Errorf("%s is not a valid rcode", rcode)
+			}
+
+			f.alternateRcodes = append(f.alternateRcodes, rc)
+		}
 	default:
 		return c.Errf("unknown property '%s'", c.Val())
 	}