Add alternate option to forward plugin

Allows the forward plugin to execute the next plugin based on the return code. Similar to the externally mainted alternate plugin https://github.com/coredns/alternate

Based on the idea of chrisohaver@ in coredns#6549 (comment)
Also incoperated the request to rename `alternate` to `next` as an option

I am having issues adding a proper test for functionality. Primarily, I do not know the code base enough and having multiple `dnstest.NewServer` with ResponseWriter does not work. From my testing these are "Singletons'' and only the last defined response writer is used for all servers

Signed-off-by: Jasper Bernhardt <jasper.bernhardt@live.de>

plugin/forward/README.md

@@ -50,6 +50,7 @@ forward FROM TO... {
     policy random|round_robin|sequential
     health_check DURATION [no_rec] [domain FQDN]
     max_concurrent MAX
+    next RCODE_1 [RCODE_2] [RCODE_3...]
 }
 ~~~
 
@@ -95,6 +96,7 @@ forward FROM TO... {
   response does not count as a health failure. When choosing a value for **MAX**, pick a number
   at least greater than the expected *upstream query rate* * *latency* of the upstream servers.
   As an upper bound for **MAX**, consider that each concurrent query will use about 2kb of memory.
+* `next` If the `RCODE` (i.e. `NXDOMAIN`) is returned by the remote then execute the next plugin. If no next plugin is defined, or the next plugin is not a `forward` plugin, this setting is ignored
 
 Also note the TLS config is "global" for the whole forwarding proxy if you need a different
 `tls_servername` for different upstreams you're out of luck.
@@ -268,6 +270,21 @@ Or when you have multiple DoT upstreams with different `tls_servername`s, you ca
 }
 ~~~
 
+The following would try 1.2.3.4 first. If the response is `NXDOMAIN`, try 5.6.7.8. If the response from 5.6.7.8 is `NXDOMAIN`, try 9.0.1.2.
+
+~~~ corefile
+. {
+  forward . 1.2.3.4 {
+    next NXDOMAIN
+  }
+  forward . 5.6.7.8 {
+    next NXDOMAIN
+  }
+  forward . 9.0.1.2 {
+  }
+}
+~~~
+
 ## See Also
 
 [RFC 7858](https://tools.ietf.org/html/rfc7858) for DNS over TLS.

plugin/forward/forward.go

@@ -43,6 +43,8 @@ type Forward struct {
 	from    string
 	ignored []string
 
+	nextAlternateRcodes []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.nextAlternateRcodes {
+			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 "next":
+		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.nextAlternateRcodes = append(f.nextAlternateRcodes, rc)
+		}
 	default:
 		return c.Errf("unknown property '%s'", c.Val())
 	}

plugin/forward/setup_test.go

@@ -342,3 +342,44 @@ func TestMultiForward(t *testing.T) {
 		t.Error("expected third plugin to be last, but Next is not nil")
 	}
 }
+func TestNextAlternate(t *testing.T) {
+	testsValid := []struct {
+		input    string
+		expected []int
+	}{
+		{"forward . 127.0.0.1 {\nnext NXDOMAIN\n}\n", []int{dns.RcodeNameError}},
+		{"forward . 127.0.0.1 {\nnext SERVFAIL\n}\n", []int{dns.RcodeServerFailure}},
+		{"forward . 127.0.0.1 {\nnext NXDOMAIN SERVFAIL\n}\n", []int{dns.RcodeNameError, dns.RcodeServerFailure}},
+		{"forward . 127.0.0.1 {\nnext NXDOMAIN SERVFAIL REFUSED\n}\n", []int{dns.RcodeNameError, dns.RcodeServerFailure, dns.RcodeRefused}},
+	}
+	for i, test := range testsValid {
+		c := caddy.NewTestController("dns", test.input)
+		f, err := parseForward(c)
+		forward := f[0]
+		if err != nil {
+			t.Errorf("Test %d: %v", i, err)
+		}
+		if len(forward.nextAlternateRcodes) != len(test.expected) {
+			t.Errorf("Test %d: expected %d next rcodes, got %d", i, len(test.expected), len(forward.nextAlternateRcodes))
+		}
+		for j, rcode := range forward.nextAlternateRcodes {
+			if rcode != test.expected[j] {
+				t.Errorf("Test %d: expected next rcode %d, got %d", i, test.expected[j], rcode)
+			}
+		}
+	}
+
+	testsInvalid := []string{
+		"forward . 127.0.0.1 {\nnext\n}\n",
+		"forward . 127.0.0.1 {\nnext INVALID\n}\n",
+		"forward . 127.0.0.1 {\nnext NXDOMAIN INVALID\n}\n",
+	}
+	for i, test := range testsInvalid {
+		c := caddy.NewTestController("dns", test)
+		_, err := parseForward(c)
+		if err == nil {
+			t.Errorf("Test %d: expected error, got nil", i)
+		}
+	}
+
+}