feat(secondary): add fallthrough support (#8041)

plugin/secondary/README.md

@@ -27,6 +27,7 @@ A working syntax would be:
 ~~~
 secondary [zones...] {
     transfer from ADDRESS [ADDRESS...]
+    fallthrough [ZONES...]
 }
 ~~~
 
@@ -34,6 +35,11 @@ secondary [zones...] {
    times; if one does not work, another will be tried. Transferring this zone outwards again can be
    done by enabling the *transfer* plugin.
 
+*  `fallthrough` If a query for a record in the zone results in NXDOMAIN, the query will be passed
+   to the next plugin in the chain. If **[ZONES...]** are listed, then only queries for those zones
+   will be subject to fallthrough. This can be useful in split DNS setups where the secondary zone
+   contains only partial records.
+
 When a zone is due to be refreshed (refresh timer fires) a random jitter of 5 seconds is applied,
 before fetching. In the case of retry this will be 2 seconds. If there are any errors during the
 transfer in, the transfer fails; this will be logged.

plugin/secondary/setup.go

@@ -7,6 +7,7 @@ import (
 	"github.com/coredns/coredns/core/dnsserver"
 	"github.com/coredns/coredns/plugin"
 	"github.com/coredns/coredns/plugin/file"
+	"github.com/coredns/coredns/plugin/pkg/fall"
 	clog "github.com/coredns/coredns/plugin/pkg/log"
 	"github.com/coredns/coredns/plugin/pkg/parse"
 	"github.com/coredns/coredns/plugin/pkg/upstream"
@@ -18,12 +19,12 @@ var log = clog.NewWithPlugin("secondary")
 func init() { plugin.Register("secondary", setup) }
 
 func setup(c *caddy.Controller) error {
-	zones, err := secondaryParse(c)
+	zones, fall, err := secondaryParse(c)
 	if err != nil {
 		return plugin.Error("secondary", err)
 	}
 
-	s := &Secondary{file.File{Zones: zones}}
+	s := &Secondary{file.File{Zones: zones, Fall: fall}}
 	var x *transfer.Transfer
 	c.OnStartup(func() error {
 		t := dnsserver.GetConfig(c).Handler("transfer")
@@ -84,9 +85,10 @@ func setup(c *caddy.Controller) error {
 	return nil
 }
 
-func secondaryParse(c *caddy.Controller) (file.Zones, error) {
+func secondaryParse(c *caddy.Controller) (file.Zones, fall.F, error) {
 	z := make(map[string]*file.Zone)
 	names := []string{}
+	fall := fall.F{}
 	for c.Next() {
 		if c.Val() == "secondary" {
 			// secondary [origin]
@@ -105,11 +107,13 @@ func secondaryParse(c *caddy.Controller) (file.Zones, error) {
 					var err error
 					f, err = parse.TransferIn(c)
 					if err != nil {
-						return file.Zones{}, err
+						return file.Zones{}, fall, err
 					}
 					hasTransfer = true
+				case "fallthrough":
+					fall.SetZonesFromArgs(c.RemainingArgs())
 				default:
-					return file.Zones{}, c.Errf("unknown property '%s'", c.Val())
+					return file.Zones{}, fall, c.Errf("unknown property '%s'", c.Val())
 				}
 
 				for _, origin := range origins {
@@ -120,9 +124,9 @@ func secondaryParse(c *caddy.Controller) (file.Zones, error) {
 				}
 			}
 			if !hasTransfer {
-				return file.Zones{}, c.Err("secondary zones require a transfer from property")
+				return file.Zones{}, fall, c.Err("secondary zones require a transfer from property")
 			}
 		}
 	}
-	return file.Zones{Z: z, Names: names}, nil
+	return file.Zones{Z: z, Names: names}, fall, nil
 }

plugin/secondary/setup_test.go

@@ -4,6 +4,7 @@ import (
 	"testing"
 
 	"github.com/coredns/caddy"
+	"github.com/coredns/coredns/plugin/pkg/fall"
 )
 
 func TestSecondaryParse(t *testing.T) {
@@ -12,6 +13,7 @@ func TestSecondaryParse(t *testing.T) {
 		shouldErr      bool
 		transferFrom   string
 		zones          []string
+		fall           fall.F
 	}{
 		{
 			`secondary {
@@ -20,6 +22,7 @@ func TestSecondaryParse(t *testing.T) {
 			false,
 			"127.0.0.1:53",
 			nil,
+			fall.F{},
 		},
 		{
 			`secondary example.org {
@@ -28,12 +31,14 @@ func TestSecondaryParse(t *testing.T) {
 			false,
 			"127.0.0.1:53",
 			[]string{"example.org."},
+			fall.F{},
 		},
 		{
 			`secondary`,
 			true,
 			"",
 			nil,
+			fall.F{},
 		},
 		{
 			`secondary example.org {
@@ -42,12 +47,35 @@ func TestSecondaryParse(t *testing.T) {
 			true,
 			"",
 			nil,
+			fall.F{},
+		},
+		// fallthrough: bare (all zones)
+		{
+			`secondary {
+				transfer from 127.0.0.1
+				fallthrough
+			}`,
+			false,
+			"127.0.0.1:53",
+			nil,
+			fall.Root,
+		},
+		// fallthrough: specific zone
+		{
+			`secondary example.org {
+				transfer from 127.0.0.1
+				fallthrough example.org
+			}`,
+			false,
+			"127.0.0.1:53",
+			[]string{"example.org."},
+			fall.F{Zones: []string{"example.org."}},
 		},
 	}
 
 	for i, test := range tests {
 		c := caddy.NewTestController("dns", test.inputFileRules)
-		s, err := secondaryParse(c)
+		s, f, err := secondaryParse(c)
 
 		if err == nil && test.shouldErr {
 			t.Fatalf("Test %d expected errors, but got no error", i)
@@ -67,5 +95,9 @@ func TestSecondaryParse(t *testing.T) {
 				t.Fatalf("Test %d transform from names don't match expected %q, but got %q", i, test.transferFrom, x)
 			}
 		}
+
+		if !f.Equal(test.fall) {
+			t.Fatalf("Test %d fallthrough not equal: expected %v, got %v", i, test.fall, f)
+		}
 	}
 }