Add middleware/erratic (#471)

This middleware allows playing with responses. Only one type is
implemented: it allows you to drop queries. I.e. withhold the response
from the client.

middleware/erratic/setup.go

@@ -0,0 +1,59 @@
+package erratic
+
+import (
+	"fmt"
+	"strconv"
+
+	"github.com/miekg/coredns/core/dnsserver"
+	"github.com/miekg/coredns/middleware"
+
+	"github.com/mholt/caddy"
+)
+
+func init() {
+	caddy.RegisterPlugin("erratic", caddy.Plugin{
+		ServerType: "dns",
+		Action:     setupErratic,
+	})
+}
+
+func setupErratic(c *caddy.Controller) error {
+	e, err := parseErratic(c)
+	if err != nil {
+		return middleware.Error("erratic", err)
+	}
+
+	dnsserver.GetConfig(c).AddMiddleware(func(next middleware.Handler) middleware.Handler {
+		return e
+	})
+
+	return nil
+}
+
+func parseErratic(c *caddy.Controller) (*Erratic, error) {
+	e := &Erratic{amount: 2}
+	for c.Next() { // 'erratic'
+		for c.NextBlock() {
+			switch c.Val() {
+			case "drop":
+				args := c.RemainingArgs()
+				if len(args) > 1 {
+					return nil, c.ArgErr()
+				}
+
+				if len(args) == 0 {
+					return nil, nil
+				}
+				amount, err := strconv.ParseInt(args[0], 10, 32)
+				if err != nil {
+					return nil, err
+				}
+				if amount < 0 {
+					return nil, fmt.Errorf("illegal amount value given %q", args[0])
+				}
+				e.amount = uint64(amount)
+			}
+		}
+	}
+	return e, nil
+}