plugin/view: Advanced routing interface and new 'view' plugin (#5538)

* introduce new interface "dnsserver.Viewer", that allows a plugin implementing it to decide if a query should be routed into its server block.
* add new plugin "view", that uses the new interface to enable a user to define expression based conditions that must be met for a query to be routed to its server block.

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

core/dnsserver/config.go

@@ -1,12 +1,14 @@
 package dnsserver
 
 import (
+	"context"
 	"crypto/tls"
 	"fmt"
 	"net/http"
 
 	"github.com/coredns/caddy"
 	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/coredns/request"
 )
 
 // Config configuration for a single server.
@@ -40,6 +42,14 @@ type Config struct {
 	// may depend on it.
 	HTTPRequestValidateFunc func(*http.Request) bool
 
+	// FilterFuncs is used to further filter access
+	// to this handler. E.g. to limit access to a reverse zone
+	// on a non-octet boundary, i.e. /17
+	FilterFuncs []FilterFunc
+
+	// ViewName is the name of the Viewer PLugin defined in the Config
+	ViewName string
+
 	// TLSConfig when listening for encrypted connections (gRPC, DNS-over-TLS).
 	TLSConfig *tls.Config
 
@@ -60,8 +70,14 @@ type Config struct {
 	// firstConfigInBlock is used to reference the first config in a server block, for the
 	// purpose of sharing single instance of each plugin among all zones in a server block.
 	firstConfigInBlock *Config
+
+	// metaCollector references the first MetadataCollector plugin, if one exists
+	metaCollector MetadataCollector
 }
 
+// FilterFunc is a function that filters requests from the Config
+type FilterFunc func(context.Context, *request.Request) bool
+
 // keyForConfig builds a key for identifying the configs during setup time
 func keyForConfig(blocIndex int, blocKeyIndex int) string {
 	return fmt.Sprintf("%d:%d", blocIndex, blocKeyIndex)