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>

plugin/pkg/expression/expression.go

@@ -0,0 +1,47 @@
+package expression
+
+import (
+	"context"
+	"errors"
+	"net"
+
+	"github.com/coredns/coredns/plugin/metadata"
+	"github.com/coredns/coredns/request"
+)
+
+// DefaultEnv returns the default set of custom state variables and functions available to for use in expression evaluation.
+func DefaultEnv(ctx context.Context, state *request.Request) map[string]interface{} {
+	return map[string]interface{}{
+		"incidr": func(ipStr, cidrStr string) (bool, error) {
+			ip := net.ParseIP(ipStr)
+			if ip == nil {
+				return false, errors.New("first argument is not an IP address")
+			}
+			_, cidr, err := net.ParseCIDR(cidrStr)
+			if err != nil {
+				return false, err
+			}
+			return cidr.Contains(ip), nil
+		},
+		"metadata": func(label string) string {
+			f := metadata.ValueFunc(ctx, label)
+			if f == nil {
+				return ""
+			}
+			return f()
+		},
+		"type":        state.Type,
+		"name":        state.Name,
+		"class":       state.Class,
+		"proto":       state.Proto,
+		"size":        state.Len,
+		"client_ip":   state.IP,
+		"port":        state.Port,
+		"id":          func() int { return int(state.Req.Id) },
+		"opcode":      func() int { return state.Req.Opcode },
+		"do":          state.Do,
+		"bufsize":     state.Size,
+		"server_ip":   state.LocalIP,
+		"server_port": state.LocalPort,
+	}
+}